Auto merge of #75653 - JohnTitor:rollup-0ejtdfo, r=JohnTitor

Rollup of 8 pull requests

Successful merges:

 - #75389 (attempt to improve span_label docs)
 - #75392 (Add `as_uninit`-like methods to pointer types and unify documentation of `as_ref` methods)
 - #75464 (Move to intra doc links for ascii.rs and panic.rs)
 - #75578 (Allowing raw ptr dereference in const fn)
 - #75613 (Add explanation for `&mut self` method call when expecting `-> Self`)
 - #75626 (Clean up E0754 explanation)
 - #75629 (Use intra-doc links in `std::env`, `std::alloc` and `std::error`)
 - #75634 (Mark x86_64-linux-kernel as *)

Failed merges:

r? @ghost

21 files changed, 702 insertions, 215 deletions

diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 763457d485d..3838fcf74cc 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -114,6 +114,7 @@
 #![feature(optin_builtin_traits)]
 #![feature(or_patterns)]
 #![feature(prelude_import)]
+#![feature(ptr_as_uninit)]
 #![feature(repr_simd, platform_intrinsics)]
 #![feature(rustc_attrs)]
 #![feature(simd_ffi)]
diff --git a/library/core/src/ptr/const_ptr.rs b/library/core/src/ptr/const_ptr.rs
index 9e8b0b41410..7d7306574a6 100644
--- a/library/core/src/ptr/const_ptr.rs
+++ b/library/core/src/ptr/const_ptr.rs
@@ -2,7 +2,7 @@ use super::*;
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics;
 use crate::mem;
-use crate::slice::SliceIndex;
+use crate::slice::{self, SliceIndex};
 
 #[lang = "const_ptr"]
 impl<T: ?Sized> *const T {
@@ -48,32 +48,33 @@ impl<T: ?Sized> *const T {
         self as _
     }
 
-    /// Returns `None` if the pointer is null, or else returns a reference to
-    /// the value wrapped in `Some`.
+    /// Returns `None` if the pointer is null, or else returns a shared reference to
+    /// the value wrapped in `Some`. If the value may be uninitialized, [`as_uninit_ref`]
+    /// must be used instead.
     ///
-    /// # Safety
+    /// [`as_uninit_ref`]: #method.as_uninit_ref
     ///
-    /// While this method and its mutable counterpart are useful for
-    /// null-safety, it is important to note that this is still an unsafe
-    /// operation because the returned value could be pointing to invalid
-    /// memory.
+    /// # Safety
     ///
     /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
     /// all of the following is true:
-    /// - it is properly aligned
-    /// - it must point to an initialized instance of T; in particular, the pointer must be
-    ///   "dereferenceable" in the sense defined [here].
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
     ///
     /// This applies even if the result of this method is unused!
     /// (The part about being initialized is not yet fully decided, but until
     /// it is, the only safe approach is to ensure that they are indeed initialized.)
     ///
-    /// Additionally, the lifetime `'a` returned is arbitrarily chosen and does
-    /// not necessarily reflect the actual lifetime of the data. *You* must enforce
-    /// Rust's aliasing rules. In particular, for the duration of this lifetime,
-    /// the memory the pointer points to must not get mutated (except inside `UnsafeCell`).
-    ///
-    /// [here]: crate::ptr#safety
+    /// [the module documentation]: crate::ptr#safety
     ///
     /// # Examples
     ///
@@ -111,6 +112,56 @@ impl<T: ?Sized> *const T {
         if self.is_null() { None } else { unsafe { Some(&*self) } }
     }
 
+    /// Returns `None` if the pointer is null, or else returns a shared reference to
+    /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// [`as_ref`]: #method.as_ref
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
+    /// all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(ptr_as_uninit)]
+    ///
+    /// let ptr: *const u8 = &10u8 as *const u8;
+    ///
+    /// unsafe {
+    ///     if let Some(val_back) = ptr.as_uninit_ref() {
+    ///         println!("We got back the value: {}!", val_back.assume_init());
+    ///     }
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
+    where
+        T: Sized,
+    {
+        // SAFETY: the caller must guarantee that `self` meets all the
+        // requirements for a reference.
+        if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) }
+    }
+
     /// Calculates the offset from a pointer.
     ///
     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
@@ -925,6 +976,55 @@ impl<T> *const [T] {
         // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
         unsafe { index.get_unchecked(self) }
     }
+
+    /// Returns `None` if the pointer is null, or else returns a shared slice to
+    /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// [`as_ref`]: #method.as_ref
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
+    /// all of the following is true:
+    ///
+    /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
+    ///   and it must be properly aligned. This means in particular:
+    ///
+    ///     * The entire memory range of this slice must be contained within a single allocated object!
+    ///       Slices can never span across multiple allocated objects.
+    ///
+    ///     * The pointer must be aligned even for zero-length slices. One
+    ///       reason for this is that enum layout optimizations may rely on references
+    ///       (including slices of any length) being aligned and non-null to distinguish
+    ///       them from other data. You can obtain a pointer that is usable as `data`
+    ///       for zero-length slices using [`NonNull::dangling()`].
+    ///
+    /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// See also [`slice::from_raw_parts`][].
+    ///
+    /// [valid]: crate::ptr#safety
+    /// [`NonNull::dangling()`]: NonNull::dangling
+    /// [`pointer::offset`]: ../std/primitive.pointer.html#method.offset
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
+        if self.is_null() {
+            None
+        } else {
+            // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
+            Some(unsafe { slice::from_raw_parts(self as *const MaybeUninit<T>, self.len()) })
+        }
+    }
 }
 
 // Equality for pointers
diff --git a/library/core/src/ptr/mut_ptr.rs b/library/core/src/ptr/mut_ptr.rs
index 811e58875d5..3daeec36041 100644
--- a/library/core/src/ptr/mut_ptr.rs
+++ b/library/core/src/ptr/mut_ptr.rs
@@ -1,7 +1,7 @@
 use super::*;
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics;
-use crate::slice::SliceIndex;
+use crate::slice::{self, SliceIndex};
 
 #[lang = "mut_ptr"]
 impl<T: ?Sized> *mut T {
@@ -47,32 +47,36 @@ impl<T: ?Sized> *mut T {
         self as _
     }
 
-    /// Returns `None` if the pointer is null, or else returns a reference to
-    /// the value wrapped in `Some`.
+    /// Returns `None` if the pointer is null, or else returns a shared reference to
+    /// the value wrapped in `Some`. If the value may be uninitialized, [`as_uninit_ref`]
+    /// must be used instead.
     ///
-    /// # Safety
+    /// For the mutable counterpart see [`as_mut`].
+    ///
+    /// [`as_uninit_ref`]: #method.as_uninit_ref-1
+    /// [`as_mut`]: #method.as_mut
     ///
-    /// While this method and its mutable counterpart are useful for
-    /// null-safety, it is important to note that this is still an unsafe
-    /// operation because the returned value could be pointing to invalid
-    /// memory.
+    /// # Safety
     ///
     /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
     /// all of the following is true:
-    /// - it is properly aligned
-    /// - it must point to an initialized instance of T; in particular, the pointer must be
-    ///   "dereferencable" in the sense defined [here].
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
     ///
     /// This applies even if the result of this method is unused!
     /// (The part about being initialized is not yet fully decided, but until
     /// it is, the only safe approach is to ensure that they are indeed initialized.)
     ///
-    /// Additionally, the lifetime `'a` returned is arbitrarily chosen and does
-    /// not necessarily reflect the actual lifetime of the data. *You* must enforce
-    /// Rust's aliasing rules. In particular, for the duration of this lifetime,
-    /// the memory the pointer points to must not get mutated (except inside `UnsafeCell`).
-    ///
-    /// [here]: crate::ptr#safety
+    /// [the module documentation]: crate::ptr#safety
     ///
     /// # Examples
     ///
@@ -110,6 +114,59 @@ impl<T: ?Sized> *mut T {
         if self.is_null() { None } else { unsafe { Some(&*self) } }
     }
 
+    /// Returns `None` if the pointer is null, or else returns a shared reference to
+    /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// For the mutable counterpart see [`as_uninit_mut`].
+    ///
+    /// [`as_ref`]: #method.as_ref-1
+    /// [`as_uninit_mut`]: #method.as_uninit_mut
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
+    /// all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(ptr_as_uninit)]
+    ///
+    /// let ptr: *mut u8 = &mut 10u8 as *mut u8;
+    ///
+    /// unsafe {
+    ///     if let Some(val_back) = ptr.as_uninit_ref() {
+    ///         println!("We got back the value: {}!", val_back.assume_init());
+    ///     }
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
+    where
+        T: Sized,
+    {
+        // SAFETY: the caller must guarantee that `self` meets all the
+        // requirements for a reference.
+        if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) }
+    }
+
     /// Calculates the offset from a pointer.
     ///
     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
@@ -235,33 +292,36 @@ impl<T: ?Sized> *mut T {
         unsafe { intrinsics::arith_offset(self, count) as *mut T }
     }
 
-    /// Returns `None` if the pointer is null, or else returns a mutable
-    /// reference to the value wrapped in `Some`.
+    /// Returns `None` if the pointer is null, or else returns a unique reference to
+    /// the value wrapped in `Some`. If the value may be uninitialized, [`as_uninit_mut`]
+    /// must be used instead.
     ///
-    /// # Safety
+    /// For the shared counterpart see [`as_ref`].
     ///
-    /// As with [`as_ref`], this is unsafe because it cannot verify the validity
-    /// of the returned pointer, nor can it ensure that the lifetime `'a`
-    /// returned is indeed a valid lifetime for the contained data.
+    /// [`as_uninit_mut`]: #method.as_uninit_mut
+    /// [`as_ref`]: #method.as_ref-1
+    ///
+    /// # Safety
     ///
     /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
     /// all of the following is true:
-    /// - it is properly aligned
-    /// - it must point to an initialized instance of T; in particular, the pointer must be
-    ///   "dereferenceable" in the sense defined [here].
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get accessed (read or written) through any other pointer.
     ///
     /// This applies even if the result of this method is unused!
     /// (The part about being initialized is not yet fully decided, but until
-    /// it is the only safe approach is to ensure that they are indeed initialized.)
-    ///
-    /// Additionally, the lifetime `'a` returned is arbitrarily chosen and does
-    /// not necessarily reflect the actual lifetime of the data. *You* must enforce
-    /// Rust's aliasing rules. In particular, for the duration of this lifetime,
-    /// the memory this pointer points to must not get accessed (read or written)
-    /// through any other pointer.
+    /// it is, the only safe approach is to ensure that they are indeed initialized.)
     ///
-    /// [here]: crate::ptr#safety
-    /// [`as_ref`]: #method.as_ref
+    /// [the module documentation]: crate::ptr#safety
     ///
     /// # Examples
     ///
@@ -272,6 +332,7 @@ impl<T: ?Sized> *mut T {
     /// let ptr: *mut u32 = s.as_mut_ptr();
     /// let first_value = unsafe { ptr.as_mut().unwrap() };
     /// *first_value = 4;
+    /// # assert_eq!(s, [4, 2, 3]);
     /// println!("{:?}", s); // It'll print: "[4, 2, 3]".
     /// ```
     ///
@@ -286,6 +347,7 @@ impl<T: ?Sized> *mut T {
     /// let ptr: *mut u32 = s.as_mut_ptr();
     /// let first_value = unsafe { &mut *ptr };
     /// *first_value = 4;
+    /// # assert_eq!(s, [4, 2, 3]);
     /// println!("{:?}", s); // It'll print: "[4, 2, 3]".
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
@@ -296,6 +358,43 @@ impl<T: ?Sized> *mut T {
         if self.is_null() { None } else { unsafe { Some(&mut *self) } }
     }
 
+    /// Returns `None` if the pointer is null, or else returns a unique reference to
+    /// the value wrapped in `Some`. In contrast to [`as_mut`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// For the shared counterpart see [`as_uninit_ref`].
+    ///
+    /// [`as_mut`]: #method.as_mut
+    /// [`as_uninit_ref`]: #method.as_uninit_ref-1
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
+    /// all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get accessed (read or written) through any other pointer.
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_mut<'a>(self) -> Option<&'a mut MaybeUninit<T>>
+    where
+        T: Sized,
+    {
+        // SAFETY: the caller must guarantee that `self` meets all the
+        // requirements for a reference.
+        if self.is_null() { None } else { Some(unsafe { &mut *(self as *mut MaybeUninit<T>) }) }
+    }
+
     /// Returns whether two pointers are guaranteed to be equal.
     ///
     /// At runtime this function behaves like `self == other`.
@@ -1131,6 +1230,110 @@ impl<T> *mut [T] {
         // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
         unsafe { index.get_unchecked_mut(self) }
     }
+
+    /// Returns `None` if the pointer is null, or else returns a shared slice to
+    /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// For the mutable counterpart see [`as_uninit_slice_mut`].
+    ///
+    /// [`as_ref`]: #method.as_ref-1
+    /// [`as_uninit_slice_mut`]: #method.as_uninit_slice_mut
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
+    /// all of the following is true:
+    ///
+    /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
+    ///   and it must be properly aligned. This means in particular:
+    ///
+    ///     * The entire memory range of this slice must be contained within a single allocated object!
+    ///       Slices can never span across multiple allocated objects.
+    ///
+    ///     * The pointer must be aligned even for zero-length slices. One
+    ///       reason for this is that enum layout optimizations may rely on references
+    ///       (including slices of any length) being aligned and non-null to distinguish
+    ///       them from other data. You can obtain a pointer that is usable as `data`
+    ///       for zero-length slices using [`NonNull::dangling()`].
+    ///
+    /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// See also [`slice::from_raw_parts`][].
+    ///
+    /// [valid]: crate::ptr#safety
+    /// [`NonNull::dangling()`]: NonNull::dangling
+    /// [`pointer::offset`]: ../std/primitive.pointer.html#method.offset
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
+        if self.is_null() {
+            None
+        } else {
+            // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
+            Some(unsafe { slice::from_raw_parts(self as *const MaybeUninit<T>, self.len()) })
+        }
+    }
+
+    /// Returns `None` if the pointer is null, or else returns a unique slice to
+    /// the value wrapped in `Some`. In contrast to [`as_mut`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// For the shared counterpart see [`as_uninit_slice`].
+    ///
+    /// [`as_mut`]: #method.as_mut
+    /// [`as_uninit_slice`]: #method.as_uninit_slice-1
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
+    /// all of the following is true:
+    ///
+    /// * The pointer must be [valid] for reads and writes for `ptr.len() * mem::size_of::<T>()`
+    ///   many bytes, and it must be properly aligned. This means in particular:
+    ///
+    ///     * The entire memory range of this slice must be contained within a single allocated object!
+    ///       Slices can never span across multiple allocated objects.
+    ///
+    ///     * The pointer must be aligned even for zero-length slices. One
+    ///       reason for this is that enum layout optimizations may rely on references
+    ///       (including slices of any length) being aligned and non-null to distinguish
+    ///       them from other data. You can obtain a pointer that is usable as `data`
+    ///       for zero-length slices using [`NonNull::dangling()`].
+    ///
+    /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get accessed (read or written) through any other pointer.
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// See also [`slice::from_raw_parts_mut`][].
+    ///
+    /// [valid]: crate::ptr#safety
+    /// [`NonNull::dangling()`]: NonNull::dangling
+    /// [`pointer::offset`]: ../std/primitive.pointer.html#method.offset
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_slice_mut<'a>(self) -> Option<&'a mut [MaybeUninit<T>]> {
+        if self.is_null() {
+            None
+        } else {
+            // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.
+            Some(unsafe { slice::from_raw_parts_mut(self as *mut MaybeUninit<T>, self.len()) })
+        }
+    }
 }
 
 // Equality for pointers
diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs
index d876ab23653..294a3173d0c 100644
--- a/library/core/src/ptr/non_null.rs
+++ b/library/core/src/ptr/non_null.rs
@@ -3,10 +3,10 @@ use crate::convert::From;
 use crate::fmt;
 use crate::hash;
 use crate::marker::Unsize;
-use crate::mem;
+use crate::mem::{self, MaybeUninit};
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
 use crate::ptr::Unique;
-use crate::slice::SliceIndex;
+use crate::slice::{self, SliceIndex};
 
 /// `*mut T` but non-zero and covariant.
 ///
@@ -76,6 +76,70 @@ impl<T: Sized> NonNull<T> {
             NonNull::new_unchecked(ptr)
         }
     }
+
+    /// Returns a shared references to the value. In contrast to [`as_ref`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// For the mutable counterpart see [`as_uninit_mut`].
+    ///
+    /// [`as_ref`]: #method.as_ref
+    /// [`as_uninit_mut`]: #method.as_uninit_mut
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_ref(&self) -> &MaybeUninit<T> {
+        // SAFETY: the caller must guarantee that `self` meets all the
+        // requirements for a reference.
+        unsafe { &*self.cast().as_ptr() }
+    }
+
+    /// Returns a unique references to the value. In contrast to [`as_mut`], this does not require
+    /// that the value has to be initialized.
+    ///
+    /// For the shared counterpart see [`as_uninit_ref`].
+    ///
+    /// [`as_mut`]: #method.as_mut
+    /// [`as_uninit_ref`]: #method.as_uninit_ref
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get accessed (read or written) through any other pointer.
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_mut(&mut self) -> &mut MaybeUninit<T> {
+        // SAFETY: the caller must guarantee that `self` meets all the
+        // requirements for a reference.
+        unsafe { &mut *self.cast().as_ptr() }
+    }
 }
 
 impl<T: ?Sized> NonNull<T> {
@@ -112,29 +176,34 @@ impl<T: ?Sized> NonNull<T> {
         self.pointer as *mut T
     }
 
-    /// Dereferences the content.
+    /// Returns a shared reference to the value. If the value may be uninitialized, [`as_uninit_ref`]
+    /// must be used instead.
+    ///
+    /// For the mutable counterpart see [`as_mut`].
     ///
-    /// The resulting lifetime is bound to self so this behaves "as if"
-    /// it were actually an instance of T that is getting borrowed. If a longer
-    /// (unbound) lifetime is needed, use `&*my_ptr.as_ptr()`.
+    /// [`as_uninit_ref`]: #method.as_uninit_ref
+    /// [`as_mut`]: #method.as_mut
     ///
     /// # Safety
     ///
     /// When calling this method, you have to ensure that all of the following is true:
-    /// - `self` is properly aligned
-    /// - `self` must point to an initialized instance of T; in particular, the pointer must be
-    ///   "dereferencable" in the sense defined [here].
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
     ///
     /// This applies even if the result of this method is unused!
     /// (The part about being initialized is not yet fully decided, but until
     /// it is, the only safe approach is to ensure that they are indeed initialized.)
     ///
-    /// Additionally, the lifetime of `self` does not necessarily reflect the actual
-    /// lifetime of the data. *You* must enforce Rust's aliasing rules. In particular,
-    /// for the duration of this lifetime, the memory the pointer points to must not
-    /// get mutated (except inside `UnsafeCell`).
-    ///
-    /// [here]: crate::ptr#safety
+    /// [the module documentation]: crate::ptr#safety
     #[stable(feature = "nonnull", since = "1.25.0")]
     #[inline]
     pub unsafe fn as_ref(&self) -> &T {
@@ -143,29 +212,34 @@ impl<T: ?Sized> NonNull<T> {
         unsafe { &*self.as_ptr() }
     }
 
-    /// Mutably dereferences the content.
+    /// Returns a unique reference to the value. If the value may be uninitialized, [`as_uninit_mut`]
+    /// must be used instead.
+    ///
+    /// For the shared counterpart see [`as_ref`].
     ///
-    /// The resulting lifetime is bound to self so this behaves "as if"
-    /// it were actually an instance of T that is getting borrowed. If a longer
-    /// (unbound) lifetime is needed, use `&mut *my_ptr.as_ptr()`.
+    /// [`as_uninit_mut`]: #method.as_uninit_mut
+    /// [`as_ref`]: #method.as_ref
     ///
     /// # Safety
     ///
     /// When calling this method, you have to ensure that all of the following is true:
-    /// - `self` is properly aligned
-    /// - `self` must point to an initialized instance of T; in particular, the pointer must be
-    ///   "dereferenceable" in the sense defined [here].
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferencable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get accessed (read or written) through any other pointer.
     ///
     /// This applies even if the result of this method is unused!
     /// (The part about being initialized is not yet fully decided, but until
-    /// it is the only safe approach is to ensure that they are indeed initialized.)
-    ///
-    /// Additionally, the lifetime of `self` does not necessarily reflect the actual
-    /// lifetime of the data. *You* must enforce Rust's aliasing rules. In particular,
-    /// for the duration of this lifetime, the memory this pointer points to must not
-    /// get accessed (read or written) through any other pointer.
+    /// it is, the only safe approach is to ensure that they are indeed initialized.)
     ///
-    /// [here]: crate::ptr#safety
+    /// [the module documentation]: crate::ptr#safety
     #[stable(feature = "nonnull", since = "1.25.0")]
     #[inline]
     pub unsafe fn as_mut(&mut self) -> &mut T {
@@ -278,6 +352,115 @@ impl<T> NonNull<[T]> {
         self.as_non_null_ptr().as_ptr()
     }
 
+    /// Returns a shared reference to a slice of possibly uninitialized values. In contrast to
+    /// [`as_ref`], this does not require that the value has to be initialized.
+    ///
+    /// For the mutable counterpart see [`as_uninit_slice_mut`].
+    ///
+    /// [`as_ref`]: #method.as_ref
+    /// [`as_uninit_slice_mut`]: #method.as_uninit_slice_mut
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that all of the following is true:
+    ///
+    /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
+    ///   and it must be properly aligned. This means in particular:
+    ///
+    ///     * The entire memory range of this slice must be contained within a single allocated object!
+    ///       Slices can never span across multiple allocated objects.
+    ///
+    ///     * The pointer must be aligned even for zero-length slices. One
+    ///       reason for this is that enum layout optimizations may rely on references
+    ///       (including slices of any length) being aligned and non-null to distinguish
+    ///       them from other data. You can obtain a pointer that is usable as `data`
+    ///       for zero-length slices using [`NonNull::dangling()`].
+    ///
+    /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// See also [`slice::from_raw_parts`][].
+    ///
+    /// [valid]: crate::ptr#safety
+    /// [`NonNull::dangling()`]: NonNull::dangling
+    /// [`pointer::offset`]: ../../std/primitive.pointer.html#method.offset
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_slice(&self) -> &[MaybeUninit<T>] {
+        // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
+        unsafe { slice::from_raw_parts(self.cast().as_ptr(), self.len()) }
+    }
+
+    /// Returns a unique reference to a slice of possibly uninitialized values. In contrast to
+    /// [`as_mut`], this does not require that the value has to be initialized.
+    ///
+    /// For the shared counterpart see [`as_uninit_slice`].
+    ///
+    /// [`as_mut`]: #method.as_mut
+    /// [`as_uninit_slice`]: #method.as_uninit_slice
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that all of the following is true:
+    ///
+    /// * The pointer must be [valid] for reads and writes for `ptr.len() * mem::size_of::<T>()`
+    ///   many bytes, and it must be properly aligned. This means in particular:
+    ///
+    ///     * The entire memory range of this slice must be contained within a single allocated object!
+    ///       Slices can never span across multiple allocated objects.
+    ///
+    ///     * The pointer must be aligned even for zero-length slices. One
+    ///       reason for this is that enum layout optimizations may rely on references
+    ///       (including slices of any length) being aligned and non-null to distinguish
+    ///       them from other data. You can obtain a pointer that is usable as `data`
+    ///       for zero-length slices using [`NonNull::dangling()`].
+    ///
+    /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
+    ///   See the safety documentation of [`pointer::offset`].
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, for the duration of this lifetime, the memory the pointer points to must
+    ///   not get accessed (read or written) through any other pointer.
+    ///
+    /// This applies even if the result of this method is unused!
+    ///
+    /// See also [`slice::from_raw_parts_mut`][].
+    ///
+    /// [valid]: crate::ptr#safety
+    /// [`NonNull::dangling()`]: NonNull::dangling
+    /// [`pointer::offset`]: ../../std/primitive.pointer.html#method.offset
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(allocator_api, ptr_as_uninit)]
+    ///
+    /// use std::alloc::{AllocRef, Layout, Global};
+    /// use std::mem::MaybeUninit;
+    /// use std::ptr::NonNull;
+    ///
+    /// let memory: NonNull<[u8]> = Global.alloc(Layout::new::<[u8; 32]>())?;
+    /// // This is safe as `memory` is valid for reads and writes for `memory.len()` many bytes.
+    /// // Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
+    /// # #[allow(unused_variables)]
+    /// let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };
+    /// # Ok::<_, std::alloc::AllocErr>(())
+    /// ```
+    #[inline]
+    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
+    pub unsafe fn as_uninit_slice_mut(&self) -> &mut [MaybeUninit<T>] {
+        // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.
+        unsafe { slice::from_raw_parts_mut(self.cast().as_ptr(), self.len()) }
+    }
+
     /// Returns a raw pointer to an element or subslice, without doing bounds
     /// checking.
     ///
diff --git a/library/std/src/alloc.rs b/library/std/src/alloc.rs
index 4712cc95b4a..37a8f514aa1 100644
--- a/library/std/src/alloc.rs
+++ b/library/std/src/alloc.rs
@@ -7,8 +7,6 @@
 //! like `cdylib`s and `staticlib`s are guaranteed to use the [`System`] by
 //! default.
 //!
-//! [`System`]: struct.System.html
-//!
 //! # The `#[global_allocator]` attribute
 //!
 //! This attribute allows configuring the choice of global allocator.
@@ -43,8 +41,6 @@
 //! The attribute is used on a `static` item whose type implements the
 //! [`GlobalAlloc`] trait. This type can be provided by an external library:
 //!
-//! [`GlobalAlloc`]: ../../core/alloc/trait.GlobalAlloc.html
-//!
 //! ```rust,ignore (demonstrates crates.io usage)
 //! extern crate jemallocator;
 //!
@@ -284,9 +280,6 @@ static HOOK: AtomicPtr<()> = AtomicPtr::new(ptr::null_mut());
 /// about the allocation that failed.
 ///
 /// The allocation error hook is a global resource.
-///
-/// [`set_alloc_error_hook`]: fn.set_alloc_error_hook.html
-/// [`take_alloc_error_hook`]: fn.take_alloc_error_hook.html
 #[unstable(feature = "alloc_error_hook", issue = "51245")]
 pub fn set_alloc_error_hook(hook: fn(Layout)) {
     HOOK.store(hook as *mut (), Ordering::SeqCst);
@@ -297,8 +290,6 @@ pub fn set_alloc_error_hook(hook: fn(Layout)) {
 /// *See also the function [`set_alloc_error_hook`].*
 ///
 /// If no custom hook is registered, the default hook will be returned.
-///
-/// [`set_alloc_error_hook`]: fn.set_alloc_error_hook.html
 #[unstable(feature = "alloc_error_hook", issue = "51245")]
 pub fn take_alloc_error_hook() -> fn(Layout) {
     let hook = HOOK.swap(ptr::null_mut(), Ordering::SeqCst);
diff --git a/library/std/src/ascii.rs b/library/std/src/ascii.rs
index 5cd2a25b117..c9106136d34 100644
--- a/library/std/src/ascii.rs
+++ b/library/std/src/ascii.rs
@@ -10,9 +10,6 @@
 //!
 //! The [`escape_default`] function provides an iterator over the bytes of an
 //! escaped version of the character given.
-//!
-//! [`AsciiExt`]: trait.AsciiExt.html
-//! [`escape_default`]: fn.escape_default.html
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
@@ -52,7 +49,7 @@ pub trait AsciiExt {
     ///
     /// # Note
     ///
-    /// This method will be deprecated in favor of the identically-named
+    /// This method is deprecated in favor of the identically-named
     /// inherent methods on `u8`, `char`, `[u8]` and `str`.
     #[stable(feature = "rust1", since = "1.0.0")]
     fn is_ascii(&self) -> bool;
@@ -69,10 +66,10 @@ pub trait AsciiExt {
     ///
     /// # Note
     ///
-    /// This method will be deprecated in favor of the identically-named
+    /// This method is deprecated in favor of the identically-named
     /// inherent methods on `u8`, `char`, `[u8]` and `str`.
     ///
-    /// [`make_ascii_uppercase`]: #tymethod.make_ascii_uppercase
+    /// [`make_ascii_uppercase`]: AsciiExt::make_ascii_uppercase
     /// [`str::to_uppercase`]: ../primitive.str.html#method.to_uppercase
     #[stable(feature = "rust1", since = "1.0.0")]
     #[allow(deprecated)]
@@ -90,10 +87,10 @@ pub trait AsciiExt {
     ///
     /// # Note
     ///
-    /// This method will be deprecated in favor of the identically-named
+    /// This method is deprecated in favor of the identically-named
     /// inherent methods on `u8`, `char`, `[u8]` and `str`.
     ///
-    /// [`make_ascii_lowercase`]: #tymethod.make_ascii_lowercase
+    /// [`make_ascii_lowercase`]: AsciiExt::make_ascii_lowercase
     /// [`str::to_lowercase`]: ../primitive.str.html#method.to_lowercase
     #[stable(feature = "rust1", since = "1.0.0")]
     #[allow(deprecated)]
@@ -106,7 +103,7 @@ pub trait AsciiExt {
     ///
     /// # Note
     ///
-    /// This method will be deprecated in favor of the identically-named
+    /// This method is deprecated in favor of the identically-named
     /// inherent methods on `u8`, `char`, `[u8]` and `str`.
     #[stable(feature = "rust1", since = "1.0.0")]
     fn eq_ignore_ascii_case(&self, other: &Self) -> bool;
@@ -121,10 +118,10 @@ pub trait AsciiExt {
     ///
     /// # Note
     ///
-    /// This method will be deprecated in favor of the identically-named
+    /// This method is deprecated in favor of the identically-named
     /// inherent methods on `u8`, `char`, `[u8]` and `str`.
     ///
-    /// [`to_ascii_uppercase`]: #tymethod.to_ascii_uppercase
+    /// [`to_ascii_uppercase`]: AsciiExt::to_ascii_uppercase
     #[stable(feature = "ascii", since = "1.9.0")]
     fn make_ascii_uppercase(&mut self);
 
@@ -138,10 +135,10 @@ pub trait AsciiExt {
     ///
     /// # Note
     ///
-    /// This method will be deprecated in favor of the identically-named
+    /// This method is deprecated in favor of the identically-named
     /// inherent methods on `u8`, `char`, `[u8]` and `str`.
     ///
-    /// [`to_ascii_lowercase`]: #tymethod.to_ascii_lowercase
+    /// [`to_ascii_lowercase`]: AsciiExt::to_ascii_lowercase
     #[stable(feature = "ascii", since = "1.9.0")]
     fn make_ascii_lowercase(&mut self);
 }
diff --git a/library/std/src/env.rs b/library/std/src/env.rs
index 6489e0709cb..387c588f4a0 100644
--- a/library/std/src/env.rs
+++ b/library/std/src/env.rs
@@ -7,9 +7,6 @@
 //! There are several functions and structs in this module that have a
 //! counterpart ending in `os`. Those ending in `os` will return an [`OsString`]
 //! and those without will return a [`String`].
-//!
-//! [`OsString`]: ../../std/ffi/struct.OsString.html
-//! [`String`]: ../string/struct.String.html
 
 #![stable(feature = "env", since = "1.0.0")]
 
@@ -31,9 +28,6 @@ use crate::sys::os as os_imp;
 /// * Current directory does not exist.
 /// * There are insufficient permissions to access the current directory.
 ///
-/// [`PathBuf`]: ../../std/path/struct.PathBuf.html
-/// [`Err`]: ../../std/result/enum.Result.html#method.err
-///
 /// # Examples
 ///
 /// ```
@@ -54,8 +48,6 @@ pub fn current_dir() -> io::Result<PathBuf> {
 ///
 /// Returns an [`Err`] if the operation fails.
 ///
-/// [`Err`]: ../../std/result/enum.Result.html#method.err
-///
 /// # Examples
 ///
 /// ```
@@ -76,7 +68,7 @@ pub fn set_current_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// This structure is created by the [`std::env::vars`] function. See its
 /// documentation for more.
 ///
-/// [`std::env::vars`]: fn.vars.html
+/// [`std::env::vars`]: vars
 #[stable(feature = "env", since = "1.0.0")]
 pub struct Vars {
     inner: VarsOs,
@@ -87,7 +79,7 @@ pub struct Vars {
 /// This structure is created by the [`std::env::vars_os`] function. See
 /// its documentation for more.
 ///
-/// [`std::env::vars_os`]: fn.vars_os.html
+/// [`std::env::vars_os`]: vars_os
 #[stable(feature = "env", since = "1.0.0")]
 pub struct VarsOs {
     inner: os_imp::Env,
@@ -106,7 +98,7 @@ pub struct VarsOs {
 /// environment is not valid unicode. If this is not desired, consider using the
 /// [`env::vars_os`] function.
 ///
-/// [`env::vars_os`]: fn.vars_os.html
+/// [`env::vars_os`]: vars_os
 ///
 /// # Examples
 ///
@@ -222,8 +214,6 @@ fn _var(key: &OsStr) -> Result<String, VarError> {
 /// Fetches the environment variable `key` from the current process, returning
 /// [`None`] if the variable isn't set.
 ///
-/// [`None`]: ../option/enum.Option.html#variant.None
-///
 /// # Panics
 ///
 /// This function may panic if `key` is empty, contains an ASCII equals sign
@@ -254,7 +244,7 @@ fn _var_os(key: &OsStr) -> Option<OsString> {
 /// The error type for operations interacting with environment variables.
 /// Possibly returned from the [`env::var`] function.
 ///
-/// [`env::var`]: fn.var.html
+/// [`env::var`]: var
 #[derive(Debug, PartialEq, Eq, Clone)]
 #[stable(feature = "env", since = "1.0.0")]
 pub enum VarError {
@@ -382,8 +372,7 @@ fn _remove_var(k: &OsStr) {
 /// This structure is created by the [`std::env::split_paths`] function. See its
 /// documentation for more.
 ///
-/// [`PathBuf`]: ../../std/path/struct.PathBuf.html
-/// [`std::env::split_paths`]: fn.split_paths.html
+/// [`std::env::split_paths`]: split_paths
 #[stable(feature = "env", since = "1.0.0")]
 pub struct SplitPaths<'a> {
     inner: os_imp::SplitPaths<'a>,
@@ -410,8 +399,6 @@ pub struct SplitPaths<'a> {
 ///     None => println!("{} is not defined in the environment.", key)
 /// }
 /// ```
-///
-/// [`PathBuf`]: ../../std/path/struct.PathBuf.html
 #[stable(feature = "env", since = "1.0.0")]
 pub fn split_paths<T: AsRef<OsStr> + ?Sized>(unparsed: &T) -> SplitPaths<'_> {
     SplitPaths { inner: os_imp::split_paths(unparsed.as_ref()) }
@@ -438,7 +425,7 @@ impl fmt::Debug for SplitPaths<'_> {
 /// The error type for operations on the `PATH` variable. Possibly returned from
 /// the [`env::join_paths`] function.
 ///
-/// [`env::join_paths`]: fn.join_paths.html
+/// [`env::join_paths`]: join_paths
 #[derive(Debug)]
 #[stable(feature = "env", since = "1.0.0")]
 pub struct JoinPathsError {
@@ -450,14 +437,10 @@ pub struct JoinPathsError {
 ///
 /// # Errors
 ///
-/// Returns an [`Err`][err] (containing an error message) if one of the input
+/// Returns an [`Err`] (containing an error message) if one of the input
 /// [`Path`]s contains an invalid character for constructing the `PATH`
 /// variable (a double quote on Windows or a colon on Unix).
 ///
-/// [`Path`]: ../../std/path/struct.Path.html
-/// [`OsString`]: ../../std/ffi/struct.OsString.html
-/// [err]: ../../std/result/enum.Result.html#variant.Err
-///
 /// # Examples
 ///
 /// Joining paths on a Unix-like platform:
@@ -508,7 +491,7 @@ pub struct JoinPathsError {
 /// }
 /// ```
 ///
-/// [`env::split_paths`]: fn.split_paths.html
+/// [`env::split_paths`]: split_paths
 #[stable(feature = "env", since = "1.0.0")]
 pub fn join_paths<I, T>(paths: I) -> Result<OsString, JoinPathsError>
 where
@@ -688,8 +671,7 @@ pub fn current_exe() -> io::Result<PathBuf> {
 /// set to arbitrary text, and may not even exist. This means this property
 /// should not be relied upon for security purposes.
 ///
-/// [`String`]: ../string/struct.String.html
-/// [`std::env::args`]: ./fn.args.html
+/// [`std::env::args`]: args
 #[stable(feature = "env", since = "1.0.0")]
 pub struct Args {
     inner: ArgsOs,
@@ -705,8 +687,7 @@ pub struct Args {
 /// set to arbitrary text, and may not even exist. This means this property
 /// should not be relied upon for security purposes.
 ///
-/// [`OsString`]: ../ffi/struct.OsString.html
-/// [`std::env::args_os`]: ./fn.args_os.html
+/// [`std::env::args_os`]: args_os
 #[stable(feature = "env", since = "1.0.0")]
 pub struct ArgsOs {
     inner: sys::args::Args,
@@ -744,8 +725,6 @@ pub struct ArgsOs {
 ///     println!("{}", argument);
 /// }
 /// ```
-///
-/// [`args_os`]: ./fn.args_os.html
 #[stable(feature = "env", since = "1.0.0")]
 pub fn args() -> Args {
     Args { inner: args_os() }
diff --git a/library/std/src/error.rs b/library/std/src/error.rs
index 3b4cb859dd4..1b7681bd4bb 100644
--- a/library/std/src/error.rs
+++ b/library/std/src/error.rs
@@ -40,10 +40,8 @@ use crate::string;
 /// provide its own errors while also revealing some of the implementation for
 /// debugging via [`source`] chains.
 ///
-/// [`Result<T, E>`]: ../result/enum.Result.html
-/// [`Display`]: ../fmt/trait.Display.html
-/// [`Debug`]: ../fmt/trait.Debug.html
-/// [`source`]: trait.Error.html#method.source
+/// [`Result<T, E>`]: Result
+/// [`source`]: Error::source
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Error: Debug + Display {
     /// The lower-level source of this error, if any.
@@ -164,8 +162,6 @@ mod private {
 impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
     /// Converts a type of [`Error`] into a box of dyn [`Error`].
     ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -199,8 +195,6 @@ impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync +
     /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
     /// dyn [`Error`] + [`Send`] + [`Sync`].
     ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -238,8 +232,6 @@ impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync +
 impl From<String> for Box<dyn Error + Send + Sync> {
     /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
     ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -283,8 +275,6 @@ impl From<String> for Box<dyn Error + Send + Sync> {
 impl From<String> for Box<dyn Error> {
     /// Converts a [`String`] into a box of dyn [`Error`].
     ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -306,8 +296,6 @@ impl From<String> for Box<dyn Error> {
 impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
     /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
     ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -329,8 +317,6 @@ impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
 impl From<&str> for Box<dyn Error> {
     /// Converts a [`str`] into a box of dyn [`Error`].
     ///
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -350,9 +336,6 @@ impl From<&str> for Box<dyn Error> {
 impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
     /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
     ///
-    /// [`Cow`]: ../borrow/enum.Cow.html
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -374,9 +357,6 @@ impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
 impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
     /// Converts a [`Cow`] into a box of dyn [`Error`].
     ///
-    /// [`Cow`]: ../borrow/enum.Cow.html
-    /// [`Error`]: ../error/trait.Error.html
-    ///
     /// # Examples
     ///
     /// ```
@@ -703,7 +683,7 @@ impl dyn Error {
     /// assert!(iter.next().is_none());
     /// ```
     ///
-    /// [`source`]: trait.Error.html#method.source
+    /// [`source`]: Error::source
     #[unstable(feature = "error_iter", issue = "58520")]
     #[inline]
     pub fn chain(&self) -> Chain<'_> {
@@ -715,8 +695,6 @@ impl dyn Error {
 ///
 /// If you want to omit the initial error and only process
 /// its sources, use `skip(1)`.
-///
-/// [`Error`]: trait.Error.html
 #[unstable(feature = "error_iter", issue = "58520")]
 #[derive(Clone, Debug)]
 pub struct Chain<'a> {
diff --git a/library/std/src/panic.rs b/library/std/src/panic.rs
index 6ad5519d34a..8fcb24033b1 100644
--- a/library/std/src/panic.rs
+++ b/library/std/src/panic.rs
@@ -30,10 +30,6 @@ pub use core::panic::{Location, PanicInfo};
 /// purpose of this trait is to encode what types are safe to cross a [`catch_unwind`]
 /// boundary with no fear of unwind safety.
 ///
-/// [`Send`]: ../marker/trait.Send.html
-/// [`Sync`]: ../marker/trait.Sync.html
-/// [`catch_unwind`]: ./fn.catch_unwind.html
-///
 /// ## What is unwind safety?
 ///
 /// In Rust a function can "return" early if it either panics or calls a
@@ -99,8 +95,6 @@ pub use core::panic::{Location, PanicInfo};
 /// above, the lack of `unsafe` means it is mostly an advisory. The
 /// [`AssertUnwindSafe`] wrapper struct can be used to force this trait to be
 /// implemented for any closed over variables passed to `catch_unwind`.
-///
-/// [`AssertUnwindSafe`]: ./struct.AssertUnwindSafe.html
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 #[rustc_on_unimplemented(
     message = "the type `{Self}` may not be safely transferred across an unwind boundary",
@@ -116,9 +110,6 @@ pub auto trait UnwindSafe {}
 ///
 /// This is a "helper marker trait" used to provide impl blocks for the
 /// [`UnwindSafe`] trait, for more information see that documentation.
-///
-/// [`UnsafeCell`]: ../cell/struct.UnsafeCell.html
-/// [`UnwindSafe`]: ./trait.UnwindSafe.html
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 #[rustc_on_unimplemented(
     message = "the type `{Self}` may contain interior mutability and a reference may not be safely \
@@ -138,7 +129,6 @@ pub auto trait RefUnwindSafe {}
 /// account. This wrapper struct is useful for a quick and lightweight
 /// annotation that a variable is indeed unwind safe.
 ///
-/// [`catch_unwind`]: ./fn.catch_unwind.html
 /// # Examples
 ///
 /// One way to use `AssertUnwindSafe` is to assert that the entire closure
@@ -352,8 +342,6 @@ impl<F: Future> Future for AssertUnwindSafe<F> {
 /// can fail on a regular basis. Additionally, this function is not guaranteed
 /// to catch all panics, see the "Notes" section below.
 ///
-/// [`Result`]: ../result/enum.Result.html
-///
 /// The closure provided is required to adhere to the [`UnwindSafe`] trait to ensure
 /// that all captured variables are safe to cross this boundary. The purpose of
 /// this bound is to encode the concept of [exception safety][rfc] in the type
@@ -362,9 +350,6 @@ impl<F: Future> Future for AssertUnwindSafe<F> {
 /// becomes a problem the [`AssertUnwindSafe`] wrapper struct can be used to quickly
 /// assert that the usage here is indeed unwind safe.
 ///
-/// [`AssertUnwindSafe`]: ./struct.AssertUnwindSafe.html
-/// [`UnwindSafe`]: ./trait.UnwindSafe.html
-///
 /// [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1236-stabilize-catch-panic.md
 ///
 /// # Notes
@@ -399,8 +384,6 @@ pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
 /// This is designed to be used in conjunction with [`catch_unwind`] to, for
 /// example, carry a panic across a layer of C code.
 ///
-/// [`catch_unwind`]: ./fn.catch_unwind.html
-///
 /// # Notes
 ///
 /// Note that panics in Rust are not always implemented via unwinding, but they
diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 61261e82147..21874853839 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -204,7 +204,7 @@ target | std | host | notes
 `thumbv4t-none-eabi` | * |  | ARMv4T T32
 `x86_64-apple-ios-macabi` | ✓[^apple] |  | Apple Catalyst
 `x86_64-apple-tvos` | *[^apple] | | x86 64-bit tvOS
-`x86_64-linux-kernel` | ? |  | Linux kernel modules
+`x86_64-linux-kernel` | * |  | Linux kernel modules
 `x86_64-pc-solaris` | ? |  |
 `x86_64-pc-windows-msvc` | ✓ |  | 64-bit Windows XP support
 `x86_64-unknown-cloudabi` | ✓ |  | 64-bit CloudABI
diff --git a/src/librustc_error_codes/error_codes/E0754.md b/src/librustc_error_codes/error_codes/E0754.md
index abdc01ed21a..57620bcd65c 100644
--- a/src/librustc_error_codes/error_codes/E0754.md
+++ b/src/librustc_error_codes/error_codes/E0754.md
@@ -1,31 +1,25 @@
 An non-ascii identifier was used in an invalid context.
 
-Erroneous code example:
+Erroneous code examples:
 
 ```compile_fail,E0754
 # #![feature(non_ascii_idents)]
 
-mod řųśť;
-// ^ error!
-fn main() {}
-```
-
-```compile_fail,E0754
-# #![feature(non_ascii_idents)]
+mod řųśť; // error!
 
 #[no_mangle]
-fn řųśť() {}
-// ^ error!
+fn řųśť() {} // error!
+
 fn main() {}
 ```
 
-Non-ascii can be used as module names if it is inline
-or a #\[path\] attribute is specified. For example:
+Non-ascii can be used as module names if it is inlined or if a `#[path]`
+attribute is specified. For example:
 
 ```
 # #![feature(non_ascii_idents)]
 
-mod řųśť {
+mod řųśť { // ok!
     const IS_GREAT: bool = true;
 }
 
diff --git a/src/librustc_errors/diagnostic.rs b/src/librustc_errors/diagnostic.rs
index cd4b5d56f36..870f7b81e21 100644
--- a/src/librustc_errors/diagnostic.rs
+++ b/src/librustc_errors/diagnostic.rs
@@ -127,14 +127,15 @@ impl Diagnostic {
     }
 
     /// Adds a span/label to be included in the resulting snippet.
-    /// This label will be shown together with the original span/label used when creating the
-    /// diagnostic, *not* a span added by one of the `span_*` methods.
     ///
-    /// This is pushed onto the `MultiSpan` that was created when the
-    /// diagnostic was first built. If you don't call this function at
-    /// all, and you just supplied a `Span` to create the diagnostic,
-    /// then the snippet will just include that `Span`, which is
-    /// called the primary span.
+    /// This is pushed onto the [`MultiSpan`] that was created when the diagnostic
+    /// was first built. That means it will be shown together with the original
+    /// span/label, *not* a span added by one of the `span_{note,warn,help,suggestions}` methods.
+    ///
+    /// This span is *not* considered a ["primary span"][`MultiSpan`]; only
+    /// the `Span` supplied when creating the diagnostic is primary.
+    ///
+    /// [`MultiSpan`]: ../rustc_span/struct.MultiSpan.html
     pub fn span_label<T: Into<String>>(&mut self, span: Span, label: T) -> &mut Self {
         self.span.push_span_label(span, label.into());
         self
diff --git a/src/librustc_errors/diagnostic_builder.rs b/src/librustc_errors/diagnostic_builder.rs
index dc52e29fa9b..d1ff6f721c4 100644
--- a/src/librustc_errors/diagnostic_builder.rs
+++ b/src/librustc_errors/diagnostic_builder.rs
@@ -184,11 +184,15 @@ impl<'a> DiagnosticBuilder<'a> {
     }
 
     /// Adds a span/label to be included in the resulting snippet.
-    /// This is pushed onto the `MultiSpan` that was created when the
-    /// diagnostic was first built. If you don't call this function at
-    /// all, and you just supplied a `Span` to create the diagnostic,
-    /// then the snippet will just include that `Span`, which is
-    /// called the primary span.
+    ///
+    /// This is pushed onto the [`MultiSpan`] that was created when the diagnostic
+    /// was first built. That means it will be shown together with the original
+    /// span/label, *not* a span added by one of the `span_{note,warn,help,suggestions}` methods.
+    ///
+    /// This span is *not* considered a ["primary span"][`MultiSpan`]; only
+    /// the `Span` supplied when creating the diagnostic is primary.
+    ///
+    /// [`MultiSpan`]: ../rustc_span/struct.MultiSpan.html
     pub fn span_label(&mut self, span: Span, label: impl Into<String>) -> &mut Self {
         self.0.diagnostic.span_label(span, label);
         self
diff --git a/src/librustc_mir/transform/check_unsafety.rs b/src/librustc_mir/transform/check_unsafety.rs
index d2a5616b8ed..6aabc1941a6 100644
--- a/src/librustc_mir/transform/check_unsafety.rs
+++ b/src/librustc_mir/transform/check_unsafety.rs
@@ -228,7 +228,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
             let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
             match base_ty.kind {
                 ty::RawPtr(..) => self.require_unsafe(
-                    UnsafetyViolationKind::General,
+                    UnsafetyViolationKind::GeneralAndConstFn,
                     UnsafetyViolationDetails::DerefOfRawPointer,
                 ),
                 ty::Adt(adt, _) => {
diff --git a/src/librustc_typeck/check/demand.rs b/src/librustc_typeck/check/demand.rs
index ad97dbe63d8..aa92d8b8b2b 100644
--- a/src/librustc_typeck/check/demand.rs
+++ b/src/librustc_typeck/check/demand.rs
@@ -36,6 +36,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.suggest_missing_await(err, expr, expected, expr_ty);
         self.suggest_missing_parentheses(err, expr);
         self.note_need_for_fn_pointer(err, expected, expr_ty);
+        self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
     }
 
     // Requires that the two types unify, and prints an error message if
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 5b8c775e74d..824e81a974c 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -5198,6 +5198,51 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    fn note_internal_mutation_in_method(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        expr: &hir::Expr<'_>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+    ) {
+        if found != self.tcx.types.unit {
+            return;
+        }
+        if let ExprKind::MethodCall(path_segment, _, [rcvr, ..], _) = expr.kind {
+            if self
+                .typeck_results
+                .borrow()
+                .expr_ty_adjusted_opt(rcvr)
+                .map_or(true, |ty| expected.peel_refs() != ty.peel_refs())
+            {
+                return;
+            }
+            let mut sp = MultiSpan::from_span(path_segment.ident.span);
+            sp.push_span_label(
+                path_segment.ident.span,
+                format!(
+                    "this call modifies {} in-place",
+                    match rcvr.kind {
+                        ExprKind::Path(QPath::Resolved(
+                            None,
+                            hir::Path { segments: [segment], .. },
+                        )) => format!("`{}`", segment.ident),
+                        _ => "its receiver".to_string(),
+                    }
+                ),
+            );
+            sp.push_span_label(
+                rcvr.span,
+                "you probably want to use this value after calling the method...".to_string(),
+            );
+            err.span_note(
+                sp,
+                &format!("method `{}` modifies its receiver in-place", path_segment.ident),
+            );
+            err.note(&format!("...instead of the `()` output of method `{}`", path_segment.ident));
+        }
+    }
+
     /// When encountering an `impl Future` where `BoxFuture` is expected, suggest `Box::pin`.
     fn suggest_calling_boxed_future_when_appropriate(
         &self,
diff --git a/src/test/ui/consts/min_const_fn/allow_raw_ptr_dereference_const_fn.rs b/src/test/ui/consts/min_const_fn/allow_raw_ptr_dereference_const_fn.rs
new file mode 100644
index 00000000000..25dc457d144
--- /dev/null
+++ b/src/test/ui/consts/min_const_fn/allow_raw_ptr_dereference_const_fn.rs
@@ -0,0 +1,11 @@
+// check-pass
+#![feature(const_raw_ptr_deref)]
+#![feature(raw_ref_macros)]
+
+use std::ptr;
+
+const fn test_fn(x: *const i32) {
+    let x2 = unsafe { ptr::raw_const!(*x) };
+}
+
+fn main() {}
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.rs b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.rs
index 0b1ab1c34ff..6462d736ad1 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.rs
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.rs
@@ -1,4 +1,4 @@
-const fn bad_const_fn_deref_raw(x: *mut usize) -> &'static usize { unsafe { &*x } } //~ is unsafe
+const fn bad_const_fn_deref_raw(x: *mut usize) -> &'static usize { unsafe { &*x } }
 //~^ dereferencing raw pointers in constant functions
 
 const unsafe fn bad_const_unsafe_deref_raw(x: *mut usize) -> usize { *x }
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.stderr b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.stderr
index 97b0a778d2c..427ecff5c6d 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.stderr
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_bad.stderr
@@ -34,15 +34,7 @@ LL |     Foo { x: () }.y
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
-error[E0133]: dereference of raw pointer is unsafe and requires unsafe function or block
-  --> $DIR/min_const_fn_unsafe_bad.rs:1:77
-   |
-LL | const fn bad_const_fn_deref_raw(x: *mut usize) -> &'static usize { unsafe { &*x } }
-   |                                                                             ^^^ dereference of raw pointer
-   |
-   = note: raw pointers may be NULL, dangling or unaligned; they can violate aliasing rules and cause data races: all of these are undefined behavior
-
-error: aborting due to 5 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0133, E0658, E0723.
-For more information about an error, try `rustc --explain E0133`.
+Some errors have detailed explanations: E0658, E0723.
+For more information about an error, try `rustc --explain E0658`.
diff --git a/src/test/ui/suggestions/chain-method-call-mutation-in-place.rs b/src/test/ui/suggestions/chain-method-call-mutation-in-place.rs
new file mode 100644
index 00000000000..cb92ab87a8f
--- /dev/null
+++ b/src/test/ui/suggestions/chain-method-call-mutation-in-place.rs
@@ -0,0 +1,4 @@
+fn main() {}
+fn foo(mut s: String) -> String {
+    s.push_str("asdf") //~ ERROR mismatched types
+}
diff --git a/src/test/ui/suggestions/chain-method-call-mutation-in-place.stderr b/src/test/ui/suggestions/chain-method-call-mutation-in-place.stderr
new file mode 100644
index 00000000000..63e3bb78954
--- /dev/null
+++ b/src/test/ui/suggestions/chain-method-call-mutation-in-place.stderr
@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/chain-method-call-mutation-in-place.rs:3:5
+   |
+LL | fn foo(mut s: String) -> String {
+   |                          ------ expected `std::string::String` because of return type
+LL |     s.push_str("asdf")
+   |     ^^^^^^^^^^^^^^^^^^ expected struct `std::string::String`, found `()`
+   |
+note: method `push_str` modifies its receiver in-place
+  --> $DIR/chain-method-call-mutation-in-place.rs:3:7
+   |
+LL |     s.push_str("asdf")
+   |     - ^^^^^^^^ this call modifies `s` in-place
+   |     |
+   |     you probably want to use this value after calling the method...
+   = note: ...instead of the `()` output of method `push_str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.