Merge pull request #2215 from Kobzol/pull

rustc pull

117 files changed, 2382 insertions, 1608 deletions

diff --git a/library/Cargo.lock b/library/Cargo.lock
index 207c744ee22..7b9081d46a0 100644
--- a/library/Cargo.lock
+++ b/library/Cargo.lock
@@ -61,9 +61,9 @@ dependencies = [
 
 [[package]]
 name = "compiler_builtins"
-version = "0.1.140"
+version = "0.1.143"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "df14d41c5d172a886df3753d54238eefb0f61c96cbd8b363c33ccc92c457bee3"
+checksum = "c85ba2077e3eab3dd81be4ece6b7fb2ad0887c1fb813e9a45400baf75c6c7c29"
 dependencies = [
  "cc",
  "rustc-std-workspace-core",
@@ -166,9 +166,9 @@ dependencies = [
 
 [[package]]
 name = "miniz_oxide"
-version = "0.8.2"
+version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4ffbe83022cedc1d264172192511ae958937694cd57ce297164951b8b3568394"
+checksum = "b8402cab7aefae129c6977bb0ff1b8fd9a04eb5b51efc50a70bea51cda0c7924"
 dependencies = [
  "adler2",
  "compiler_builtins",
diff --git a/library/alloc/Cargo.toml b/library/alloc/Cargo.toml
index 07596fa16f9..96caac890a3 100644
--- a/library/alloc/Cargo.toml
+++ b/library/alloc/Cargo.toml
@@ -10,7 +10,7 @@ edition = "2021"
 
 [dependencies]
 core = { path = "../core" }
-compiler_builtins = { version = "=0.1.140", features = ['rustc-dep-of-std'] }
+compiler_builtins = { version = "=0.1.143", features = ['rustc-dep-of-std'] }
 
 [dev-dependencies]
 rand = { version = "0.8.5", default-features = false, features = ["alloc"] }
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 0f66217b5cb..1b5e44a9134 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -2028,8 +2028,7 @@ impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
     }
 }
 
-#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+#[stable(feature = "async_closure", since = "1.85.0")]
 impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args> for Box<F, A> {
     type Output = F::Output;
     type CallOnceFuture = F::CallOnceFuture;
@@ -2039,8 +2038,7 @@ impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args>
     }
 }
 
-#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+#[stable(feature = "async_closure", since = "1.85.0")]
 impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> for Box<F, A> {
     type CallRefFuture<'a>
         = F::CallRefFuture<'a>
@@ -2052,8 +2050,7 @@ impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> f
     }
 }
 
-#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+#[stable(feature = "async_closure", since = "1.85.0")]
 impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box<F, A> {
     extern "rust-call" fn async_call(&self, args: Args) -> Self::CallRefFuture<'_> {
         F::async_call(self, args)
diff --git a/library/alloc/src/collections/btree/node.rs b/library/alloc/src/collections/btree/node.rs
index 0c93eff0d20..4057657632b 100644
--- a/library/alloc/src/collections/btree/node.rs
+++ b/library/alloc/src/collections/btree/node.rs
@@ -383,9 +383,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
     /// Borrows a view into the keys stored in the node.
     pub fn keys(&self) -> &[K] {
         let leaf = self.into_leaf();
-        unsafe {
-            MaybeUninit::slice_assume_init_ref(leaf.keys.get_unchecked(..usize::from(leaf.len)))
-        }
+        unsafe { leaf.keys.get_unchecked(..usize::from(leaf.len)).assume_init_ref() }
     }
 }
 
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index aff90f5abb3..b4f08debc93 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -91,7 +91,6 @@
 //
 // Library features:
 // tidy-alphabetical-start
-#![cfg_attr(bootstrap, feature(async_closure))]
 #![cfg_attr(test, feature(str_as_str))]
 #![feature(alloc_layout_extra)]
 #![feature(allocator_api)]
@@ -127,6 +126,7 @@
 #![feature(local_waker)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array_transpose)]
+#![feature(nonnull_provenance)]
 #![feature(panic_internals)]
 #![feature(pattern)]
 #![feature(pin_coerce_unsized_trait)]
@@ -143,6 +143,7 @@
 #![feature(slice_range)]
 #![feature(std_internals)]
 #![feature(str_internals)]
+#![feature(temporary_niche_types)]
 #![feature(trusted_fused)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]
diff --git a/library/alloc/src/macros.rs b/library/alloc/src/macros.rs
index 6ee3907cc8e..c000fd6f4ef 100644
--- a/library/alloc/src/macros.rs
+++ b/library/alloc/src/macros.rs
@@ -48,8 +48,8 @@ macro_rules! vec {
     );
     ($($x:expr),+ $(,)?) => (
         <[_]>::into_vec(
-            // Using the intrinsic produces a dramatic improvement in compile
-            // time when constructing arrays with many elements.
+            // Using the intrinsic produces a dramatic improvement in stack usage for
+            // unoptimized programs using this code path to construct large Vecs.
             $crate::boxed::box_new([$($x),+])
         )
     );
diff --git a/library/alloc/src/raw_vec.rs b/library/alloc/src/raw_vec.rs
index e93ff2f9023..ad86bf4bf07 100644
--- a/library/alloc/src/raw_vec.rs
+++ b/library/alloc/src/raw_vec.rs
@@ -33,21 +33,15 @@ enum AllocInit {
     Zeroed,
 }
 
-#[repr(transparent)]
-#[cfg_attr(target_pointer_width = "16", rustc_layout_scalar_valid_range_end(0x7fff))]
-#[cfg_attr(target_pointer_width = "32", rustc_layout_scalar_valid_range_end(0x7fff_ffff))]
-#[cfg_attr(target_pointer_width = "64", rustc_layout_scalar_valid_range_end(0x7fff_ffff_ffff_ffff))]
-struct Cap(usize);
+type Cap = core::num::niche_types::UsizeNoHighBit;
 
-impl Cap {
-    const ZERO: Cap = unsafe { Cap(0) };
+const ZERO_CAP: Cap = unsafe { Cap::new_unchecked(0) };
 
-    /// `Cap(cap)`, except if `T` is a ZST then `Cap::ZERO`.
-    ///
-    /// # Safety: cap must be <= `isize::MAX`.
-    unsafe fn new<T>(cap: usize) -> Self {
-        if T::IS_ZST { Cap::ZERO } else { unsafe { Self(cap) } }
-    }
+/// `Cap(cap)`, except if `T` is a ZST then `Cap::ZERO`.
+///
+/// # Safety: cap must be <= `isize::MAX`.
+unsafe fn new_cap<T>(cap: usize) -> Cap {
+    if T::IS_ZST { ZERO_CAP } else { unsafe { Cap::new_unchecked(cap) } }
 }
 
 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
@@ -257,7 +251,7 @@ impl<T, A: Allocator> RawVec<T, A> {
         // SAFETY: Precondition passed to the caller
         unsafe {
             let ptr = ptr.cast();
-            let capacity = Cap::new::<T>(capacity);
+            let capacity = new_cap::<T>(capacity);
             Self {
                 inner: RawVecInner::from_raw_parts_in(ptr, capacity, alloc),
                 _marker: PhantomData,
@@ -275,7 +269,7 @@ impl<T, A: Allocator> RawVec<T, A> {
         // SAFETY: Precondition passed to the caller
         unsafe {
             let ptr = ptr.cast();
-            let capacity = Cap::new::<T>(capacity);
+            let capacity = new_cap::<T>(capacity);
             Self { inner: RawVecInner::from_nonnull_in(ptr, capacity, alloc), _marker: PhantomData }
         }
     }
@@ -410,7 +404,7 @@ impl<A: Allocator> RawVecInner<A> {
     const fn new_in(alloc: A, align: usize) -> Self {
         let ptr = unsafe { core::mem::transmute(align) };
         // `cap: 0` means "unallocated". zero-sized types are ignored.
-        Self { ptr, cap: Cap::ZERO, alloc }
+        Self { ptr, cap: ZERO_CAP, alloc }
     }
 
     #[cfg(not(no_global_oom_handling))]
@@ -483,7 +477,11 @@ impl<A: Allocator> RawVecInner<A> {
         // Allocators currently return a `NonNull<[u8]>` whose length
         // matches the size requested. If that ever changes, the capacity
         // here should change to `ptr.len() / mem::size_of::<T>()`.
-        Ok(Self { ptr: Unique::from(ptr.cast()), cap: unsafe { Cap(capacity) }, alloc })
+        Ok(Self {
+            ptr: Unique::from(ptr.cast()),
+            cap: unsafe { Cap::new_unchecked(capacity) },
+            alloc,
+        })
     }
 
     #[inline]
@@ -508,7 +506,7 @@ impl<A: Allocator> RawVecInner<A> {
 
     #[inline]
     const fn capacity(&self, elem_size: usize) -> usize {
-        if elem_size == 0 { usize::MAX } else { self.cap.0 }
+        if elem_size == 0 { usize::MAX } else { self.cap.as_inner() }
     }
 
     #[inline]
@@ -518,7 +516,7 @@ impl<A: Allocator> RawVecInner<A> {
 
     #[inline]
     fn current_memory(&self, elem_layout: Layout) -> Option<(NonNull<u8>, Layout)> {
-        if elem_layout.size() == 0 || self.cap.0 == 0 {
+        if elem_layout.size() == 0 || self.cap.as_inner() == 0 {
             None
         } else {
             // We could use Layout::array here which ensures the absence of isize and usize overflows
@@ -526,7 +524,7 @@ impl<A: Allocator> RawVecInner<A> {
             // has already been allocated so we know it can't overflow and currently Rust does not
             // support such types. So we can do better by skipping some checks and avoid an unwrap.
             unsafe {
-                let alloc_size = elem_layout.size().unchecked_mul(self.cap.0);
+                let alloc_size = elem_layout.size().unchecked_mul(self.cap.as_inner());
                 let layout = Layout::from_size_align_unchecked(alloc_size, elem_layout.align());
                 Some((self.ptr.into(), layout))
             }
@@ -562,7 +560,7 @@ impl<A: Allocator> RawVecInner<A> {
     #[inline]
     #[track_caller]
     fn grow_one(&mut self, elem_layout: Layout) {
-        if let Err(err) = self.grow_amortized(self.cap.0, 1, elem_layout) {
+        if let Err(err) = self.grow_amortized(self.cap.as_inner(), 1, elem_layout) {
             handle_error(err);
         }
     }
@@ -627,7 +625,7 @@ impl<A: Allocator> RawVecInner<A> {
         // the size requested. If that ever changes, the capacity here should
         // change to `ptr.len() / mem::size_of::<T>()`.
         self.ptr = Unique::from(ptr.cast());
-        self.cap = unsafe { Cap(cap) };
+        self.cap = unsafe { Cap::new_unchecked(cap) };
     }
 
     fn grow_amortized(
@@ -650,7 +648,7 @@ impl<A: Allocator> RawVecInner<A> {
 
         // This guarantees exponential growth. The doubling cannot overflow
         // because `cap <= isize::MAX` and the type of `cap` is `usize`.
-        let cap = cmp::max(self.cap.0 * 2, required_cap);
+        let cap = cmp::max(self.cap.as_inner() * 2, required_cap);
         let cap = cmp::max(min_non_zero_cap(elem_layout.size()), cap);
 
         let new_layout = layout_array(cap, elem_layout)?;
@@ -719,7 +717,7 @@ impl<A: Allocator> RawVecInner<A> {
             unsafe { self.alloc.deallocate(ptr, layout) };
             self.ptr =
                 unsafe { Unique::new_unchecked(ptr::without_provenance_mut(elem_layout.align())) };
-            self.cap = Cap::ZERO;
+            self.cap = ZERO_CAP;
         } else {
             let ptr = unsafe {
                 // Layout cannot overflow here because it would have
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index e014404eff3..ae3318b839d 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -252,6 +252,7 @@ use core::intrinsics::abort;
 use core::iter;
 use core::marker::{PhantomData, Unsize};
 use core::mem::{self, ManuallyDrop, align_of_val_raw};
+use core::num::NonZeroUsize;
 use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, LegacyReceiver};
 use core::panic::{RefUnwindSafe, UnwindSafe};
 #[cfg(not(no_global_oom_handling))]
@@ -1789,7 +1790,7 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     /// let x: Rc<&str> = Rc::new("Hello, world!");
     /// {
     ///     let s = String::from("Oh, no!");
-    ///     let mut y: Rc<&str> = x.clone().into();
+    ///     let mut y: Rc<&str> = x.clone();
     ///     unsafe {
     ///         // this is Undefined Behavior, because x's inner type
     ///         // is &'long str, not &'short str
@@ -3027,12 +3028,7 @@ impl<T> Weak<T> {
     #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")]
     #[must_use]
     pub const fn new() -> Weak<T> {
-        Weak {
-            ptr: unsafe {
-                NonNull::new_unchecked(ptr::without_provenance_mut::<RcInner<T>>(usize::MAX))
-            },
-            alloc: Global,
-        }
+        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc: Global }
     }
 }
 
@@ -3054,12 +3050,7 @@ impl<T, A: Allocator> Weak<T, A> {
     #[inline]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn new_in(alloc: A) -> Weak<T, A> {
-        Weak {
-            ptr: unsafe {
-                NonNull::new_unchecked(ptr::without_provenance_mut::<RcInner<T>>(usize::MAX))
-            },
-            alloc,
-        }
+        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc }
     }
 }
 
@@ -3717,7 +3708,11 @@ pub struct UniqueRc<
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
 > {
     ptr: NonNull<RcInner<T>>,
-    phantom: PhantomData<RcInner<T>>,
+    // Define the ownership of `RcInner<T>` for drop-check
+    _marker: PhantomData<RcInner<T>>,
+    // Invariance is necessary for soundness: once other `Weak`
+    // references exist, we already have a form of shared mutability!
+    _marker2: PhantomData<*mut T>,
     alloc: A,
 }
 
@@ -4003,7 +3998,7 @@ impl<T, A: Allocator> UniqueRc<T, A> {
             },
             alloc,
         ));
-        Self { ptr: ptr.into(), phantom: PhantomData, alloc }
+        Self { ptr: ptr.into(), _marker: PhantomData, _marker2: PhantomData, alloc }
     }
 }
 
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index b34a6d3f660..8eee7cff208 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -18,6 +18,7 @@ use core::intrinsics::abort;
 use core::iter;
 use core::marker::{PhantomData, Unsize};
 use core::mem::{self, ManuallyDrop, align_of_val_raw};
+use core::num::NonZeroUsize;
 use core::ops::{CoerceUnsized, Deref, DerefPure, DispatchFromDyn, LegacyReceiver};
 use core::panic::{RefUnwindSafe, UnwindSafe};
 use core::pin::{Pin, PinCoerceUnsized};
@@ -1396,6 +1397,8 @@ impl<T: ?Sized> Arc<T> {
     /// different types. See [`mem::transmute`][transmute] for more information
     /// on what restrictions apply in this case.
     ///
+    /// The raw pointer must point to a block of memory allocated by the global allocator.
+    ///
     /// The user of `from_raw` has to make sure a specific value of `T` is only
     /// dropped once.
     ///
@@ -1451,7 +1454,8 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// The pointer must have been obtained through `Arc::into_raw`, and the
     /// associated `Arc` instance must be valid (i.e. the strong count must be at
-    /// least 1) for the duration of this method.
+    /// least 1) for the duration of this method, and `ptr` must point to a block of memory
+    /// allocated by the global allocator.
     ///
     /// # Examples
     ///
@@ -1485,7 +1489,8 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// The pointer must have been obtained through `Arc::into_raw`, and the
     /// associated `Arc` instance must be valid (i.e. the strong count must be at
-    /// least 1) when invoking this method. This method can be used to release the final
+    /// least 1) when invoking this method, and `ptr` must point to a block of memory
+    /// allocated by the global allocator. This method can be used to release the final
     /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
     /// released.
     ///
@@ -2468,7 +2473,7 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
     /// let x: Arc<&str> = Arc::new("Hello, world!");
     /// {
     ///     let s = String::from("Oh, no!");
-    ///     let mut y: Arc<&str> = x.clone().into();
+    ///     let mut y: Arc<&str> = x.clone();
     ///     unsafe {
     ///         // this is Undefined Behavior, because x's inner type
     ///         // is &'long str, not &'short str
@@ -2687,12 +2692,7 @@ impl<T> Weak<T> {
     #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")]
     #[must_use]
     pub const fn new() -> Weak<T> {
-        Weak {
-            ptr: unsafe {
-                NonNull::new_unchecked(ptr::without_provenance_mut::<ArcInner<T>>(usize::MAX))
-            },
-            alloc: Global,
-        }
+        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc: Global }
     }
 }
 
@@ -2717,12 +2717,7 @@ impl<T, A: Allocator> Weak<T, A> {
     #[inline]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn new_in(alloc: A) -> Weak<T, A> {
-        Weak {
-            ptr: unsafe {
-                NonNull::new_unchecked(ptr::without_provenance_mut::<ArcInner<T>>(usize::MAX))
-            },
-            alloc,
-        }
+        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc }
     }
 }
 
diff --git a/library/alloc/src/vec/is_zero.rs b/library/alloc/src/vec/is_zero.rs
index ba57d940d8c..a3ddd6f6e23 100644
--- a/library/alloc/src/vec/is_zero.rs
+++ b/library/alloc/src/vec/is_zero.rs
@@ -40,19 +40,8 @@ impl_is_zero!(char, |x| x == '\0');
 impl_is_zero!(f32, |x: f32| x.to_bits() == 0);
 impl_is_zero!(f64, |x: f64| x.to_bits() == 0);
 
-unsafe impl<T> IsZero for *const T {
-    #[inline]
-    fn is_zero(&self) -> bool {
-        (*self).is_null()
-    }
-}
-
-unsafe impl<T> IsZero for *mut T {
-    #[inline]
-    fn is_zero(&self) -> bool {
-        (*self).is_null()
-    }
-}
+// `IsZero` cannot be soundly implemented for pointers because of provenance
+// (see #135338).
 
 unsafe impl<T: IsZero, const N: usize> IsZero for [T; N] {
     #[inline]
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 3a706d5f36b..cd2afd7a473 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -3587,7 +3587,7 @@ impl<T, A: Allocator> Vec<T, A> {
     /// with the given `replace_with` iterator and yields the removed items.
     /// `replace_with` does not need to be the same length as `range`.
     ///
-    /// `range` is removed even if the iterator is not consumed until the end.
+    /// `range` is removed even if the `Splice` iterator is not consumed before it is dropped.
     ///
     /// It is unspecified how many elements are removed from the vector
     /// if the `Splice` value is leaked.
@@ -3613,8 +3613,18 @@ impl<T, A: Allocator> Vec<T, A> {
     /// let mut v = vec![1, 2, 3, 4];
     /// let new = [7, 8, 9];
     /// let u: Vec<_> = v.splice(1..3, new).collect();
-    /// assert_eq!(v, &[1, 7, 8, 9, 4]);
-    /// assert_eq!(u, &[2, 3]);
+    /// assert_eq!(v, [1, 7, 8, 9, 4]);
+    /// assert_eq!(u, [2, 3]);
+    /// ```
+    ///
+    /// Using `splice` to insert new items into a vector efficiently at a specific position
+    /// indicated by an empty range:
+    ///
+    /// ```
+    /// let mut v = vec![1, 5];
+    /// let new = [2, 3, 4];
+    /// v.splice(1..1, new);
+    /// assert_eq!(v, [1, 2, 3, 4, 5]);
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
diff --git a/library/alloc/tests/vec.rs b/library/alloc/tests/vec.rs
index 2e654d3d1ff..b24daec2968 100644
--- a/library/alloc/tests/vec.rs
+++ b/library/alloc/tests/vec.rs
@@ -2742,3 +2742,13 @@ fn max_swap_remove() {
     let mut v = vec![0];
     v.swap_remove(usize::MAX);
 }
+
+// Regression test for #135338
+#[test]
+fn vec_null_ptr_roundtrip() {
+    let ptr = std::ptr::from_ref(&42);
+    let zero = ptr.with_addr(0);
+    let roundtripped = vec![zero; 1].pop().unwrap();
+    let new = roundtripped.with_addr(ptr.addr());
+    unsafe { new.read() };
+}
diff --git a/library/core/src/alloc/layout.rs b/library/core/src/alloc/layout.rs
index e6f39db9dce..17f4d68867e 100644
--- a/library/core/src/alloc/layout.rs
+++ b/library/core/src/alloc/layout.rs
@@ -178,7 +178,7 @@ impl Layout {
     /// allocate backing structure for `T` (which could be a trait
     /// or other unsized type like a slice).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.85.0")]
     #[must_use]
     #[inline]
     pub const fn for_value<T: ?Sized>(t: &T) -> Self {
@@ -233,8 +233,7 @@ impl Layout {
     #[must_use]
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
-        // SAFETY: align is guaranteed to be non-zero
-        unsafe { NonNull::new_unchecked(crate::ptr::without_provenance_mut::<u8>(self.align())) }
+        NonNull::without_provenance(self.align.as_nonzero())
     }
 
     /// Creates a layout describing the record that can hold a value
@@ -252,7 +251,7 @@ impl Layout {
     /// Returns an error if the combination of `self.size()` and the given
     /// `align` violates the conditions listed in [`Layout::from_size_align`].
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.85.0")]
     #[inline]
     pub const fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
         if let Some(align) = Alignment::new(align) {
@@ -327,7 +326,7 @@ impl Layout {
     /// This is equivalent to adding the result of `padding_needed_for`
     /// to the layout's current size.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.85.0")]
     #[must_use = "this returns a new `Layout`, \
                   without modifying the original"]
     #[inline]
@@ -426,7 +425,7 @@ impl Layout {
     /// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));
     /// ```
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.85.0")]
     #[inline]
     pub const fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
         let new_align = Alignment::max(self.align, next.align);
@@ -489,7 +488,7 @@ impl Layout {
     /// On arithmetic overflow or when the total size would exceed
     /// `isize::MAX`, returns `LayoutError`.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
-    #[rustc_const_stable(feature = "const_alloc_layout", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.85.0")]
     #[inline]
     pub const fn array<T>(n: usize) -> Result<Self, LayoutError> {
         // Reduce the amount of code we need to monomorphize per `T`.
diff --git a/library/core/src/arch.rs b/library/core/src/arch.rs
index 95d88c7f679..cb130f60cec 100644
--- a/library/core/src/arch.rs
+++ b/library/core/src/arch.rs
@@ -65,7 +65,6 @@ pub macro global_asm("assembly template", $(operands,)* $(options($(option),*))?
 // When stabilizing this, update the comment on `core::intrinsics::breakpoint`.
 #[unstable(feature = "breakpoint", issue = "133724")]
 #[inline(always)]
-#[cfg(not(bootstrap))]
 pub fn breakpoint() {
     core::intrinsics::breakpoint();
 }
diff --git a/library/core/src/array/iter.rs b/library/core/src/array/iter.rs
index 9ce0eb61e08..1edade41597 100644
--- a/library/core/src/array/iter.rs
+++ b/library/core/src/array/iter.rs
@@ -214,7 +214,7 @@ impl<T, const N: usize> IntoIter<T, N> {
         // SAFETY: We know that all elements within `alive` are properly initialized.
         unsafe {
             let slice = self.data.get_unchecked(self.alive.clone());
-            MaybeUninit::slice_assume_init_ref(slice)
+            slice.assume_init_ref()
         }
     }
 
@@ -224,7 +224,7 @@ impl<T, const N: usize> IntoIter<T, N> {
         // SAFETY: We know that all elements within `alive` are properly initialized.
         unsafe {
             let slice = self.data.get_unchecked_mut(self.alive.clone());
-            MaybeUninit::slice_assume_init_mut(slice)
+            slice.assume_init_mut()
         }
     }
 }
@@ -285,7 +285,7 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
         // SAFETY: These elements are currently initialized, so it's fine to drop them.
         unsafe {
             let slice = self.data.get_unchecked_mut(range_to_drop);
-            ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
+            slice.assume_init_drop();
         }
 
         NonZero::new(remaining).map_or(Ok(()), Err)
@@ -340,7 +340,7 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
         // SAFETY: These elements are currently initialized, so it's fine to drop them.
         unsafe {
             let slice = self.data.get_unchecked_mut(range_to_drop);
-            ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
+            slice.assume_init_drop();
         }
 
         NonZero::new(remaining).map_or(Ok(()), Err)
diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index 95c1eb460cd..2ae5ded1fd5 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -911,9 +911,7 @@ impl<T> Drop for Guard<'_, T> {
 
         // SAFETY: this slice will contain only initialized objects.
         unsafe {
-            crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
-                self.array_mut.get_unchecked_mut(..self.initialized),
-            ));
+            self.array_mut.get_unchecked_mut(..self.initialized).assume_init_drop();
         }
     }
 }
diff --git a/library/core/src/ffi/c_str.rs b/library/core/src/ffi/c_str.rs
index 8831443a10f..7180593edf0 100644
--- a/library/core/src/ffi/c_str.rs
+++ b/library/core/src/ffi/c_str.rs
@@ -124,37 +124,25 @@ pub struct CStr {
 ///
 /// let _: FromBytesWithNulError = CStr::from_bytes_with_nul(b"f\0oo").unwrap_err();
 /// ```
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
 #[stable(feature = "core_c_str", since = "1.64.0")]
-pub struct FromBytesWithNulError {
-    kind: FromBytesWithNulErrorKind,
-}
-
-#[derive(Clone, PartialEq, Eq, Debug)]
-enum FromBytesWithNulErrorKind {
-    InteriorNul(usize),
+pub enum FromBytesWithNulError {
+    /// Data provided contains an interior nul byte at byte `position`.
+    InteriorNul {
+        /// The position of the interior nul byte.
+        position: usize,
+    },
+    /// Data provided is not nul terminated.
     NotNulTerminated,
 }
 
-// FIXME: const stability attributes should not be required here, I think
-impl FromBytesWithNulError {
-    const fn interior_nul(pos: usize) -> FromBytesWithNulError {
-        FromBytesWithNulError { kind: FromBytesWithNulErrorKind::InteriorNul(pos) }
-    }
-    const fn not_nul_terminated() -> FromBytesWithNulError {
-        FromBytesWithNulError { kind: FromBytesWithNulErrorKind::NotNulTerminated }
-    }
-}
-
 #[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
 impl Error for FromBytesWithNulError {
     #[allow(deprecated)]
     fn description(&self) -> &str {
-        match self.kind {
-            FromBytesWithNulErrorKind::InteriorNul(..) => {
-                "data provided contains an interior nul byte"
-            }
-            FromBytesWithNulErrorKind::NotNulTerminated => "data provided is not nul terminated",
+        match self {
+            Self::InteriorNul { .. } => "data provided contains an interior nul byte",
+            Self::NotNulTerminated => "data provided is not nul terminated",
         }
     }
 }
@@ -199,8 +187,8 @@ impl fmt::Display for FromBytesWithNulError {
     #[allow(deprecated, deprecated_in_future)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str(self.description())?;
-        if let FromBytesWithNulErrorKind::InteriorNul(pos) = self.kind {
-            write!(f, " at byte pos {pos}")?;
+        if let Self::InteriorNul { position } = self {
+            write!(f, " at byte pos {position}")?;
         }
         Ok(())
     }
@@ -349,25 +337,25 @@ impl CStr {
     /// use std::ffi::CStr;
     ///
     /// let cstr = CStr::from_bytes_with_nul(b"hello\0");
-    /// assert!(cstr.is_ok());
+    /// assert_eq!(cstr, Ok(c"hello"));
     /// ```
     ///
     /// Creating a `CStr` without a trailing nul terminator is an error:
     ///
     /// ```
-    /// use std::ffi::CStr;
+    /// use std::ffi::{CStr, FromBytesWithNulError};
     ///
     /// let cstr = CStr::from_bytes_with_nul(b"hello");
-    /// assert!(cstr.is_err());
+    /// assert_eq!(cstr, Err(FromBytesWithNulError::NotNulTerminated));
     /// ```
     ///
     /// Creating a `CStr` with an interior nul byte is an error:
     ///
     /// ```
-    /// use std::ffi::CStr;
+    /// use std::ffi::{CStr, FromBytesWithNulError};
     ///
     /// let cstr = CStr::from_bytes_with_nul(b"he\0llo\0");
-    /// assert!(cstr.is_err());
+    /// assert_eq!(cstr, Err(FromBytesWithNulError::InteriorNul { position: 2 }));
     /// ```
     #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
     #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
@@ -379,8 +367,8 @@ impl CStr {
                 // of the byte slice.
                 Ok(unsafe { Self::from_bytes_with_nul_unchecked(bytes) })
             }
-            Some(nul_pos) => Err(FromBytesWithNulError::interior_nul(nul_pos)),
-            None => Err(FromBytesWithNulError::not_nul_terminated()),
+            Some(position) => Err(FromBytesWithNulError::InteriorNul { position }),
+            None => Err(FromBytesWithNulError::NotNulTerminated),
         }
     }
 
diff --git a/library/core/src/future/async_drop.rs b/library/core/src/future/async_drop.rs
index ea6d3b800e6..f1778a4d782 100644
--- a/library/core/src/future/async_drop.rs
+++ b/library/core/src/future/async_drop.rs
@@ -133,9 +133,8 @@ pub trait AsyncDrop {
 }
 
 #[lang = "async_destruct"]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 trait AsyncDestruct {
     type AsyncDestructor: Future<Output = ()>;
 }
diff --git a/library/core/src/hash/mod.rs b/library/core/src/hash/mod.rs
index 84bbf985e8b..7a6630c82d0 100644
--- a/library/core/src/hash/mod.rs
+++ b/library/core/src/hash/mod.rs
@@ -752,11 +752,8 @@ pub struct BuildHasherDefault<H>(marker::PhantomData<fn() -> H>);
 
 impl<H> BuildHasherDefault<H> {
     /// Creates a new BuildHasherDefault for Hasher `H`.
-    #[stable(feature = "build_hasher_default_const_new", since = "CURRENT_RUSTC_VERSION")]
-    #[rustc_const_stable(
-        feature = "build_hasher_default_const_new",
-        since = "CURRENT_RUSTC_VERSION"
-    )]
+    #[stable(feature = "build_hasher_default_const_new", since = "1.85.0")]
+    #[rustc_const_stable(feature = "build_hasher_default_const_new", since = "1.85.0")]
     pub const fn new() -> Self {
         BuildHasherDefault(marker::PhantomData)
     }
diff --git a/library/core/src/intrinsics/fallback.rs b/library/core/src/intrinsics/fallback.rs
index 1779126b180..70484e4d0f2 100644
--- a/library/core/src/intrinsics/fallback.rs
+++ b/library/core/src/intrinsics/fallback.rs
@@ -8,6 +8,7 @@
 #![allow(missing_docs)]
 
 #[const_trait]
+#[rustc_const_unstable(feature = "core_intrinsics_fallbacks", issue = "none")]
 pub trait CarryingMulAdd: Copy + 'static {
     type Unsigned: Copy + 'static;
     fn carrying_mul_add(
diff --git a/library/core/src/intrinsics/mir.rs b/library/core/src/intrinsics/mir.rs
index 834f44c7790..55dcf7cd47e 100644
--- a/library/core/src/intrinsics/mir.rs
+++ b/library/core/src/intrinsics/mir.rs
@@ -233,7 +233,7 @@
 //!
 //!  - Operands implicitly convert to `Use` rvalues.
 //!  - `&`, `&mut`, `addr_of!`, and `addr_of_mut!` all work to create their associated rvalue.
-//!  - [`Discriminant`] and [`CopyForDeref`] have associated functions.
+//!  - [`Discriminant`], [`Len`], and [`CopyForDeref`] have associated functions.
 //!  - Unary and binary operations use their normal Rust syntax - `a * b`, `!c`, etc.
 //!  - The binary operation `Offset` can be created via [`Offset`].
 //!  - Checked binary operations are represented by wrapping the associated binop in [`Checked`].
@@ -401,6 +401,7 @@ define!("mir_storage_dead", fn StorageDead<T>(local: T));
 define!("mir_assume", fn Assume(operand: bool));
 define!("mir_deinit", fn Deinit<T>(place: T));
 define!("mir_checked", fn Checked<T>(binop: T) -> (T, bool));
+define!("mir_len", fn Len<T>(place: T) -> usize);
 define!(
     "mir_ptr_metadata",
     fn PtrMetadata<P: ?Sized>(place: *const P) -> <P as ::core::ptr::Pointee>::Metadata
diff --git a/library/core/src/intrinsics/mod.rs b/library/core/src/intrinsics/mod.rs
index 7d44ea7a66b..41b2ffad668 100644
--- a/library/core/src/intrinsics/mod.rs
+++ b/library/core/src/intrinsics/mod.rs
@@ -1382,22 +1382,10 @@ pub unsafe fn prefetch_write_instruction<T>(_data: *const T, _locality: i32) {
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
 #[rustc_nounwind]
-#[cfg(not(bootstrap))]
 pub fn breakpoint() {
     unreachable!()
 }
 
-/// Executes a breakpoint trap, for inspection by a debugger.
-///
-/// This intrinsic does not have a stable counterpart.
-#[rustc_intrinsic]
-#[rustc_intrinsic_must_be_overridden]
-#[rustc_nounwind]
-#[cfg(bootstrap)]
-pub unsafe fn breakpoint() {
-    unreachable!()
-}
-
 /// Magic intrinsic that derives its meaning from attributes
 /// attached to the function.
 ///
@@ -1909,7 +1897,11 @@ pub const fn forget<T: ?Sized>(_: T) {
 /// }
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_allowed_through_unstable_modules]
+#[cfg_attr(bootstrap, rustc_allowed_through_unstable_modules)]
+#[cfg_attr(
+    not(bootstrap),
+    rustc_allowed_through_unstable_modules = "import this function via `std::mem` instead"
+)]
 #[rustc_const_stable(feature = "const_transmute", since = "1.56.0")]
 #[rustc_diagnostic_item = "transmute"]
 #[rustc_nounwind]
@@ -3323,8 +3315,8 @@ pub const fn mul_with_overflow<T: Copy>(_x: T, _y: T) -> (T, bool) {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_const_unstable(feature = "const_carrying_mul_add", issue = "85532")]
 #[rustc_nounwind]
-#[cfg_attr(not(bootstrap), rustc_intrinsic)]
-#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_is_spec)]
+#[rustc_intrinsic]
+#[miri::intrinsic_fallback_is_spec]
 pub const fn carrying_mul_add<T: ~const fallback::CarryingMulAdd<Unsigned = U>, U>(
     multiplier: T,
     multiplicand: T,
@@ -3969,21 +3961,6 @@ pub const fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
     false
 }
 
-#[rustc_nounwind]
-#[inline]
-#[rustc_intrinsic]
-#[rustc_intrinsic_const_stable_indirect]
-#[rustc_allow_const_fn_unstable(const_swap_nonoverlapping)] // this is anyway not called since CTFE implements the intrinsic
-#[cfg(bootstrap)]
-pub const unsafe fn typed_swap<T>(x: *mut T, y: *mut T) {
-    // SAFETY: The caller provided single non-overlapping items behind
-    // pointers, so swapping them with `count: 1` is fine.
-    unsafe { ptr::swap_nonoverlapping(x, y, 1) };
-}
-
-#[cfg(bootstrap)]
-pub use typed_swap as typed_swap_nonoverlapping;
-
 /// Non-overlapping *typed* swap of a single value.
 ///
 /// The codegen backends will replace this with a better implementation when
@@ -3999,7 +3976,6 @@ pub use typed_swap as typed_swap_nonoverlapping;
 #[rustc_intrinsic]
 #[rustc_intrinsic_const_stable_indirect]
 #[rustc_allow_const_fn_unstable(const_swap_nonoverlapping)] // this is anyway not called since CTFE implements the intrinsic
-#[cfg(not(bootstrap))]
 pub const unsafe fn typed_swap_nonoverlapping<T>(x: *mut T, y: *mut T) {
     // SAFETY: The caller provided single non-overlapping items behind
     // pointers, so swapping them with `count: 1` is fine.
@@ -4353,7 +4329,11 @@ pub const fn ptr_metadata<P: ptr::Pointee<Metadata = M> + ?Sized, M>(_ptr: *cons
 /// [`Vec::append`]: ../../std/vec/struct.Vec.html#method.append
 #[doc(alias = "memcpy")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_allowed_through_unstable_modules]
+#[cfg_attr(bootstrap, rustc_allowed_through_unstable_modules)]
+#[cfg_attr(
+    not(bootstrap),
+    rustc_allowed_through_unstable_modules = "import this function via `std::mem` instead"
+)]
 #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.83.0")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
@@ -4457,7 +4437,11 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
 /// ```
 #[doc(alias = "memmove")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_allowed_through_unstable_modules]
+#[cfg_attr(bootstrap, rustc_allowed_through_unstable_modules)]
+#[cfg_attr(
+    not(bootstrap),
+    rustc_allowed_through_unstable_modules = "import this function via `std::mem` instead"
+)]
 #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.83.0")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
@@ -4540,7 +4524,11 @@ pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
 /// ```
 #[doc(alias = "memset")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_allowed_through_unstable_modules]
+#[cfg_attr(bootstrap, rustc_allowed_through_unstable_modules)]
+#[cfg_attr(
+    not(bootstrap),
+    rustc_allowed_through_unstable_modules = "import this function via `std::mem` instead"
+)]
 #[rustc_const_stable(feature = "const_ptr_write", since = "1.83.0")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
diff --git a/library/core/src/intrinsics/simd.rs b/library/core/src/intrinsics/simd.rs
index f80a60d471c..d03d801b936 100644
--- a/library/core/src/intrinsics/simd.rs
+++ b/library/core/src/intrinsics/simd.rs
@@ -623,7 +623,6 @@ extern "rust-intrinsic" {
     /// and others do not.
     ///
     /// `T` must be a vector of floats.
-    #[cfg(not(bootstrap))]
     #[rustc_nounwind]
     pub fn simd_relaxed_fma<T>(x: T, y: T, z: T) -> T;
 
diff --git a/library/core/src/io/borrowed_buf.rs b/library/core/src/io/borrowed_buf.rs
index 4227e503ba7..f86abf7f1e9 100644
--- a/library/core/src/io/borrowed_buf.rs
+++ b/library/core/src/io/borrowed_buf.rs
@@ -94,7 +94,7 @@ impl<'data> BorrowedBuf<'data> {
         // SAFETY: We only slice the filled part of the buffer, which is always valid
         unsafe {
             let buf = self.buf.get_unchecked(..self.filled);
-            MaybeUninit::slice_assume_init_ref(buf)
+            buf.assume_init_ref()
         }
     }
 
@@ -104,7 +104,7 @@ impl<'data> BorrowedBuf<'data> {
         // SAFETY: We only slice the filled part of the buffer, which is always valid
         unsafe {
             let buf = self.buf.get_unchecked_mut(..self.filled);
-            MaybeUninit::slice_assume_init_mut(buf)
+            buf.assume_init_mut()
         }
     }
 
@@ -114,7 +114,7 @@ impl<'data> BorrowedBuf<'data> {
         // SAFETY: We only slice the filled part of the buffer, which is always valid
         unsafe {
             let buf = self.buf.get_unchecked(..self.filled);
-            MaybeUninit::slice_assume_init_ref(buf)
+            buf.assume_init_ref()
         }
     }
 
@@ -124,7 +124,7 @@ impl<'data> BorrowedBuf<'data> {
         // SAFETY: We only slice the filled part of the buffer, which is always valid
         unsafe {
             let buf = self.buf.get_unchecked_mut(..self.filled);
-            MaybeUninit::slice_assume_init_mut(buf)
+            buf.assume_init_mut()
         }
     }
 
@@ -233,7 +233,7 @@ impl<'a> BorrowedCursor<'a> {
         // SAFETY: We only slice the initialized part of the buffer, which is always valid
         unsafe {
             let buf = self.buf.buf.get_unchecked(self.buf.filled..self.buf.init);
-            MaybeUninit::slice_assume_init_ref(buf)
+            buf.assume_init_ref()
         }
     }
 
@@ -243,7 +243,7 @@ impl<'a> BorrowedCursor<'a> {
         // SAFETY: We only slice the initialized part of the buffer, which is always valid
         unsafe {
             let buf = self.buf.buf.get_unchecked_mut(self.buf.filled..self.buf.init);
-            MaybeUninit::slice_assume_init_mut(buf)
+            buf.assume_init_mut()
         }
     }
 
@@ -344,7 +344,7 @@ impl<'a> BorrowedCursor<'a> {
 
         // SAFETY: we do not de-initialize any of the elements of the slice
         unsafe {
-            MaybeUninit::copy_from_slice(&mut self.as_mut()[..buf.len()], buf);
+            self.as_mut()[..buf.len()].write_copy_of_slice(buf);
         }
 
         // SAFETY: We just added the entire contents of buf to the filled section.
diff --git a/library/core/src/iter/adapters/filter_map.rs b/library/core/src/iter/adapters/filter_map.rs
index cc64ceb13f7..24ec6b1741c 100644
--- a/library/core/src/iter/adapters/filter_map.rs
+++ b/library/core/src/iter/adapters/filter_map.rs
@@ -81,9 +81,7 @@ where
                 if const { crate::mem::needs_drop::<T>() } {
                     // SAFETY: self.initialized is always <= N, which also is the length of the array.
                     unsafe {
-                        core::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
-                            self.array.get_unchecked_mut(..self.initialized),
-                        ));
+                        self.array.get_unchecked_mut(..self.initialized).assume_init_drop();
                     }
                 }
             }
diff --git a/library/core/src/iter/traits/collect.rs b/library/core/src/iter/traits/collect.rs
index 73e6d931060..97bb21c8a36 100644
--- a/library/core/src/iter/traits/collect.rs
+++ b/library/core/src/iter/traits/collect.rs
@@ -472,7 +472,7 @@ macro_rules! spec_tuple_impl {
             #[doc(fake_variadic)]
             #[doc = "This trait is implemented for tuples up to twelve items long. The `impl`s for \
                      1- and 3- through 12-ary tuples were stabilized after 2-tuples, in \
-                     CURRENT_RUSTC_VERSION."]
+                     1.85.0."]
             => ($ty_name, $var_name, $extend_ty_name, $cnt),
         );
     };
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index e31957a1fa7..e845bb34426 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -107,7 +107,6 @@
 //
 // Library features:
 // tidy-alphabetical-start
-#![cfg_attr(bootstrap, feature(do_not_recommend))]
 #![feature(array_ptr_get)]
 #![feature(asm_experimental_arch)]
 #![feature(bigint_helper_methods)]
diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index 402b436d28e..5c04e5a40df 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -224,6 +224,7 @@ pub macro assert_matches {
 ///     }
 /// }
 /// ```
+#[cfg(bootstrap)]
 #[unstable(feature = "cfg_match", issue = "115585")]
 #[rustc_diagnostic_item = "cfg_match"]
 pub macro cfg_match {
@@ -284,6 +285,57 @@ pub macro cfg_match {
     }
 }
 
+/// A macro for defining `#[cfg]` match-like statements.
+///
+/// It is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade of
+/// `#[cfg]` cases, emitting the implementation which matches first.
+///
+/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
+/// without having to rewrite each clause multiple times.
+///
+/// Trailing `_` wildcard match arms are **optional** and they indicate a fallback branch when
+/// all previous declarations do not evaluate to true.
+///
+/// # Example
+///
+/// ```
+/// #![feature(cfg_match)]
+///
+/// cfg_match! {
+///     unix => {
+///         fn foo() { /* unix specific functionality */ }
+///     }
+///     target_pointer_width = "32" => {
+///         fn foo() { /* non-unix, 32-bit functionality */ }
+///     }
+///     _ => {
+///         fn foo() { /* fallback implementation */ }
+///     }
+/// }
+/// ```
+#[cfg(not(bootstrap))]
+#[unstable(feature = "cfg_match", issue = "115585")]
+#[rustc_diagnostic_item = "cfg_match"]
+pub macro cfg_match {
+    ({ $($tt:tt)* }) => {{
+        cfg_match! { $($tt)* }
+    }},
+    (_ => { $($output:tt)* }) => {
+        $($output)*
+    },
+    (
+        $cfg:meta => $output:tt
+        $($( $rest:tt )+)?
+    ) => {
+        #[cfg($cfg)]
+        cfg_match! { _ => $output }
+        $(
+            #[cfg(not($cfg))]
+            cfg_match! { $($rest)+ }
+        )?
+    },
+}
+
 /// Asserts that a boolean expression is `true` at runtime.
 ///
 /// This will invoke the [`panic!`] macro if the provided expression cannot be
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 29fc01d37fe..01af964a83e 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -141,9 +141,8 @@ unsafe impl<T: Sync + ?Sized> Send for &T {}
 )]
 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
 #[rustc_specialization_trait]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 #[rustc_coinductive]
 pub trait Sized {
     // Empty.
@@ -183,33 +182,27 @@ pub trait Sized {
 /// [^1]: Formerly known as *object safe*.
 #[unstable(feature = "unsize", issue = "18598")]
 #[lang = "unsize"]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 pub trait Unsize<T: ?Sized> {
     // Empty.
 }
 
 /// Required trait for constants used in pattern matches.
 ///
-/// Any type that derives `PartialEq` automatically implements this trait,
-/// *regardless* of whether its type-parameters implement `PartialEq`.
+/// Constants are only allowed as patterns if (a) their type implements
+/// `PartialEq`, and (b) interpreting the value of the constant as a pattern
+/// is equialent to calling `PartialEq`. This ensures that constants used as
+/// patterns cannot expose implementation details in an unexpected way or
+/// cause semver hazards.
 ///
-/// If a `const` item contains some type that does not implement this trait,
-/// then that type either (1.) does not implement `PartialEq` (which means the
-/// constant will not provide that comparison method, which code generation
-/// assumes is available), or (2.) it implements *its own* version of
-/// `PartialEq` (which we assume does not conform to a structural-equality
-/// comparison).
+/// This trait ensures point (b).
+/// Any type that derives `PartialEq` automatically implements this trait.
 ///
-/// In either of the two scenarios above, we reject usage of such a constant in
-/// a pattern match.
-///
-/// See also the [structural match RFC][RFC1445], and [issue 63438] which
-/// motivated migrating from an attribute-based design to this trait.
-///
-/// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
-/// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
+/// Implementing this trait (which is unstable) is a way for type authors to explicitly allow
+/// comparing const values of this type; that operation will recursively compare all fields
+/// (including private fields), even if that behavior differs from `PartialEq`. This can make it
+/// semver-breaking to add further private fields to a type.
 #[unstable(feature = "structural_match", issue = "31434")]
 #[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
 #[lang = "structural_peq"]
@@ -819,9 +812,8 @@ impl<T: ?Sized> StructuralPartialEq for PhantomData<T> {}
     reason = "this trait is unlikely to ever be stabilized, use `mem::discriminant` instead"
 )]
 #[lang = "discriminant_kind"]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 pub trait DiscriminantKind {
     /// The type of the discriminant, which must satisfy the trait
     /// bounds required by `mem::Discriminant`.
@@ -960,12 +952,12 @@ marker_impls! {
 /// This should be used for `~const` bounds,
 /// as non-const bounds will always hold for every type.
 #[unstable(feature = "const_destruct", issue = "133214")]
+#[rustc_const_unstable(feature = "const_destruct", issue = "133214")]
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
-#[cfg_attr(not(bootstrap), const_trait)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
+#[const_trait]
 pub trait Destruct {}
 
 /// A marker for tuple types.
@@ -975,9 +967,8 @@ pub trait Destruct {}
 #[unstable(feature = "tuple_trait", issue = "none")]
 #[lang = "tuple_trait"]
 #[diagnostic::on_unimplemented(message = "`{Self}` is not a tuple")]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 pub trait Tuple {}
 
 /// A marker for pointer-like types.
@@ -996,10 +987,9 @@ pub trait Tuple {}
     message = "`{Self}` needs to have the same ABI as a pointer",
     label = "`{Self}` needs to be a pointer-like type"
 )]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_do_not_implement_via_object]
 pub trait PointerLike {}
 
-#[cfg(not(bootstrap))]
 marker_impls! {
     #[unstable(feature = "pointer_like_trait", issue = "none")]
     PointerLike for
@@ -1086,9 +1076,8 @@ marker_impls! {
     reason = "internal trait for implementing various traits for all function pointers"
 )]
 #[lang = "fn_ptr_trait"]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 pub trait FnPtr: Copy + Clone {
     /// Returns the address of the function pointer.
     #[lang = "fn_ptr_addr"]
@@ -1099,7 +1088,6 @@ pub trait FnPtr: Copy + Clone {
 #[rustc_builtin_macro(CoercePointee, attributes(pointee))]
 #[allow_internal_unstable(dispatch_from_dyn, coerce_unsized, unsize)]
 #[unstable(feature = "derive_coerce_pointee", issue = "123430")]
-#[cfg(not(bootstrap))]
 pub macro CoercePointee($item:item) {
     /* compiler built-in */
 }
diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs
index 58fb5be5812..ac5307a671d 100644
--- a/library/core/src/mem/maybe_uninit.rs
+++ b/library/core/src/mem/maybe_uninit.rs
@@ -1,5 +1,5 @@
 use crate::any::type_name;
-use crate::mem::{self, ManuallyDrop};
+use crate::mem::ManuallyDrop;
 use crate::{fmt, intrinsics, ptr, slice};
 
 /// A wrapper type to construct uninitialized instances of `T`.
@@ -354,7 +354,7 @@ impl<T> MaybeUninit<T> {
     /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
     ///     unsafe {
     ///         let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
-    ///         MaybeUninit::slice_assume_init_ref(&buf[..len])
+    ///         buf[..len].assume_init_ref()
     ///     }
     /// }
     ///
@@ -507,7 +507,7 @@ impl<T> MaybeUninit<T> {
     /// ```
     #[inline(always)]
     #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
-    #[rustc_const_stable(feature = "const_maybe_uninit_write", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_maybe_uninit_write", since = "1.85.0")]
     pub const fn write(&mut self, val: T) -> &mut T {
         *self = MaybeUninit::new(val);
         // SAFETY: We just initialized this value.
@@ -740,7 +740,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// On top of that, all additional invariants of the type `T` must be
     /// satisfied, as the `Drop` implementation of `T` (or its members) may
-    /// rely on this. For example, setting a [`Vec<T>`] to an invalid but
+    /// rely on this. For example, setting a `Vec<T>` to an invalid but
     /// non-null address makes it initialized (under the current implementation;
     /// this does not constitute a stable guarantee), because the only
     /// requirement the compiler knows about it is that the data pointer must be
@@ -748,7 +748,6 @@ impl<T> MaybeUninit<T> {
     /// behavior.
     ///
     /// [`assume_init`]: MaybeUninit::assume_init
-    /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
     #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
     pub unsafe fn assume_init_drop(&mut self) {
         // SAFETY: the caller must guarantee that `self` is initialized and
@@ -982,44 +981,87 @@ impl<T> MaybeUninit<T> {
         }
     }
 
-    /// Assuming all the elements are initialized, get a slice to them.
+    /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
     ///
-    /// # Safety
+    /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
+    /// contain padding bytes which are left uninitialized.
     ///
-    /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
-    /// really are in an initialized state.
-    /// Calling this when the content is not yet fully initialized causes undefined behavior.
+    /// # Examples
     ///
-    /// See [`assume_init_ref`] for more details and examples.
+    /// ```
+    /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
+    /// use std::mem::MaybeUninit;
     ///
-    /// [`assume_init_ref`]: MaybeUninit::assume_init_ref
-    #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
-    #[inline(always)]
-    pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
-        // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
-        // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
-        // The pointer obtained is valid since it refers to memory owned by `slice` which is a
-        // reference and thus guaranteed to be valid for reads.
-        unsafe { &*(slice as *const [Self] as *const [T]) }
+    /// let val = 0x12345678_i32;
+    /// let uninit = MaybeUninit::new(val);
+    /// let uninit_bytes = uninit.as_bytes();
+    /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
+    /// assert_eq!(bytes, val.to_ne_bytes());
+    /// ```
+    #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
+    pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
+        // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
+        unsafe {
+            slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of::<T>())
+        }
     }
 
-    /// Assuming all the elements are initialized, get a mutable slice to them.
+    /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
+    /// bytes.
     ///
-    /// # Safety
+    /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
+    /// contain padding bytes which are left uninitialized.
     ///
-    /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
-    /// really are in an initialized state.
-    /// Calling this when the content is not yet fully initialized causes undefined behavior.
+    /// # Examples
     ///
-    /// See [`assume_init_mut`] for more details and examples.
+    /// ```
+    /// #![feature(maybe_uninit_as_bytes)]
+    /// use std::mem::MaybeUninit;
     ///
-    /// [`assume_init_mut`]: MaybeUninit::assume_init_mut
+    /// let val = 0x12345678_i32;
+    /// let mut uninit = MaybeUninit::new(val);
+    /// let uninit_bytes = uninit.as_bytes_mut();
+    /// if cfg!(target_endian = "little") {
+    ///     uninit_bytes[0].write(0xcd);
+    /// } else {
+    ///     uninit_bytes[3].write(0xcd);
+    /// }
+    /// let val2 = unsafe { uninit.assume_init() };
+    /// assert_eq!(val2, 0x123456cd);
+    /// ```
+    #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
+    pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
+        // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
+        unsafe {
+            slice::from_raw_parts_mut(
+                self.as_mut_ptr().cast::<MaybeUninit<u8>>(),
+                super::size_of::<T>(),
+            )
+        }
+    }
+
+    /// Deprecated version of [`slice::assume_init_ref`].
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
-    #[inline(always)]
+    #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[deprecated(
+        note = "replaced by inherent assume_init_ref method; will eventually be removed",
+        since = "1.83.0"
+    )]
+    pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
+        // SAFETY: Same for both methods.
+        unsafe { slice.assume_init_ref() }
+    }
+
+    /// Deprecated version of [`slice::assume_init_mut`].
+    #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[deprecated(
+        note = "replaced by inherent assume_init_mut method; will eventually be removed",
+        since = "1.83.0"
+    )]
     pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
-        // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
-        // mutable reference which is also guaranteed to be valid for writes.
-        unsafe { &mut *(slice as *mut [Self] as *mut [T]) }
+        // SAFETY: Same for both methods.
+        unsafe { slice.assume_init_mut() }
     }
 
     /// Gets a pointer to the first element of the array.
@@ -1036,142 +1078,34 @@ impl<T> MaybeUninit<T> {
         this.as_mut_ptr() as *mut T
     }
 
-    /// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
-    ///
-    /// If `T` does not implement `Copy`, use [`clone_from_slice`]
-    ///
-    /// This is similar to [`slice::copy_from_slice`].
-    ///
-    /// # Panics
-    ///
-    /// This function will panic if the two slices have different lengths.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut dst = [MaybeUninit::uninit(); 32];
-    /// let src = [0; 32];
-    ///
-    /// let init = MaybeUninit::copy_from_slice(&mut dst, &src);
-    ///
-    /// assert_eq!(init, src);
-    /// ```
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut vec = Vec::with_capacity(32);
-    /// let src = [0; 16];
-    ///
-    /// MaybeUninit::copy_from_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
-    ///
-    /// // SAFETY: we have just copied all the elements of len into the spare capacity
-    /// // the first src.len() elements of the vec are valid now.
-    /// unsafe {
-    ///     vec.set_len(src.len());
-    /// }
-    ///
-    /// assert_eq!(vec, src);
-    /// ```
-    ///
-    /// [`clone_from_slice`]: MaybeUninit::clone_from_slice
+    /// Deprecated version of [`slice::write_copy_of_slice`].
     #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
+    #[deprecated(
+        note = "replaced by inherent write_copy_of_slice method; will eventually be removed",
+        since = "1.83.0"
+    )]
     pub fn copy_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
     where
         T: Copy,
     {
-        // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
-        let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
-
-        this.copy_from_slice(uninit_src);
-
-        // SAFETY: Valid elements have just been copied into `this` so it is initialized
-        unsafe { MaybeUninit::slice_assume_init_mut(this) }
+        this.write_copy_of_slice(src)
     }
 
-    /// Clones the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
-    /// Any already initialized elements will not be dropped.
-    ///
-    /// If `T` implements `Copy`, use [`copy_from_slice`]
-    ///
-    /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
-    ///
-    /// # Panics
-    ///
-    /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
-    ///
-    /// If there is a panic, the already cloned elements will be dropped.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()];
-    /// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()];
-    ///
-    /// let init = MaybeUninit::clone_from_slice(&mut dst, &src);
-    ///
-    /// assert_eq!(init, src);
-    /// # // Prevent leaks for Miri
-    /// # unsafe { std::ptr::drop_in_place(init); }
-    /// ```
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut vec = Vec::with_capacity(32);
-    /// let src = ["rust", "is", "a", "pretty", "cool", "language"];
-    ///
-    /// MaybeUninit::clone_from_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
-    ///
-    /// // SAFETY: we have just cloned all the elements of len into the spare capacity
-    /// // the first src.len() elements of the vec are valid now.
-    /// unsafe {
-    ///     vec.set_len(src.len());
-    /// }
-    ///
-    /// assert_eq!(vec, src);
-    /// ```
-    ///
-    /// [`copy_from_slice`]: MaybeUninit::copy_from_slice
+    /// Deprecated version of [`slice::write_clone_of_slice`].
     #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
+    #[deprecated(
+        note = "replaced by inherent write_clone_of_slice method; will eventually be removed",
+        since = "1.83.0"
+    )]
     pub fn clone_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
     where
         T: Clone,
     {
-        // unlike copy_from_slice this does not call clone_from_slice on the slice
-        // this is because `MaybeUninit<T: Clone>` does not implement Clone.
-
-        assert_eq!(this.len(), src.len(), "destination and source slices have different lengths");
-        // NOTE: We need to explicitly slice them to the same length
-        // for bounds checking to be elided, and the optimizer will
-        // generate memcpy for simple cases (for example T = u8).
-        let len = this.len();
-        let src = &src[..len];
-
-        // guard is needed b/c panic might happen during a clone
-        let mut guard = Guard { slice: this, initialized: 0 };
-
-        for i in 0..len {
-            guard.slice[i].write(src[i].clone());
-            guard.initialized += 1;
-        }
-
-        super::forget(guard);
-
-        // SAFETY: Valid elements have just been written into `this` so it is initialized
-        unsafe { MaybeUninit::slice_assume_init_mut(this) }
+        this.write_clone_of_slice(src)
     }
 
-    /// Fills `this` with elements by cloning `value`, returning a mutable reference to the now
-    /// initialized contents of `this`.
+    /// Fills a slice with elements by cloning `value`, returning a mutable reference to the now
+    /// initialized contents of the slice.
     /// Any previously initialized elements will not be dropped.
     ///
     /// This is similar to [`slice::fill`].
@@ -1185,27 +1119,26 @@ impl<T> MaybeUninit<T> {
     ///
     /// # Examples
     ///
-    /// Fill an uninit vec with 1.
     /// ```
     /// #![feature(maybe_uninit_fill)]
     /// use std::mem::MaybeUninit;
     ///
-    /// let mut buf = vec![MaybeUninit::uninit(); 10];
-    /// let initialized = MaybeUninit::fill(buf.as_mut_slice(), 1);
+    /// let mut buf = [const { MaybeUninit::uninit() }; 10];
+    /// let initialized = MaybeUninit::fill(&mut buf, 1);
     /// assert_eq!(initialized, &mut [1; 10]);
     /// ```
     #[doc(alias = "memset")]
     #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
-    pub fn fill<'a>(this: &'a mut [MaybeUninit<T>], value: T) -> &'a mut [T]
+    pub fn fill(this: &mut [MaybeUninit<T>], value: T) -> &mut [T]
     where
         T: Clone,
     {
         SpecFill::spec_fill(this, value);
         // SAFETY: Valid elements have just been filled into `this` so it is initialized
-        unsafe { MaybeUninit::slice_assume_init_mut(this) }
+        unsafe { this.assume_init_mut() }
     }
 
-    /// Fills `this` with elements returned by calling a closure repeatedly.
+    /// Fills a slice with elements returned by calling a closure repeatedly.
     ///
     /// This method uses a closure to create new values.  If you'd rather `Clone` a given value, use
     /// [`MaybeUninit::fill`].  If you want to use the `Default` trait to generate values, you can
@@ -1220,17 +1153,16 @@ impl<T> MaybeUninit<T> {
     ///
     /// # Examples
     ///
-    /// Fill an uninit vec with the default value.
     /// ```
     /// #![feature(maybe_uninit_fill)]
     /// use std::mem::MaybeUninit;
     ///
-    /// let mut buf = vec![MaybeUninit::<i32>::uninit(); 10];
-    /// let initialized = MaybeUninit::fill_with(buf.as_mut_slice(), Default::default);
+    /// let mut buf = [const { MaybeUninit::<i32>::uninit() }; 10];
+    /// let initialized = MaybeUninit::fill_with(&mut buf, Default::default);
     /// assert_eq!(initialized, &mut [0; 10]);
     /// ```
     #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
-    pub fn fill_with<'a, F>(this: &'a mut [MaybeUninit<T>], mut f: F) -> &'a mut [T]
+    pub fn fill_with<F>(this: &mut [MaybeUninit<T>], mut f: F) -> &mut [T]
     where
         F: FnMut() -> T,
     {
@@ -1244,13 +1176,13 @@ impl<T> MaybeUninit<T> {
         super::forget(guard);
 
         // SAFETY: Valid elements have just been written into `this` so it is initialized
-        unsafe { MaybeUninit::slice_assume_init_mut(this) }
+        unsafe { this.assume_init_mut() }
     }
 
-    /// Fills `this` with elements yielded by an iterator until either all elements have been
+    /// Fills a slice with elements yielded by an iterator until either all elements have been
     /// initialized or the iterator is empty.
     ///
-    /// Returns two slices.  The first slice contains the initialized portion of the original slice.
+    /// Returns two slices. The first slice contains the initialized portion of the original slice.
     /// The second slice is the still-uninitialized remainder of the original slice.
     ///
     /// # Panics
@@ -1262,37 +1194,51 @@ impl<T> MaybeUninit<T> {
     ///
     /// # Examples
     ///
-    /// Fill an uninit vec with a cycling iterator.
+    /// Completely filling the slice:
+    ///
     /// ```
     /// #![feature(maybe_uninit_fill)]
     /// use std::mem::MaybeUninit;
     ///
-    /// let mut buf = vec![MaybeUninit::uninit(); 5];
+    /// let mut buf = [const { MaybeUninit::uninit() }; 5];
     ///
     /// let iter = [1, 2, 3].into_iter().cycle();
     /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter);
     ///
     /// assert_eq!(initialized, &mut [1, 2, 3, 1, 2]);
-    /// assert_eq!(0, remainder.len());
+    /// assert_eq!(remainder.len(), 0);
     /// ```
     ///
-    /// Fill an uninit vec, but not completely.
+    /// Partially filling the slice:
+    ///
     /// ```
     /// #![feature(maybe_uninit_fill)]
     /// use std::mem::MaybeUninit;
     ///
-    /// let mut buf = vec![MaybeUninit::uninit(); 5];
+    /// let mut buf = [const { MaybeUninit::uninit() }; 5];
     /// let iter = [1, 2];
     /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter);
     ///
     /// assert_eq!(initialized, &mut [1, 2]);
     /// assert_eq!(remainder.len(), 3);
     /// ```
+    ///
+    /// Checking an iterator after filling a slice:
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_fill)]
+    /// use std::mem::MaybeUninit;
+    ///
+    /// let mut buf = [const { MaybeUninit::uninit() }; 3];
+    /// let mut iter = [1, 2, 3, 4, 5].into_iter();
+    /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter.by_ref());
+    ///
+    /// assert_eq!(initialized, &mut [1, 2, 3]);
+    /// assert_eq!(remainder.len(), 0);
+    /// assert_eq!(iter.as_slice(), &[4, 5]);
+    /// ```
     #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
-    pub fn fill_from<'a, I>(
-        this: &'a mut [MaybeUninit<T>],
-        it: I,
-    ) -> (&'a mut [T], &'a mut [MaybeUninit<T>])
+    pub fn fill_from<I>(this: &mut [MaybeUninit<T>], it: I) -> (&mut [T], &mut [MaybeUninit<T>])
     where
         I: IntoIterator<Item = T>,
     {
@@ -1312,70 +1258,169 @@ impl<T> MaybeUninit<T> {
 
         // SAFETY: Valid elements have just been written into `init`, so that portion
         // of `this` is initialized.
-        (unsafe { MaybeUninit::slice_assume_init_mut(initted) }, remainder)
+        (unsafe { initted.assume_init_mut() }, remainder)
     }
 
-    /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
+    /// Deprecated version of [`slice::as_bytes`].
+    #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
+    #[deprecated(
+        note = "replaced by inherent as_bytes method; will eventually be removed",
+        since = "1.83.0"
+    )]
+    pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
+        this.as_bytes()
+    }
+
+    /// Deprecated version of [`slice::as_bytes_mut`].
+    #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
+    #[deprecated(
+        note = "replaced by inherent as_bytes_mut method; will eventually be removed",
+        since = "1.83.0"
+    )]
+    pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
+        this.as_bytes_mut()
+    }
+}
+
+impl<T> [MaybeUninit<T>] {
+    /// Copies the elements from `src` to `self`,
+    /// returning a mutable reference to the now initialized contents of `self`.
     ///
-    /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
-    /// contain padding bytes which are left uninitialized.
+    /// If `T` does not implement `Copy`, use [`write_clone_of_slice`] instead.
+    ///
+    /// This is similar to [`slice::copy_from_slice`].
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the two slices have different lengths.
     ///
     /// # Examples
     ///
     /// ```
-    /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
+    /// #![feature(maybe_uninit_write_slice)]
     /// use std::mem::MaybeUninit;
     ///
-    /// let val = 0x12345678_i32;
-    /// let uninit = MaybeUninit::new(val);
-    /// let uninit_bytes = uninit.as_bytes();
-    /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(uninit_bytes) };
-    /// assert_eq!(bytes, val.to_ne_bytes());
+    /// let mut dst = [MaybeUninit::uninit(); 32];
+    /// let src = [0; 32];
+    ///
+    /// let init = dst.write_copy_of_slice(&src);
+    ///
+    /// assert_eq!(init, src);
     /// ```
-    #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
-    pub fn as_bytes(&self) -> &[MaybeUninit<u8>] {
-        // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
-        unsafe {
-            slice::from_raw_parts(self.as_ptr() as *const MaybeUninit<u8>, mem::size_of::<T>())
-        }
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_write_slice)]
+    ///
+    /// let mut vec = Vec::with_capacity(32);
+    /// let src = [0; 16];
+    ///
+    /// vec.spare_capacity_mut()[..src.len()].write_copy_of_slice(&src);
+    ///
+    /// // SAFETY: we have just copied all the elements of len into the spare capacity
+    /// // the first src.len() elements of the vec are valid now.
+    /// unsafe {
+    ///     vec.set_len(src.len());
+    /// }
+    ///
+    /// assert_eq!(vec, src);
+    /// ```
+    ///
+    /// [`write_clone_of_slice`]: slice::write_clone_of_slice
+    #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
+    #[rustc_const_unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
+    pub const fn write_copy_of_slice(&mut self, src: &[T]) -> &mut [T]
+    where
+        T: Copy,
+    {
+        // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
+        let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
+
+        self.copy_from_slice(uninit_src);
+
+        // SAFETY: Valid elements have just been copied into `self` so it is initialized
+        unsafe { self.assume_init_mut() }
     }
 
-    /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
-    /// bytes.
+    /// Clones the elements from `src` to `self`,
+    /// returning a mutable reference to the now initialized contents of `self`.
+    /// Any already initialized elements will not be dropped.
     ///
-    /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
-    /// contain padding bytes which are left uninitialized.
+    /// If `T` implements `Copy`, use [`write_copy_of_slice`] instead.
+    ///
+    /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
+    ///
+    /// If there is a panic, the already cloned elements will be dropped.
     ///
     /// # Examples
     ///
     /// ```
-    /// #![feature(maybe_uninit_as_bytes)]
+    /// #![feature(maybe_uninit_write_slice)]
     /// use std::mem::MaybeUninit;
     ///
-    /// let val = 0x12345678_i32;
-    /// let mut uninit = MaybeUninit::new(val);
-    /// let uninit_bytes = uninit.as_bytes_mut();
-    /// if cfg!(target_endian = "little") {
-    ///     uninit_bytes[0].write(0xcd);
-    /// } else {
-    ///     uninit_bytes[3].write(0xcd);
+    /// let mut dst = [const { MaybeUninit::uninit() }; 5];
+    /// let src = ["wibbly", "wobbly", "timey", "wimey", "stuff"].map(|s| s.to_string());
+    ///
+    /// let init = dst.write_clone_of_slice(&src);
+    ///
+    /// assert_eq!(init, src);
+    ///
+    /// # // Prevent leaks for Miri
+    /// # unsafe { std::ptr::drop_in_place(init); }
+    /// ```
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_write_slice)]
+    ///
+    /// let mut vec = Vec::with_capacity(32);
+    /// let src = ["rust", "is", "a", "pretty", "cool", "language"].map(|s| s.to_string());
+    ///
+    /// vec.spare_capacity_mut()[..src.len()].write_clone_of_slice(&src);
+    ///
+    /// // SAFETY: we have just cloned all the elements of len into the spare capacity
+    /// // the first src.len() elements of the vec are valid now.
+    /// unsafe {
+    ///     vec.set_len(src.len());
     /// }
-    /// let val2 = unsafe { uninit.assume_init() };
-    /// assert_eq!(val2, 0x123456cd);
+    ///
+    /// assert_eq!(vec, src);
     /// ```
-    #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
-    pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
-        // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
-        unsafe {
-            slice::from_raw_parts_mut(
-                self.as_mut_ptr() as *mut MaybeUninit<u8>,
-                mem::size_of::<T>(),
-            )
+    ///
+    /// [`write_copy_of_slice`]: slice::write_copy_of_slice
+    #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
+    pub fn write_clone_of_slice(&mut self, src: &[T]) -> &mut [T]
+    where
+        T: Clone,
+    {
+        // unlike copy_from_slice this does not call clone_from_slice on the slice
+        // this is because `MaybeUninit<T: Clone>` does not implement Clone.
+
+        assert_eq!(self.len(), src.len(), "destination and source slices have different lengths");
+
+        // NOTE: We need to explicitly slice them to the same length
+        // for bounds checking to be elided, and the optimizer will
+        // generate memcpy for simple cases (for example T = u8).
+        let len = self.len();
+        let src = &src[..len];
+
+        // guard is needed b/c panic might happen during a clone
+        let mut guard = Guard { slice: self, initialized: 0 };
+
+        for i in 0..len {
+            guard.slice[i].write(src[i].clone());
+            guard.initialized += 1;
         }
+
+        super::forget(guard);
+
+        // SAFETY: Valid elements have just been written into `self` so it is initialized
+        unsafe { self.assume_init_mut() }
     }
 
-    /// Returns the contents of this slice of `MaybeUninit` as a slice of potentially uninitialized
-    /// bytes.
+    /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
     ///
     /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
     /// contain padding bytes which are left uninitialized.
@@ -1387,21 +1432,22 @@ impl<T> MaybeUninit<T> {
     /// use std::mem::MaybeUninit;
     ///
     /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
-    /// let uninit_bytes = MaybeUninit::slice_as_bytes(&uninit);
-    /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(&uninit_bytes) };
+    /// let uninit_bytes = uninit.as_bytes();
+    /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
     /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
     /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
     /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
     /// ```
     #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
-    pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
-        let bytes = mem::size_of_val(this);
+    pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
         // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
-        unsafe { slice::from_raw_parts(this.as_ptr() as *const MaybeUninit<u8>, bytes) }
+        unsafe {
+            slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of_val(self))
+        }
     }
 
-    /// Returns the contents of this mutable slice of `MaybeUninit` as a mutable slice of
-    /// potentially uninitialized bytes.
+    /// Returns the contents of this `MaybeUninit` slice as a mutable slice of potentially
+    /// uninitialized bytes.
     ///
     /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
     /// contain padding bytes which are left uninitialized.
@@ -1414,8 +1460,8 @@ impl<T> MaybeUninit<T> {
     ///
     /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
     /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
-    /// MaybeUninit::copy_from_slice(uninit_bytes, &[0x12, 0x34, 0x56, 0x78]);
-    /// let vals = unsafe { MaybeUninit::slice_assume_init_ref(&uninit) };
+    /// uninit_bytes.write_copy_of_slice(&[0x12, 0x34, 0x56, 0x78]);
+    /// let vals = unsafe { uninit.assume_init_ref() };
     /// if cfg!(target_endian = "little") {
     ///     assert_eq!(vals, &[0x3412u16, 0x7856u16]);
     /// } else {
@@ -1423,10 +1469,74 @@ impl<T> MaybeUninit<T> {
     /// }
     /// ```
     #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
-    pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
-        let bytes = mem::size_of_val(this);
+    pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
         // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
-        unsafe { slice::from_raw_parts_mut(this.as_mut_ptr() as *mut MaybeUninit<u8>, bytes) }
+        unsafe {
+            slice::from_raw_parts_mut(
+                self.as_mut_ptr() as *mut MaybeUninit<u8>,
+                super::size_of_val(self),
+            )
+        }
+    }
+
+    /// Drops the contained values in place.
+    ///
+    /// # Safety
+    ///
+    /// It is up to the caller to guarantee that every `MaybeUninit<T>` in the slice
+    /// really is in an initialized state. Calling this when the content is not yet
+    /// fully initialized causes undefined behavior.
+    ///
+    /// On top of that, all additional invariants of the type `T` must be
+    /// satisfied, as the `Drop` implementation of `T` (or its members) may
+    /// rely on this. For example, setting a `Vec<T>` to an invalid but
+    /// non-null address makes it initialized (under the current implementation;
+    /// this does not constitute a stable guarantee), because the only
+    /// requirement the compiler knows about it is that the data pointer must be
+    /// non-null. Dropping such a `Vec<T>` however will cause undefined
+    /// behaviour.
+    #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[inline(always)]
+    pub unsafe fn assume_init_drop(&mut self) {
+        if !self.is_empty() {
+            // SAFETY: the caller must guarantee that every element of `self`
+            // is initialized and satisfies all invariants of `T`.
+            // Dropping the value in place is safe if that is the case.
+            unsafe { ptr::drop_in_place(self as *mut [MaybeUninit<T>] as *mut [T]) }
+        }
+    }
+
+    /// Gets a shared reference to the contained value.
+    ///
+    /// # Safety
+    ///
+    /// Calling this when the content is not yet fully initialized causes undefined
+    /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in
+    /// the slice really is in an initialized state.
+    #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[inline(always)]
+    pub const unsafe fn assume_init_ref(&self) -> &[T] {
+        // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
+        // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
+        // The pointer obtained is valid since it refers to memory owned by `slice` which is a
+        // reference and thus guaranteed to be valid for reads.
+        unsafe { &*(self as *const Self as *const [T]) }
+    }
+
+    /// Gets a mutable (unique) reference to the contained value.
+    ///
+    /// # Safety
+    ///
+    /// Calling this when the content is not yet fully initialized causes undefined
+    /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in the
+    /// slice really is in an initialized state. For instance, `.assume_init_mut()` cannot
+    /// be used to initialize a `MaybeUninit` slice.
+    #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[inline(always)]
+    pub const unsafe fn assume_init_mut(&mut self) -> &mut [T] {
+        // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
+        // mutable reference which is also guaranteed to be valid for writes.
+        unsafe { &mut *(self as *mut Self as *mut [T]) }
     }
 }
 
@@ -1479,7 +1589,7 @@ impl<'a, T> Drop for Guard<'a, T> {
         let initialized_part = &mut self.slice[..self.initialized];
         // SAFETY: this raw sub-slice will contain only initialized objects.
         unsafe {
-            crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(initialized_part));
+            initialized_part.assume_init_drop();
         }
     }
 }
diff --git a/library/core/src/mem/mod.rs b/library/core/src/mem/mod.rs
index 2d66e5c2f2a..b9bb6d6a13f 100644
--- a/library/core/src/mem/mod.rs
+++ b/library/core/src/mem/mod.rs
@@ -333,7 +333,7 @@ pub const fn size_of<T>() -> usize {
 #[inline]
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_stable(feature = "const_size_of_val", since = "CURRENT_RUSTC_VERSION")]
+#[rustc_const_stable(feature = "const_size_of_val", since = "1.85.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_size_of_val")]
 pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
     // SAFETY: `val` is a reference, so it's a valid raw pointer
@@ -484,7 +484,7 @@ pub const fn align_of<T>() -> usize {
 #[inline]
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_stable(feature = "const_align_of_val", since = "CURRENT_RUSTC_VERSION")]
+#[rustc_const_stable(feature = "const_align_of_val", since = "1.85.0")]
 #[allow(deprecated)]
 pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
     // SAFETY: val is a reference, so it's a valid raw pointer
@@ -725,7 +725,7 @@ pub unsafe fn uninitialized<T>() -> T {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_stable(feature = "const_swap", since = "CURRENT_RUSTC_VERSION")]
+#[rustc_const_stable(feature = "const_swap", since = "1.85.0")]
 #[rustc_diagnostic_item = "mem_swap"]
 pub const fn swap<T>(x: &mut T, y: &mut T) {
     // SAFETY: `&mut` guarantees these are typed readable and writable
diff --git a/library/core/src/mem/transmutability.rs b/library/core/src/mem/transmutability.rs
index 9cf587650d9..6a4f84c849c 100644
--- a/library/core/src/mem/transmutability.rs
+++ b/library/core/src/mem/transmutability.rs
@@ -84,9 +84,8 @@ use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
 /// `usize` is stable, but not portable.
 #[unstable(feature = "transmutability", issue = "99571")]
 #[lang = "transmute_trait"]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 #[rustc_coinductive]
 pub unsafe trait TransmuteFrom<Src, const ASSUME: Assume = { Assume::NOTHING }>
 where
diff --git a/library/core/src/net/display_buffer.rs b/library/core/src/net/display_buffer.rs
index bab84a97308..a7d12217081 100644
--- a/library/core/src/net/display_buffer.rs
+++ b/library/core/src/net/display_buffer.rs
@@ -18,7 +18,7 @@ impl<const SIZE: usize> DisplayBuffer<SIZE> {
         // SAFETY: `buf` is only written to by the `fmt::Write::write_str` implementation
         // which writes a valid UTF-8 string to `buf` and correctly sets `len`.
         unsafe {
-            let s = MaybeUninit::slice_assume_init_ref(&self.buf[..self.len]);
+            let s = self.buf[..self.len].assume_init_ref();
             str::from_utf8_unchecked(s)
         }
     }
@@ -29,7 +29,7 @@ impl<const SIZE: usize> fmt::Write for DisplayBuffer<SIZE> {
         let bytes = s.as_bytes();
 
         if let Some(buf) = self.buf.get_mut(self.len..(self.len + bytes.len())) {
-            MaybeUninit::copy_from_slice(buf, bytes);
+            buf.write_copy_of_slice(bytes);
             self.len += bytes.len();
             Ok(())
         } else {
diff --git a/library/core/src/num/f128.rs b/library/core/src/num/f128.rs
index 4ebeaf04611..8fb1588e60b 100644
--- a/library/core/src/num/f128.rs
+++ b/library/core/src/num/f128.rs
@@ -504,7 +504,6 @@ impl f128 {
     ///
     /// ```rust
     /// #![feature(f128)]
-    /// #![feature(float_next_up_down)]
     /// # // FIXME(f16_f128): remove when `eqtf2` is available
     /// # #[cfg(all(target_arch = "x86_64", target_os = "linux"))] {
     ///
@@ -516,13 +515,15 @@ impl f128 {
     /// # }
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextUp`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
+    #[doc(alias = "nextUp")]
     #[unstable(feature = "f128", issue = "116909")]
-    // #[unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_up(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
@@ -558,7 +559,6 @@ impl f128 {
     ///
     /// ```rust
     /// #![feature(f128)]
-    /// #![feature(float_next_up_down)]
     /// # // FIXME(f16_f128): remove when `eqtf2` is available
     /// # #[cfg(all(target_arch = "x86_64", target_os = "linux"))] {
     ///
@@ -570,13 +570,15 @@ impl f128 {
     /// # }
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextDown`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
+    #[doc(alias = "nextDown")]
     #[unstable(feature = "f128", issue = "116909")]
-    // #[unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_down(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
diff --git a/library/core/src/num/f16.rs b/library/core/src/num/f16.rs
index c82f0d7cd4a..8c2af74b8f8 100644
--- a/library/core/src/num/f16.rs
+++ b/library/core/src/num/f16.rs
@@ -497,7 +497,6 @@ impl f16 {
     ///
     /// ```rust
     /// #![feature(f16)]
-    /// #![feature(float_next_up_down)]
     /// # // FIXME(f16_f128): ABI issues on MSVC
     /// # #[cfg(all(target_arch = "x86_64", target_os = "linux"))] {
     ///
@@ -509,13 +508,15 @@ impl f16 {
     /// # }
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextUp`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
+    #[doc(alias = "nextUp")]
     #[unstable(feature = "f16", issue = "116909")]
-    // #[unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_up(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
@@ -551,7 +552,6 @@ impl f16 {
     ///
     /// ```rust
     /// #![feature(f16)]
-    /// #![feature(float_next_up_down)]
     /// # // FIXME(f16_f128): ABI issues on MSVC
     /// # #[cfg(all(target_arch = "x86_64", target_os = "linux"))] {
     ///
@@ -563,13 +563,15 @@ impl f16 {
     /// # }
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextDown`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
+    #[doc(alias = "nextDown")]
     #[unstable(feature = "f16", issue = "116909")]
-    // #[unstable(feature = "float_next_up_down", issue = "91399")]
     pub const fn next_down(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
diff --git a/library/core/src/num/f32.rs b/library/core/src/num/f32.rs
index 4c0d95f95e5..817bedbd44f 100644
--- a/library/core/src/num/f32.rs
+++ b/library/core/src/num/f32.rs
@@ -726,7 +726,6 @@ impl f32 {
     /// is finite `x == x.next_up().next_down()` also holds.
     ///
     /// ```rust
-    /// #![feature(float_next_up_down)]
     /// // f32::EPSILON is the difference between 1.0 and the next number up.
     /// assert_eq!(1.0f32.next_up(), 1.0 + f32::EPSILON);
     /// // But not for most numbers.
@@ -734,12 +733,16 @@ impl f32 {
     /// assert_eq!(16777216f32.next_up(), 16777218.0);
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextUp`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
-    #[unstable(feature = "float_next_up_down", issue = "91399")]
+    #[doc(alias = "nextUp")]
+    #[stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
     pub const fn next_up(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
@@ -774,7 +777,6 @@ impl f32 {
     /// is finite `x == x.next_down().next_up()` also holds.
     ///
     /// ```rust
-    /// #![feature(float_next_up_down)]
     /// let x = 1.0f32;
     /// // Clamp value into range [0, 1).
     /// let clamped = x.clamp(0.0, 1.0f32.next_down());
@@ -782,12 +784,16 @@ impl f32 {
     /// assert_eq!(clamped.next_up(), 1.0);
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextDown`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
-    #[unstable(feature = "float_next_up_down", issue = "91399")]
+    #[doc(alias = "nextDown")]
+    #[stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
     pub const fn next_down(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
@@ -818,7 +824,7 @@ impl f32 {
     /// ```
     #[must_use = "this returns the result of the operation, without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn recip(self) -> f32 {
         1.0 / self
@@ -836,7 +842,7 @@ impl f32 {
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
     #[stable(feature = "f32_deg_rad_conversions", since = "1.7.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn to_degrees(self) -> f32 {
         // Use a constant for better precision.
@@ -856,7 +862,7 @@ impl f32 {
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
     #[stable(feature = "f32_deg_rad_conversions", since = "1.7.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn to_radians(self) -> f32 {
         const RADS_PER_DEG: f32 = consts::PI / 180.0;
@@ -878,7 +884,7 @@ impl f32 {
     /// ```
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn max(self, other: f32) -> f32 {
         intrinsics::maxnumf32(self, other)
@@ -899,7 +905,7 @@ impl f32 {
     /// ```
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn min(self, other: f32) -> f32 {
         intrinsics::minnumf32(self, other)
@@ -988,8 +994,8 @@ impl f32 {
     /// assert_eq!((-5.5f32).midpoint(8.0), 1.25);
     /// ```
     #[inline]
-    #[stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
-    #[rustc_const_stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "num_midpoint", since = "1.85.0")]
+    #[rustc_const_stable(feature = "num_midpoint", since = "1.85.0")]
     pub const fn midpoint(self, other: f32) -> f32 {
         cfg_if! {
             // Allow faster implementation that have known good 64-bit float
@@ -1396,7 +1402,7 @@ impl f32 {
     /// ```
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[stable(feature = "clamp", since = "1.50.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn clamp(mut self, min: f32, max: f32) -> f32 {
         const_assert!(
@@ -1433,7 +1439,7 @@ impl f32 {
     /// ```
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn abs(self) -> f32 {
         // SAFETY: this is actually a safe intrinsic
@@ -1458,7 +1464,7 @@ impl f32 {
     /// ```
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn signum(self) -> f32 {
         if self.is_nan() { Self::NAN } else { 1.0_f32.copysign(self) }
@@ -1493,7 +1499,7 @@ impl f32 {
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[inline]
     #[stable(feature = "copysign", since = "1.35.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     pub const fn copysign(self, sign: f32) -> f32 {
         // SAFETY: this is actually a safe intrinsic
         unsafe { intrinsics::copysignf32(self, sign) }
diff --git a/library/core/src/num/f64.rs b/library/core/src/num/f64.rs
index 77ca56df067..1b0651a0def 100644
--- a/library/core/src/num/f64.rs
+++ b/library/core/src/num/f64.rs
@@ -743,7 +743,6 @@ impl f64 {
     /// is finite `x == x.next_up().next_down()` also holds.
     ///
     /// ```rust
-    /// #![feature(float_next_up_down)]
     /// // f64::EPSILON is the difference between 1.0 and the next number up.
     /// assert_eq!(1.0f64.next_up(), 1.0 + f64::EPSILON);
     /// // But not for most numbers.
@@ -751,12 +750,16 @@ impl f64 {
     /// assert_eq!(9007199254740992f64.next_up(), 9007199254740994.0);
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextUp`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
-    #[unstable(feature = "float_next_up_down", issue = "91399")]
+    #[doc(alias = "nextUp")]
+    #[stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
     pub const fn next_up(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
@@ -791,7 +794,6 @@ impl f64 {
     /// is finite `x == x.next_down().next_up()` also holds.
     ///
     /// ```rust
-    /// #![feature(float_next_up_down)]
     /// let x = 1.0f64;
     /// // Clamp value into range [0, 1).
     /// let clamped = x.clamp(0.0, 1.0f64.next_down());
@@ -799,12 +801,16 @@ impl f64 {
     /// assert_eq!(clamped.next_up(), 1.0);
     /// ```
     ///
+    /// This operation corresponds to IEEE-754 `nextDown`.
+    ///
     /// [`NEG_INFINITY`]: Self::NEG_INFINITY
     /// [`INFINITY`]: Self::INFINITY
     /// [`MIN`]: Self::MIN
     /// [`MAX`]: Self::MAX
     #[inline]
-    #[unstable(feature = "float_next_up_down", issue = "91399")]
+    #[doc(alias = "nextDown")]
+    #[stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "float_next_up_down", since = "CURRENT_RUSTC_VERSION")]
     pub const fn next_down(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
@@ -835,7 +841,7 @@ impl f64 {
     /// ```
     #[must_use = "this returns the result of the operation, without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn recip(self) -> f64 {
         1.0 / self
@@ -853,7 +859,7 @@ impl f64 {
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn to_degrees(self) -> f64 {
         // The division here is correctly rounded with respect to the true
@@ -874,7 +880,7 @@ impl f64 {
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn to_radians(self) -> f64 {
         const RADS_PER_DEG: f64 = consts::PI / 180.0;
@@ -896,7 +902,7 @@ impl f64 {
     /// ```
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn max(self, other: f64) -> f64 {
         intrinsics::maxnumf64(self, other)
@@ -917,7 +923,7 @@ impl f64 {
     /// ```
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn min(self, other: f64) -> f64 {
         intrinsics::minnumf64(self, other)
@@ -1006,8 +1012,8 @@ impl f64 {
     /// assert_eq!((-5.5f64).midpoint(8.0), 1.25);
     /// ```
     #[inline]
-    #[stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
-    #[rustc_const_stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "num_midpoint", since = "1.85.0")]
+    #[rustc_const_stable(feature = "num_midpoint", since = "1.85.0")]
     pub const fn midpoint(self, other: f64) -> f64 {
         const LO: f64 = f64::MIN_POSITIVE * 2.;
         const HI: f64 = f64::MAX / 2.;
@@ -1396,7 +1402,7 @@ impl f64 {
     /// ```
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[stable(feature = "clamp", since = "1.50.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn clamp(mut self, min: f64, max: f64) -> f64 {
         const_assert!(
@@ -1433,7 +1439,7 @@ impl f64 {
     /// ```
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn abs(self) -> f64 {
         // SAFETY: this is actually a safe intrinsic
@@ -1458,7 +1464,7 @@ impl f64 {
     /// ```
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn signum(self) -> f64 {
         if self.is_nan() { Self::NAN } else { 1.0_f64.copysign(self) }
@@ -1492,7 +1498,7 @@ impl f64 {
     /// ```
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[stable(feature = "copysign", since = "1.35.0")]
-    #[rustc_const_stable(feature = "const_float_methods", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn copysign(self, sign: f64) -> f64 {
         // SAFETY: this is actually a safe intrinsic
diff --git a/library/core/src/num/flt2dec/mod.rs b/library/core/src/num/flt2dec/mod.rs
index d6413fadc33..7601e3e2c58 100644
--- a/library/core/src/num/flt2dec/mod.rs
+++ b/library/core/src/num/flt2dec/mod.rs
@@ -210,10 +210,10 @@ fn digits_to_dec_str<'a>(
         if frac_digits > buf.len() && frac_digits - buf.len() > minus_exp {
             parts[3] = MaybeUninit::new(Part::Zero((frac_digits - buf.len()) - minus_exp));
             // SAFETY: we just initialized the elements `..4`.
-            unsafe { MaybeUninit::slice_assume_init_ref(&parts[..4]) }
+            unsafe { parts[..4].assume_init_ref() }
         } else {
             // SAFETY: we just initialized the elements `..3`.
-            unsafe { MaybeUninit::slice_assume_init_ref(&parts[..3]) }
+            unsafe { parts[..3].assume_init_ref() }
         }
     } else {
         let exp = exp as usize;
@@ -225,10 +225,10 @@ fn digits_to_dec_str<'a>(
             if frac_digits > buf.len() - exp {
                 parts[3] = MaybeUninit::new(Part::Zero(frac_digits - (buf.len() - exp)));
                 // SAFETY: we just initialized the elements `..4`.
-                unsafe { MaybeUninit::slice_assume_init_ref(&parts[..4]) }
+                unsafe { parts[..4].assume_init_ref() }
             } else {
                 // SAFETY: we just initialized the elements `..3`.
-                unsafe { MaybeUninit::slice_assume_init_ref(&parts[..3]) }
+                unsafe { parts[..3].assume_init_ref() }
             }
         } else {
             // the decimal point is after rendered digits: [1234][____0000] or [1234][__][.][__].
@@ -238,10 +238,10 @@ fn digits_to_dec_str<'a>(
                 parts[2] = MaybeUninit::new(Part::Copy(b"."));
                 parts[3] = MaybeUninit::new(Part::Zero(frac_digits));
                 // SAFETY: we just initialized the elements `..4`.
-                unsafe { MaybeUninit::slice_assume_init_ref(&parts[..4]) }
+                unsafe { parts[..4].assume_init_ref() }
             } else {
                 // SAFETY: we just initialized the elements `..2`.
-                unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) }
+                unsafe { parts[..2].assume_init_ref() }
             }
         }
     }
@@ -292,7 +292,7 @@ fn digits_to_exp_str<'a>(
         parts[n + 1] = MaybeUninit::new(Part::Num(exp as u16));
     }
     // SAFETY: we just initialized the elements `..n + 2`.
-    unsafe { MaybeUninit::slice_assume_init_ref(&parts[..n + 2]) }
+    unsafe { parts[..n + 2].assume_init_ref() }
 }
 
 /// Sign formatting options.
@@ -366,12 +366,12 @@ where
         FullDecoded::Nan => {
             parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Infinite => {
             parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Zero => {
             if frac_digits > 0 {
@@ -381,14 +381,14 @@ where
                 Formatted {
                     sign,
                     // SAFETY: we just initialized the elements `..2`.
-                    parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) },
+                    parts: unsafe { parts[..2].assume_init_ref() },
                 }
             } else {
                 parts[0] = MaybeUninit::new(Part::Copy(b"0"));
                 Formatted {
                     sign,
                     // SAFETY: we just initialized the elements `..1`.
-                    parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+                    parts: unsafe { parts[..1].assume_init_ref() },
                 }
             }
         }
@@ -442,12 +442,12 @@ where
         FullDecoded::Nan => {
             parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Infinite => {
             parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Zero => {
             parts[0] = if dec_bounds.0 <= 0 && 0 < dec_bounds.1 {
@@ -456,7 +456,7 @@ where
                 MaybeUninit::new(Part::Copy(if upper { b"0E0" } else { b"0e0" }))
             };
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Finite(ref decoded) => {
             let (buf, exp) = format_shortest(decoded, buf);
@@ -533,12 +533,12 @@ where
         FullDecoded::Nan => {
             parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Infinite => {
             parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Zero => {
             if ndigits > 1 {
@@ -549,14 +549,14 @@ where
                 Formatted {
                     sign,
                     // SAFETY: we just initialized the elements `..3`.
-                    parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..3]) },
+                    parts: unsafe { parts[..3].assume_init_ref() },
                 }
             } else {
                 parts[0] = MaybeUninit::new(Part::Copy(if upper { b"0E0" } else { b"0e0" }));
                 Formatted {
                     sign,
                     // SAFETY: we just initialized the elements `..1`.
-                    parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+                    parts: unsafe { parts[..1].assume_init_ref() },
                 }
             }
         }
@@ -607,12 +607,12 @@ where
         FullDecoded::Nan => {
             parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Infinite => {
             parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
             // SAFETY: we just initialized the elements `..1`.
-            Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+            Formatted { sign, parts: unsafe { parts[..1].assume_init_ref() } }
         }
         FullDecoded::Zero => {
             if frac_digits > 0 {
@@ -622,14 +622,14 @@ where
                 Formatted {
                     sign,
                     // SAFETY: we just initialized the elements `..2`.
-                    parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) },
+                    parts: unsafe { parts[..2].assume_init_ref() },
                 }
             } else {
                 parts[0] = MaybeUninit::new(Part::Copy(b"0"));
                 Formatted {
                     sign,
                     // SAFETY: we just initialized the elements `..1`.
-                    parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+                    parts: unsafe { parts[..1].assume_init_ref() },
                 }
             }
         }
@@ -654,14 +654,14 @@ where
                     Formatted {
                         sign,
                         // SAFETY: we just initialized the elements `..2`.
-                        parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) },
+                        parts: unsafe { parts[..2].assume_init_ref() },
                     }
                 } else {
                     parts[0] = MaybeUninit::new(Part::Copy(b"0"));
                     Formatted {
                         sign,
                         // SAFETY: we just initialized the elements `..1`.
-                        parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+                        parts: unsafe { parts[..1].assume_init_ref() },
                     }
                 }
             } else {
diff --git a/library/core/src/num/flt2dec/strategy/dragon.rs b/library/core/src/num/flt2dec/strategy/dragon.rs
index e801f07b3bc..dd73e4b4846 100644
--- a/library/core/src/num/flt2dec/strategy/dragon.rs
+++ b/library/core/src/num/flt2dec/strategy/dragon.rs
@@ -247,7 +247,7 @@ pub fn format_shortest<'a>(
         // it seems that this condition is very hard to satisfy (possibly impossible),
         // but we are just being safe and consistent here.
         // SAFETY: we initialized that memory above.
-        if let Some(c) = round_up(unsafe { MaybeUninit::slice_assume_init_mut(&mut buf[..i]) }) {
+        if let Some(c) = round_up(unsafe { buf[..i].assume_init_mut() }) {
             buf[i] = MaybeUninit::new(c);
             i += 1;
             k += 1;
@@ -255,7 +255,7 @@ pub fn format_shortest<'a>(
     }
 
     // SAFETY: we initialized that memory above.
-    (unsafe { MaybeUninit::slice_assume_init_ref(&buf[..i]) }, k)
+    (unsafe { buf[..i].assume_init_ref() }, k)
 }
 
 /// The exact and fixed mode implementation for Dragon.
@@ -333,7 +333,7 @@ pub fn format_exact<'a>(
                     *c = MaybeUninit::new(b'0');
                 }
                 // SAFETY: we initialized that memory above.
-                return (unsafe { MaybeUninit::slice_assume_init_ref(&buf[..len]) }, k);
+                return (unsafe { buf[..len].assume_init_ref() }, k);
             }
 
             let mut d = 0;
@@ -372,7 +372,7 @@ pub fn format_exact<'a>(
         // if rounding up changes the length, the exponent should also change.
         // but we've been requested a fixed number of digits, so do not alter the buffer...
         // SAFETY: we initialized that memory above.
-        if let Some(c) = round_up(unsafe { MaybeUninit::slice_assume_init_mut(&mut buf[..len]) }) {
+        if let Some(c) = round_up(unsafe { buf[..len].assume_init_mut() }) {
             // ...unless we've been requested the fixed precision instead.
             // we also need to check that, if the original buffer was empty,
             // the additional digit can only be added when `k == limit` (edge case).
@@ -385,5 +385,5 @@ pub fn format_exact<'a>(
     }
 
     // SAFETY: we initialized that memory above.
-    (unsafe { MaybeUninit::slice_assume_init_ref(&buf[..len]) }, k)
+    (unsafe { buf[..len].assume_init_ref() }, k)
 }
diff --git a/library/core/src/num/flt2dec/strategy/grisu.rs b/library/core/src/num/flt2dec/strategy/grisu.rs
index bdf544a4133..2816de4c633 100644
--- a/library/core/src/num/flt2dec/strategy/grisu.rs
+++ b/library/core/src/num/flt2dec/strategy/grisu.rs
@@ -275,7 +275,7 @@ pub fn format_shortest_opt<'a>(
             let ten_kappa = (ten_kappa as u64) << e; // scale 10^kappa back to the shared exponent
             return round_and_weed(
                 // SAFETY: we initialized that memory above.
-                unsafe { MaybeUninit::slice_assume_init_mut(&mut buf[..i]) },
+                unsafe { buf[..i].assume_init_mut() },
                 exp,
                 plus1rem,
                 delta1,
@@ -324,7 +324,7 @@ pub fn format_shortest_opt<'a>(
             let ten_kappa = 1 << e; // implicit divisor
             return round_and_weed(
                 // SAFETY: we initialized that memory above.
-                unsafe { MaybeUninit::slice_assume_init_mut(&mut buf[..i]) },
+                unsafe { buf[..i].assume_init_mut() },
                 exp,
                 r,
                 threshold,
@@ -713,7 +713,7 @@ pub fn format_exact_opt<'a>(
         // `10^kappa` did not overflow after all, the second check is fine.
         if ten_kappa - remainder > remainder && ten_kappa - 2 * remainder >= 2 * ulp {
             // SAFETY: our caller initialized that memory.
-            return Some((unsafe { MaybeUninit::slice_assume_init_ref(&buf[..len]) }, exp));
+            return Some((unsafe { buf[..len].assume_init_ref() }, exp));
         }
 
         //   :<------- remainder ------>|   :
@@ -736,7 +736,7 @@ pub fn format_exact_opt<'a>(
         if remainder > ulp && ten_kappa - (remainder - ulp) <= remainder - ulp {
             if let Some(c) =
                 // SAFETY: our caller must have initialized that memory.
-                round_up(unsafe { MaybeUninit::slice_assume_init_mut(&mut buf[..len]) })
+                round_up(unsafe { buf[..len].assume_init_mut() })
             {
                 // only add an additional digit when we've been requested the fixed precision.
                 // we also need to check that, if the original buffer was empty,
@@ -748,7 +748,7 @@ pub fn format_exact_opt<'a>(
                 }
             }
             // SAFETY: we and our caller initialized that memory.
-            return Some((unsafe { MaybeUninit::slice_assume_init_ref(&buf[..len]) }, exp));
+            return Some((unsafe { buf[..len].assume_init_ref() }, exp));
         }
 
         // otherwise we are doomed (i.e., some values between `v - 1 ulp` and `v + 1 ulp` are
diff --git a/library/core/src/num/mod.rs b/library/core/src/num/mod.rs
index 1f5b8ce6c6e..6c1b568e231 100644
--- a/library/core/src/num/mod.rs
+++ b/library/core/src/num/mod.rs
@@ -51,6 +51,10 @@ mod overflow_panic;
 mod saturating;
 mod wrapping;
 
+/// 100% perma-unstable
+#[doc(hidden)]
+pub mod niche_types;
+
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg(not(no_fp_fmt_parse))]
 pub use dec2flt::ParseFloatError;
@@ -138,8 +142,8 @@ macro_rules! midpoint_impl {
         #[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(4), 2);")]
         #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".midpoint(4), 2);")]
         /// ```
-        #[stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
-        #[rustc_const_stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
+        #[stable(feature = "num_midpoint", since = "1.85.0")]
+        #[rustc_const_stable(feature = "num_midpoint", since = "1.85.0")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline]
@@ -192,8 +196,8 @@ macro_rules! midpoint_impl {
         #[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(4), 2);")]
         #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".midpoint(4), 2);")]
         /// ```
-        #[stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
-        #[rustc_const_stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
+        #[stable(feature = "num_midpoint", since = "1.85.0")]
+        #[rustc_const_stable(feature = "num_midpoint", since = "1.85.0")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline]
diff --git a/library/core/src/num/niche_types.rs b/library/core/src/num/niche_types.rs
new file mode 100644
index 00000000000..096713c318f
--- /dev/null
+++ b/library/core/src/num/niche_types.rs
@@ -0,0 +1,168 @@
+#![unstable(
+    feature = "temporary_niche_types",
+    issue = "none",
+    reason = "for core, alloc, and std internals until pattern types are further along"
+)]
+
+use crate::cmp::Ordering;
+use crate::fmt;
+use crate::hash::{Hash, Hasher};
+use crate::marker::StructuralPartialEq;
+
+macro_rules! define_valid_range_type {
+    ($(
+        $(#[$m:meta])*
+        $vis:vis struct $name:ident($int:ident as $uint:ident in $low:literal..=$high:literal);
+    )+) => {$(
+        #[derive(Clone, Copy, Eq)]
+        #[repr(transparent)]
+        #[rustc_layout_scalar_valid_range_start($low)]
+        #[rustc_layout_scalar_valid_range_end($high)]
+        $(#[$m])*
+        $vis struct $name($int);
+
+        const _: () = {
+            // With the `valid_range` attributes, it's always specified as unsigned
+            assert!(<$uint>::MIN == 0);
+            let ulow: $uint = $low;
+            let uhigh: $uint = $high;
+            assert!(ulow <= uhigh);
+
+            assert!(size_of::<$int>() == size_of::<$uint>());
+        };
+
+        impl $name {
+            /// Constructs an instance of this type from the underlying integer
+            /// primitive without checking whether its zero.
+            ///
+            /// # Safety
+            /// Immediate language UB if `val == 0`, as it violates the validity
+            /// invariant of this type.
+            #[inline]
+            pub const unsafe fn new_unchecked(val: $int) -> Self {
+                // SAFETY: Caller promised that `val` is non-zero.
+                unsafe { $name(val) }
+            }
+
+            #[inline]
+            pub const fn as_inner(self) -> $int {
+                // SAFETY: This is a transparent wrapper, so unwrapping it is sound
+                // (Not using `.0` due to MCP#807.)
+                unsafe { crate::mem::transmute(self) }
+            }
+        }
+
+        // This is required to allow matching a constant.  We don't get it from a derive
+        // because the derived `PartialEq` would do a field projection, which is banned
+        // by <https://github.com/rust-lang/compiler-team/issues/807>.
+        impl StructuralPartialEq for $name {}
+
+        impl PartialEq for $name {
+            #[inline]
+            fn eq(&self, other: &Self) -> bool {
+                self.as_inner() == other.as_inner()
+            }
+        }
+
+        impl Ord for $name {
+            #[inline]
+            fn cmp(&self, other: &Self) -> Ordering {
+                Ord::cmp(&self.as_inner(), &other.as_inner())
+            }
+        }
+
+        impl PartialOrd for $name {
+            #[inline]
+            fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+                Some(Ord::cmp(self, other))
+            }
+        }
+
+        impl Hash for $name {
+            // Required method
+            fn hash<H: Hasher>(&self, state: &mut H) {
+                Hash::hash(&self.as_inner(), state);
+            }
+        }
+
+        impl fmt::Debug for $name {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                <$int as fmt::Debug>::fmt(&self.as_inner(), f)
+            }
+        }
+    )+};
+}
+
+define_valid_range_type! {
+    pub struct Nanoseconds(u32 as u32 in 0..=999_999_999);
+}
+
+impl Nanoseconds {
+    // SAFETY: 0 is within the valid range
+    pub const ZERO: Self = unsafe { Nanoseconds::new_unchecked(0) };
+}
+
+impl Default for Nanoseconds {
+    #[inline]
+    fn default() -> Self {
+        Self::ZERO
+    }
+}
+
+define_valid_range_type! {
+    pub struct NonZeroU8Inner(u8 as u8 in 1..=0xff);
+    pub struct NonZeroU16Inner(u16 as u16 in 1..=0xff_ff);
+    pub struct NonZeroU32Inner(u32 as u32 in 1..=0xffff_ffff);
+    pub struct NonZeroU64Inner(u64 as u64 in 1..=0xffffffff_ffffffff);
+    pub struct NonZeroU128Inner(u128 as u128 in 1..=0xffffffffffffffff_ffffffffffffffff);
+
+    pub struct NonZeroI8Inner(i8 as u8 in 1..=0xff);
+    pub struct NonZeroI16Inner(i16 as u16 in 1..=0xff_ff);
+    pub struct NonZeroI32Inner(i32 as u32 in 1..=0xffff_ffff);
+    pub struct NonZeroI64Inner(i64 as u64 in 1..=0xffffffff_ffffffff);
+    pub struct NonZeroI128Inner(i128 as u128 in 1..=0xffffffffffffffff_ffffffffffffffff);
+}
+
+#[cfg(target_pointer_width = "16")]
+define_valid_range_type! {
+    pub struct UsizeNoHighBit(usize as usize in 0..=0x7fff);
+    pub struct NonZeroUsizeInner(usize as usize in 1..=0xffff);
+    pub struct NonZeroIsizeInner(isize as usize in 1..=0xffff);
+}
+#[cfg(target_pointer_width = "32")]
+define_valid_range_type! {
+    pub struct UsizeNoHighBit(usize as usize in 0..=0x7fff_ffff);
+    pub struct NonZeroUsizeInner(usize as usize in 1..=0xffff_ffff);
+    pub struct NonZeroIsizeInner(isize as usize in 1..=0xffff_ffff);
+}
+#[cfg(target_pointer_width = "64")]
+define_valid_range_type! {
+    pub struct UsizeNoHighBit(usize as usize in 0..=0x7fff_ffff_ffff_ffff);
+    pub struct NonZeroUsizeInner(usize as usize in 1..=0xffff_ffff_ffff_ffff);
+    pub struct NonZeroIsizeInner(isize as usize in 1..=0xffff_ffff_ffff_ffff);
+}
+
+define_valid_range_type! {
+    pub struct U32NotAllOnes(u32 as u32 in 0..=0xffff_fffe);
+    pub struct I32NotAllOnes(i32 as u32 in 0..=0xffff_fffe);
+
+    pub struct U64NotAllOnes(u64 as u64 in 0..=0xffff_ffff_ffff_fffe);
+    pub struct I64NotAllOnes(i64 as u64 in 0..=0xffff_ffff_ffff_fffe);
+}
+
+pub trait NotAllOnesHelper {
+    type Type;
+}
+pub type NotAllOnes<T> = <T as NotAllOnesHelper>::Type;
+impl NotAllOnesHelper for u32 {
+    type Type = U32NotAllOnes;
+}
+impl NotAllOnesHelper for i32 {
+    type Type = I32NotAllOnes;
+}
+impl NotAllOnesHelper for u64 {
+    type Type = U64NotAllOnes;
+}
+impl NotAllOnesHelper for i64 {
+    type Type = I64NotAllOnes;
+}
diff --git a/library/core/src/num/nonzero.rs b/library/core/src/num/nonzero.rs
index a9294306b1b..dbce64420ac 100644
--- a/library/core/src/num/nonzero.rs
+++ b/library/core/src/num/nonzero.rs
@@ -37,41 +37,12 @@ pub unsafe trait ZeroablePrimitive: Sized + Copy + private::Sealed {
 macro_rules! impl_zeroable_primitive {
     ($($NonZeroInner:ident ( $primitive:ty )),+ $(,)?) => {
         mod private {
-            use super::*;
-
             #[unstable(
                 feature = "nonzero_internals",
                 reason = "implementation detail which may disappear or be replaced at any time",
                 issue = "none"
             )]
             pub trait Sealed {}
-
-            $(
-                // This inner type is never shown directly, so intentionally does not have Debug
-                #[expect(missing_debug_implementations)]
-                // Since this struct is non-generic and derives Copy,
-                // the derived Clone is `*self` and thus doesn't field-project.
-                #[derive(Clone, Copy)]
-                #[repr(transparent)]
-                #[rustc_layout_scalar_valid_range_start(1)]
-                #[rustc_nonnull_optimization_guaranteed]
-                #[unstable(
-                    feature = "nonzero_internals",
-                    reason = "implementation detail which may disappear or be replaced at any time",
-                    issue = "none"
-                )]
-                pub struct $NonZeroInner($primitive);
-
-                // This is required to allow matching a constant.  We don't get it from a derive
-                // because the derived `PartialEq` would do a field projection, which is banned
-                // by <https://github.com/rust-lang/compiler-team/issues/807>.
-                #[unstable(
-                    feature = "nonzero_internals",
-                    reason = "implementation detail which may disappear or be replaced at any time",
-                    issue = "none"
-                )]
-                impl StructuralPartialEq for $NonZeroInner {}
-            )+
         }
 
         $(
@@ -88,7 +59,7 @@ macro_rules! impl_zeroable_primitive {
                 issue = "none"
             )]
             unsafe impl ZeroablePrimitive for $primitive {
-                type NonZeroInner = private::$NonZeroInner;
+                type NonZeroInner = super::niche_types::$NonZeroInner;
             }
         )+
     };
@@ -1543,8 +1514,8 @@ macro_rules! nonzero_integer_signedness_dependent_methods {
         /// # Some(())
         /// # }
         /// ```
-        #[stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
-        #[rustc_const_stable(feature = "num_midpoint", since = "CURRENT_RUSTC_VERSION")]
+        #[stable(feature = "num_midpoint", since = "1.85.0")]
+        #[rustc_const_stable(feature = "num_midpoint", since = "1.85.0")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline]
diff --git a/library/core/src/ops/arith.rs b/library/core/src/ops/arith.rs
index 565bccf5898..fe7ff2d9ede 100644
--- a/library/core/src/ops/arith.rs
+++ b/library/core/src/ops/arith.rs
@@ -65,6 +65,7 @@
 /// ```
 #[lang = "add"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_ops", issue = "90080")]
 #[rustc_on_unimplemented(
     on(all(_Self = "{integer}", Rhs = "{float}"), message = "cannot add a float to an integer",),
     on(all(_Self = "{float}", Rhs = "{integer}"), message = "cannot add an integer to a float",),
@@ -73,7 +74,7 @@
     append_const_msg
 )]
 #[doc(alias = "+")]
-#[cfg_attr(not(bootstrap), const_trait)]
+#[const_trait]
 pub trait Add<Rhs = Self> {
     /// The resulting type after applying the `+` operator.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -95,18 +96,6 @@ pub trait Add<Rhs = Self> {
 macro_rules! add_impl {
     ($($t:ty)*) => ($(
         #[stable(feature = "rust1", since = "1.0.0")]
-        #[cfg(bootstrap)]
-        impl Add for $t {
-            type Output = $t;
-
-            #[inline]
-            #[track_caller]
-            #[rustc_inherit_overflow_checks]
-            fn add(self, other: $t) -> $t { self + other }
-        }
-
-        #[stable(feature = "rust1", since = "1.0.0")]
-        #[cfg(not(bootstrap))]
         impl const Add for $t {
             type Output = $t;
 
diff --git a/library/core/src/ops/async_function.rs b/library/core/src/ops/async_function.rs
index 0073afd4960..c90ae7babbd 100644
--- a/library/core/src/ops/async_function.rs
+++ b/library/core/src/ops/async_function.rs
@@ -4,8 +4,7 @@ use crate::marker::Tuple;
 /// An async-aware version of the [`Fn`](crate::ops::Fn) trait.
 ///
 /// All `async fn` and functions returning futures implement this trait.
-#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+#[stable(feature = "async_closure", since = "1.85.0")]
 #[rustc_paren_sugar]
 #[fundamental]
 #[must_use = "async closures are lazy and do nothing unless called"]
@@ -19,8 +18,7 @@ pub trait AsyncFn<Args: Tuple>: AsyncFnMut<Args> {
 /// An async-aware version of the [`FnMut`](crate::ops::FnMut) trait.
 ///
 /// All `async fn` and functions returning futures implement this trait.
-#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+#[stable(feature = "async_closure", since = "1.85.0")]
 #[rustc_paren_sugar]
 #[fundamental]
 #[must_use = "async closures are lazy and do nothing unless called"]
@@ -41,8 +39,7 @@ pub trait AsyncFnMut<Args: Tuple>: AsyncFnOnce<Args> {
 /// An async-aware version of the [`FnOnce`](crate::ops::FnOnce) trait.
 ///
 /// All `async fn` and functions returning futures implement this trait.
-#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+#[stable(feature = "async_closure", since = "1.85.0")]
 #[rustc_paren_sugar]
 #[fundamental]
 #[must_use = "async closures are lazy and do nothing unless called"]
@@ -67,8 +64,7 @@ mod impls {
     use super::{AsyncFn, AsyncFnMut, AsyncFnOnce};
     use crate::marker::Tuple;
 
-    #[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-    #[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+    #[stable(feature = "async_closure", since = "1.85.0")]
     impl<A: Tuple, F: ?Sized> AsyncFn<A> for &F
     where
         F: AsyncFn<A>,
@@ -78,8 +74,7 @@ mod impls {
         }
     }
 
-    #[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-    #[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+    #[stable(feature = "async_closure", since = "1.85.0")]
     impl<A: Tuple, F: ?Sized> AsyncFnMut<A> for &F
     where
         F: AsyncFn<A>,
@@ -94,8 +89,7 @@ mod impls {
         }
     }
 
-    #[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-    #[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+    #[stable(feature = "async_closure", since = "1.85.0")]
     impl<'a, A: Tuple, F: ?Sized> AsyncFnOnce<A> for &'a F
     where
         F: AsyncFn<A>,
@@ -108,8 +102,7 @@ mod impls {
         }
     }
 
-    #[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-    #[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+    #[stable(feature = "async_closure", since = "1.85.0")]
     impl<A: Tuple, F: ?Sized> AsyncFnMut<A> for &mut F
     where
         F: AsyncFnMut<A>,
@@ -124,8 +117,7 @@ mod impls {
         }
     }
 
-    #[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-    #[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+    #[stable(feature = "async_closure", since = "1.85.0")]
     impl<'a, A: Tuple, F: ?Sized> AsyncFnOnce<A> for &'a mut F
     where
         F: AsyncFnMut<A>,
diff --git a/library/core/src/ops/deref.rs b/library/core/src/ops/deref.rs
index c36d55fe2a8..11490ea2bfc 100644
--- a/library/core/src/ops/deref.rs
+++ b/library/core/src/ops/deref.rs
@@ -133,7 +133,8 @@
 #[doc(alias = "&*")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "Deref"]
-#[cfg_attr(not(bootstrap), const_trait)]
+#[const_trait]
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
 pub trait Deref {
     /// The resulting type after dereferencing.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -148,18 +149,6 @@ pub trait Deref {
     fn deref(&self) -> &Self::Target;
 }
 
-#[cfg(bootstrap)]
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for &T {
-    type Target = T;
-
-    #[rustc_diagnostic_item = "noop_method_deref"]
-    fn deref(&self) -> &T {
-        *self
-    }
-}
-
-#[cfg(not(bootstrap))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> const Deref for &T {
     type Target = T;
@@ -173,17 +162,6 @@ impl<T: ?Sized> const Deref for &T {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> !DerefMut for &T {}
 
-#[cfg(bootstrap)]
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for &mut T {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        *self
-    }
-}
-
-#[cfg(not(bootstrap))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> const Deref for &mut T {
     type Target = T;
@@ -282,11 +260,11 @@ impl<T: ?Sized> const Deref for &mut T {
 /// *x = 'b';
 /// assert_eq!('b', x.value);
 /// ```
-#[cfg(not(bootstrap))]
 #[lang = "deref_mut"]
 #[doc(alias = "*")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[const_trait]
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
 pub trait DerefMut: ~const Deref {
     /// Mutably dereferences the value.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -294,27 +272,6 @@ pub trait DerefMut: ~const Deref {
     fn deref_mut(&mut self) -> &mut Self::Target;
 }
 
-/// Bootstrap
-#[lang = "deref_mut"]
-#[doc(alias = "*")]
-#[stable(feature = "rust1", since = "1.0.0")]
-#[cfg(bootstrap)]
-pub trait DerefMut: Deref {
-    /// Mutably dereferences the value.
-    #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_diagnostic_item = "deref_mut_method"]
-    fn deref_mut(&mut self) -> &mut Self::Target;
-}
-
-#[cfg(bootstrap)]
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> DerefMut for &mut T {
-    fn deref_mut(&mut self) -> &mut T {
-        *self
-    }
-}
-
-#[cfg(not(bootstrap))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> const DerefMut for &mut T {
     fn deref_mut(&mut self) -> &mut T {
diff --git a/library/core/src/ops/drop.rs b/library/core/src/ops/drop.rs
index f3314364e54..e024b7fb4d3 100644
--- a/library/core/src/ops/drop.rs
+++ b/library/core/src/ops/drop.rs
@@ -203,7 +203,8 @@
 /// [nomicon]: ../../nomicon/phantom-data.html#an-exception-the-special-case-of-the-standard-library-and-its-unstable-may_dangle
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[cfg_attr(not(bootstrap), const_trait)]
+#[const_trait]
+#[rustc_const_unstable(feature = "const_destruct", issue = "133214")]
 pub trait Drop {
     /// Executes the destructor for this type.
     ///
diff --git a/library/core/src/pin.rs b/library/core/src/pin.rs
index 83730285636..71044190f0c 100644
--- a/library/core/src/pin.rs
+++ b/library/core/src/pin.rs
@@ -331,7 +331,7 @@
 //!
 //! Note that this invariant is enforced by simply making it impossible to call code that would
 //! perform a move on the pinned value. This is the case since the only way to access that pinned
-//! value is through the pinning <code>[Pin]<[&mut] T>></code>, which in turn restricts our access.
+//! value is through the pinning <code>[Pin]<[&mut] T></code>, which in turn restricts our access.
 //!
 //! ## [`Unpin`]
 //!
@@ -379,7 +379,7 @@
 //!
 //! Exposing access to the inner field which you want to remain pinned must then be carefully
 //! considered as well! Remember, exposing a method that gives access to a
-//! <code>[Pin]<[&mut] InnerT>></code> where <code>InnerT: [Unpin]</code> would allow safe code to
+//! <code>[Pin]<[&mut] InnerT></code> where <code>InnerT: [Unpin]</code> would allow safe code to
 //! trivially move the inner value out of that pinning pointer, which is precisely what you're
 //! seeking to prevent! Exposing a field of a pinned value through a pinning pointer is called
 //! "projecting" a pin, and the more general case of deciding in which cases a pin should be able
diff --git a/library/core/src/prelude/mod.rs b/library/core/src/prelude/mod.rs
index 9b23874bcf7..d3fda1cd273 100644
--- a/library/core/src/prelude/mod.rs
+++ b/library/core/src/prelude/mod.rs
@@ -71,7 +71,7 @@ pub mod rust_2021 {
 /// The 2024 version of the core prelude.
 ///
 /// See the [module-level documentation](self) for more.
-#[stable(feature = "prelude_2024", since = "CURRENT_RUSTC_VERSION")]
+#[stable(feature = "prelude_2024", since = "1.85.0")]
 pub mod rust_2024 {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::common::*;
@@ -84,7 +84,7 @@ pub mod rust_2024 {
     #[doc(no_inline)]
     pub use crate::convert::{TryFrom, TryInto};
 
-    #[stable(feature = "prelude_2024", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "prelude_2024", since = "1.85.0")]
     #[doc(no_inline)]
     pub use crate::future::{Future, IntoFuture};
 }
diff --git a/library/core/src/ptr/alignment.rs b/library/core/src/ptr/alignment.rs
index 74a1d40f4e7..2da94e72566 100644
--- a/library/core/src/ptr/alignment.rs
+++ b/library/core/src/ptr/alignment.rs
@@ -42,9 +42,10 @@ impl Alignment {
     /// but in an `Alignment` instead of a `usize`.
     #[unstable(feature = "ptr_alignment_type", issue = "102070")]
     #[inline]
+    #[must_use]
     pub const fn of<T>() -> Self {
-        // SAFETY: rustc ensures that type alignment is always a power of two.
-        unsafe { Alignment::new_unchecked(mem::align_of::<T>()) }
+        // This can't actually panic since type alignment is always a power of two.
+        const { Alignment::new(mem::align_of::<T>()).unwrap() }
     }
 
     /// Creates an `Alignment` from a `usize`, or returns `None` if it's
@@ -95,8 +96,13 @@ impl Alignment {
     #[unstable(feature = "ptr_alignment_type", issue = "102070")]
     #[inline]
     pub const fn as_nonzero(self) -> NonZero<usize> {
+        // This transmutes directly to avoid the UbCheck in `NonZero::new_unchecked`
+        // since there's no way for the user to trip that check anyway -- the
+        // validity invariant of the type would have to have been broken earlier --
+        // and emitting it in an otherwise simple method is bad for compile time.
+
         // SAFETY: All the discriminants are non-zero.
-        unsafe { NonZero::new_unchecked(self.as_usize()) }
+        unsafe { mem::transmute::<Alignment, NonZero<usize>>(self) }
     }
 
     /// Returns the base-2 logarithm of the alignment.
diff --git a/library/core/src/ptr/metadata.rs b/library/core/src/ptr/metadata.rs
index b1d5e1fa3a0..e93b5658e24 100644
--- a/library/core/src/ptr/metadata.rs
+++ b/library/core/src/ptr/metadata.rs
@@ -53,9 +53,8 @@ use crate::ptr::NonNull;
 ///
 /// [`to_raw_parts`]: *const::to_raw_parts
 #[lang = "pointee_trait"]
-#[cfg_attr(bootstrap, rustc_deny_explicit_impl(implement_via_object = false))]
-#[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
-#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
+#[rustc_deny_explicit_impl]
+#[rustc_do_not_implement_via_object]
 pub trait Pointee {
     /// The type for metadata in pointers and references to `Self`.
     #[lang = "metadata_type"]
diff --git a/library/core/src/ptr/mod.rs b/library/core/src/ptr/mod.rs
index a70af793004..e1348552b65 100644
--- a/library/core/src/ptr/mod.rs
+++ b/library/core/src/ptr/mod.rs
@@ -596,12 +596,7 @@ pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {
 #[stable(feature = "strict_provenance", since = "1.84.0")]
 #[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
 pub const fn without_provenance<T>(addr: usize) -> *const T {
-    // An int-to-pointer transmute currently has exactly the intended semantics: it creates a
-    // pointer without provenance. Note that this is *not* a stable guarantee about transmute
-    // semantics, it relies on sysroot crates having special status.
-    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
-    // pointer).
-    unsafe { mem::transmute(addr) }
+    without_provenance_mut(addr)
 }
 
 /// Creates a new pointer that is dangling, but non-null and well-aligned.
@@ -618,7 +613,7 @@ pub const fn without_provenance<T>(addr: usize) -> *const T {
 #[stable(feature = "strict_provenance", since = "1.84.0")]
 #[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
 pub const fn dangling<T>() -> *const T {
-    without_provenance(mem::align_of::<T>())
+    dangling_mut()
 }
 
 /// Creates a pointer with the given address and no [provenance][crate::ptr#provenance].
@@ -661,7 +656,7 @@ pub const fn without_provenance_mut<T>(addr: usize) -> *mut T {
 #[stable(feature = "strict_provenance", since = "1.84.0")]
 #[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
 pub const fn dangling_mut<T>() -> *mut T {
-    without_provenance_mut(mem::align_of::<T>())
+    NonNull::dangling().as_ptr()
 }
 
 /// Converts an address back to a pointer, picking up some previously 'exposed'
@@ -777,7 +772,7 @@ pub fn with_exposed_provenance_mut<T>(addr: usize) -> *mut T {
 /// # type T = i32;
 /// # fn foo() -> T { 42 }
 /// // The temporary holding the return value of `foo` does *not* have its lifetime extended,
-/// // because the surrounding expression involves no function call.
+/// // because the surrounding expression involves a function call.
 /// let p = ptr::from_ref(&foo());
 /// unsafe { p.read() }; // UB! Reading from a dangling pointer ⚠️
 /// ```
@@ -828,7 +823,7 @@ pub const fn from_ref<T: ?Sized>(r: &T) -> *const T {
 /// # type T = i32;
 /// # fn foo() -> T { 42 }
 /// // The temporary holding the return value of `foo` does *not* have its lifetime extended,
-/// // because the surrounding expression involves no function call.
+/// // because the surrounding expression involves a function call.
 /// let p = ptr::from_mut(&mut foo());
 /// unsafe { p.write(T::default()) }; // UB! Writing to a dangling pointer ⚠️
 /// ```
@@ -1009,7 +1004,7 @@ pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_stable(feature = "const_swap", since = "CURRENT_RUSTC_VERSION")]
+#[rustc_const_stable(feature = "const_swap", since = "1.85.0")]
 #[rustc_diagnostic_item = "ptr_swap"]
 pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
     // Give ourselves some scratch space to work with.
@@ -2150,7 +2145,7 @@ pub fn addr_eq<T: ?Sized, U: ?Sized>(p: *const T, q: *const U) -> bool {
 /// ```
 ///
 /// [subtype]: https://doc.rust-lang.org/reference/subtyping.html
-#[stable(feature = "ptr_fn_addr_eq", since = "CURRENT_RUSTC_VERSION")]
+#[stable(feature = "ptr_fn_addr_eq", since = "1.85.0")]
 #[inline(always)]
 #[must_use = "function pointer comparison produces a value"]
 pub fn fn_addr_eq<T: FnPtr, U: FnPtr>(f: T, g: U) -> bool {
diff --git a/library/core/src/ptr/mut_ptr.rs b/library/core/src/ptr/mut_ptr.rs
index d75d570a969..5d9d337f101 100644
--- a/library/core/src/ptr/mut_ptr.rs
+++ b/library/core/src/ptr/mut_ptr.rs
@@ -1594,7 +1594,7 @@ impl<T: ?Sized> *mut T {
     ///
     /// [`ptr::swap`]: crate::ptr::swap()
     #[stable(feature = "pointer_methods", since = "1.26.0")]
-    #[rustc_const_stable(feature = "const_swap", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_swap", since = "1.85.0")]
     #[inline(always)]
     pub const unsafe fn swap(self, with: *mut T)
     where
diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs
index 1058fa42cc1..d93069d384e 100644
--- a/library/core/src/ptr/non_null.rs
+++ b/library/core/src/ptr/non_null.rs
@@ -85,6 +85,20 @@ impl<T: ?Sized> !Send for NonNull<T> {}
 impl<T: ?Sized> !Sync for NonNull<T> {}
 
 impl<T: Sized> NonNull<T> {
+    /// Creates a pointer with the given address and no [provenance][crate::ptr#provenance].
+    ///
+    /// For more details, see the equivalent method on a raw pointer, [`ptr::without_provenance_mut`].
+    ///
+    /// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
+    #[unstable(feature = "nonnull_provenance", issue = "135243")]
+    #[must_use]
+    #[inline]
+    pub const fn without_provenance(addr: NonZero<usize>) -> Self {
+        let pointer = crate::ptr::without_provenance(addr.get());
+        // SAFETY: we know `addr` is non-zero.
+        unsafe { NonNull { pointer } }
+    }
+
     /// Creates a new `NonNull` that is dangling, but well-aligned.
     ///
     /// This is useful for initializing types which lazily allocate, like
@@ -109,9 +123,22 @@ impl<T: Sized> NonNull<T> {
     #[must_use]
     #[inline]
     pub const fn dangling() -> Self {
-        // SAFETY: ptr::dangling_mut() returns a non-null well-aligned pointer.
+        let align = crate::ptr::Alignment::of::<T>();
+        NonNull::without_provenance(align.as_nonzero())
+    }
+
+    /// Converts an address back to a mutable pointer, picking up some previously 'exposed'
+    /// [provenance][crate::ptr#provenance].
+    ///
+    /// For more details, see the equivalent method on a raw pointer, [`ptr::with_exposed_provenance_mut`].
+    ///
+    /// This is an [Exposed Provenance][crate::ptr#exposed-provenance] API.
+    #[unstable(feature = "nonnull_provenance", issue = "135243")]
+    #[inline]
+    pub fn with_exposed_provenance(addr: NonZero<usize>) -> Self {
+        // SAFETY: we know `addr` is non-zero.
         unsafe {
-            let ptr = crate::ptr::dangling_mut::<T>();
+            let ptr = crate::ptr::with_exposed_provenance_mut(addr.get());
             NonNull::new_unchecked(ptr)
         }
     }
@@ -224,7 +251,7 @@ impl<T: ?Sized> NonNull<T> {
     /// }
     /// ```
     #[stable(feature = "nonnull", since = "1.25.0")]
-    #[rustc_const_stable(feature = "const_nonnull_new", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_nonnull_new", since = "1.85.0")]
     #[inline]
     pub const fn new(ptr: *mut T) -> Option<Self> {
         if !ptr.is_null() {
@@ -282,7 +309,7 @@ impl<T: ?Sized> NonNull<T> {
 
     /// Gets the "address" portion of the pointer.
     ///
-    /// For more details see the equivalent method on a raw pointer, [`pointer::addr`].
+    /// For more details, see the equivalent method on a raw pointer, [`pointer::addr`].
     ///
     /// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
     #[must_use]
@@ -294,10 +321,23 @@ impl<T: ?Sized> NonNull<T> {
         unsafe { NonZero::new_unchecked(self.as_ptr().addr()) }
     }
 
+    /// Exposes the ["provenance"][crate::ptr#provenance] part of the pointer for future use in
+    /// [`with_exposed_provenance`][NonNull::with_exposed_provenance] and returns the "address" portion.
+    ///
+    /// For more details, see the equivalent method on a raw pointer, [`pointer::expose_provenance`].
+    ///
+    /// This is an [Exposed Provenance][crate::ptr#exposed-provenance] API.
+    #[unstable(feature = "nonnull_provenance", issue = "135243")]
+    pub fn expose_provenance(self) -> NonZero<usize> {
+        // SAFETY: The pointer is guaranteed by the type to be non-null,
+        // meaning that the address will be non-zero.
+        unsafe { NonZero::new_unchecked(self.as_ptr().expose_provenance()) }
+    }
+
     /// Creates a new pointer with the given address and the [provenance][crate::ptr#provenance] of
     /// `self`.
     ///
-    /// For more details see the equivalent method on a raw pointer, [`pointer::with_addr`].
+    /// For more details, see the equivalent method on a raw pointer, [`pointer::with_addr`].
     ///
     /// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
     #[must_use]
@@ -311,7 +351,7 @@ impl<T: ?Sized> NonNull<T> {
     /// Creates a new pointer by mapping `self`'s address to a new one, preserving the
     /// [provenance][crate::ptr#provenance] of `self`.
     ///
-    /// For more details see the equivalent method on a raw pointer, [`pointer::map_addr`].
+    /// For more details, see the equivalent method on a raw pointer, [`pointer::map_addr`].
     ///
     /// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
     #[must_use]
@@ -1146,7 +1186,7 @@ impl<T: ?Sized> NonNull<T> {
     /// [`ptr::swap`]: crate::ptr::swap()
     #[inline(always)]
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
-    #[rustc_const_stable(feature = "const_swap", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_swap", since = "1.85.0")]
     pub const unsafe fn swap(self, with: NonNull<T>)
     where
         T: Sized,
@@ -1556,7 +1596,6 @@ impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: U
 unsafe impl<T: ?Sized> PinCoerceUnsized for NonNull<T> {}
 
 #[unstable(feature = "pointer_like_trait", issue = "none")]
-#[cfg(not(bootstrap))]
 impl<T> core::marker::PointerLike for NonNull<T> {}
 
 #[stable(feature = "nonnull", since = "1.25.0")]
diff --git a/library/core/src/slice/iter.rs b/library/core/src/slice/iter.rs
index d2842f69008..a687ed7129d 100644
--- a/library/core/src/slice/iter.rs
+++ b/library/core/src/slice/iter.rs
@@ -49,7 +49,7 @@ impl<'a, T> IntoIterator for &'a mut [T] {
 /// // First, we need a slice to call the `iter` method on:
 /// let slice = &[1, 2, 3];
 ///
-/// // Then we call `iter` on the slice to get the `Iter` struct,
+/// // Then we call `iter` on the slice to get the `Iter` iterator,
 /// // and iterate over it:
 /// for element in slice.iter() {
 ///     println!("{element}");
@@ -107,24 +107,20 @@ impl<'a, T> Iter<'a, T> {
 
     /// Views the underlying data as a subslice of the original data.
     ///
-    /// This has the same lifetime as the original slice, and so the
-    /// iterator can continue to be used while this exists.
-    ///
     /// # Examples
     ///
     /// Basic usage:
     ///
     /// ```
     /// // First, we need a slice to call the `iter` method on:
-    /// // struct (`&[usize]` here):
     /// let slice = &[1, 2, 3];
     ///
-    /// // Then we call `iter` on the slice to get the `Iter` struct:
+    /// // Then we call `iter` on the slice to get the `Iter` iterator:
     /// let mut iter = slice.iter();
     /// // Here `as_slice` still returns the whole slice, so this prints "[1, 2, 3]":
     /// println!("{:?}", iter.as_slice());
     ///
-    /// // Now, we call the `next` method to remove the first element of the iterator:
+    /// // Now, we call the `next` method to remove the first element from the iterator:
     /// iter.next();
     /// // Here the iterator does not contain the first element of the slice any more,
     /// // so `as_slice` only returns the last two elements of the slice,
@@ -181,7 +177,7 @@ impl<T> AsRef<[T]> for Iter<'_, T> {
 /// // First, we need a slice to call the `iter_mut` method on:
 /// let slice = &mut [1, 2, 3];
 ///
-/// // Then we call `iter_mut` on the slice to get the `IterMut` struct,
+/// // Then we call `iter_mut` on the slice to get the `IterMut` iterator,
 /// // iterate over it and increment each element value:
 /// for element in slice.iter_mut() {
 ///     *element += 1;
@@ -286,25 +282,30 @@ impl<'a, T> IterMut<'a, T> {
 
     /// Views the underlying data as a subslice of the original data.
     ///
-    /// To avoid creating `&mut [T]` references that alias, the returned slice
-    /// borrows its lifetime from the iterator the method is applied on.
-    ///
     /// # Examples
     ///
     /// Basic usage:
     ///
     /// ```
-    /// let mut slice: &mut [usize] = &mut [1, 2, 3];
+    /// // First, we need a slice to call the `iter_mut` method on:
+    /// let slice = &mut [1, 2, 3];
     ///
-    /// // First, we get the iterator:
+    /// // Then we call `iter_mut` on the slice to get the `IterMut` iterator:
     /// let mut iter = slice.iter_mut();
-    /// // So if we check what the `as_slice` method returns here, we have "[1, 2, 3]":
-    /// assert_eq!(iter.as_slice(), &[1, 2, 3]);
+    /// // Here `as_slice` still returns the whole slice, so this prints "[1, 2, 3]":
+    /// println!("{:?}", iter.as_slice());
     ///
-    /// // Next, we move to the second element of the slice:
-    /// iter.next();
-    /// // Now `as_slice` returns "[2, 3]":
-    /// assert_eq!(iter.as_slice(), &[2, 3]);
+    /// // Now, we call the `next` method to remove the first element from the iterator
+    /// // and increment its value:
+    /// *iter.next().unwrap() += 1;
+    /// // Here the iterator does not contain the first element of the slice any more,
+    /// // so `as_slice` only returns the last two elements of the slice,
+    /// // and so this prints "[2, 3]":
+    /// println!("{:?}", iter.as_slice());
+    ///
+    /// // The underlying slice still contains three elements, but its first element
+    /// // was increased by 1, so this prints "[2, 2, 3]":
+    /// println!("{:?}", slice);
     /// ```
     #[must_use]
     #[stable(feature = "slice_iter_mut_as_slice", since = "1.53.0")]
@@ -315,9 +316,6 @@ impl<'a, T> IterMut<'a, T> {
 
     /// Views the underlying data as a mutable subslice of the original data.
     ///
-    /// To avoid creating `&mut [T]` references that alias, the returned slice
-    /// borrows its lifetime from the iterator the method is applied on.
-    ///
     /// # Examples
     ///
     /// Basic usage:
diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index f800e0d391c..ba5746d0ade 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -913,7 +913,7 @@ impl<T> [T] {
     /// assert!(v == ["a", "b", "e", "d", "c"]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "const_swap", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "const_swap", since = "1.85.0")]
     #[inline]
     #[track_caller]
     pub const fn swap(&mut self, a: usize, b: usize) {
@@ -3071,19 +3071,21 @@ impl<T> [T] {
         sort::unstable::sort(self, &mut |a, b| f(a).lt(&f(b)));
     }
 
-    /// Reorders the slice such that the element at `index` after the reordering is at its final
-    /// sorted position.
+    /// Reorders the slice such that the element at `index` is at a sort-order position. All
+    /// elements before `index` will be `<=` to this value, and all elements after will be `>=` to
+    /// it.
     ///
-    /// This reordering has the additional property that any value at position `i < index` will be
-    /// less than or equal to any value at a position `j > index`. Additionally, this reordering is
-    /// unstable (i.e. any number of equal elements may end up at position `index`), in-place (i.e.
-    /// does not allocate), and runs in *O*(*n*) time. This function is also known as "kth element"
-    /// in other libraries.
+    /// This reordering is unstable (i.e. any element that compares equal to the nth element may end
+    /// up at that position), in-place (i.e.  does not allocate), and runs in *O*(*n*) time. This
+    /// function is also known as "kth element" in other libraries.
+    ///
+    /// Returns a triple that partitions the reordered slice:
+    ///
+    /// * The unsorted subslice before `index`, whose elements all satisfy `x <= self[index]`.
+    ///
+    /// * The element at `index`.
     ///
-    /// It returns a triplet of the following from the reordered slice: the subslice prior to
-    /// `index`, the element at `index`, and the subslice after `index`; accordingly, the values in
-    /// those two subslices will respectively all be less-than-or-equal-to and
-    /// greater-than-or-equal-to the value of the element at `index`.
+    /// * The unsorted subslice after `index`, whose elements all satisfy `x >= self[index]`.
     ///
     /// # Current implementation
     ///
@@ -3096,7 +3098,7 @@ impl<T> [T] {
     ///
     /// # Panics
     ///
-    /// Panics when `index >= len()`, meaning it always panics on empty slices.
+    /// Panics when `index >= len()`, and so always panics on empty slices.
     ///
     /// May panic if the implementation of [`Ord`] for `T` does not implement a [total order].
     ///
@@ -3105,8 +3107,7 @@ impl<T> [T] {
     /// ```
     /// let mut v = [-5i32, 4, 2, -3, 1];
     ///
-    /// // Find the items less than or equal to the median, the median, and greater than or equal to
-    /// // the median.
+    /// // Find the items `<=` to the median, the median itself, and the items `>=` to it.
     /// let (lesser, median, greater) = v.select_nth_unstable(2);
     ///
     /// assert!(lesser == [-3, -5] || lesser == [-5, -3]);
@@ -3132,19 +3133,23 @@ impl<T> [T] {
         sort::select::partition_at_index(self, index, T::lt)
     }
 
-    /// Reorders the slice with a comparator function such that the element at `index` after the
-    /// reordering is at its final sorted position.
+    /// Reorders the slice with a comparator function such that the element at `index` is at a
+    /// sort-order position. All elements before `index` will be `<=` to this value, and all
+    /// elements after will be `>=` to it, according to the comparator function.
     ///
-    /// This reordering has the additional property that any value at position `i < index` will be
-    /// less than or equal to any value at a position `j > index` using the comparator function.
-    /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
-    /// position `index`), in-place (i.e. does not allocate), and runs in *O*(*n*) time. This
+    /// This reordering is unstable (i.e. any element that compares equal to the nth element may end
+    /// up at that position), in-place (i.e.  does not allocate), and runs in *O*(*n*) time. This
     /// function is also known as "kth element" in other libraries.
     ///
-    /// It returns a triplet of the following from the slice reordered according to the provided
-    /// comparator function: the subslice prior to `index`, the element at `index`, and the subslice
-    /// after `index`; accordingly, the values in those two subslices will respectively all be
-    /// less-than-or-equal-to and greater-than-or-equal-to the value of the element at `index`.
+    /// Returns a triple partitioning the reordered slice:
+    ///
+    /// * The unsorted subslice before `index`, whose elements all satisfy
+    ///   `compare(x, self[index]).is_le()`.
+    ///
+    /// * The element at `index`.
+    ///
+    /// * The unsorted subslice after `index`, whose elements all satisfy
+    ///   `compare(x, self[index]).is_ge()`.
     ///
     /// # Current implementation
     ///
@@ -3157,7 +3162,7 @@ impl<T> [T] {
     ///
     /// # Panics
     ///
-    /// Panics when `index >= len()`, meaning it always panics on empty slices.
+    /// Panics when `index >= len()`, and so always panics on empty slices.
     ///
     /// May panic if `compare` does not implement a [total order].
     ///
@@ -3166,13 +3171,13 @@ impl<T> [T] {
     /// ```
     /// let mut v = [-5i32, 4, 2, -3, 1];
     ///
-    /// // Find the items less than or equal to the median, the median, and greater than or equal to
-    /// // the median as if the slice were sorted in descending order.
-    /// let (lesser, median, greater) = v.select_nth_unstable_by(2, |a, b| b.cmp(a));
+    /// // Find the items `>=` to the median, the median itself, and the items `<=` to it, by using
+    /// // a reversed comparator.
+    /// let (before, median, after) = v.select_nth_unstable_by(2, |a, b| b.cmp(a));
     ///
-    /// assert!(lesser == [4, 2] || lesser == [2, 4]);
+    /// assert!(before == [4, 2] || before == [2, 4]);
     /// assert_eq!(median, &mut 1);
-    /// assert!(greater == [-3, -5] || greater == [-5, -3]);
+    /// assert!(after == [-3, -5] || after == [-5, -3]);
     ///
     /// // We are only guaranteed the slice will be one of the following, based on the way we sort
     /// // about the specified index.
@@ -3197,19 +3202,21 @@ impl<T> [T] {
         sort::select::partition_at_index(self, index, |a: &T, b: &T| compare(a, b) == Less)
     }
 
-    /// Reorders the slice with a key extraction function such that the element at `index` after the
-    /// reordering is at its final sorted position.
+    /// Reorders the slice with a key extraction function such that the element at `index` is at a
+    /// sort-order position. All elements before `index` will have keys `<=` to the key at `index`,
+    /// and all elements after will have keys `>=` to it.
     ///
-    /// This reordering has the additional property that any value at position `i < index` will be
-    /// less than or equal to any value at a position `j > index` using the key extraction function.
-    /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
-    /// position `index`), in-place (i.e. does not allocate), and runs in *O*(*n*) time. This
+    /// This reordering is unstable (i.e. any element that compares equal to the nth element may end
+    /// up at that position), in-place (i.e.  does not allocate), and runs in *O*(*n*) time. This
     /// function is also known as "kth element" in other libraries.
     ///
-    /// It returns a triplet of the following from the slice reordered according to the provided key
-    /// extraction function: the subslice prior to `index`, the element at `index`, and the subslice
-    /// after `index`; accordingly, the values in those two subslices will respectively all be
-    /// less-than-or-equal-to and greater-than-or-equal-to the value of the element at `index`.
+    /// Returns a triple partitioning the reordered slice:
+    ///
+    /// * The unsorted subslice before `index`, whose elements all satisfy `f(x) <= f(self[index])`.
+    ///
+    /// * The element at `index`.
+    ///
+    /// * The unsorted subslice after `index`, whose elements all satisfy `f(x) >= f(self[index])`.
     ///
     /// # Current implementation
     ///
@@ -3231,8 +3238,8 @@ impl<T> [T] {
     /// ```
     /// let mut v = [-5i32, 4, 1, -3, 2];
     ///
-    /// // Find the items less than or equal to the median, the median, and greater than or equal to
-    /// // the median as if the slice were sorted according to absolute value.
+    /// // Find the items `<=` to the absolute median, the absolute median itself, and the items
+    /// // `>=` to it.
     /// let (lesser, median, greater) = v.select_nth_unstable_by_key(2, |a| a.abs());
     ///
     /// assert!(lesser == [1, 2] || lesser == [2, 1]);
@@ -3703,6 +3710,7 @@ impl<T> [T] {
     /// [`clone_from_slice`]: slice::clone_from_slice
     /// [`split_at_mut`]: slice::split_at_mut
     #[doc(alias = "memcpy")]
+    #[inline]
     #[stable(feature = "copy_from_slice", since = "1.9.0")]
     #[rustc_const_unstable(feature = "const_copy_from_slice", issue = "131415")]
     #[track_caller]
diff --git a/library/core/src/task/wake.rs b/library/core/src/task/wake.rs
index ba429005fab..4c51ca0a5e4 100644
--- a/library/core/src/task/wake.rs
+++ b/library/core/src/task/wake.rs
@@ -60,7 +60,7 @@ impl RawWaker {
         RawWaker { data, vtable }
     }
 
-    #[stable(feature = "noop_waker", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "noop_waker", since = "1.85.0")]
     const NOOP: RawWaker = {
         const VTABLE: RawWakerVTable = RawWakerVTable::new(
             // Cloning just returns a new no-op raw waker
@@ -564,8 +564,8 @@ impl Waker {
     /// ```
     #[inline]
     #[must_use]
-    #[stable(feature = "noop_waker", since = "CURRENT_RUSTC_VERSION")]
-    #[rustc_const_stable(feature = "noop_waker", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "noop_waker", since = "1.85.0")]
+    #[rustc_const_stable(feature = "noop_waker", since = "1.85.0")]
     pub const fn noop() -> &'static Waker {
         const WAKER: &Waker = &Waker { waker: RawWaker::NOOP };
         WAKER
diff --git a/library/core/src/time.rs b/library/core/src/time.rs
index 2d93148bac0..22bd46c567e 100644
--- a/library/core/src/time.rs
+++ b/library/core/src/time.rs
@@ -21,6 +21,7 @@
 
 use crate::fmt;
 use crate::iter::Sum;
+use crate::num::niche_types::Nanoseconds;
 use crate::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
 
 const NANOS_PER_SEC: u32 = 1_000_000_000;
@@ -37,24 +38,6 @@ const HOURS_PER_DAY: u64 = 24;
 #[unstable(feature = "duration_units", issue = "120301")]
 const DAYS_PER_WEEK: u64 = 7;
 
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
-#[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-#[rustc_layout_scalar_valid_range_end(999_999_999)]
-struct Nanoseconds(u32);
-
-impl Nanoseconds {
-    // SAFETY: 0 is within the valid range
-    const ZERO: Self = unsafe { Nanoseconds(0) };
-}
-
-impl Default for Nanoseconds {
-    #[inline]
-    fn default() -> Self {
-        Self::ZERO
-    }
-}
-
 /// A `Duration` type to represent a span of time, typically used for system
 /// timeouts.
 ///
@@ -211,14 +194,14 @@ impl Duration {
     pub const fn new(secs: u64, nanos: u32) -> Duration {
         if nanos < NANOS_PER_SEC {
             // SAFETY: nanos < NANOS_PER_SEC, therefore nanos is within the valid range
-            Duration { secs, nanos: unsafe { Nanoseconds(nanos) } }
+            Duration { secs, nanos: unsafe { Nanoseconds::new_unchecked(nanos) } }
         } else {
             let secs = secs
                 .checked_add((nanos / NANOS_PER_SEC) as u64)
                 .expect("overflow in Duration::new");
             let nanos = nanos % NANOS_PER_SEC;
             // SAFETY: nanos % NANOS_PER_SEC < NANOS_PER_SEC, therefore nanos is within the valid range
-            Duration { secs, nanos: unsafe { Nanoseconds(nanos) } }
+            Duration { secs, nanos: unsafe { Nanoseconds::new_unchecked(nanos) } }
         }
     }
 
@@ -263,7 +246,7 @@ impl Duration {
         let subsec_millis = (millis % MILLIS_PER_SEC) as u32;
         // SAFETY: (x % 1_000) * 1_000_000 < 1_000_000_000
         //         => x % 1_000 < 1_000
-        let subsec_nanos = unsafe { Nanoseconds(subsec_millis * NANOS_PER_MILLI) };
+        let subsec_nanos = unsafe { Nanoseconds::new_unchecked(subsec_millis * NANOS_PER_MILLI) };
 
         Duration { secs, nanos: subsec_nanos }
     }
@@ -289,7 +272,7 @@ impl Duration {
         let subsec_micros = (micros % MICROS_PER_SEC) as u32;
         // SAFETY: (x % 1_000_000) * 1_000 < 1_000_000_000
         //         => x % 1_000_000 < 1_000_000
-        let subsec_nanos = unsafe { Nanoseconds(subsec_micros * NANOS_PER_MICRO) };
+        let subsec_nanos = unsafe { Nanoseconds::new_unchecked(subsec_micros * NANOS_PER_MICRO) };
 
         Duration { secs, nanos: subsec_nanos }
     }
@@ -320,7 +303,7 @@ impl Duration {
         let secs = nanos / NANOS_PER_SEC;
         let subsec_nanos = (nanos % NANOS_PER_SEC) as u32;
         // SAFETY: x % 1_000_000_000 < 1_000_000_000
-        let subsec_nanos = unsafe { Nanoseconds(subsec_nanos) };
+        let subsec_nanos = unsafe { Nanoseconds::new_unchecked(subsec_nanos) };
 
         Duration { secs, nanos: subsec_nanos }
     }
@@ -458,7 +441,7 @@ impl Duration {
     #[rustc_const_stable(feature = "duration_zero", since = "1.53.0")]
     #[inline]
     pub const fn is_zero(&self) -> bool {
-        self.secs == 0 && self.nanos.0 == 0
+        self.secs == 0 && self.nanos.as_inner() == 0
     }
 
     /// Returns the number of _whole_ seconds contained by this `Duration`.
@@ -509,7 +492,7 @@ impl Duration {
     #[must_use]
     #[inline]
     pub const fn subsec_millis(&self) -> u32 {
-        self.nanos.0 / NANOS_PER_MILLI
+        self.nanos.as_inner() / NANOS_PER_MILLI
     }
 
     /// Returns the fractional part of this `Duration`, in whole microseconds.
@@ -532,7 +515,7 @@ impl Duration {
     #[must_use]
     #[inline]
     pub const fn subsec_micros(&self) -> u32 {
-        self.nanos.0 / NANOS_PER_MICRO
+        self.nanos.as_inner() / NANOS_PER_MICRO
     }
 
     /// Returns the fractional part of this `Duration`, in nanoseconds.
@@ -555,7 +538,7 @@ impl Duration {
     #[must_use]
     #[inline]
     pub const fn subsec_nanos(&self) -> u32 {
-        self.nanos.0
+        self.nanos.as_inner()
     }
 
     /// Returns the total number of whole milliseconds contained by this `Duration`.
@@ -573,7 +556,8 @@ impl Duration {
     #[must_use]
     #[inline]
     pub const fn as_millis(&self) -> u128 {
-        self.secs as u128 * MILLIS_PER_SEC as u128 + (self.nanos.0 / NANOS_PER_MILLI) as u128
+        self.secs as u128 * MILLIS_PER_SEC as u128
+            + (self.nanos.as_inner() / NANOS_PER_MILLI) as u128
     }
 
     /// Returns the total number of whole microseconds contained by this `Duration`.
@@ -591,7 +575,8 @@ impl Duration {
     #[must_use]
     #[inline]
     pub const fn as_micros(&self) -> u128 {
-        self.secs as u128 * MICROS_PER_SEC as u128 + (self.nanos.0 / NANOS_PER_MICRO) as u128
+        self.secs as u128 * MICROS_PER_SEC as u128
+            + (self.nanos.as_inner() / NANOS_PER_MICRO) as u128
     }
 
     /// Returns the total number of nanoseconds contained by this `Duration`.
@@ -609,7 +594,7 @@ impl Duration {
     #[must_use]
     #[inline]
     pub const fn as_nanos(&self) -> u128 {
-        self.secs as u128 * NANOS_PER_SEC as u128 + self.nanos.0 as u128
+        self.secs as u128 * NANOS_PER_SEC as u128 + self.nanos.as_inner() as u128
     }
 
     /// Computes the absolute difference between `self` and `other`.
@@ -649,7 +634,7 @@ impl Duration {
     #[rustc_const_stable(feature = "duration_consts_2", since = "1.58.0")]
     pub const fn checked_add(self, rhs: Duration) -> Option<Duration> {
         if let Some(mut secs) = self.secs.checked_add(rhs.secs) {
-            let mut nanos = self.nanos.0 + rhs.nanos.0;
+            let mut nanos = self.nanos.as_inner() + rhs.nanos.as_inner();
             if nanos >= NANOS_PER_SEC {
                 nanos -= NANOS_PER_SEC;
                 if let Some(new_secs) = secs.checked_add(1) {
@@ -707,11 +692,11 @@ impl Duration {
     #[rustc_const_stable(feature = "duration_consts_2", since = "1.58.0")]
     pub const fn checked_sub(self, rhs: Duration) -> Option<Duration> {
         if let Some(mut secs) = self.secs.checked_sub(rhs.secs) {
-            let nanos = if self.nanos.0 >= rhs.nanos.0 {
-                self.nanos.0 - rhs.nanos.0
+            let nanos = if self.nanos.as_inner() >= rhs.nanos.as_inner() {
+                self.nanos.as_inner() - rhs.nanos.as_inner()
             } else if let Some(sub_secs) = secs.checked_sub(1) {
                 secs = sub_secs;
-                self.nanos.0 + NANOS_PER_SEC - rhs.nanos.0
+                self.nanos.as_inner() + NANOS_PER_SEC - rhs.nanos.as_inner()
             } else {
                 return None;
             };
@@ -763,7 +748,7 @@ impl Duration {
     #[rustc_const_stable(feature = "duration_consts_2", since = "1.58.0")]
     pub const fn checked_mul(self, rhs: u32) -> Option<Duration> {
         // Multiply nanoseconds as u64, because it cannot overflow that way.
-        let total_nanos = self.nanos.0 as u64 * rhs as u64;
+        let total_nanos = self.nanos.as_inner() as u64 * rhs as u64;
         let extra_secs = total_nanos / (NANOS_PER_SEC as u64);
         let nanos = (total_nanos % (NANOS_PER_SEC as u64)) as u32;
         // FIXME(const-hack): use `and_then` once that is possible.
@@ -820,7 +805,8 @@ impl Duration {
     pub const fn checked_div(self, rhs: u32) -> Option<Duration> {
         if rhs != 0 {
             let (secs, extra_secs) = (self.secs / (rhs as u64), self.secs % (rhs as u64));
-            let (mut nanos, extra_nanos) = (self.nanos.0 / rhs, self.nanos.0 % rhs);
+            let (mut nanos, extra_nanos) =
+                (self.nanos.as_inner() / rhs, self.nanos.as_inner() % rhs);
             nanos +=
                 ((extra_secs * (NANOS_PER_SEC as u64) + extra_nanos as u64) / (rhs as u64)) as u32;
             debug_assert!(nanos < NANOS_PER_SEC);
@@ -846,7 +832,7 @@ impl Duration {
     #[inline]
     #[rustc_const_stable(feature = "duration_consts_float", since = "1.83.0")]
     pub const fn as_secs_f64(&self) -> f64 {
-        (self.secs as f64) + (self.nanos.0 as f64) / (NANOS_PER_SEC as f64)
+        (self.secs as f64) + (self.nanos.as_inner() as f64) / (NANOS_PER_SEC as f64)
     }
 
     /// Returns the number of seconds contained by this `Duration` as `f32`.
@@ -865,7 +851,7 @@ impl Duration {
     #[inline]
     #[rustc_const_stable(feature = "duration_consts_float", since = "1.83.0")]
     pub const fn as_secs_f32(&self) -> f32 {
-        (self.secs as f32) + (self.nanos.0 as f32) / (NANOS_PER_SEC as f32)
+        (self.secs as f32) + (self.nanos.as_inner() as f32) / (NANOS_PER_SEC as f32)
     }
 
     /// Returns the number of milliseconds contained by this `Duration` as `f64`.
@@ -885,7 +871,7 @@ impl Duration {
     #[inline]
     pub const fn as_millis_f64(&self) -> f64 {
         (self.secs as f64) * (MILLIS_PER_SEC as f64)
-            + (self.nanos.0 as f64) / (NANOS_PER_MILLI as f64)
+            + (self.nanos.as_inner() as f64) / (NANOS_PER_MILLI as f64)
     }
 
     /// Returns the number of milliseconds contained by this `Duration` as `f32`.
@@ -905,7 +891,7 @@ impl Duration {
     #[inline]
     pub const fn as_millis_f32(&self) -> f32 {
         (self.secs as f32) * (MILLIS_PER_SEC as f32)
-            + (self.nanos.0 as f32) / (NANOS_PER_MILLI as f32)
+            + (self.nanos.as_inner() as f32) / (NANOS_PER_MILLI as f32)
     }
 
     /// Creates a new `Duration` from the specified number of seconds represented
@@ -1084,8 +1070,9 @@ impl Duration {
     #[inline]
     #[rustc_const_stable(feature = "duration_consts_float", since = "1.83.0")]
     pub const fn div_duration_f64(self, rhs: Duration) -> f64 {
-        let self_nanos = (self.secs as f64) * (NANOS_PER_SEC as f64) + (self.nanos.0 as f64);
-        let rhs_nanos = (rhs.secs as f64) * (NANOS_PER_SEC as f64) + (rhs.nanos.0 as f64);
+        let self_nanos =
+            (self.secs as f64) * (NANOS_PER_SEC as f64) + (self.nanos.as_inner() as f64);
+        let rhs_nanos = (rhs.secs as f64) * (NANOS_PER_SEC as f64) + (rhs.nanos.as_inner() as f64);
         self_nanos / rhs_nanos
     }
 
@@ -1105,8 +1092,9 @@ impl Duration {
     #[inline]
     #[rustc_const_stable(feature = "duration_consts_float", since = "1.83.0")]
     pub const fn div_duration_f32(self, rhs: Duration) -> f32 {
-        let self_nanos = (self.secs as f32) * (NANOS_PER_SEC as f32) + (self.nanos.0 as f32);
-        let rhs_nanos = (rhs.secs as f32) * (NANOS_PER_SEC as f32) + (rhs.nanos.0 as f32);
+        let self_nanos =
+            (self.secs as f32) * (NANOS_PER_SEC as f32) + (self.nanos.as_inner() as f32);
+        let rhs_nanos = (rhs.secs as f32) * (NANOS_PER_SEC as f32) + (rhs.nanos.as_inner() as f32);
         self_nanos / rhs_nanos
     }
 }
@@ -1201,13 +1189,13 @@ macro_rules! sum_durations {
         for entry in $iter {
             total_secs =
                 total_secs.checked_add(entry.secs).expect("overflow in iter::sum over durations");
-            total_nanos = match total_nanos.checked_add(entry.nanos.0 as u64) {
+            total_nanos = match total_nanos.checked_add(entry.nanos.as_inner() as u64) {
                 Some(n) => n,
                 None => {
                     total_secs = total_secs
                         .checked_add(total_nanos / NANOS_PER_SEC as u64)
                         .expect("overflow in iter::sum over durations");
-                    (total_nanos % NANOS_PER_SEC as u64) + entry.nanos.0 as u64
+                    (total_nanos % NANOS_PER_SEC as u64) + entry.nanos.as_inner() as u64
                 }
             };
         }
@@ -1399,27 +1387,27 @@ impl fmt::Debug for Duration {
         let prefix = if f.sign_plus() { "+" } else { "" };
 
         if self.secs > 0 {
-            fmt_decimal(f, self.secs, self.nanos.0, NANOS_PER_SEC / 10, prefix, "s")
-        } else if self.nanos.0 >= NANOS_PER_MILLI {
+            fmt_decimal(f, self.secs, self.nanos.as_inner(), NANOS_PER_SEC / 10, prefix, "s")
+        } else if self.nanos.as_inner() >= NANOS_PER_MILLI {
             fmt_decimal(
                 f,
-                (self.nanos.0 / NANOS_PER_MILLI) as u64,
-                self.nanos.0 % NANOS_PER_MILLI,
+                (self.nanos.as_inner() / NANOS_PER_MILLI) as u64,
+                self.nanos.as_inner() % NANOS_PER_MILLI,
                 NANOS_PER_MILLI / 10,
                 prefix,
                 "ms",
             )
-        } else if self.nanos.0 >= NANOS_PER_MICRO {
+        } else if self.nanos.as_inner() >= NANOS_PER_MICRO {
             fmt_decimal(
                 f,
-                (self.nanos.0 / NANOS_PER_MICRO) as u64,
-                self.nanos.0 % NANOS_PER_MICRO,
+                (self.nanos.as_inner() / NANOS_PER_MICRO) as u64,
+                self.nanos.as_inner() % NANOS_PER_MICRO,
                 NANOS_PER_MICRO / 10,
                 prefix,
                 "µs",
             )
         } else {
-            fmt_decimal(f, self.nanos.0 as u64, 0, 1, prefix, "ns")
+            fmt_decimal(f, self.nanos.as_inner() as u64, 0, 1, prefix, "ns")
         }
     }
 }
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 18feee9fb25..a8980c5f30a 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -153,7 +153,10 @@ mod intrinsics;
 mod io;
 mod iter;
 mod lazy;
+#[cfg(not(bootstrap))]
 mod macros;
+#[cfg(bootstrap)]
+mod macros_bootstrap;
 mod manually_drop;
 mod mem;
 mod net;
diff --git a/library/core/tests/macros.rs b/library/core/tests/macros.rs
index fdb4ea29412..b30a40b7df2 100644
--- a/library/core/tests/macros.rs
+++ b/library/core/tests/macros.rs
@@ -10,7 +10,7 @@ struct Struct;
 
 impl Trait for Struct {
     cfg_match! {
-        cfg(feature = "blah") => {
+        feature = "blah" => {
             fn blah(&self) {
                 unimplemented!();
             }
@@ -47,21 +47,21 @@ fn matches_leading_pipe() {
 #[test]
 fn cfg_match_basic() {
     cfg_match! {
-        cfg(target_pointer_width = "64") => { fn f0_() -> bool { true }}
+        target_pointer_width = "64" => { fn f0_() -> bool { true }}
     }
 
     cfg_match! {
-        cfg(unix) => { fn f1_() -> bool { true }}
-        cfg(any(target_os = "macos", target_os = "linux")) => { fn f1_() -> bool { false }}
+        unix => { fn f1_() -> bool { true } }
+        any(target_os = "macos", target_os = "linux") => { fn f1_() -> bool { false }}
     }
 
     cfg_match! {
-        cfg(target_pointer_width = "32") => { fn f2_() -> bool { false }}
-        cfg(target_pointer_width = "64") => { fn f2_() -> bool { true }}
+        target_pointer_width = "32" => { fn f2_() -> bool { false } }
+        target_pointer_width = "64" => { fn f2_() -> bool { true } }
     }
 
     cfg_match! {
-        cfg(target_pointer_width = "16") => { fn f3_() -> i32 { 1 }}
+        target_pointer_width = "16" => { fn f3_() -> i32 { 1 } }
         _ => { fn f3_() -> i32 { 2 }}
     }
 
@@ -83,7 +83,7 @@ fn cfg_match_basic() {
 #[test]
 fn cfg_match_debug_assertions() {
     cfg_match! {
-        cfg(debug_assertions) => {
+        debug_assertions => {
             assert!(cfg!(debug_assertions));
             assert_eq!(4, 2+2);
         }
@@ -98,13 +98,13 @@ fn cfg_match_debug_assertions() {
 #[test]
 fn cfg_match_no_duplication_on_64() {
     cfg_match! {
-        cfg(windows) => {
+        windows => {
             fn foo() {}
         }
-        cfg(unix) => {
+        unix => {
             fn foo() {}
         }
-        cfg(target_pointer_width = "64") => {
+        target_pointer_width = "64" => {
             fn foo() {}
         }
     }
@@ -114,7 +114,7 @@ fn cfg_match_no_duplication_on_64() {
 #[test]
 fn cfg_match_options() {
     cfg_match! {
-        cfg(test) => {
+        test => {
             use core::option::Option as Option2;
             fn works1() -> Option2<u32> { Some(1) }
         }
@@ -122,26 +122,26 @@ fn cfg_match_options() {
     }
 
     cfg_match! {
-        cfg(feature = "foo") => { fn works2() -> bool { false } }
-        cfg(test) => { fn works2() -> bool { true } }
+        feature = "foo" => { fn works2() -> bool { false } }
+        test => { fn works2() -> bool { true } }
         _ => { fn works2() -> bool { false } }
     }
 
     cfg_match! {
-        cfg(feature = "foo") => { fn works3() -> bool { false } }
+        feature = "foo" => { fn works3() -> bool { false } }
         _ => { fn works3() -> bool { true } }
     }
 
     cfg_match! {
-        cfg(test) => {
+        test => {
             use core::option::Option as Option3;
             fn works4() -> Option3<u32> { Some(1) }
         }
     }
 
     cfg_match! {
-        cfg(feature = "foo") => { fn works5() -> bool { false } }
-        cfg(test) => { fn works5() -> bool { true } }
+        feature = "foo" => { fn works5() -> bool { false } }
+        test => { fn works5() -> bool { true } }
     }
 
     assert!(works1().is_some());
@@ -154,7 +154,7 @@ fn cfg_match_options() {
 #[test]
 fn cfg_match_two_functions() {
     cfg_match! {
-        cfg(target_pointer_width = "64") => {
+        target_pointer_width = "64" => {
             fn foo1() {}
             fn bar1() {}
         }
@@ -178,7 +178,7 @@ fn cfg_match_two_functions() {
 
 fn _accepts_expressions() -> i32 {
     cfg_match! {
-        cfg(unix) => { 1 }
+        unix => { 1 }
         _ => { 2 }
     }
 }
@@ -189,7 +189,18 @@ fn _allows_stmt_expr_attributes() {
     let one = 1;
     let two = 2;
     cfg_match! {
-        cfg(unix) => { one * two; }
+        unix => { one * two; }
         _ => { one + two; }
     }
 }
+
+fn _expression() {
+    let _ = cfg_match!({
+        windows => {
+            " XP"
+        }
+        _ => {
+            ""
+        }
+    });
+}
diff --git a/library/core/tests/macros_bootstrap.rs b/library/core/tests/macros_bootstrap.rs
new file mode 100644
index 00000000000..f10ef862c5d
--- /dev/null
+++ b/library/core/tests/macros_bootstrap.rs
@@ -0,0 +1,193 @@
+#![allow(unused_must_use)]
+
+#[allow(dead_code)]
+trait Trait {
+    fn blah(&self);
+}
+
+#[allow(dead_code)]
+struct Struct;
+
+impl Trait for Struct {
+    cfg_match! {
+        cfg(feature = "blah") => {
+            fn blah(&self) {
+                unimplemented!();
+            }
+        }
+        _ => {
+            fn blah(&self) {
+                unimplemented!();
+            }
+        }
+    }
+}
+
+#[test]
+fn assert_eq_trailing_comma() {
+    assert_eq!(1, 1,);
+}
+
+#[test]
+fn assert_escape() {
+    assert!(r#"☃\backslash"#.contains("\\"));
+}
+
+#[test]
+fn assert_ne_trailing_comma() {
+    assert_ne!(1, 2,);
+}
+
+#[rustfmt::skip]
+#[test]
+fn matches_leading_pipe() {
+    matches!(1, | 1 | 2 | 3);
+}
+
+#[test]
+fn cfg_match_basic() {
+    cfg_match! {
+        cfg(target_pointer_width = "64") => { fn f0_() -> bool { true }}
+    }
+
+    cfg_match! {
+        cfg(unix) => { fn f1_() -> bool { true }}
+        cfg(any(target_os = "macos", target_os = "linux")) => { fn f1_() -> bool { false }}
+    }
+
+    cfg_match! {
+        cfg(target_pointer_width = "32") => { fn f2_() -> bool { false }}
+        cfg(target_pointer_width = "64") => { fn f2_() -> bool { true }}
+    }
+
+    cfg_match! {
+        cfg(target_pointer_width = "16") => { fn f3_() -> i32 { 1 }}
+        _ => { fn f3_() -> i32 { 2 }}
+    }
+
+    #[cfg(target_pointer_width = "64")]
+    assert!(f0_());
+
+    #[cfg(unix)]
+    assert!(f1_());
+
+    #[cfg(target_pointer_width = "32")]
+    assert!(!f2_());
+    #[cfg(target_pointer_width = "64")]
+    assert!(f2_());
+
+    #[cfg(not(target_pointer_width = "16"))]
+    assert_eq!(f3_(), 2);
+}
+
+#[test]
+fn cfg_match_debug_assertions() {
+    cfg_match! {
+        cfg(debug_assertions) => {
+            assert!(cfg!(debug_assertions));
+            assert_eq!(4, 2+2);
+        }
+        _ => {
+            assert!(cfg!(not(debug_assertions)));
+            assert_eq!(10, 5+5);
+        }
+    }
+}
+
+#[cfg(target_pointer_width = "64")]
+#[test]
+fn cfg_match_no_duplication_on_64() {
+    cfg_match! {
+        cfg(windows) => {
+            fn foo() {}
+        }
+        cfg(unix) => {
+            fn foo() {}
+        }
+        cfg(target_pointer_width = "64") => {
+            fn foo() {}
+        }
+    }
+    foo();
+}
+
+#[test]
+fn cfg_match_options() {
+    cfg_match! {
+        cfg(test) => {
+            use core::option::Option as Option2;
+            fn works1() -> Option2<u32> { Some(1) }
+        }
+        _ => { fn works1() -> Option<u32> { None } }
+    }
+
+    cfg_match! {
+        cfg(feature = "foo") => { fn works2() -> bool { false } }
+        cfg(test) => { fn works2() -> bool { true } }
+        _ => { fn works2() -> bool { false } }
+    }
+
+    cfg_match! {
+        cfg(feature = "foo") => { fn works3() -> bool { false } }
+        _ => { fn works3() -> bool { true } }
+    }
+
+    cfg_match! {
+        cfg(test) => {
+            use core::option::Option as Option3;
+            fn works4() -> Option3<u32> { Some(1) }
+        }
+    }
+
+    cfg_match! {
+        cfg(feature = "foo") => { fn works5() -> bool { false } }
+        cfg(test) => { fn works5() -> bool { true } }
+    }
+
+    assert!(works1().is_some());
+    assert!(works2());
+    assert!(works3());
+    assert!(works4().is_some());
+    assert!(works5());
+}
+
+#[test]
+fn cfg_match_two_functions() {
+    cfg_match! {
+        cfg(target_pointer_width = "64") => {
+            fn foo1() {}
+            fn bar1() {}
+        }
+        _ => {
+            fn foo2() {}
+            fn bar2() {}
+        }
+    }
+
+    #[cfg(target_pointer_width = "64")]
+    {
+        foo1();
+        bar1();
+    }
+    #[cfg(not(target_pointer_width = "64"))]
+    {
+        foo2();
+        bar2();
+    }
+}
+
+fn _accepts_expressions() -> i32 {
+    cfg_match! {
+        cfg(unix) => { 1 }
+        _ => { 2 }
+    }
+}
+
+fn _allows_stmt_expr_attributes() {
+    let one = 1;
+    let two = 2;
+    cfg_match! {
+        cfg(unix) => { one * two; }
+        _ => { one + two; }
+    }
+}
diff --git a/library/core/tests/mem.rs b/library/core/tests/mem.rs
index f3b4387f6a8..1b5c5fc82a6 100644
--- a/library/core/tests/mem.rs
+++ b/library/core/tests/mem.rs
@@ -200,60 +200,60 @@ fn uninit_array_assume_init() {
 }
 
 #[test]
-fn uninit_write_slice() {
+fn uninit_write_copy_of_slice() {
     let mut dst = [MaybeUninit::new(255); 64];
     let src = [0; 64];
 
-    assert_eq!(MaybeUninit::copy_from_slice(&mut dst, &src), &src);
+    assert_eq!(dst.write_copy_of_slice(&src), &src);
 }
 
 #[test]
 #[should_panic(expected = "source slice length (32) does not match destination slice length (64)")]
-fn uninit_write_slice_panic_lt() {
+fn uninit_write_copy_of_slice_panic_lt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 32];
 
-    MaybeUninit::copy_from_slice(&mut dst, &src);
+    dst.write_copy_of_slice(&src);
 }
 
 #[test]
 #[should_panic(expected = "source slice length (128) does not match destination slice length (64)")]
-fn uninit_write_slice_panic_gt() {
+fn uninit_write_copy_of_slice_panic_gt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 128];
 
-    MaybeUninit::copy_from_slice(&mut dst, &src);
+    dst.write_copy_of_slice(&src);
 }
 
 #[test]
-fn uninit_clone_from_slice() {
+fn uninit_write_clone_of_slice() {
     let mut dst = [MaybeUninit::new(255); 64];
     let src = [0; 64];
 
-    assert_eq!(MaybeUninit::clone_from_slice(&mut dst, &src), &src);
+    assert_eq!(dst.write_clone_of_slice(&src), &src);
 }
 
 #[test]
 #[should_panic(expected = "destination and source slices have different lengths")]
-fn uninit_write_slice_cloned_panic_lt() {
+fn uninit_write_clone_of_slice_panic_lt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 32];
 
-    MaybeUninit::clone_from_slice(&mut dst, &src);
+    dst.write_clone_of_slice(&src);
 }
 
 #[test]
 #[should_panic(expected = "destination and source slices have different lengths")]
-fn uninit_write_slice_cloned_panic_gt() {
+fn uninit_write_clone_of_slice_panic_gt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 128];
 
-    MaybeUninit::clone_from_slice(&mut dst, &src);
+    dst.write_clone_of_slice(&src);
 }
 
 #[test]
 #[cfg(panic = "unwind")]
-fn uninit_write_slice_cloned_mid_panic() {
+fn uninit_write_clone_of_slice_mid_panic() {
     use std::panic;
 
     enum IncrementOrPanic {
@@ -289,7 +289,7 @@ fn uninit_write_slice_cloned_mid_panic() {
     ];
 
     let err = panic::catch_unwind(panic::AssertUnwindSafe(|| {
-        MaybeUninit::clone_from_slice(&mut dst, &src);
+        dst.write_clone_of_slice(&src);
     }));
 
     drop(src);
@@ -317,11 +317,11 @@ impl Drop for Bomb {
 }
 
 #[test]
-fn uninit_write_slice_cloned_no_drop() {
+fn uninit_write_clone_of_slice_no_drop() {
     let mut dst = [MaybeUninit::uninit()];
     let src = [Bomb];
 
-    MaybeUninit::clone_from_slice(&mut dst, &src);
+    dst.write_clone_of_slice(&src);
 
     forget(src);
 }
diff --git a/library/core/tests/ptr.rs b/library/core/tests/ptr.rs
index e6825d8e39e..7cefb615d03 100644
--- a/library/core/tests/ptr.rs
+++ b/library/core/tests/ptr.rs
@@ -304,7 +304,7 @@ fn test_const_nonnull_new() {
 #[test]
 #[cfg(unix)] // printf may not be available on other platforms
 #[allow(deprecated)] // For SipHasher
-#[cfg_attr(not(bootstrap), allow(unpredictable_function_pointer_comparisons))]
+#[allow(unpredictable_function_pointer_comparisons)]
 pub fn test_variadic_fnptr() {
     use core::ffi;
     use core::hash::{Hash, SipHasher};
diff --git a/library/proc_macro/src/bridge/arena.rs b/library/proc_macro/src/bridge/arena.rs
index 1d5986093c8..29636e793f6 100644
--- a/library/proc_macro/src/bridge/arena.rs
+++ b/library/proc_macro/src/bridge/arena.rs
@@ -102,7 +102,7 @@ impl Arena {
     #[allow(clippy::mut_from_ref)] // arena allocator
     pub(crate) fn alloc_str<'a>(&'a self, string: &str) -> &'a mut str {
         let alloc = self.alloc_raw(string.len());
-        let bytes = MaybeUninit::copy_from_slice(alloc, string.as_bytes());
+        let bytes = alloc.write_copy_of_slice(string.as_bytes());
 
         // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
         // and immediately convert the clone back to `&str`.
diff --git a/library/proc_macro/src/lib.rs b/library/proc_macro/src/lib.rs
index 26a09f0daca..b19c9cee75a 100644
--- a/library/proc_macro/src/lib.rs
+++ b/library/proc_macro/src/lib.rs
@@ -419,7 +419,7 @@ pub mod token_stream {
 /// Unquoting is done with `$`, and works by taking the single next ident as the unquoted term.
 /// To quote `$` itself, use `$$`.
 #[unstable(feature = "proc_macro_quote", issue = "54722")]
-#[allow_internal_unstable(proc_macro_def_site, proc_macro_internals)]
+#[allow_internal_unstable(proc_macro_def_site, proc_macro_internals, proc_macro_totokens)]
 #[rustc_builtin_macro]
 pub macro quote($($t:tt)*) {
     /* compiler built-in */
diff --git a/library/proc_macro/src/quote.rs b/library/proc_macro/src/quote.rs
index 04fa696d5e6..bcb15912bb6 100644
--- a/library/proc_macro/src/quote.rs
+++ b/library/proc_macro/src/quote.rs
@@ -4,12 +4,14 @@
 //! This quasiquoter uses macros 2.0 hygiene to reliably access
 //! items from `proc_macro`, to build a `proc_macro::TokenStream`.
 
-use crate::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
+use crate::{
+    Delimiter, Group, Ident, Literal, Punct, Spacing, Span, ToTokens, TokenStream, TokenTree,
+};
 
-macro_rules! quote_tt {
-    (($($t:tt)*)) => { Group::new(Delimiter::Parenthesis, quote!($($t)*)) };
-    ([$($t:tt)*]) => { Group::new(Delimiter::Bracket, quote!($($t)*)) };
-    ({$($t:tt)*}) => { Group::new(Delimiter::Brace, quote!($($t)*)) };
+macro_rules! minimal_quote_tt {
+    (($($t:tt)*)) => { Group::new(Delimiter::Parenthesis, minimal_quote!($($t)*)) };
+    ([$($t:tt)*]) => { Group::new(Delimiter::Bracket, minimal_quote!($($t)*)) };
+    ({$($t:tt)*}) => { Group::new(Delimiter::Brace, minimal_quote!($($t)*)) };
     (,) => { Punct::new(',', Spacing::Alone) };
     (.) => { Punct::new('.', Spacing::Alone) };
     (;) => { Punct::new(';', Spacing::Alone) };
@@ -21,21 +23,20 @@ macro_rules! quote_tt {
     ($i:ident) => { Ident::new(stringify!($i), Span::def_site()) };
 }
 
-macro_rules! quote_ts {
+macro_rules! minimal_quote_ts {
     ((@ $($t:tt)*)) => { $($t)* };
     (::) => {
-        [
-            TokenTree::from(Punct::new(':', Spacing::Joint)),
-            TokenTree::from(Punct::new(':', Spacing::Alone)),
-        ].iter()
-            .cloned()
-            .map(|mut x| {
-                x.set_span(Span::def_site());
-                x
-            })
-            .collect::<TokenStream>()
+        {
+            let mut c = (
+                TokenTree::from(Punct::new(':', Spacing::Joint)),
+                TokenTree::from(Punct::new(':', Spacing::Alone))
+            );
+            c.0.set_span(Span::def_site());
+            c.1.set_span(Span::def_site());
+            [c.0, c.1].into_iter().collect::<TokenStream>()
+        }
     };
-    ($t:tt) => { TokenTree::from(quote_tt!($t)) };
+    ($t:tt) => { TokenTree::from(minimal_quote_tt!($t)) };
 }
 
 /// Simpler version of the real `quote!` macro, implemented solely
@@ -46,12 +47,14 @@ macro_rules! quote_ts {
 ///
 /// Note: supported tokens are a subset of the real `quote!`, but
 /// unquoting is different: instead of `$x`, this uses `(@ expr)`.
-macro_rules! quote {
-    () => { TokenStream::new() };
+macro_rules! minimal_quote {
     ($($t:tt)*) => {
-        [
-            $(TokenStream::from(quote_ts!($t)),)*
-        ].iter().cloned().collect::<TokenStream>()
+        {
+            #[allow(unused_mut)] // In case the expansion is empty
+            let mut ts = TokenStream::new();
+            $(ToTokens::to_tokens(&minimal_quote_ts!($t), &mut ts);)*
+            ts
+        }
     };
 }
 
@@ -62,52 +65,66 @@ macro_rules! quote {
 #[unstable(feature = "proc_macro_quote", issue = "54722")]
 pub fn quote(stream: TokenStream) -> TokenStream {
     if stream.is_empty() {
-        return quote!(crate::TokenStream::new());
+        return minimal_quote!(crate::TokenStream::new());
     }
-    let proc_macro_crate = quote!(crate);
+    let proc_macro_crate = minimal_quote!(crate);
     let mut after_dollar = false;
-    let tokens = stream
-        .into_iter()
-        .filter_map(|tree| {
-            if after_dollar {
-                after_dollar = false;
-                match tree {
-                    TokenTree::Ident(_) => {
-                        return Some(quote!(Into::<crate::TokenStream>::into(
-                        Clone::clone(&(@ tree))),));
-                    }
-                    TokenTree::Punct(ref tt) if tt.as_char() == '$' => {}
-                    _ => panic!("`$` must be followed by an ident or `$` in `quote!`"),
-                }
-            } else if let TokenTree::Punct(ref tt) = tree {
-                if tt.as_char() == '$' {
-                    after_dollar = true;
-                    return None;
+
+    let mut tokens = crate::TokenStream::new();
+    for tree in stream {
+        if after_dollar {
+            after_dollar = false;
+            match tree {
+                TokenTree::Ident(_) => {
+                    minimal_quote!(crate::ToTokens::to_tokens(&(@ tree), &mut ts);)
+                        .to_tokens(&mut tokens);
+                    continue;
                 }
+                TokenTree::Punct(ref tt) if tt.as_char() == '$' => {}
+                _ => panic!("`$` must be followed by an ident or `$` in `quote!`"),
             }
+        } else if let TokenTree::Punct(ref tt) = tree {
+            if tt.as_char() == '$' {
+                after_dollar = true;
+                continue;
+            }
+        }
 
-            Some(quote!(crate::TokenStream::from((@ match tree {
-                TokenTree::Punct(tt) => quote!(crate::TokenTree::Punct(crate::Punct::new(
+        match tree {
+            TokenTree::Punct(tt) => {
+                minimal_quote!(crate::ToTokens::to_tokens(&crate::TokenTree::Punct(crate::Punct::new(
                     (@ TokenTree::from(Literal::character(tt.as_char()))),
                     (@ match tt.spacing() {
-                        Spacing::Alone => quote!(crate::Spacing::Alone),
-                        Spacing::Joint => quote!(crate::Spacing::Joint),
+                        Spacing::Alone => minimal_quote!(crate::Spacing::Alone),
+                        Spacing::Joint => minimal_quote!(crate::Spacing::Joint),
                     }),
-                ))),
-                TokenTree::Group(tt) => quote!(crate::TokenTree::Group(crate::Group::new(
+                )), &mut ts);)
+            }
+            TokenTree::Group(tt) => {
+                minimal_quote!(crate::ToTokens::to_tokens(&crate::TokenTree::Group(crate::Group::new(
                     (@ match tt.delimiter() {
-                        Delimiter::Parenthesis => quote!(crate::Delimiter::Parenthesis),
-                        Delimiter::Brace => quote!(crate::Delimiter::Brace),
-                        Delimiter::Bracket => quote!(crate::Delimiter::Bracket),
-                        Delimiter::None => quote!(crate::Delimiter::None),
+                        Delimiter::Parenthesis => minimal_quote!(crate::Delimiter::Parenthesis),
+                        Delimiter::Brace => minimal_quote!(crate::Delimiter::Brace),
+                        Delimiter::Bracket => minimal_quote!(crate::Delimiter::Bracket),
+                        Delimiter::None => minimal_quote!(crate::Delimiter::None),
                     }),
                     (@ quote(tt.stream())),
-                ))),
-                TokenTree::Ident(tt) => quote!(crate::TokenTree::Ident(crate::Ident::new(
-                    (@ TokenTree::from(Literal::string(&tt.to_string()))),
+                )), &mut ts);)
+            }
+            TokenTree::Ident(tt) => {
+                let literal = tt.to_string();
+                let (literal, ctor) = if let Some(stripped) = literal.strip_prefix("r#") {
+                    (stripped, minimal_quote!(crate::Ident::new_raw))
+                } else {
+                    (literal.as_str(), minimal_quote!(crate::Ident::new))
+                };
+                minimal_quote!(crate::ToTokens::to_tokens(&crate::TokenTree::Ident((@ ctor)(
+                    (@ TokenTree::from(Literal::string(literal))),
                     (@ quote_span(proc_macro_crate.clone(), tt.span())),
-                ))),
-                TokenTree::Literal(tt) => quote!(crate::TokenTree::Literal({
+                )), &mut ts);)
+            }
+            TokenTree::Literal(tt) => {
+                minimal_quote!(crate::ToTokens::to_tokens(&crate::TokenTree::Literal({
                     let mut iter = (@ TokenTree::from(Literal::string(&tt.to_string())))
                         .parse::<crate::TokenStream>()
                         .unwrap()
@@ -120,16 +137,22 @@ pub fn quote(stream: TokenStream) -> TokenStream {
                     } else {
                         unreachable!()
                     }
-                }))
-            })),))
-        })
-        .collect::<TokenStream>();
-
+                }), &mut ts);)
+            }
+        }
+        .to_tokens(&mut tokens);
+    }
     if after_dollar {
         panic!("unexpected trailing `$` in `quote!`");
     }
 
-    quote!([(@ tokens)].iter().cloned().collect::<crate::TokenStream>())
+    minimal_quote! {
+        {
+            let mut ts = crate::TokenStream::new();
+            (@ tokens)
+            ts
+        }
+    }
 }
 
 /// Quote a `Span` into a `TokenStream`.
@@ -137,5 +160,5 @@ pub fn quote(stream: TokenStream) -> TokenStream {
 #[unstable(feature = "proc_macro_quote", issue = "54722")]
 pub fn quote_span(proc_macro_crate: TokenStream, span: Span) -> TokenStream {
     let id = span.save_span();
-    quote!((@ proc_macro_crate ) ::Span::recover_proc_macro_span((@ TokenTree::from(Literal::usize_unsuffixed(id)))))
+    minimal_quote!((@ proc_macro_crate ) ::Span::recover_proc_macro_span((@ TokenTree::from(Literal::usize_unsuffixed(id)))))
 }
diff --git a/library/std/Cargo.toml b/library/std/Cargo.toml
index e7f7f38cb41..da58d7c13bd 100644
--- a/library/std/Cargo.toml
+++ b/library/std/Cargo.toml
@@ -17,7 +17,7 @@ cfg-if = { version = "1.0", features = ['rustc-dep-of-std'] }
 panic_unwind = { path = "../panic_unwind", optional = true }
 panic_abort = { path = "../panic_abort" }
 core = { path = "../core", public = true }
-compiler_builtins = { version = "=0.1.140" }
+compiler_builtins = { version = "=0.1.143" }
 unwind = { path = "../unwind" }
 hashbrown = { version = "0.15", default-features = false, features = [
     'rustc-dep-of-std',
diff --git a/library/std/src/collections/hash/map.rs b/library/std/src/collections/hash/map.rs
index 56d734ba2fb..d2342d8fd51 100644
--- a/library/std/src/collections/hash/map.rs
+++ b/library/std/src/collections/hash/map.rs
@@ -296,10 +296,7 @@ impl<K, V, S> HashMap<K, V, S> {
     /// ```
     #[inline]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
-    #[rustc_const_stable(
-        feature = "const_collections_with_hasher",
-        since = "CURRENT_RUSTC_VERSION"
-    )]
+    #[rustc_const_stable(feature = "const_collections_with_hasher", since = "1.85.0")]
     pub const fn with_hasher(hash_builder: S) -> HashMap<K, V, S> {
         HashMap { base: base::HashMap::with_hasher(hash_builder) }
     }
diff --git a/library/std/src/collections/hash/set.rs b/library/std/src/collections/hash/set.rs
index bcf6d6b4ccf..bbb6ca23521 100644
--- a/library/std/src/collections/hash/set.rs
+++ b/library/std/src/collections/hash/set.rs
@@ -388,10 +388,7 @@ impl<T, S> HashSet<T, S> {
     /// ```
     #[inline]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
-    #[rustc_const_stable(
-        feature = "const_collections_with_hasher",
-        since = "CURRENT_RUSTC_VERSION"
-    )]
+    #[rustc_const_stable(feature = "const_collections_with_hasher", since = "1.85.0")]
     pub const fn with_hasher(hasher: S) -> HashSet<T, S> {
         HashSet { base: base::HashSet::with_hasher(hasher) }
     }
diff --git a/library/std/src/env.rs b/library/std/src/env.rs
index 535236ff89a..11a29cdae62 100644
--- a/library/std/src/env.rs
+++ b/library/std/src/env.rs
@@ -622,7 +622,7 @@ impl Error for JoinPathsError {
 ///
 /// In UWP (Universal Windows Platform) targets this function is unimplemented and always returns `None`.
 ///
-/// Before Rust CURRENT_RUSTC_VERSION, this function used to return the value of the 'HOME' environment variable
+/// Before Rust 1.85.0, this function used to return the value of the 'HOME' environment variable
 /// on Windows, which in Cygwin or Mingw environments could return non-standard paths like `/home/you`
 /// instead of `C:\Users\you`.
 ///
diff --git a/library/std/src/ffi/os_str/tests.rs b/library/std/src/ffi/os_str/tests.rs
index cbec44c8626..2572b71fd9a 100644
--- a/library/std/src/ffi/os_str/tests.rs
+++ b/library/std/src/ffi/os_str/tests.rs
@@ -295,7 +295,7 @@ fn clone_to_uninit() {
 
     let mut storage = vec![MaybeUninit::<u8>::uninit(); size_of_val::<OsStr>(a)];
     unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()).cast()) };
-    assert_eq!(a.as_encoded_bytes(), unsafe { MaybeUninit::slice_assume_init_ref(&storage) });
+    assert_eq!(a.as_encoded_bytes(), unsafe { storage.assume_init_ref() });
 
     let mut b: Box<OsStr> = OsStr::new("world.exe").into();
     assert_eq!(size_of_val::<OsStr>(a), size_of_val::<OsStr>(&b));
diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
index 17721090db5..5251cc302cb 100644
--- a/library/std/src/io/buffered/bufreader/buffer.rs
+++ b/library/std/src/io/buffered/bufreader/buffer.rs
@@ -50,7 +50,7 @@ impl Buffer {
     pub fn buffer(&self) -> &[u8] {
         // SAFETY: self.pos and self.cap are valid, and self.cap => self.pos, and
         // that region is initialized because those are all invariants of this type.
-        unsafe { MaybeUninit::slice_assume_init_ref(self.buf.get_unchecked(self.pos..self.filled)) }
+        unsafe { self.buf.get_unchecked(self.pos..self.filled).assume_init_ref() }
     }
 
     #[inline]
diff --git a/library/std/src/io/error.rs b/library/std/src/io/error.rs
index 476c403c21f..38b72336617 100644
--- a/library/std/src/io/error.rs
+++ b/library/std/src/io/error.rs
@@ -339,7 +339,7 @@ pub enum ErrorKind {
     #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
     NotSeekable,
     /// Filesystem quota or some other kind of quota was exceeded.
-    #[stable(feature = "io_error_quota_exceeded", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "io_error_quota_exceeded", since = "1.85.0")]
     QuotaExceeded,
     /// File larger than allowed or supported.
     ///
@@ -364,7 +364,7 @@ pub enum ErrorKind {
     #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
     Deadlock,
     /// Cross-device or cross-filesystem (hard) link or rename.
-    #[stable(feature = "io_error_crosses_devices", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "io_error_crosses_devices", since = "1.85.0")]
     CrossesDevices,
     /// Too many (hard) links to the same filesystem object.
     ///
diff --git a/library/std/src/io/error/repr_bitpacked.rs b/library/std/src/io/error/repr_bitpacked.rs
index f958a938646..716da37168d 100644
--- a/library/std/src/io/error/repr_bitpacked.rs
+++ b/library/std/src/io/error/repr_bitpacked.rs
@@ -103,7 +103,8 @@
 //! the time.
 
 use core::marker::PhantomData;
-use core::ptr::{self, NonNull};
+use core::num::NonZeroUsize;
+use core::ptr::NonNull;
 
 use super::{Custom, ErrorData, ErrorKind, RawOsError, SimpleMessage};
 
@@ -176,7 +177,7 @@ impl Repr {
         let utagged = ((code as usize) << 32) | TAG_OS;
         // Safety: `TAG_OS` is not zero, so the result of the `|` is not 0.
         let res = Self(
-            unsafe { NonNull::new_unchecked(ptr::without_provenance_mut(utagged)) },
+            NonNull::without_provenance(unsafe { NonZeroUsize::new_unchecked(utagged) }),
             PhantomData,
         );
         // quickly smoke-check we encoded the right thing (This generally will
@@ -193,7 +194,7 @@ impl Repr {
         let utagged = ((kind as usize) << 32) | TAG_SIMPLE;
         // Safety: `TAG_SIMPLE` is not zero, so the result of the `|` is not 0.
         let res = Self(
-            unsafe { NonNull::new_unchecked(ptr::without_provenance_mut(utagged)) },
+            NonNull::without_provenance(unsafe { NonZeroUsize::new_unchecked(utagged) }),
             PhantomData,
         );
         // quickly smoke-check we encoded the right thing (This generally will
diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index 7912f969bbd..231c8712ebd 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -330,6 +330,7 @@ pub use self::{
 };
 use crate::mem::take;
 use crate::ops::{Deref, DerefMut};
+use crate::sys::anonymous_pipe::{AnonPipe, pipe as pipe_inner};
 use crate::{cmp, fmt, slice, str, sys};
 
 mod buffered;
@@ -3250,3 +3251,251 @@ impl<B: BufRead> Iterator for Lines<B> {
         }
     }
 }
+
+/// Create anonymous pipe that is close-on-exec and blocking.
+///
+/// # Behavior
+///
+/// A pipe is a synchronous, unidirectional data channel between two or more processes, like an
+/// interprocess [`mpsc`](crate::sync::mpsc) provided by the OS. In particular:
+///
+/// * A read on a [`PipeReader`] blocks until the pipe is non-empty.
+/// * A write on a [`PipeWriter`] blocks when the pipe is full.
+/// * When all copies of a [`PipeWriter`] are closed, a read on the corresponding [`PipeReader`]
+///   returns EOF.
+/// * [`PipeReader`] can be shared, but only one process will consume the data in the pipe.
+///
+/// # Capacity
+///
+/// Pipe capacity is platform dependent. To quote the Linux [man page]:
+///
+/// > Different implementations have different limits for the pipe capacity. Applications should
+/// > not rely on a particular capacity: an application should be designed so that a reading process
+/// > consumes data as soon as it is available, so that a writing process does not remain blocked.
+///
+/// # Examples
+///
+/// ```no_run
+/// #![feature(anonymous_pipe)]
+/// # #[cfg(miri)] fn main() {}
+/// # #[cfg(not(miri))]
+/// # fn main() -> std::io::Result<()> {
+/// # use std::process::Command;
+/// # use std::io::{Read, Write};
+/// let (ping_rx, mut ping_tx) = std::io::pipe()?;
+/// let (mut pong_rx, pong_tx) = std::io::pipe()?;
+///
+/// // Spawn a process that echoes its input.
+/// let mut echo_server = Command::new("cat").stdin(ping_rx).stdout(pong_tx).spawn()?;
+///
+/// ping_tx.write_all(b"hello")?;
+/// // Close to unblock echo_server's reader.
+/// drop(ping_tx);
+///
+/// let mut buf = String::new();
+/// // Block until echo_server's writer is closed.
+/// pong_rx.read_to_string(&mut buf)?;
+/// assert_eq!(&buf, "hello");
+///
+/// echo_server.wait()?;
+/// # Ok(())
+/// # }
+/// ```
+/// [pipe]: https://man7.org/linux/man-pages/man2/pipe.2.html
+/// [CreatePipe]: https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-createpipe
+/// [man page]: https://man7.org/linux/man-pages/man7/pipe.7.html
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+#[inline]
+pub fn pipe() -> Result<(PipeReader, PipeWriter)> {
+    pipe_inner().map(|(reader, writer)| (PipeReader(reader), PipeWriter(writer)))
+}
+
+/// Read end of the anonymous pipe.
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+#[derive(Debug)]
+pub struct PipeReader(pub(crate) AnonPipe);
+
+/// Write end of the anonymous pipe.
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+#[derive(Debug)]
+pub struct PipeWriter(pub(crate) AnonPipe);
+
+impl PipeReader {
+    /// Create a new [`PipeReader`] instance that shares the same underlying file description.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(anonymous_pipe)]
+    /// # #[cfg(miri)] fn main() {}
+    /// # #[cfg(not(miri))]
+    /// # fn main() -> std::io::Result<()> {
+    /// # use std::fs;
+    /// # use std::io::Write;
+    /// # use std::process::Command;
+    /// const NUM_SLOT: u8 = 2;
+    /// const NUM_PROC: u8 = 5;
+    /// const OUTPUT: &str = "work.txt";
+    ///
+    /// let mut jobs = vec![];
+    /// let (reader, mut writer) = std::io::pipe()?;
+    ///
+    /// // Write NUM_SLOT characters the pipe.
+    /// writer.write_all(&[b'|'; NUM_SLOT as usize])?;
+    ///
+    /// // Spawn several processes that read a character from the pipe, do some work, then
+    /// // write back to the pipe. When the pipe is empty, the processes block, so only
+    /// // NUM_SLOT processes can be working at any given time.
+    /// for _ in 0..NUM_PROC {
+    ///     jobs.push(
+    ///         Command::new("bash")
+    ///             .args(["-c",
+    ///                 &format!(
+    ///                      "read -n 1\n\
+    ///                       echo -n 'x' >> '{OUTPUT}'\n\
+    ///                       echo -n '|'",
+    ///                 ),
+    ///             ])
+    ///             .stdin(reader.try_clone()?)
+    ///             .stdout(writer.try_clone()?)
+    ///             .spawn()?,
+    ///     );
+    /// }
+    ///
+    /// // Wait for all jobs to finish.
+    /// for mut job in jobs {
+    ///     job.wait()?;
+    /// }
+    ///
+    /// // Check our work and clean up.
+    /// let xs = fs::read_to_string(OUTPUT)?;
+    /// fs::remove_file(OUTPUT)?;
+    /// assert_eq!(xs, "x".repeat(NUM_PROC.into()));
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[unstable(feature = "anonymous_pipe", issue = "127154")]
+    pub fn try_clone(&self) -> Result<Self> {
+        self.0.try_clone().map(Self)
+    }
+}
+
+impl PipeWriter {
+    /// Create a new [`PipeWriter`] instance that shares the same underlying file description.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(anonymous_pipe)]
+    /// # #[cfg(miri)] fn main() {}
+    /// # #[cfg(not(miri))]
+    /// # fn main() -> std::io::Result<()> {
+    /// # use std::process::Command;
+    /// # use std::io::Read;
+    /// let (mut reader, writer) = std::io::pipe()?;
+    ///
+    /// // Spawn a process that writes to stdout and stderr.
+    /// let mut peer = Command::new("bash")
+    ///     .args([
+    ///         "-c",
+    ///         "echo -n foo\n\
+    ///          echo -n bar >&2"
+    ///     ])
+    ///     .stdout(writer.try_clone()?)
+    ///     .stderr(writer)
+    ///     .spawn()?;
+    ///
+    /// // Read and check the result.
+    /// let mut msg = String::new();
+    /// reader.read_to_string(&mut msg)?;
+    /// assert_eq!(&msg, "foobar");
+    ///
+    /// peer.wait()?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[unstable(feature = "anonymous_pipe", issue = "127154")]
+    pub fn try_clone(&self) -> Result<Self> {
+        self.0.try_clone().map(Self)
+    }
+}
+
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+impl Read for &PipeReader {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        self.0.read(buf)
+    }
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
+        self.0.read_vectored(bufs)
+    }
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        self.0.is_read_vectored()
+    }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
+        self.0.read_to_end(buf)
+    }
+    fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()> {
+        self.0.read_buf(buf)
+    }
+}
+
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+impl Read for PipeReader {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        self.0.read(buf)
+    }
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
+        self.0.read_vectored(bufs)
+    }
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        self.0.is_read_vectored()
+    }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
+        self.0.read_to_end(buf)
+    }
+    fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()> {
+        self.0.read_buf(buf)
+    }
+}
+
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+impl Write for &PipeWriter {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        self.0.write(buf)
+    }
+    #[inline]
+    fn flush(&mut self) -> Result<()> {
+        Ok(())
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
+        self.0.write_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.0.is_write_vectored()
+    }
+}
+
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+impl Write for PipeWriter {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        self.0.write(buf)
+    }
+    #[inline]
+    fn flush(&mut self) -> Result<()> {
+        Ok(())
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
+        self.0.write_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.0.is_write_vectored()
+    }
+}
diff --git a/library/std/src/io/tests.rs b/library/std/src/io/tests.rs
index 47cbb9614af..85098b3bb18 100644
--- a/library/std/src/io/tests.rs
+++ b/library/std/src/io/tests.rs
@@ -823,3 +823,20 @@ fn try_oom_error() {
     let io_err = io::Error::from(reserve_err);
     assert_eq!(io::ErrorKind::OutOfMemory, io_err.kind());
 }
+
+#[test]
+#[cfg(all(windows, unix, not(miri)))]
+fn pipe_creation_clone_and_rw() {
+    let (rx, tx) = std::io::pipe().unwrap();
+
+    tx.try_clone().unwrap().write_all(b"12345").unwrap();
+    drop(tx);
+
+    let mut rx2 = rx.try_clone().unwrap();
+    drop(rx);
+
+    let mut s = String::new();
+    rx2.read_to_string(&mut s).unwrap();
+    drop(rx2);
+    assert_eq!(s, "12345");
+}
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 2f8f5c5c581..39f234e4ba6 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -333,7 +333,6 @@
 #![feature(extend_one)]
 #![feature(float_gamma)]
 #![feature(float_minimum_maximum)]
-#![feature(float_next_up_down)]
 #![feature(fmt_internals)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(hashmap_internals)]
@@ -342,6 +341,7 @@
 #![feature(lazy_get)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_write_slice)]
+#![feature(nonnull_provenance)]
 #![feature(panic_can_unwind)]
 #![feature(panic_internals)]
 #![feature(pin_coerce_unsized_trait)]
@@ -357,6 +357,7 @@
 #![feature(str_internals)]
 #![feature(strict_provenance_atomic_ptr)]
 #![feature(sync_unsafe_cell)]
+#![feature(temporary_niche_types)]
 #![feature(ub_checks)]
 #![feature(used_with_arg)]
 // tidy-alphabetical-end
@@ -594,8 +595,6 @@ pub mod panic;
 #[unstable(feature = "pattern_type_macro", issue = "123646")]
 pub mod pat;
 pub mod path;
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-pub mod pipe;
 pub mod process;
 #[unstable(feature = "random", issue = "130703")]
 pub mod random;
diff --git a/library/std/src/os/fd/owned.rs b/library/std/src/os/fd/owned.rs
index abb13b75f50..1e814eca3c1 100644
--- a/library/std/src/os/fd/owned.rs
+++ b/library/std/src/os/fd/owned.rs
@@ -11,6 +11,8 @@ use crate::sys::cvt;
 use crate::sys_common::{AsInner, FromInner, IntoInner};
 use crate::{fmt, fs, io};
 
+type ValidRawFd = core::num::niche_types::NotAllOnes<RawFd>;
+
 /// A borrowed file descriptor.
 ///
 /// This has a lifetime parameter to tie it to the lifetime of something that owns the file
@@ -32,15 +34,10 @@ use crate::{fmt, fs, io};
 /// instead, but this is not supported on all platforms.
 #[derive(Copy, Clone)]
 #[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// libstd/os/raw/mod.rs assures me that every libstd-supported platform has a
-// 32-bit c_int. Below is -2, in two's complement, but that only works out
-// because c_int is 32 bits.
-#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
 #[rustc_nonnull_optimization_guaranteed]
 #[stable(feature = "io_safety", since = "1.63.0")]
 pub struct BorrowedFd<'fd> {
-    fd: RawFd,
+    fd: ValidRawFd,
     _phantom: PhantomData<&'fd OwnedFd>,
 }
 
@@ -56,15 +53,10 @@ pub struct BorrowedFd<'fd> {
 ///
 /// You can use [`AsFd::as_fd`] to obtain a [`BorrowedFd`].
 #[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// libstd/os/raw/mod.rs assures me that every libstd-supported platform has a
-// 32-bit c_int. Below is -2, in two's complement, but that only works out
-// because c_int is 32 bits.
-#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
 #[rustc_nonnull_optimization_guaranteed]
 #[stable(feature = "io_safety", since = "1.63.0")]
 pub struct OwnedFd {
-    fd: RawFd,
+    fd: ValidRawFd,
 }
 
 impl BorrowedFd<'_> {
@@ -80,7 +72,8 @@ impl BorrowedFd<'_> {
     pub const unsafe fn borrow_raw(fd: RawFd) -> Self {
         assert!(fd != u32::MAX as RawFd);
         // SAFETY: we just asserted that the value is in the valid range and isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
-        unsafe { Self { fd, _phantom: PhantomData } }
+        let fd = unsafe { ValidRawFd::new_unchecked(fd) };
+        Self { fd, _phantom: PhantomData }
     }
 }
 
@@ -130,7 +123,7 @@ impl BorrowedFd<'_> {
 impl AsRawFd for BorrowedFd<'_> {
     #[inline]
     fn as_raw_fd(&self) -> RawFd {
-        self.fd
+        self.fd.as_inner()
     }
 }
 
@@ -138,7 +131,7 @@ impl AsRawFd for BorrowedFd<'_> {
 impl AsRawFd for OwnedFd {
     #[inline]
     fn as_raw_fd(&self) -> RawFd {
-        self.fd
+        self.fd.as_inner()
     }
 }
 
@@ -146,7 +139,7 @@ impl AsRawFd for OwnedFd {
 impl IntoRawFd for OwnedFd {
     #[inline]
     fn into_raw_fd(self) -> RawFd {
-        ManuallyDrop::new(self).fd
+        ManuallyDrop::new(self).fd.as_inner()
     }
 }
 
@@ -164,7 +157,8 @@ impl FromRawFd for OwnedFd {
     unsafe fn from_raw_fd(fd: RawFd) -> Self {
         assert_ne!(fd, u32::MAX as RawFd);
         // SAFETY: we just asserted that the value is in the valid range and isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
-        unsafe { Self { fd } }
+        let fd = unsafe { ValidRawFd::new_unchecked(fd) };
+        Self { fd }
     }
 }
 
@@ -187,12 +181,12 @@ impl Drop for OwnedFd {
             #[cfg(not(target_os = "hermit"))]
             {
                 #[cfg(unix)]
-                crate::sys::fs::debug_assert_fd_is_open(self.fd);
+                crate::sys::fs::debug_assert_fd_is_open(self.fd.as_inner());
 
-                let _ = libc::close(self.fd);
+                let _ = libc::close(self.fd.as_inner());
             }
             #[cfg(target_os = "hermit")]
-            let _ = hermit_abi::close(self.fd);
+            let _ = hermit_abi::close(self.fd.as_inner());
         }
     }
 }
diff --git a/library/std/src/os/fd/raw.rs b/library/std/src/os/fd/raw.rs
index 22f5528248a..03dff94350d 100644
--- a/library/std/src/os/fd/raw.rs
+++ b/library/std/src/os/fd/raw.rs
@@ -19,11 +19,9 @@ use crate::sys_common::{AsInner, IntoInner};
 use crate::{fs, io};
 
 /// Raw file descriptors.
-#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg(not(target_os = "hermit"))]
 pub type RawFd = raw::c_int;
-#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg(target_os = "hermit")]
 pub type RawFd = i32;
@@ -33,7 +31,6 @@ pub type RawFd = i32;
 /// This is only available on unix and WASI platforms and must be imported in
 /// order to call the method. Windows platforms have a corresponding
 /// `AsRawHandle` and `AsRawSocket` set of traits.
-#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait AsRawFd {
     /// Extracts the raw file descriptor.
@@ -67,7 +64,6 @@ pub trait AsRawFd {
 
 /// A trait to express the ability to construct an object from a raw file
 /// descriptor.
-#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "from_raw_os", since = "1.1.0")]
 pub trait FromRawFd {
     /// Constructs a new instance of `Self` from the given raw file
@@ -112,7 +108,6 @@ pub trait FromRawFd {
 
 /// A trait to express the ability to consume an object and acquire ownership of
 /// its raw file descriptor.
-#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "into_raw_os", since = "1.4.0")]
 pub trait IntoRawFd {
     /// Consumes this object, returning the raw underlying file descriptor.
diff --git a/library/std/src/os/solid/io.rs b/library/std/src/os/solid/io.rs
index 2d18f339615..b8601b533fe 100644
--- a/library/std/src/os/solid/io.rs
+++ b/library/std/src/os/solid/io.rs
@@ -54,6 +54,9 @@ use crate::{fmt, net, sys};
 /// Raw file descriptors.
 pub type RawFd = i32;
 
+// The max of this is -2, in two's complement. -1 is `SOLID_NET_INVALID_FD`.
+type ValidRawFd = core::num::niche_types::NotAllOnes<RawFd>;
+
 /// A borrowed SOLID Sockets file descriptor.
 ///
 /// This has a lifetime parameter to tie it to the lifetime of something that
@@ -69,12 +72,9 @@ pub type RawFd = i32;
 /// socket, which is then borrowed under the same lifetime.
 #[derive(Copy, Clone)]
 #[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// This is -2, in two's complement. -1 is `SOLID_NET_INVALID_FD`.
-#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
 #[rustc_nonnull_optimization_guaranteed]
 pub struct BorrowedFd<'socket> {
-    fd: RawFd,
+    fd: ValidRawFd,
     _phantom: PhantomData<&'socket OwnedFd>,
 }
 
@@ -87,12 +87,9 @@ pub struct BorrowedFd<'socket> {
 /// an argument, it is not captured or consumed, and it never has the value
 /// `SOLID_NET_INVALID_FD`.
 #[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// This is -2, in two's complement. -1 is `SOLID_NET_INVALID_FD`.
-#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
 #[rustc_nonnull_optimization_guaranteed]
 pub struct OwnedFd {
-    fd: RawFd,
+    fd: ValidRawFd,
 }
 
 impl BorrowedFd<'_> {
@@ -108,7 +105,8 @@ impl BorrowedFd<'_> {
         assert!(fd != -1 as RawFd);
         // SAFETY: we just asserted that the value is in the valid range and
         // isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
-        unsafe { Self { fd, _phantom: PhantomData } }
+        let fd = unsafe { ValidRawFd::new_unchecked(fd) };
+        Self { fd, _phantom: PhantomData }
     }
 }
 
@@ -132,21 +130,21 @@ impl BorrowedFd<'_> {
 impl AsRawFd for BorrowedFd<'_> {
     #[inline]
     fn as_raw_fd(&self) -> RawFd {
-        self.fd
+        self.fd.as_inner()
     }
 }
 
 impl AsRawFd for OwnedFd {
     #[inline]
     fn as_raw_fd(&self) -> RawFd {
-        self.fd
+        self.fd.as_inner()
     }
 }
 
 impl IntoRawFd for OwnedFd {
     #[inline]
     fn into_raw_fd(self) -> RawFd {
-        ManuallyDrop::new(self).fd
+        ManuallyDrop::new(self).fd.as_inner()
     }
 }
 
@@ -162,14 +160,15 @@ impl FromRawFd for OwnedFd {
         assert_ne!(fd, -1 as RawFd);
         // SAFETY: we just asserted that the value is in the valid range and
         // isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
-        unsafe { Self { fd } }
+        let fd = unsafe { ValidRawFd::new_unchecked(fd) };
+        Self { fd }
     }
 }
 
 impl Drop for OwnedFd {
     #[inline]
     fn drop(&mut self) {
-        unsafe { sys::net::netc::close(self.fd) };
+        unsafe { sys::net::netc::close(self.fd.as_inner()) };
     }
 }
 
diff --git a/library/std/src/os/wasi/io/fd.rs b/library/std/src/os/wasi/io/fd.rs
deleted file mode 100644
index 930aca887e3..00000000000
--- a/library/std/src/os/wasi/io/fd.rs
+++ /dev/null
@@ -1,9 +0,0 @@
-//! Owned and borrowed file descriptors.
-
-#![unstable(feature = "wasi_ext", issue = "71213")]
-
-// Tests for this module
-#[cfg(test)]
-mod tests;
-
-pub use crate::os::fd::owned::*;
diff --git a/library/std/src/os/wasi/io/mod.rs b/library/std/src/os/wasi/io/mod.rs
index 4e123a1eec8..5f9a735db08 100644
--- a/library/std/src/os/wasi/io/mod.rs
+++ b/library/std/src/os/wasi/io/mod.rs
@@ -4,3 +4,7 @@
 
 #[stable(feature = "io_safety_wasi", since = "1.65.0")]
 pub use crate::os::fd::*;
+
+// Tests for this module
+#[cfg(test)]
+mod tests;
diff --git a/library/std/src/os/wasi/io/raw.rs b/library/std/src/os/wasi/io/raw.rs
deleted file mode 100644
index da3b36adad4..00000000000
--- a/library/std/src/os/wasi/io/raw.rs
+++ /dev/null
@@ -1,20 +0,0 @@
-//! WASI-specific extensions to general I/O primitives.
-
-#![unstable(feature = "wasi_ext", issue = "71213")]
-
-// NOTE: despite the fact that this module is unstable,
-// stable Rust had the capability to access the stable
-// re-exported items from os::fd::raw through this
-// unstable module.
-// In PR #95956 the stability checker was changed to check
-// all path segments of an item rather than just the last,
-// which caused the aforementioned stable usage to regress
-// (see issue #99502).
-// As a result, the items in os::fd::raw were given the
-// rustc_allowed_through_unstable_modules attribute.
-// No regression tests were added to ensure this property,
-// as CI is not configured to test wasm32-wasi.
-// If this module is stabilized,
-// you may want to remove those attributes
-// (assuming no other unstable modules need them).
-pub use crate::os::fd::raw::*;
diff --git a/library/std/src/os/wasi/io/fd/tests.rs b/library/std/src/os/wasi/io/tests.rs
index 418274752b0..418274752b0 100644
--- a/library/std/src/os/wasi/io/fd/tests.rs
+++ b/library/std/src/os/wasi/io/tests.rs
diff --git a/library/std/src/os/windows/io/socket.rs b/library/std/src/os/windows/io/socket.rs
index c6d7bad9440..6e13a8b502a 100644
--- a/library/std/src/os/windows/io/socket.rs
+++ b/library/std/src/os/windows/io/socket.rs
@@ -9,6 +9,9 @@ use crate::mem::{self, ManuallyDrop};
 use crate::sys::cvt;
 use crate::{fmt, io, sys};
 
+// The max here is -2, in two's complement. -1 is `INVALID_SOCKET`.
+type ValidRawSocket = core::num::niche_types::NotAllOnes<RawSocket>;
+
 /// A borrowed socket.
 ///
 /// This has a lifetime parameter to tie it to the lifetime of something that
@@ -24,17 +27,10 @@ use crate::{fmt, io, sys};
 /// socket, which is then borrowed under the same lifetime.
 #[derive(Copy, Clone)]
 #[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// This is -2, in two's complement. -1 is `INVALID_SOCKET`.
-#[cfg_attr(target_pointer_width = "32", rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE))]
-#[cfg_attr(
-    target_pointer_width = "64",
-    rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FF_FF_FF_FF_FE)
-)]
 #[rustc_nonnull_optimization_guaranteed]
 #[stable(feature = "io_safety", since = "1.63.0")]
 pub struct BorrowedSocket<'socket> {
-    socket: RawSocket,
+    socket: ValidRawSocket,
     _phantom: PhantomData<&'socket OwnedSocket>,
 }
 
@@ -47,17 +43,10 @@ pub struct BorrowedSocket<'socket> {
 /// argument or returned as an owned value, and it never has the value
 /// `INVALID_SOCKET`.
 #[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// This is -2, in two's complement. -1 is `INVALID_SOCKET`.
-#[cfg_attr(target_pointer_width = "32", rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE))]
-#[cfg_attr(
-    target_pointer_width = "64",
-    rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FF_FF_FF_FF_FE)
-)]
 #[rustc_nonnull_optimization_guaranteed]
 #[stable(feature = "io_safety", since = "1.63.0")]
 pub struct OwnedSocket {
-    socket: RawSocket,
+    socket: ValidRawSocket,
 }
 
 impl BorrowedSocket<'_> {
@@ -73,7 +62,8 @@ impl BorrowedSocket<'_> {
     #[stable(feature = "io_safety", since = "1.63.0")]
     pub const unsafe fn borrow_raw(socket: RawSocket) -> Self {
         assert!(socket != sys::c::INVALID_SOCKET as RawSocket);
-        unsafe { Self { socket, _phantom: PhantomData } }
+        let socket = unsafe { ValidRawSocket::new_unchecked(socket) };
+        Self { socket, _phantom: PhantomData }
     }
 }
 
@@ -172,7 +162,7 @@ fn last_error() -> io::Error {
 impl AsRawSocket for BorrowedSocket<'_> {
     #[inline]
     fn as_raw_socket(&self) -> RawSocket {
-        self.socket
+        self.socket.as_inner()
     }
 }
 
@@ -180,7 +170,7 @@ impl AsRawSocket for BorrowedSocket<'_> {
 impl AsRawSocket for OwnedSocket {
     #[inline]
     fn as_raw_socket(&self) -> RawSocket {
-        self.socket
+        self.socket.as_inner()
     }
 }
 
@@ -188,7 +178,7 @@ impl AsRawSocket for OwnedSocket {
 impl IntoRawSocket for OwnedSocket {
     #[inline]
     fn into_raw_socket(self) -> RawSocket {
-        ManuallyDrop::new(self).socket
+        ManuallyDrop::new(self).socket.as_inner()
     }
 }
 
@@ -196,10 +186,9 @@ impl IntoRawSocket for OwnedSocket {
 impl FromRawSocket for OwnedSocket {
     #[inline]
     unsafe fn from_raw_socket(socket: RawSocket) -> Self {
-        unsafe {
-            debug_assert_ne!(socket, sys::c::INVALID_SOCKET as RawSocket);
-            Self { socket }
-        }
+        debug_assert_ne!(socket, sys::c::INVALID_SOCKET as RawSocket);
+        let socket = unsafe { ValidRawSocket::new_unchecked(socket) };
+        Self { socket }
     }
 }
 
@@ -208,7 +197,7 @@ impl Drop for OwnedSocket {
     #[inline]
     fn drop(&mut self) {
         unsafe {
-            let _ = sys::c::closesocket(self.socket as sys::c::SOCKET);
+            let _ = sys::c::closesocket(self.socket.as_inner() as sys::c::SOCKET);
         }
     }
 }
diff --git a/library/std/src/panicking.rs b/library/std/src/panicking.rs
index 3b02254548b..8e50bf11dd0 100644
--- a/library/std/src/panicking.rs
+++ b/library/std/src/panicking.rs
@@ -258,31 +258,34 @@ fn default_hook(info: &PanicHookInfo<'_>) {
     let location = info.location().unwrap();
 
     let msg = payload_as_str(info.payload());
-    let thread = thread::try_current();
-    let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
 
     let write = #[optimize(size)]
     |err: &mut dyn crate::io::Write| {
         // Use a lock to prevent mixed output in multithreading context.
         // Some platforms also require it when printing a backtrace, like `SymFromAddr` on Windows.
         let mut lock = backtrace::lock();
-        // Try to write the panic message to a buffer first to prevent other concurrent outputs
-        // interleaving with it.
-        let mut buffer = [0u8; 512];
-        let mut cursor = crate::io::Cursor::new(&mut buffer[..]);
-
-        let write_msg = |dst: &mut dyn crate::io::Write| {
-            // We add a newline to ensure the panic message appears at the start of a line.
-            writeln!(dst, "\nthread '{name}' panicked at {location}:\n{msg}")
-        };
 
-        if write_msg(&mut cursor).is_ok() {
-            let pos = cursor.position() as usize;
-            let _ = err.write_all(&buffer[0..pos]);
-        } else {
-            // The message did not fit into the buffer, write it directly instead.
-            let _ = write_msg(err);
-        };
+        thread::with_current_name(|name| {
+            let name = name.unwrap_or("<unnamed>");
+
+            // Try to write the panic message to a buffer first to prevent other concurrent outputs
+            // interleaving with it.
+            let mut buffer = [0u8; 512];
+            let mut cursor = crate::io::Cursor::new(&mut buffer[..]);
+
+            let write_msg = |dst: &mut dyn crate::io::Write| {
+                // We add a newline to ensure the panic message appears at the start of a line.
+                writeln!(dst, "\nthread '{name}' panicked at {location}:\n{msg}")
+            };
+
+            if write_msg(&mut cursor).is_ok() {
+                let pos = cursor.position() as usize;
+                let _ = err.write_all(&buffer[0..pos]);
+            } else {
+                // The message did not fit into the buffer, write it directly instead.
+                let _ = write_msg(err);
+            };
+        });
 
         static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 
diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 35e920ab344..7fd08a97f1f 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -298,7 +298,7 @@ where
 }
 
 // Detect scheme on Redox
-fn has_redox_scheme(s: &[u8]) -> bool {
+pub(crate) fn has_redox_scheme(s: &[u8]) -> bool {
     cfg!(target_os = "redox") && s.contains(&b':')
 }
 
@@ -2155,7 +2155,7 @@ impl Path {
         unsafe { Path::new(OsStr::from_encoded_bytes_unchecked(s)) }
     }
     // The following (private!) function reveals the byte encoding used for OsStr.
-    fn as_u8_slice(&self) -> &[u8] {
+    pub(crate) fn as_u8_slice(&self) -> &[u8] {
         self.inner.as_encoded_bytes()
     }
 
@@ -2323,14 +2323,7 @@ impl Path {
     #[must_use]
     #[allow(deprecated)]
     pub fn is_absolute(&self) -> bool {
-        if cfg!(target_os = "redox") {
-            // FIXME: Allow Redox prefixes
-            self.has_root() || has_redox_scheme(self.as_u8_slice())
-        } else {
-            self.has_root()
-                && (cfg!(any(unix, target_os = "hermit", target_os = "wasi"))
-                    || self.prefix().is_some())
-        }
+        sys::path::is_absolute(self)
     }
 
     /// Returns `true` if the `Path` is relative, i.e., not absolute.
@@ -2353,7 +2346,7 @@ impl Path {
         !self.is_absolute()
     }
 
-    fn prefix(&self) -> Option<Prefix<'_>> {
+    pub(crate) fn prefix(&self) -> Option<Prefix<'_>> {
         self.components().prefix
     }
 
diff --git a/library/std/src/path/tests.rs b/library/std/src/path/tests.rs
index ff3f7151bb8..3f96ac4672a 100644
--- a/library/std/src/path/tests.rs
+++ b/library/std/src/path/tests.rs
@@ -2069,9 +2069,7 @@ fn clone_to_uninit() {
 
     let mut storage = vec![MaybeUninit::<u8>::uninit(); size_of_val::<Path>(a)];
     unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()).cast()) };
-    assert_eq!(a.as_os_str().as_encoded_bytes(), unsafe {
-        MaybeUninit::slice_assume_init_ref(&storage)
-    });
+    assert_eq!(a.as_os_str().as_encoded_bytes(), unsafe { storage.assume_init_ref() });
 
     let mut b: Box<Path> = Path::new("world.exe").into();
     assert_eq!(size_of_val::<Path>(a), size_of_val::<Path>(&b));
diff --git a/library/std/src/pipe.rs b/library/std/src/pipe.rs
deleted file mode 100644
index 913c22588a7..00000000000
--- a/library/std/src/pipe.rs
+++ /dev/null
@@ -1,258 +0,0 @@
-//!  A cross-platform anonymous pipe.
-//!
-//! This module provides support for anonymous OS pipes, like [pipe] on Linux or [CreatePipe] on
-//! Windows.
-//!
-//! # Behavior
-//!
-//! A pipe is a synchronous, unidirectional data channel between two or more processes, like an
-//! interprocess [`mpsc`](crate::sync::mpsc) provided by the OS. In particular:
-//!
-//! * A read on a [`PipeReader`] blocks until the pipe is non-empty.
-//! * A write on a [`PipeWriter`] blocks when the pipe is full.
-//! * When all copies of a [`PipeWriter`] are closed, a read on the corresponding [`PipeReader`]
-//!   returns EOF.
-//! * [`PipeReader`] can be shared, but only one process will consume the data in the pipe.
-//!
-//! # Capacity
-//!
-//! Pipe capacity is platform dependent. To quote the Linux [man page]:
-//!
-//! > Different implementations have different limits for the pipe capacity. Applications should
-//! > not rely on a particular capacity: an application should be designed so that a reading process
-//! > consumes data as soon as it is available, so that a writing process does not remain blocked.
-//!
-//! # Examples
-//!
-//! ```no_run
-//! #![feature(anonymous_pipe)]
-//! # #[cfg(miri)] fn main() {}
-//! # #[cfg(not(miri))]
-//! # fn main() -> std::io::Result<()> {
-//! # use std::process::Command;
-//! # use std::io::{Read, Write};
-//! let (ping_rx, mut ping_tx) = std::pipe::pipe()?;
-//! let (mut pong_rx, pong_tx) = std::pipe::pipe()?;
-//!
-//! // Spawn a process that echoes its input.
-//! let mut echo_server = Command::new("cat").stdin(ping_rx).stdout(pong_tx).spawn()?;
-//!
-//! ping_tx.write_all(b"hello")?;
-//! // Close to unblock echo_server's reader.
-//! drop(ping_tx);
-//!
-//! let mut buf = String::new();
-//! // Block until echo_server's writer is closed.
-//! pong_rx.read_to_string(&mut buf)?;
-//! assert_eq!(&buf, "hello");
-//!
-//! echo_server.wait()?;
-//! # Ok(())
-//! # }
-//! ```
-//! [pipe]: https://man7.org/linux/man-pages/man2/pipe.2.html
-//! [CreatePipe]: https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-createpipe
-//! [man page]: https://man7.org/linux/man-pages/man7/pipe.7.html
-use crate::io;
-use crate::sys::anonymous_pipe::{AnonPipe, pipe as pipe_inner};
-
-/// Create anonymous pipe that is close-on-exec and blocking.
-///
-/// # Examples
-///
-/// See the [module-level](crate::pipe) documentation for examples.
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-#[inline]
-pub fn pipe() -> io::Result<(PipeReader, PipeWriter)> {
-    pipe_inner().map(|(reader, writer)| (PipeReader(reader), PipeWriter(writer)))
-}
-
-/// Read end of the anonymous pipe.
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-#[derive(Debug)]
-pub struct PipeReader(pub(crate) AnonPipe);
-
-/// Write end of the anonymous pipe.
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-#[derive(Debug)]
-pub struct PipeWriter(pub(crate) AnonPipe);
-
-impl PipeReader {
-    /// Create a new [`PipeReader`] instance that shares the same underlying file description.
-    ///
-    /// # Examples
-    ///
-    /// ```no_run
-    /// #![feature(anonymous_pipe)]
-    /// # #[cfg(miri)] fn main() {}
-    /// # #[cfg(not(miri))]
-    /// # fn main() -> std::io::Result<()> {
-    /// # use std::fs;
-    /// # use std::io::Write;
-    /// # use std::process::Command;
-    /// const NUM_SLOT: u8 = 2;
-    /// const NUM_PROC: u8 = 5;
-    /// const OUTPUT: &str = "work.txt";
-    ///
-    /// let mut jobs = vec![];
-    /// let (reader, mut writer) = std::pipe::pipe()?;
-    ///
-    /// // Write NUM_SLOT characters the pipe.
-    /// writer.write_all(&[b'|'; NUM_SLOT as usize])?;
-    ///
-    /// // Spawn several processes that read a character from the pipe, do some work, then
-    /// // write back to the pipe. When the pipe is empty, the processes block, so only
-    /// // NUM_SLOT processes can be working at any given time.
-    /// for _ in 0..NUM_PROC {
-    ///     jobs.push(
-    ///         Command::new("bash")
-    ///             .args(["-c",
-    ///                 &format!(
-    ///                      "read -n 1\n\
-    ///                       echo -n 'x' >> '{OUTPUT}'\n\
-    ///                       echo -n '|'",
-    ///                 ),
-    ///             ])
-    ///             .stdin(reader.try_clone()?)
-    ///             .stdout(writer.try_clone()?)
-    ///             .spawn()?,
-    ///     );
-    /// }
-    ///
-    /// // Wait for all jobs to finish.
-    /// for mut job in jobs {
-    ///     job.wait()?;
-    /// }
-    ///
-    /// // Check our work and clean up.
-    /// let xs = fs::read_to_string(OUTPUT)?;
-    /// fs::remove_file(OUTPUT)?;
-    /// assert_eq!(xs, "x".repeat(NUM_PROC.into()));
-    /// # Ok(())
-    /// # }
-    /// ```
-    #[unstable(feature = "anonymous_pipe", issue = "127154")]
-    pub fn try_clone(&self) -> io::Result<Self> {
-        self.0.try_clone().map(Self)
-    }
-}
-
-impl PipeWriter {
-    /// Create a new [`PipeWriter`] instance that shares the same underlying file description.
-    ///
-    /// # Examples
-    ///
-    /// ```no_run
-    /// #![feature(anonymous_pipe)]
-    /// # #[cfg(miri)] fn main() {}
-    /// # #[cfg(not(miri))]
-    /// # fn main() -> std::io::Result<()> {
-    /// # use std::process::Command;
-    /// # use std::io::Read;
-    /// let (mut reader, writer) = std::pipe::pipe()?;
-    ///
-    /// // Spawn a process that writes to stdout and stderr.
-    /// let mut peer = Command::new("bash")
-    ///     .args([
-    ///         "-c",
-    ///         "echo -n foo\n\
-    ///          echo -n bar >&2"
-    ///     ])
-    ///     .stdout(writer.try_clone()?)
-    ///     .stderr(writer)
-    ///     .spawn()?;
-    ///
-    /// // Read and check the result.
-    /// let mut msg = String::new();
-    /// reader.read_to_string(&mut msg)?;
-    /// assert_eq!(&msg, "foobar");
-    ///
-    /// peer.wait()?;
-    /// # Ok(())
-    /// # }
-    /// ```
-    #[unstable(feature = "anonymous_pipe", issue = "127154")]
-    pub fn try_clone(&self) -> io::Result<Self> {
-        self.0.try_clone().map(Self)
-    }
-}
-
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-impl io::Read for &PipeReader {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        self.0.read(buf)
-    }
-    fn read_vectored(&mut self, bufs: &mut [io::IoSliceMut<'_>]) -> io::Result<usize> {
-        self.0.read_vectored(bufs)
-    }
-    #[inline]
-    fn is_read_vectored(&self) -> bool {
-        self.0.is_read_vectored()
-    }
-    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
-        self.0.read_to_end(buf)
-    }
-    fn read_buf(&mut self, buf: io::BorrowedCursor<'_>) -> io::Result<()> {
-        self.0.read_buf(buf)
-    }
-}
-
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-impl io::Read for PipeReader {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        self.0.read(buf)
-    }
-    fn read_vectored(&mut self, bufs: &mut [io::IoSliceMut<'_>]) -> io::Result<usize> {
-        self.0.read_vectored(bufs)
-    }
-    #[inline]
-    fn is_read_vectored(&self) -> bool {
-        self.0.is_read_vectored()
-    }
-    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
-        self.0.read_to_end(buf)
-    }
-    fn read_buf(&mut self, buf: io::BorrowedCursor<'_>) -> io::Result<()> {
-        self.0.read_buf(buf)
-    }
-}
-
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-impl io::Write for &PipeWriter {
-    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        self.0.write(buf)
-    }
-    #[inline]
-    fn flush(&mut self) -> io::Result<()> {
-        Ok(())
-    }
-
-    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
-        self.0.write_vectored(bufs)
-    }
-
-    #[inline]
-    fn is_write_vectored(&self) -> bool {
-        self.0.is_write_vectored()
-    }
-}
-
-#[unstable(feature = "anonymous_pipe", issue = "127154")]
-impl io::Write for PipeWriter {
-    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        self.0.write(buf)
-    }
-    #[inline]
-    fn flush(&mut self) -> io::Result<()> {
-        Ok(())
-    }
-
-    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
-        self.0.write_vectored(bufs)
-    }
-
-    #[inline]
-    fn is_write_vectored(&self) -> bool {
-        self.0.is_write_vectored()
-    }
-}
diff --git a/library/std/src/pipe/tests.rs b/library/std/src/pipe/tests.rs
deleted file mode 100644
index 9c38e106787..00000000000
--- a/library/std/src/pipe/tests.rs
+++ /dev/null
@@ -1,19 +0,0 @@
-use crate::io::{Read, Write};
-use crate::pipe::pipe;
-
-#[test]
-#[cfg(all(windows, unix, not(miri)))]
-fn pipe_creation_clone_and_rw() {
-    let (rx, tx) = pipe().unwrap();
-
-    tx.try_clone().unwrap().write_all(b"12345").unwrap();
-    drop(tx);
-
-    let mut rx2 = rx.try_clone().unwrap();
-    drop(rx);
-
-    let mut s = String::new();
-    rx2.read_to_string(&mut s).unwrap();
-    drop(rx2);
-    assert_eq!(s, "12345");
-}
diff --git a/library/std/src/prelude/common.rs b/library/std/src/prelude/common.rs
index 22a364074c5..0f2d8334fca 100644
--- a/library/std/src/prelude/common.rs
+++ b/library/std/src/prelude/common.rs
@@ -12,8 +12,7 @@ pub use crate::marker::{Send, Sized, Sync, Unpin};
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]
 pub use crate::ops::{Drop, Fn, FnMut, FnOnce};
-#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
-#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "CURRENT_RUSTC_VERSION"))]
+#[stable(feature = "async_closure", since = "1.85.0")]
 #[doc(no_inline)]
 pub use crate::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
 
diff --git a/library/std/src/prelude/mod.rs b/library/std/src/prelude/mod.rs
index 64349987fcf..4ec328208f0 100644
--- a/library/std/src/prelude/mod.rs
+++ b/library/std/src/prelude/mod.rs
@@ -169,12 +169,12 @@ pub mod rust_2021 {
 /// The 2024 version of the prelude of The Rust Standard Library.
 ///
 /// See the [module-level documentation](self) for more.
-#[stable(feature = "prelude_2024", since = "CURRENT_RUSTC_VERSION")]
+#[stable(feature = "prelude_2024", since = "1.85.0")]
 pub mod rust_2024 {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::common::*;
 
-    #[stable(feature = "prelude_2024", since = "CURRENT_RUSTC_VERSION")]
+    #[stable(feature = "prelude_2024", since = "1.85.0")]
     #[doc(no_inline)]
     pub use core::prelude::rust_2024::*;
 }
diff --git a/library/std/src/rt.rs b/library/std/src/rt.rs
index b2492238bd3..3a22a16cb16 100644
--- a/library/std/src/rt.rs
+++ b/library/std/src/rt.rs
@@ -23,7 +23,7 @@ pub use core::panicking::{panic_display, panic_fmt};
 #[rustfmt::skip]
 use crate::any::Any;
 use crate::sync::Once;
-use crate::thread::{self, Thread};
+use crate::thread::{self, main_thread};
 use crate::{mem, panic, sys};
 
 // Prints to the "panic output", depending on the platform this may be:
@@ -32,9 +32,14 @@ use crate::{mem, panic, sys};
 // - nothing (so this macro is a no-op)
 macro_rules! rtprintpanic {
     ($($t:tt)*) => {
+        #[cfg(not(feature = "panic_immediate_abort"))]
         if let Some(mut out) = crate::sys::stdio::panic_output() {
             let _ = crate::io::Write::write_fmt(&mut out, format_args!($($t)*));
         }
+        #[cfg(feature = "panic_immediate_abort")]
+        {
+            let _ = format_args!($($t)*);
+        }
     }
 }
 
@@ -67,7 +72,7 @@ macro_rules! rtunwrap {
     };
 }
 
-fn handle_rt_panic(e: Box<dyn Any + Send>) {
+fn handle_rt_panic<T>(e: Box<dyn Any + Send>) -> T {
     mem::forget(e);
     rtabort!("initialization or cleanup bug");
 }
@@ -102,24 +107,9 @@ unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
         sys::init(argc, argv, sigpipe)
     };
 
-    // Set up the current thread handle to give it the right name.
-    //
-    // When code running before main uses `ReentrantLock` (for example by
-    // using `println!`), the thread ID can become initialized before we
-    // create this handle. Since `set_current` fails when the ID of the
-    // handle does not match the current ID, we should attempt to use the
-    // current thread ID here instead of unconditionally creating a new
-    // one. Also see #130210.
-    let thread = unsafe { Thread::new_main(thread::current_id()) };
-    if let Err(_thread) = thread::set_current(thread) {
-        // `thread::current` will create a new handle if none has been set yet.
-        // Thus, if someone uses it before main, this call will fail. That's a
-        // bad idea though, as we then cannot set the main thread name here.
-        //
-        // FIXME: detect the main thread in `thread::current` and use the
-        //        correct name there.
-        rtabort!("code running before main must not use thread::current");
-    }
+    // Remember the main thread ID to give it the correct name.
+    // SAFETY: this is the only time and place where we call this function.
+    unsafe { main_thread::set(thread::current_id()) };
 }
 
 /// Clean up the thread-local runtime state. This *should* be run after all other
@@ -157,7 +147,7 @@ fn lang_start_internal(
     argc: isize,
     argv: *const *const u8,
     sigpipe: u8,
-) -> Result<isize, !> {
+) -> isize {
     // Guard against the code called by this function from unwinding outside of the Rust-controlled
     // code, which is UB. This is a requirement imposed by a combination of how the
     // `#[lang="start"]` attribute is implemented as well as by the implementation of the panicking
@@ -168,19 +158,33 @@ fn lang_start_internal(
     // panic is a std implementation bug. A quite likely one too, as there isn't any way to
     // prevent std from accidentally introducing a panic to these functions. Another is from
     // user code from `main` or, more nefariously, as described in e.g. issue #86030.
-    // SAFETY: Only called once during runtime initialization.
-    panic::catch_unwind(move || unsafe { init(argc, argv, sigpipe) })
-        .unwrap_or_else(handle_rt_panic);
-    let ret_code = panic::catch_unwind(move || panic::catch_unwind(main).unwrap_or(101) as isize)
-        .map_err(move |e| {
-            mem::forget(e);
-            rtabort!("drop of the panic payload panicked");
+    //
+    // We use `catch_unwind` with `handle_rt_panic` instead of `abort_unwind` to make the error in
+    // case of a panic a bit nicer.
+    panic::catch_unwind(move || {
+        // SAFETY: Only called once during runtime initialization.
+        unsafe { init(argc, argv, sigpipe) };
+
+        let ret_code = panic::catch_unwind(main).unwrap_or_else(move |payload| {
+            // Carefully dispose of the panic payload.
+            let payload = panic::AssertUnwindSafe(payload);
+            panic::catch_unwind(move || drop({ payload }.0)).unwrap_or_else(move |e| {
+                mem::forget(e); // do *not* drop the 2nd payload
+                rtabort!("drop of the panic payload panicked");
+            });
+            // Return error code for panicking programs.
+            101
         });
-    panic::catch_unwind(cleanup).unwrap_or_else(handle_rt_panic);
-    // Guard against multiple threads calling `libc::exit` concurrently.
-    // See the documentation for `unique_thread_exit` for more information.
-    panic::catch_unwind(crate::sys::exit_guard::unique_thread_exit).unwrap_or_else(handle_rt_panic);
-    ret_code
+        let ret_code = ret_code as isize;
+
+        cleanup();
+        // Guard against multiple threads calling `libc::exit` concurrently.
+        // See the documentation for `unique_thread_exit` for more information.
+        crate::sys::exit_guard::unique_thread_exit();
+
+        ret_code
+    })
+    .unwrap_or_else(handle_rt_panic)
 }
 
 #[cfg(not(any(test, doctest)))]
@@ -191,11 +195,10 @@ fn lang_start<T: crate::process::Termination + 'static>(
     argv: *const *const u8,
     sigpipe: u8,
 ) -> isize {
-    let Ok(v) = lang_start_internal(
+    lang_start_internal(
         &move || crate::sys::backtrace::__rust_begin_short_backtrace(main).report().to_i32(),
         argc,
         argv,
         sigpipe,
-    );
-    v
+    )
 }
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index 1e4f9b79e0f..98c83d8d326 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -31,7 +31,7 @@ union Data<T, F> {
 /// ```
 /// use std::sync::LazyLock;
 ///
-/// // n.b. static items do not call [`Drop`] on program termination, so this won't be deallocated.
+/// // Note: static items do not call [`Drop`] on program termination, so this won't be deallocated.
 /// // this is fine, as the OS can deallocate the terminated program faster than we can free memory
 /// // but tools like valgrind might report "memory leaks" as it isn't obvious this is intentional.
 /// static DEEP_THOUGHT: LazyLock<String> = LazyLock::new(|| {
diff --git a/library/std/src/sys/anonymous_pipe/unix.rs b/library/std/src/sys/anonymous_pipe/unix.rs
index 9168024730e..9e398765634 100644
--- a/library/std/src/sys/anonymous_pipe/unix.rs
+++ b/library/std/src/sys/anonymous_pipe/unix.rs
@@ -1,6 +1,5 @@
-use crate::io;
+use crate::io::{self, PipeReader, PipeWriter};
 use crate::os::fd::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
-use crate::pipe::{PipeReader, PipeWriter};
 use crate::process::Stdio;
 use crate::sys::fd::FileDesc;
 use crate::sys::pipe::anon_pipe;
diff --git a/library/std/src/sys/anonymous_pipe/unsupported.rs b/library/std/src/sys/anonymous_pipe/unsupported.rs
index dd51e70315e..4e79ac9c21a 100644
--- a/library/std/src/sys/anonymous_pipe/unsupported.rs
+++ b/library/std/src/sys/anonymous_pipe/unsupported.rs
@@ -1,5 +1,4 @@
-use crate::io;
-use crate::pipe::{PipeReader, PipeWriter};
+use crate::io::{self, PipeReader, PipeWriter};
 use crate::process::Stdio;
 pub use crate::sys::pipe::AnonPipe;
 
diff --git a/library/std/src/sys/anonymous_pipe/windows.rs b/library/std/src/sys/anonymous_pipe/windows.rs
index a48198f8a81..eb7fa8ec1c9 100644
--- a/library/std/src/sys/anonymous_pipe/windows.rs
+++ b/library/std/src/sys/anonymous_pipe/windows.rs
@@ -1,12 +1,12 @@
+use crate::io::{self, PipeReader, PipeWriter};
 use crate::os::windows::io::{
     AsHandle, AsRawHandle, BorrowedHandle, FromRawHandle, IntoRawHandle, OwnedHandle, RawHandle,
 };
-use crate::pipe::{PipeReader, PipeWriter};
 use crate::process::Stdio;
+use crate::ptr;
 use crate::sys::c;
 use crate::sys::handle::Handle;
 use crate::sys_common::{FromInner, IntoInner};
-use crate::{io, ptr};
 
 pub type AnonPipe = Handle;
 
diff --git a/library/std/src/sys/pal/solid/fs.rs b/library/std/src/sys/pal/solid/fs.rs
index 04dd10ad806..fa2e470d6b6 100644
--- a/library/std/src/sys/pal/solid/fs.rs
+++ b/library/std/src/sys/pal/solid/fs.rs
@@ -12,15 +12,12 @@ use crate::sys::unsupported;
 pub use crate::sys_common::fs::exists;
 use crate::sys_common::ignore_notfound;
 
+type CIntNotMinusOne = core::num::niche_types::NotAllOnes<c_int>;
+
 /// A file descriptor.
 #[derive(Clone, Copy)]
-#[rustc_layout_scalar_valid_range_start(0)]
-// libstd/os/raw/mod.rs assures me that every libstd-supported platform has a
-// 32-bit c_int. Below is -2, in two's complement, but that only works out
-// because c_int is 32 bits.
-#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
 struct FileDesc {
-    fd: c_int,
+    fd: CIntNotMinusOne,
 }
 
 impl FileDesc {
@@ -29,12 +26,13 @@ impl FileDesc {
         assert_ne!(fd, -1i32);
         // Safety: we just asserted that the value is in the valid range and
         // isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
-        unsafe { FileDesc { fd } }
+        let fd = unsafe { CIntNotMinusOne::new_unchecked(fd) };
+        FileDesc { fd }
     }
 
     #[inline]
     fn raw(&self) -> c_int {
-        self.fd
+        self.fd.as_inner()
     }
 }
 
diff --git a/library/std/src/sys/pal/uefi/fs.rs b/library/std/src/sys/pal/uefi/fs.rs
new file mode 100644
index 00000000000..9585ec24f68
--- /dev/null
+++ b/library/std/src/sys/pal/uefi/fs.rs
@@ -0,0 +1,344 @@
+use crate::ffi::OsString;
+use crate::fmt;
+use crate::hash::{Hash, Hasher};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut, SeekFrom};
+use crate::path::{Path, PathBuf};
+use crate::sys::time::SystemTime;
+use crate::sys::unsupported;
+
+pub struct File(!);
+
+pub struct FileAttr(!);
+
+pub struct ReadDir(!);
+
+pub struct DirEntry(!);
+
+#[derive(Clone, Debug)]
+pub struct OpenOptions {}
+
+#[derive(Copy, Clone, Debug, Default)]
+pub struct FileTimes {}
+
+pub struct FilePermissions(!);
+
+pub struct FileType(!);
+
+#[derive(Debug)]
+pub struct DirBuilder {}
+
+impl FileAttr {
+    pub fn size(&self) -> u64 {
+        self.0
+    }
+
+    pub fn perm(&self) -> FilePermissions {
+        self.0
+    }
+
+    pub fn file_type(&self) -> FileType {
+        self.0
+    }
+
+    pub fn modified(&self) -> io::Result<SystemTime> {
+        self.0
+    }
+
+    pub fn accessed(&self) -> io::Result<SystemTime> {
+        self.0
+    }
+
+    pub fn created(&self) -> io::Result<SystemTime> {
+        self.0
+    }
+}
+
+impl Clone for FileAttr {
+    fn clone(&self) -> FileAttr {
+        self.0
+    }
+}
+
+impl FilePermissions {
+    pub fn readonly(&self) -> bool {
+        self.0
+    }
+
+    pub fn set_readonly(&mut self, _readonly: bool) {
+        self.0
+    }
+}
+
+impl Clone for FilePermissions {
+    fn clone(&self) -> FilePermissions {
+        self.0
+    }
+}
+
+impl PartialEq for FilePermissions {
+    fn eq(&self, _other: &FilePermissions) -> bool {
+        self.0
+    }
+}
+
+impl Eq for FilePermissions {}
+
+impl fmt::Debug for FilePermissions {
+    fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0
+    }
+}
+
+impl FileTimes {
+    pub fn set_accessed(&mut self, _t: SystemTime) {}
+    pub fn set_modified(&mut self, _t: SystemTime) {}
+}
+
+impl FileType {
+    pub fn is_dir(&self) -> bool {
+        self.0
+    }
+
+    pub fn is_file(&self) -> bool {
+        self.0
+    }
+
+    pub fn is_symlink(&self) -> bool {
+        self.0
+    }
+}
+
+impl Clone for FileType {
+    fn clone(&self) -> FileType {
+        self.0
+    }
+}
+
+impl Copy for FileType {}
+
+impl PartialEq for FileType {
+    fn eq(&self, _other: &FileType) -> bool {
+        self.0
+    }
+}
+
+impl Eq for FileType {}
+
+impl Hash for FileType {
+    fn hash<H: Hasher>(&self, _h: &mut H) {
+        self.0
+    }
+}
+
+impl fmt::Debug for FileType {
+    fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0
+    }
+}
+
+impl fmt::Debug for ReadDir {
+    fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0
+    }
+}
+
+impl Iterator for ReadDir {
+    type Item = io::Result<DirEntry>;
+
+    fn next(&mut self) -> Option<io::Result<DirEntry>> {
+        self.0
+    }
+}
+
+impl DirEntry {
+    pub fn path(&self) -> PathBuf {
+        self.0
+    }
+
+    pub fn file_name(&self) -> OsString {
+        self.0
+    }
+
+    pub fn metadata(&self) -> io::Result<FileAttr> {
+        self.0
+    }
+
+    pub fn file_type(&self) -> io::Result<FileType> {
+        self.0
+    }
+}
+
+impl OpenOptions {
+    pub fn new() -> OpenOptions {
+        OpenOptions {}
+    }
+
+    pub fn read(&mut self, _read: bool) {}
+    pub fn write(&mut self, _write: bool) {}
+    pub fn append(&mut self, _append: bool) {}
+    pub fn truncate(&mut self, _truncate: bool) {}
+    pub fn create(&mut self, _create: bool) {}
+    pub fn create_new(&mut self, _create_new: bool) {}
+}
+
+impl File {
+    pub fn open(_path: &Path, _opts: &OpenOptions) -> io::Result<File> {
+        unsupported()
+    }
+
+    pub fn file_attr(&self) -> io::Result<FileAttr> {
+        self.0
+    }
+
+    pub fn fsync(&self) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn datasync(&self) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn lock(&self) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn lock_shared(&self) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn try_lock(&self) -> io::Result<bool> {
+        self.0
+    }
+
+    pub fn try_lock_shared(&self) -> io::Result<bool> {
+        self.0
+    }
+
+    pub fn unlock(&self) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn truncate(&self, _size: u64) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn read(&self, _buf: &mut [u8]) -> io::Result<usize> {
+        self.0
+    }
+
+    pub fn read_vectored(&self, _bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        self.0
+    }
+
+    pub fn is_read_vectored(&self) -> bool {
+        self.0
+    }
+
+    pub fn read_buf(&self, _cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn write(&self, _buf: &[u8]) -> io::Result<usize> {
+        self.0
+    }
+
+    pub fn write_vectored(&self, _bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        self.0
+    }
+
+    pub fn is_write_vectored(&self) -> bool {
+        self.0
+    }
+
+    pub fn flush(&self) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn seek(&self, _pos: SeekFrom) -> io::Result<u64> {
+        self.0
+    }
+
+    pub fn duplicate(&self) -> io::Result<File> {
+        self.0
+    }
+
+    pub fn set_permissions(&self, _perm: FilePermissions) -> io::Result<()> {
+        self.0
+    }
+
+    pub fn set_times(&self, _times: FileTimes) -> io::Result<()> {
+        self.0
+    }
+}
+
+impl DirBuilder {
+    pub fn new() -> DirBuilder {
+        DirBuilder {}
+    }
+
+    pub fn mkdir(&self, _p: &Path) -> io::Result<()> {
+        unsupported()
+    }
+}
+
+impl fmt::Debug for File {
+    fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0
+    }
+}
+
+pub fn readdir(_p: &Path) -> io::Result<ReadDir> {
+    unsupported()
+}
+
+pub fn unlink(_p: &Path) -> io::Result<()> {
+    unsupported()
+}
+
+pub fn rename(_old: &Path, _new: &Path) -> io::Result<()> {
+    unsupported()
+}
+
+pub fn set_perm(_p: &Path, perm: FilePermissions) -> io::Result<()> {
+    match perm.0 {}
+}
+
+pub fn rmdir(_p: &Path) -> io::Result<()> {
+    unsupported()
+}
+
+pub fn remove_dir_all(_path: &Path) -> io::Result<()> {
+    unsupported()
+}
+
+pub fn exists(_path: &Path) -> io::Result<bool> {
+    unsupported()
+}
+
+pub fn readlink(_p: &Path) -> io::Result<PathBuf> {
+    unsupported()
+}
+
+pub fn symlink(_original: &Path, _link: &Path) -> io::Result<()> {
+    unsupported()
+}
+
+pub fn link(_src: &Path, _dst: &Path) -> io::Result<()> {
+    unsupported()
+}
+
+pub fn stat(_p: &Path) -> io::Result<FileAttr> {
+    unsupported()
+}
+
+pub fn lstat(_p: &Path) -> io::Result<FileAttr> {
+    unsupported()
+}
+
+pub fn canonicalize(_p: &Path) -> io::Result<PathBuf> {
+    unsupported()
+}
+
+pub fn copy(_from: &Path, _to: &Path) -> io::Result<u64> {
+    unsupported()
+}
diff --git a/library/std/src/sys/pal/uefi/helpers.rs b/library/std/src/sys/pal/uefi/helpers.rs
index 47d9add72b5..7504a0f7ad7 100644
--- a/library/std/src/sys/pal/uefi/helpers.rs
+++ b/library/std/src/sys/pal/uefi/helpers.rs
@@ -222,14 +222,14 @@ pub(crate) fn runtime_services() -> Option<NonNull<r_efi::efi::RuntimeServices>>
     NonNull::new(runtime_services)
 }
 
-pub(crate) struct DevicePath(NonNull<r_efi::protocols::device_path::Protocol>);
+pub(crate) struct OwnedDevicePath(NonNull<r_efi::protocols::device_path::Protocol>);
 
-impl DevicePath {
+impl OwnedDevicePath {
     pub(crate) fn from_text(p: &OsStr) -> io::Result<Self> {
         fn inner(
             p: &OsStr,
             protocol: NonNull<r_efi::protocols::device_path_from_text::Protocol>,
-        ) -> io::Result<DevicePath> {
+        ) -> io::Result<OwnedDevicePath> {
             let path_vec = p.encode_wide().chain(Some(0)).collect::<Vec<u16>>();
             if path_vec[..path_vec.len() - 1].contains(&0) {
                 return Err(const_error!(
@@ -242,7 +242,7 @@ impl DevicePath {
                 unsafe { ((*protocol.as_ptr()).convert_text_to_device_path)(path_vec.as_ptr()) };
 
             NonNull::new(path)
-                .map(DevicePath)
+                .map(OwnedDevicePath)
                 .ok_or_else(|| const_error!(io::ErrorKind::InvalidFilename, "Invalid Device Path"))
         }
 
@@ -275,12 +275,12 @@ impl DevicePath {
         ))
     }
 
-    pub(crate) fn as_ptr(&self) -> *mut r_efi::protocols::device_path::Protocol {
+    pub(crate) const fn as_ptr(&self) -> *mut r_efi::protocols::device_path::Protocol {
         self.0.as_ptr()
     }
 }
 
-impl Drop for DevicePath {
+impl Drop for OwnedDevicePath {
     fn drop(&mut self) {
         if let Some(bt) = boot_services() {
             let bt: NonNull<r_efi::efi::BootServices> = bt.cast();
@@ -291,6 +291,15 @@ impl Drop for DevicePath {
     }
 }
 
+impl crate::fmt::Debug for OwnedDevicePath {
+    fn fmt(&self, f: &mut crate::fmt::Formatter<'_>) -> crate::fmt::Result {
+        match device_path_to_text(self.0) {
+            Ok(p) => p.fmt(f),
+            Err(_) => f.debug_struct("OwnedDevicePath").finish_non_exhaustive(),
+        }
+    }
+}
+
 pub(crate) struct OwnedProtocol<T> {
     guid: r_efi::efi::Guid,
     handle: NonNull<crate::ffi::c_void>,
diff --git a/library/std/src/sys/pal/uefi/mod.rs b/library/std/src/sys/pal/uefi/mod.rs
index f29c91f3bfe..111bed7a7eb 100644
--- a/library/std/src/sys/pal/uefi/mod.rs
+++ b/library/std/src/sys/pal/uefi/mod.rs
@@ -15,7 +15,6 @@
 
 pub mod args;
 pub mod env;
-#[path = "../unsupported/fs.rs"]
 pub mod fs;
 pub mod helpers;
 #[path = "../unsupported/io.rs"]
diff --git a/library/std/src/sys/pal/uefi/process.rs b/library/std/src/sys/pal/uefi/process.rs
index 95707ebb7f0..1a0754134df 100644
--- a/library/std/src/sys/pal/uefi/process.rs
+++ b/library/std/src/sys/pal/uefi/process.rs
@@ -326,7 +326,7 @@ mod uefi_command_internal {
 
     impl Image {
         pub fn load_image(p: &OsStr) -> io::Result<Self> {
-            let path = helpers::DevicePath::from_text(p)?;
+            let path = helpers::OwnedDevicePath::from_text(p)?;
             let boot_services: NonNull<r_efi::efi::BootServices> = boot_services()
                 .ok_or_else(|| const_error!(io::ErrorKind::NotFound, "Boot Services not found"))?
                 .cast();
diff --git a/library/std/src/sys/pal/unix/fs.rs b/library/std/src/sys/pal/unix/fs.rs
index 54fa31620ac..fdf011c1948 100644
--- a/library/std/src/sys/pal/unix/fs.rs
+++ b/library/std/src/sys/pal/unix/fs.rs
@@ -709,7 +709,7 @@ impl Iterator for ReadDir {
                 // thread safety for readdir() as long an individual DIR* is not accessed
                 // concurrently, which is sufficient for Rust.
                 super::os::set_errno(0);
-                let entry_ptr = readdir64(self.inner.dirp.0);
+                let entry_ptr: *const dirent64 = readdir64(self.inner.dirp.0);
                 if entry_ptr.is_null() {
                     // We either encountered an error, or reached the end. Either way,
                     // the next call to next() should return None.
@@ -735,29 +735,19 @@ impl Iterator for ReadDir {
                 // contents were "simply" partially initialized data.
                 //
                 // Like for uninitialized contents, converting entry_ptr to `&dirent64`
-                // would not be legal. However, unique to dirent64 is that we don't even
-                // get to use `&raw const (*entry_ptr).d_name` because that operation
-                // requires the full extent of *entry_ptr to be in bounds of the same
-                // allocation, which is not necessarily the case here.
-                //
-                // Instead we must access fields individually through their offsets.
-                macro_rules! offset_ptr {
-                    ($entry_ptr:expr, $field:ident) => {{
-                        const OFFSET: isize = mem::offset_of!(dirent64, $field) as isize;
-                        if true {
-                            // Cast to the same type determined by the else branch.
-                            $entry_ptr.byte_offset(OFFSET).cast::<_>()
-                        } else {
-                            #[allow(deref_nullptr)]
-                            {
-                                &raw const (*ptr::null::<dirent64>()).$field
-                            }
-                        }
-                    }};
+                // would not be legal. However, we can use `&raw const (*entry_ptr).d_name`
+                // to refer the fields individually, because that operation is equivalent
+                // to `byte_offset` and thus does not require the full extent of `*entry_ptr`
+                // to be in bounds of the same allocation, only the offset of the field
+                // being referenced.
+                macro_rules! entry_field_ptr {
+                    ($field:ident) => {
+                        &raw const (*entry_ptr).$field
+                    };
                 }
 
                 // d_name is guaranteed to be null-terminated.
-                let name = CStr::from_ptr(offset_ptr!(entry_ptr, d_name).cast());
+                let name = CStr::from_ptr(entry_field_ptr!(d_name).cast());
                 let name_bytes = name.to_bytes();
                 if name_bytes == b"." || name_bytes == b".." {
                     continue;
@@ -765,14 +755,14 @@ impl Iterator for ReadDir {
 
                 #[cfg(not(target_os = "vita"))]
                 let entry = dirent64_min {
-                    d_ino: *offset_ptr!(entry_ptr, d_ino) as u64,
+                    d_ino: *entry_field_ptr!(d_ino) as u64,
                     #[cfg(not(any(
                         target_os = "solaris",
                         target_os = "illumos",
                         target_os = "aix",
                         target_os = "nto",
                     )))]
-                    d_type: *offset_ptr!(entry_ptr, d_type) as u8,
+                    d_type: *entry_field_ptr!(d_type) as u8,
                 };
 
                 #[cfg(target_os = "vita")]
diff --git a/library/std/src/sys/pal/unix/kernel_copy.rs b/library/std/src/sys/pal/unix/kernel_copy.rs
index 36823a503b1..bbf29f32523 100644
--- a/library/std/src/sys/pal/unix/kernel_copy.rs
+++ b/library/std/src/sys/pal/unix/kernel_copy.rs
@@ -52,15 +52,14 @@ use crate::cmp::min;
 use crate::fs::{File, Metadata};
 use crate::io::copy::generic_copy;
 use crate::io::{
-    BufRead, BufReader, BufWriter, Error, Read, Result, StderrLock, StdinLock, StdoutLock, Take,
-    Write,
+    BufRead, BufReader, BufWriter, Error, PipeReader, PipeWriter, Read, Result, StderrLock,
+    StdinLock, StdoutLock, Take, Write,
 };
 use crate::mem::ManuallyDrop;
 use crate::net::TcpStream;
 use crate::os::unix::fs::FileTypeExt;
 use crate::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use crate::os::unix::net::UnixStream;
-use crate::pipe::{PipeReader, PipeWriter};
 use crate::process::{ChildStderr, ChildStdin, ChildStdout};
 use crate::ptr;
 use crate::sync::atomic::{AtomicBool, AtomicU8, Ordering};
diff --git a/library/std/src/sys/pal/unix/stack_overflow.rs b/library/std/src/sys/pal/unix/stack_overflow.rs
index 69b31da427f..db5c6bd3a1c 100644
--- a/library/std/src/sys/pal/unix/stack_overflow.rs
+++ b/library/std/src/sys/pal/unix/stack_overflow.rs
@@ -100,10 +100,11 @@ mod imp {
         // If the faulting address is within the guard page, then we print a
         // message saying so and abort.
         if start <= addr && addr < end {
-            rtprintpanic!(
-                "\nthread '{}' has overflowed its stack\n",
-                thread::current().name().unwrap_or("<unknown>")
-            );
+            thread::with_current_name(|name| {
+                let name = name.unwrap_or("<unknown>");
+                rtprintpanic!("\nthread '{name}' has overflowed its stack\n");
+            });
+
             rtabort!("stack overflow");
         } else {
             // Unregister ourselves by reverting back to the default behavior.
diff --git a/library/std/src/sys/pal/unix/thread.rs b/library/std/src/sys/pal/unix/thread.rs
index e360ba0f6d7..f657f82e6e3 100644
--- a/library/std/src/sys/pal/unix/thread.rs
+++ b/library/std/src/sys/pal/unix/thread.rs
@@ -130,25 +130,27 @@ impl Thread {
         }
     }
 
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "dragonfly"))]
     pub fn set_name(name: &CStr) {
-        const TASK_COMM_LEN: usize = 16;
-
         unsafe {
-            // Available since glibc 2.12, musl 1.1.16, and uClibc 1.0.20.
-            let name = truncate_cstr::<{ TASK_COMM_LEN }>(name);
+            cfg_if::cfg_if! {
+                if #[cfg(target_os = "linux")] {
+                    // Linux limits the allowed length of the name.
+                    const TASK_COMM_LEN: usize = 16;
+                    let name = truncate_cstr::<{ TASK_COMM_LEN }>(name);
+                } else {
+                    // FreeBSD and DragonFly BSD do not enforce length limits.
+                }
+            };
+            // Available since glibc 2.12, musl 1.1.16, and uClibc 1.0.20 for Linux,
+            // FreeBSD 12.2 and 13.0, and DragonFly BSD 6.0.
             let res = libc::pthread_setname_np(libc::pthread_self(), name.as_ptr());
             // We have no good way of propagating errors here, but in debug-builds let's check that this actually worked.
             debug_assert_eq!(res, 0);
         }
     }
 
-    #[cfg(any(
-        target_os = "freebsd",
-        target_os = "dragonfly",
-        target_os = "openbsd",
-        target_os = "nuttx"
-    ))]
+    #[cfg(any(target_os = "openbsd", target_os = "nuttx"))]
     pub fn set_name(name: &CStr) {
         unsafe {
             libc::pthread_set_name_np(libc::pthread_self(), name.as_ptr());
diff --git a/library/std/src/sys/pal/unix/time.rs b/library/std/src/sys/pal/unix/time.rs
index 343864d0b3f..e224980e95f 100644
--- a/library/std/src/sys/pal/unix/time.rs
+++ b/library/std/src/sys/pal/unix/time.rs
@@ -1,3 +1,5 @@
+use core::num::niche_types::Nanoseconds;
+
 use crate::time::Duration;
 use crate::{fmt, io};
 
@@ -16,12 +18,6 @@ pub(in crate::sys) const TIMESPEC_MAX_CAPPED: libc::timespec = libc::timespec {
 };
 
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
-#[repr(transparent)]
-#[rustc_layout_scalar_valid_range_start(0)]
-#[rustc_layout_scalar_valid_range_end(999_999_999)]
-struct Nanoseconds(u32);
-
-#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct SystemTime {
     pub(crate) t: Timespec,
 }
@@ -59,14 +55,14 @@ impl fmt::Debug for SystemTime {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("SystemTime")
             .field("tv_sec", &self.t.tv_sec)
-            .field("tv_nsec", &self.t.tv_nsec.0)
+            .field("tv_nsec", &self.t.tv_nsec)
             .finish()
     }
 }
 
 impl Timespec {
     const unsafe fn new_unchecked(tv_sec: i64, tv_nsec: i64) -> Timespec {
-        Timespec { tv_sec, tv_nsec: unsafe { Nanoseconds(tv_nsec as u32) } }
+        Timespec { tv_sec, tv_nsec: unsafe { Nanoseconds::new_unchecked(tv_nsec as u32) } }
     }
 
     pub const fn zero() -> Timespec {
@@ -147,12 +143,15 @@ impl Timespec {
             //
             // Ideally this code could be rearranged such that it more
             // directly expresses the lower-cost behavior we want from it.
-            let (secs, nsec) = if self.tv_nsec.0 >= other.tv_nsec.0 {
-                ((self.tv_sec - other.tv_sec) as u64, self.tv_nsec.0 - other.tv_nsec.0)
+            let (secs, nsec) = if self.tv_nsec.as_inner() >= other.tv_nsec.as_inner() {
+                (
+                    (self.tv_sec - other.tv_sec) as u64,
+                    self.tv_nsec.as_inner() - other.tv_nsec.as_inner(),
+                )
             } else {
                 (
                     (self.tv_sec - other.tv_sec - 1) as u64,
-                    self.tv_nsec.0 + (NSEC_PER_SEC as u32) - other.tv_nsec.0,
+                    self.tv_nsec.as_inner() + (NSEC_PER_SEC as u32) - other.tv_nsec.as_inner(),
                 )
             };
 
@@ -170,7 +169,7 @@ impl Timespec {
 
         // Nano calculations can't overflow because nanos are <1B which fit
         // in a u32.
-        let mut nsec = other.subsec_nanos() + self.tv_nsec.0;
+        let mut nsec = other.subsec_nanos() + self.tv_nsec.as_inner();
         if nsec >= NSEC_PER_SEC as u32 {
             nsec -= NSEC_PER_SEC as u32;
             secs = secs.checked_add(1)?;
@@ -182,7 +181,7 @@ impl Timespec {
         let mut secs = self.tv_sec.checked_sub_unsigned(other.as_secs())?;
 
         // Similar to above, nanos can't overflow.
-        let mut nsec = self.tv_nsec.0 as i32 - other.subsec_nanos() as i32;
+        let mut nsec = self.tv_nsec.as_inner() as i32 - other.subsec_nanos() as i32;
         if nsec < 0 {
             nsec += NSEC_PER_SEC as i32;
             secs = secs.checked_sub(1)?;
@@ -194,7 +193,7 @@ impl Timespec {
     pub fn to_timespec(&self) -> Option<libc::timespec> {
         Some(libc::timespec {
             tv_sec: self.tv_sec.try_into().ok()?,
-            tv_nsec: self.tv_nsec.0.try_into().ok()?,
+            tv_nsec: self.tv_nsec.as_inner().try_into().ok()?,
         })
     }
 
@@ -203,7 +202,7 @@ impl Timespec {
     #[cfg(target_os = "nto")]
     pub(in crate::sys) fn to_timespec_capped(&self) -> Option<libc::timespec> {
         // Check if timeout in nanoseconds would fit into an u64
-        if (self.tv_nsec.0 as u64)
+        if (self.tv_nsec.as_inner() as u64)
             .checked_add((self.tv_sec as u64).checked_mul(NSEC_PER_SEC)?)
             .is_none()
         {
@@ -219,7 +218,7 @@ impl Timespec {
         not(target_arch = "riscv32")
     ))]
     pub fn to_timespec64(&self) -> __timespec64 {
-        __timespec64::new(self.tv_sec, self.tv_nsec.0 as _)
+        __timespec64::new(self.tv_sec, self.tv_nsec.as_inner() as _)
     }
 }
 
@@ -293,7 +292,7 @@ impl fmt::Debug for Instant {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Instant")
             .field("tv_sec", &self.t.tv_sec)
-            .field("tv_nsec", &self.t.tv_nsec.0)
+            .field("tv_nsec", &self.t.tv_nsec)
             .finish()
     }
 }
diff --git a/library/std/src/sys/pal/windows/mod.rs b/library/std/src/sys/pal/windows/mod.rs
index 88e6def7a75..4282dbb5493 100644
--- a/library/std/src/sys/pal/windows/mod.rs
+++ b/library/std/src/sys/pal/windows/mod.rs
@@ -272,7 +272,7 @@ where
                 unreachable!();
             } else {
                 // Safety: First `k` values are initialized.
-                let slice: &[u16] = MaybeUninit::slice_assume_init_ref(&buf[..k]);
+                let slice: &[u16] = buf[..k].assume_init_ref();
                 return Ok(f2(slice));
             }
         }
diff --git a/library/std/src/sys/pal/windows/stack_overflow.rs b/library/std/src/sys/pal/windows/stack_overflow.rs
index 467e21ab56a..734cd30bed0 100644
--- a/library/std/src/sys/pal/windows/stack_overflow.rs
+++ b/library/std/src/sys/pal/windows/stack_overflow.rs
@@ -18,10 +18,10 @@ unsafe extern "system" fn vectored_handler(ExceptionInfo: *mut c::EXCEPTION_POIN
         let code = rec.ExceptionCode;
 
         if code == c::EXCEPTION_STACK_OVERFLOW {
-            rtprintpanic!(
-                "\nthread '{}' has overflowed its stack\n",
-                thread::current().name().unwrap_or("<unknown>")
-            );
+            thread::with_current_name(|name| {
+                let name = name.unwrap_or("<unknown>");
+                rtprintpanic!("\nthread '{name}' has overflowed its stack\n");
+            });
         }
         c::EXCEPTION_CONTINUE_SEARCH
     }
diff --git a/library/std/src/sys/pal/windows/stdio.rs b/library/std/src/sys/pal/windows/stdio.rs
index 1b735e7f0cb..fd3f559ba19 100644
--- a/library/std/src/sys/pal/windows/stdio.rs
+++ b/library/std/src/sys/pal/windows/stdio.rs
@@ -207,7 +207,7 @@ fn write_valid_utf8_to_console(handle: c::HANDLE, utf8: &str) -> io::Result<usiz
         assert!(result != 0, "Unexpected error in MultiByteToWideChar");
 
         // Safety: MultiByteToWideChar initializes `result` values.
-        MaybeUninit::slice_assume_init_ref(&utf16[..result as usize])
+        utf16[..result as usize].assume_init_ref()
     };
 
     let mut written = write_u16s(handle, utf16)?;
@@ -283,7 +283,7 @@ impl io::Read for Stdin {
             let read = read_u16s_fixup_surrogates(handle, &mut utf16_buf, 1, &mut self.surrogate)?;
             // Read bytes, using the (now-empty) self.incomplete_utf8 as extra space.
             let read_bytes = utf16_to_utf8(
-                unsafe { MaybeUninit::slice_assume_init_ref(&utf16_buf[..read]) },
+                unsafe { utf16_buf[..read].assume_init_ref() },
                 &mut self.incomplete_utf8.bytes,
             )?;
 
@@ -303,7 +303,7 @@ impl io::Read for Stdin {
                 read_u16s_fixup_surrogates(handle, &mut utf16_buf, amount, &mut self.surrogate)?;
             // Safety `read_u16s_fixup_surrogates` returns the number of items
             // initialized.
-            let utf16s = unsafe { MaybeUninit::slice_assume_init_ref(&utf16_buf[..read]) };
+            let utf16s = unsafe { utf16_buf[..read].assume_init_ref() };
             match utf16_to_utf8(utf16s, buf) {
                 Ok(value) => return Ok(bytes_copied + value),
                 Err(e) => return Err(e),
diff --git a/library/std/src/sys/path/sgx.rs b/library/std/src/sys/path/sgx.rs
index c805c15e702..32c7752f605 100644
--- a/library/std/src/sys/path/sgx.rs
+++ b/library/std/src/sys/path/sgx.rs
@@ -23,3 +23,7 @@ pub const MAIN_SEP: char = '/';
 pub(crate) fn absolute(_path: &Path) -> io::Result<PathBuf> {
     unsupported()
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    path.has_root() && path.prefix().is_some()
+}
diff --git a/library/std/src/sys/path/unix.rs b/library/std/src/sys/path/unix.rs
index 2a7c025c3c4..361e99964f1 100644
--- a/library/std/src/sys/path/unix.rs
+++ b/library/std/src/sys/path/unix.rs
@@ -60,3 +60,14 @@ pub(crate) fn absolute(path: &Path) -> io::Result<PathBuf> {
 
     Ok(normalized)
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    if cfg!(target_os = "redox") {
+        // FIXME: Allow Redox prefixes
+        path.has_root() || crate::path::has_redox_scheme(path.as_u8_slice())
+    } else if cfg!(any(unix, target_os = "hermit", target_os = "wasi")) {
+        path.has_root()
+    } else {
+        path.has_root() && path.prefix().is_some()
+    }
+}
diff --git a/library/std/src/sys/path/unsupported_backslash.rs b/library/std/src/sys/path/unsupported_backslash.rs
index 855f443678c..30b06c132c9 100644
--- a/library/std/src/sys/path/unsupported_backslash.rs
+++ b/library/std/src/sys/path/unsupported_backslash.rs
@@ -24,3 +24,7 @@ pub const MAIN_SEP: char = '\\';
 pub(crate) fn absolute(_path: &Path) -> io::Result<PathBuf> {
     unsupported()
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    path.has_root() && path.prefix().is_some()
+}
diff --git a/library/std/src/sys/path/windows.rs b/library/std/src/sys/path/windows.rs
index de042fa3f82..1c534721916 100644
--- a/library/std/src/sys/path/windows.rs
+++ b/library/std/src/sys/path/windows.rs
@@ -346,3 +346,7 @@ pub(crate) fn absolute(path: &Path) -> io::Result<PathBuf> {
         os2path,
     )
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    path.has_root() && path.prefix().is_some()
+}
diff --git a/library/std/src/thread/current.rs b/library/std/src/thread/current.rs
index 3d2c288b360..414711298f0 100644
--- a/library/std/src/thread/current.rs
+++ b/library/std/src/thread/current.rs
@@ -15,7 +15,7 @@ local_pointer! {
 ///
 /// We store the thread ID so that it never gets destroyed during the lifetime
 /// of a thread, either using `#[thread_local]` or multiple `local_pointer!`s.
-mod id {
+pub(super) mod id {
     use super::*;
 
     cfg_if::cfg_if! {
@@ -27,7 +27,7 @@ mod id {
 
             pub(super) const CHEAP: bool = true;
 
-            pub(super) fn get() -> Option<ThreadId> {
+            pub(crate) fn get() -> Option<ThreadId> {
                 ID.get()
             }
 
@@ -44,7 +44,7 @@ mod id {
 
             pub(super) const CHEAP: bool = false;
 
-            pub(super) fn get() -> Option<ThreadId> {
+            pub(crate) fn get() -> Option<ThreadId> {
                 let id0 = ID0.get().addr() as u64;
                 let id16 = ID16.get().addr() as u64;
                 let id32 = ID32.get().addr() as u64;
@@ -67,7 +67,7 @@ mod id {
 
             pub(super) const CHEAP: bool = false;
 
-            pub(super) fn get() -> Option<ThreadId> {
+            pub(crate) fn get() -> Option<ThreadId> {
                 let id0 = ID0.get().addr() as u64;
                 let id32 = ID32.get().addr() as u64;
                 ThreadId::from_u64((id32 << 32) + id0)
@@ -85,7 +85,7 @@ mod id {
 
             pub(super) const CHEAP: bool = true;
 
-            pub(super) fn get() -> Option<ThreadId> {
+            pub(crate) fn get() -> Option<ThreadId> {
                 let id = ID.get().addr() as u64;
                 ThreadId::from_u64(id)
             }
@@ -112,7 +112,7 @@ mod id {
 
 /// Tries to set the thread handle for the current thread. Fails if a handle was
 /// already set or if the thread ID of `thread` would change an already-set ID.
-pub(crate) fn set_current(thread: Thread) -> Result<(), Thread> {
+pub(super) fn set_current(thread: Thread) -> Result<(), Thread> {
     if CURRENT.get() != NONE {
         return Err(thread);
     }
@@ -140,28 +140,31 @@ pub(crate) fn current_id() -> ThreadId {
     // to retrieve it from the current thread handle, which will only take one
     // TLS access.
     if !id::CHEAP {
-        let current = CURRENT.get();
-        if current > DESTROYED {
-            unsafe {
-                let current = ManuallyDrop::new(Thread::from_raw(current));
-                return current.id();
-            }
+        if let Some(id) = try_with_current(|t| t.map(|t| t.id())) {
+            return id;
         }
     }
 
     id::get_or_init()
 }
 
-/// Gets a handle to the thread that invokes it, if the handle has been initialized.
-pub(crate) fn try_current() -> Option<Thread> {
+/// Gets a reference to the handle of the thread that invokes it, if the handle
+/// has been initialized.
+pub(super) fn try_with_current<F, R>(f: F) -> R
+where
+    F: FnOnce(Option<&Thread>) -> R,
+{
     let current = CURRENT.get();
     if current > DESTROYED {
+        // SAFETY: `Arc` does not contain interior mutability, so it does not
+        // matter that the address of the handle might be different depending
+        // on where this is called.
         unsafe {
             let current = ManuallyDrop::new(Thread::from_raw(current));
-            Some((*current).clone())
+            f(Some(&current))
         }
     } else {
-        None
+        f(None)
     }
 }
 
@@ -176,7 +179,7 @@ pub(crate) fn current_or_unnamed() -> Thread {
             (*current).clone()
         }
     } else if current == DESTROYED {
-        Thread::new_unnamed(id::get_or_init())
+        Thread::new(id::get_or_init(), None)
     } else {
         init_current(current)
     }
@@ -221,7 +224,7 @@ fn init_current(current: *mut ()) -> Thread {
         CURRENT.set(BUSY);
         // If the thread ID was initialized already, use it.
         let id = id::get_or_init();
-        let thread = Thread::new_unnamed(id);
+        let thread = Thread::new(id, None);
 
         // Make sure that `crate::rt::thread_cleanup` will be run, which will
         // call `drop_current`.
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
index 85ee369ca2b..59b395336f2 100644
--- a/library/std/src/thread/mod.rs
+++ b/library/std/src/thread/mod.rs
@@ -158,12 +158,9 @@
 #[cfg(all(test, not(any(target_os = "emscripten", target_os = "wasi"))))]
 mod tests;
 
-use core::cell::SyncUnsafeCell;
-use core::ffi::CStr;
-use core::mem::MaybeUninit;
-
 use crate::any::Any;
 use crate::cell::UnsafeCell;
+use crate::ffi::CStr;
 use crate::marker::PhantomData;
 use crate::mem::{self, ManuallyDrop, forget};
 use crate::num::NonZero;
@@ -186,7 +183,8 @@ mod current;
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use current::current;
-pub(crate) use current::{current_id, current_or_unnamed, drop_current, set_current, try_current};
+pub(crate) use current::{current_id, current_or_unnamed, drop_current};
+use current::{set_current, try_with_current};
 
 mod spawnhook;
 
@@ -501,10 +499,7 @@ impl Builder {
         });
 
         let id = ThreadId::new();
-        let my_thread = match name {
-            Some(name) => Thread::new(id, name.into()),
-            None => Thread::new_unnamed(id),
-        };
+        let my_thread = Thread::new(id, name);
 
         let hooks = if no_hooks {
             spawnhook::ChildSpawnHooks::default()
@@ -1235,7 +1230,7 @@ impl ThreadId {
         }
     }
 
-    #[cfg(not(target_thread_local))]
+    #[cfg(any(not(target_thread_local), target_has_atomic = "64"))]
     fn from_u64(v: u64) -> Option<ThreadId> {
         NonZero::new(v).map(ThreadId)
     }
@@ -1261,29 +1256,14 @@ impl ThreadId {
 
 // This module ensures private fields are kept private, which is necessary to enforce the safety requirements.
 mod thread_name_string {
-    use core::str;
-
     use crate::ffi::{CStr, CString};
+    use crate::str;
 
     /// Like a `String` it's guaranteed UTF-8 and like a `CString` it's null terminated.
     pub(crate) struct ThreadNameString {
         inner: CString,
     }
 
-    impl ThreadNameString {
-        pub fn as_str(&self) -> &str {
-            // SAFETY: `self.inner` is only initialised via `String`, which upholds the validity invariant of `str`.
-            unsafe { str::from_utf8_unchecked(self.inner.to_bytes()) }
-        }
-    }
-
-    impl core::ops::Deref for ThreadNameString {
-        type Target = CStr;
-        fn deref(&self) -> &CStr {
-            &self.inner
-        }
-    }
-
     impl From<String> for ThreadNameString {
         fn from(s: String) -> Self {
             Self {
@@ -1291,82 +1271,124 @@ mod thread_name_string {
             }
         }
     }
-}
-pub(crate) use thread_name_string::ThreadNameString;
-
-static MAIN_THREAD_INFO: SyncUnsafeCell<(MaybeUninit<ThreadId>, MaybeUninit<Parker>)> =
-    SyncUnsafeCell::new((MaybeUninit::uninit(), MaybeUninit::uninit()));
-
-/// The internal representation of a `Thread` that is not the main thread.
-struct OtherInner {
-    name: Option<ThreadNameString>,
-    id: ThreadId,
-    parker: Parker,
-}
-
-/// The internal representation of a `Thread` handle.
-#[derive(Clone)]
-enum Inner {
-    /// Represents the main thread. May only be constructed by Thread::new_main.
-    Main(&'static (ThreadId, Parker)),
-    /// Represents any other thread.
-    Other(Pin<Arc<OtherInner>>),
-}
-
-impl Inner {
-    fn id(&self) -> ThreadId {
-        match self {
-            Self::Main((thread_id, _)) => *thread_id,
-            Self::Other(other) => other.id,
-        }
-    }
 
-    fn cname(&self) -> Option<&CStr> {
-        match self {
-            Self::Main(_) => Some(c"main"),
-            Self::Other(other) => other.name.as_deref(),
+    impl ThreadNameString {
+        pub fn as_cstr(&self) -> &CStr {
+            &self.inner
         }
-    }
 
-    fn name(&self) -> Option<&str> {
-        match self {
-            Self::Main(_) => Some("main"),
-            Self::Other(other) => other.name.as_ref().map(ThreadNameString::as_str),
+        pub fn as_str(&self) -> &str {
+            // SAFETY: `ThreadNameString` is guaranteed to be UTF-8.
+            unsafe { str::from_utf8_unchecked(self.inner.to_bytes()) }
         }
     }
+}
 
-    fn into_raw(self) -> *const () {
-        match self {
-            // Just return the pointer to `MAIN_THREAD_INFO`.
-            Self::Main(ptr) => crate::ptr::from_ref(ptr).cast(),
-            Self::Other(arc) => {
-                // Safety: We only expose an opaque pointer, which maintains the `Pin` invariant.
-                let inner = unsafe { Pin::into_inner_unchecked(arc) };
-                Arc::into_raw(inner) as *const ()
+use thread_name_string::ThreadNameString;
+
+/// Store the ID of the main thread.
+///
+/// The thread handle for the main thread is created lazily, and this might even
+/// happen pre-main. Since not every platform has a way to identify the main
+/// thread when that happens – macOS's `pthread_main_np` function being a notable
+/// exception – we cannot assign it the right name right then. Instead, in our
+/// runtime startup code, we remember the thread ID of the main thread (through
+/// this modules `set` function) and use it to identify the main thread from then
+/// on. This works reliably and has the additional advantage that we can report
+/// the right thread name on main even after the thread handle has been destroyed.
+/// Note however that this also means that the name reported in pre-main functions
+/// will be incorrect, but that's just something we have to live with.
+pub(crate) mod main_thread {
+    cfg_if::cfg_if! {
+        if #[cfg(target_has_atomic = "64")] {
+            use super::ThreadId;
+            use crate::sync::atomic::AtomicU64;
+            use crate::sync::atomic::Ordering::Relaxed;
+
+            static MAIN: AtomicU64 = AtomicU64::new(0);
+
+            pub(super) fn get() -> Option<ThreadId> {
+                ThreadId::from_u64(MAIN.load(Relaxed))
             }
-        }
-    }
 
-    /// # Safety
-    ///
-    /// See [`Thread::from_raw`].
-    unsafe fn from_raw(ptr: *const ()) -> Self {
-        // If the pointer is to `MAIN_THREAD_INFO`, we know it is the `Main` variant.
-        if crate::ptr::eq(ptr.cast(), &MAIN_THREAD_INFO) {
-            Self::Main(unsafe { &*ptr.cast() })
+            /// # Safety
+            /// May only be called once.
+            pub(crate) unsafe fn set(id: ThreadId) {
+                MAIN.store(id.as_u64().get(), Relaxed)
+            }
         } else {
-            // Safety: Upheld by caller
-            Self::Other(unsafe { Pin::new_unchecked(Arc::from_raw(ptr as *const OtherInner)) })
+            use super::ThreadId;
+            use crate::mem::MaybeUninit;
+            use crate::sync::atomic::AtomicBool;
+            use crate::sync::atomic::Ordering::{Acquire, Release};
+
+            static INIT: AtomicBool = AtomicBool::new(false);
+            static mut MAIN: MaybeUninit<ThreadId> = MaybeUninit::uninit();
+
+            pub(super) fn get() -> Option<ThreadId> {
+                if INIT.load(Acquire) {
+                    Some(unsafe { MAIN.assume_init() })
+                } else {
+                    None
+                }
+            }
+
+            /// # Safety
+            /// May only be called once.
+            pub(crate) unsafe fn set(id: ThreadId) {
+                unsafe { MAIN = MaybeUninit::new(id) };
+                INIT.store(true, Release);
+            }
         }
     }
+}
 
-    fn parker(&self) -> Pin<&Parker> {
-        match self {
-            Self::Main((_, parker_ref)) => Pin::static_ref(parker_ref),
-            Self::Other(inner) => unsafe {
-                Pin::map_unchecked(inner.as_ref(), |inner| &inner.parker)
-            },
+/// Run a function with the current thread's name.
+///
+/// Modulo thread local accesses, this function is safe to call from signal
+/// handlers and in similar circumstances where allocations are not possible.
+pub(crate) fn with_current_name<F, R>(f: F) -> R
+where
+    F: FnOnce(Option<&str>) -> R,
+{
+    try_with_current(|thread| {
+        if let Some(thread) = thread {
+            // If there is a current thread handle, try to use the name stored
+            // there.
+            if let Some(name) = &thread.inner.name {
+                return f(Some(name.as_str()));
+            } else if Some(thread.inner.id) == main_thread::get() {
+                // The main thread doesn't store its name in the handle, we must
+                // identify it through its ID. Since we already have the `Thread`,
+                // we can retrieve the ID from it instead of going through another
+                // thread local.
+                return f(Some("main"));
+            }
+        } else if let Some(main) = main_thread::get()
+            && let Some(id) = current::id::get()
+            && id == main
+        {
+            // The main thread doesn't always have a thread handle, we must
+            // identify it through its ID instead. The checks are ordered so
+            // that the current ID is only loaded if it is actually needed,
+            // since loading it from TLS might need multiple expensive accesses.
+            return f(Some("main"));
         }
+
+        f(None)
+    })
+}
+
+/// The internal representation of a `Thread` handle
+struct Inner {
+    name: Option<ThreadNameString>,
+    id: ThreadId,
+    parker: Parker,
+}
+
+impl Inner {
+    fn parker(self: Pin<&Self>) -> Pin<&Parker> {
+        unsafe { Pin::map_unchecked(self, |inner| &inner.parker) }
     }
 }
 
@@ -1390,47 +1412,21 @@ impl Inner {
 /// docs of [`Builder`] and [`spawn`] for more details.
 ///
 /// [`thread::current`]: current::current
-pub struct Thread(Inner);
+pub struct Thread {
+    inner: Pin<Arc<Inner>>,
+}
 
 impl Thread {
-    /// Used only internally to construct a thread object without spawning.
-    pub(crate) fn new(id: ThreadId, name: String) -> Thread {
-        Self::new_inner(id, Some(ThreadNameString::from(name)))
-    }
+    pub(crate) fn new(id: ThreadId, name: Option<String>) -> Thread {
+        let name = name.map(ThreadNameString::from);
 
-    pub(crate) fn new_unnamed(id: ThreadId) -> Thread {
-        Self::new_inner(id, None)
-    }
-
-    /// Used in runtime to construct main thread
-    ///
-    /// # Safety
-    ///
-    /// This must only ever be called once, and must be called on the main thread.
-    pub(crate) unsafe fn new_main(thread_id: ThreadId) -> Thread {
-        // Safety: As this is only called once and on the main thread, nothing else is accessing MAIN_THREAD_INFO
-        // as the only other read occurs in `main_thread_info` *after* the main thread has been constructed,
-        // and this function is the only one that constructs the main thread.
-        //
-        // Pre-main thread spawning cannot hit this either, as the caller promises that this is only called on the main thread.
-        let main_thread_info = unsafe { &mut *MAIN_THREAD_INFO.get() };
-
-        unsafe { Parker::new_in_place((&raw mut main_thread_info.1).cast()) };
-        main_thread_info.0.write(thread_id);
-
-        // Store a `'static` ref to the initialised ThreadId and Parker,
-        // to avoid having to repeatedly prove initialisation.
-        Self(Inner::Main(unsafe { &*MAIN_THREAD_INFO.get().cast() }))
-    }
-
-    fn new_inner(id: ThreadId, name: Option<ThreadNameString>) -> Thread {
         // We have to use `unsafe` here to construct the `Parker` in-place,
         // which is required for the UNIX implementation.
         //
         // SAFETY: We pin the Arc immediately after creation, so its address never
         // changes.
         let inner = unsafe {
-            let mut arc = Arc::<OtherInner>::new_uninit();
+            let mut arc = Arc::<Inner>::new_uninit();
             let ptr = Arc::get_mut_unchecked(&mut arc).as_mut_ptr();
             (&raw mut (*ptr).name).write(name);
             (&raw mut (*ptr).id).write(id);
@@ -1438,7 +1434,7 @@ impl Thread {
             Pin::new_unchecked(arc.assume_init())
         };
 
-        Self(Inner::Other(inner))
+        Thread { inner }
     }
 
     /// Like the public [`park`], but callable on any handle. This is used to
@@ -1447,7 +1443,7 @@ impl Thread {
     /// # Safety
     /// May only be called from the thread to which this handle belongs.
     pub(crate) unsafe fn park(&self) {
-        unsafe { self.0.parker().park() }
+        unsafe { self.inner.as_ref().parker().park() }
     }
 
     /// Like the public [`park_timeout`], but callable on any handle. This is
@@ -1456,7 +1452,7 @@ impl Thread {
     /// # Safety
     /// May only be called from the thread to which this handle belongs.
     pub(crate) unsafe fn park_timeout(&self, dur: Duration) {
-        unsafe { self.0.parker().park_timeout(dur) }
+        unsafe { self.inner.as_ref().parker().park_timeout(dur) }
     }
 
     /// Atomically makes the handle's token available if it is not already.
@@ -1492,7 +1488,7 @@ impl Thread {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn unpark(&self) {
-        self.0.parker().unpark();
+        self.inner.as_ref().parker().unpark();
     }
 
     /// Gets the thread's unique identifier.
@@ -1512,7 +1508,7 @@ impl Thread {
     #[stable(feature = "thread_id", since = "1.19.0")]
     #[must_use]
     pub fn id(&self) -> ThreadId {
-        self.0.id()
+        self.inner.id
     }
 
     /// Gets the thread's name.
@@ -1555,11 +1551,13 @@ impl Thread {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     pub fn name(&self) -> Option<&str> {
-        self.0.name()
-    }
-
-    fn cname(&self) -> Option<&CStr> {
-        self.0.cname()
+        if let Some(name) = &self.inner.name {
+            Some(name.as_str())
+        } else if main_thread::get() == Some(self.inner.id) {
+            Some("main")
+        } else {
+            None
+        }
     }
 
     /// Consumes the `Thread`, returning a raw pointer.
@@ -1583,7 +1581,9 @@ impl Thread {
     /// ```
     #[unstable(feature = "thread_raw", issue = "97523")]
     pub fn into_raw(self) -> *const () {
-        self.0.into_raw()
+        // Safety: We only expose an opaque pointer, which maintains the `Pin` invariant.
+        let inner = unsafe { Pin::into_inner_unchecked(self.inner) };
+        Arc::into_raw(inner) as *const ()
     }
 
     /// Constructs a `Thread` from a raw pointer.
@@ -1605,7 +1605,17 @@ impl Thread {
     #[unstable(feature = "thread_raw", issue = "97523")]
     pub unsafe fn from_raw(ptr: *const ()) -> Thread {
         // Safety: Upheld by caller.
-        unsafe { Thread(Inner::from_raw(ptr)) }
+        unsafe { Thread { inner: Pin::new_unchecked(Arc::from_raw(ptr as *const Inner)) } }
+    }
+
+    fn cname(&self) -> Option<&CStr> {
+        if let Some(name) = &self.inner.name {
+            Some(name.as_cstr())
+        } else if main_thread::get() == Some(self.inner.id) {
+            Some(c"main")
+        } else {
+            None
+        }
     }
 }
 
diff --git a/library/std/tests/pipe_subprocess.rs b/library/std/tests/pipe_subprocess.rs
index 1535742a83a..df946cdcf2b 100644
--- a/library/std/tests/pipe_subprocess.rs
+++ b/library/std/tests/pipe_subprocess.rs
@@ -3,8 +3,7 @@
 fn main() {
     #[cfg(all(not(miri), any(unix, windows)))]
     {
-        use std::io::Read;
-        use std::pipe::pipe;
+        use std::io::{Read, pipe};
         use std::{env, process};
 
         if env::var("I_AM_THE_CHILD").is_ok() {