Auto merge of #92556 - matthiaskrgr:rollup-s9vopuj, r=matthiaskrgr

Rollup of 7 pull requests

Successful merges:

 - #91754 (Modifications to `std::io::Stdin` on Windows so that there is no longer a 4-byte buffer minimum in read().)
 - #91884 (Constify `Box<T, A>` methods)
 - #92107 (Actually set IMAGE_SCN_LNK_REMOVE for .rmeta)
 - #92456 (Make the documentation of builtin macro attributes accessible)
 - #92507 (Suggest single quotes when char expected, str provided)
 - #92525 (intra-doc: Make `Receiver::into_iter` into a clickable link)
 - #92532 (revert #92254 "Bump gsgdt to 0.1.3")

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

24 files changed, 458 insertions, 86 deletions

diff --git a/Cargo.lock b/Cargo.lock
index a912eee97db..1fe03a06c79 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1505,9 +1505,9 @@ dependencies = [
 
 [[package]]
 name = "gsgdt"
-version = "0.1.3"
+version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cb958139bb971f37d2f5423436f137768f88b9c616b4c21d4f634dd129508d60"
+checksum = "a0d876ce7262df96262a2a19531da6ff9a86048224d49580a585fc5c04617825"
 dependencies = [
  "serde",
 ]
diff --git a/compiler/rustc_codegen_ssa/src/back/metadata.rs b/compiler/rustc_codegen_ssa/src/back/metadata.rs
index 7c97143e807..79c24f0f172 100644
--- a/compiler/rustc_codegen_ssa/src/back/metadata.rs
+++ b/compiler/rustc_codegen_ssa/src/back/metadata.rs
@@ -6,8 +6,8 @@ use std::path::Path;
 
 use object::write::{self, StandardSegment, Symbol, SymbolSection};
 use object::{
-    elf, Architecture, BinaryFormat, Endianness, FileFlags, Object, ObjectSection, SectionFlags,
-    SectionKind, SymbolFlags, SymbolKind, SymbolScope,
+    elf, pe, Architecture, BinaryFormat, Endianness, FileFlags, Object, ObjectSection,
+    SectionFlags, SectionKind, SymbolFlags, SymbolKind, SymbolScope,
 };
 
 use snap::write::FrameEncoder;
@@ -216,13 +216,12 @@ pub fn create_rmeta_file(sess: &Session, metadata: &[u8]) -> Vec<u8> {
     );
     match file.format() {
         BinaryFormat::Coff => {
-            const IMAGE_SCN_LNK_REMOVE: u32 = 0;
             file.section_mut(section).flags =
-                SectionFlags::Coff { characteristics: IMAGE_SCN_LNK_REMOVE };
+                SectionFlags::Coff { characteristics: pe::IMAGE_SCN_LNK_REMOVE };
         }
         BinaryFormat::Elf => {
-            const SHF_EXCLUDE: u64 = 0x80000000;
-            file.section_mut(section).flags = SectionFlags::Elf { sh_flags: SHF_EXCLUDE };
+            file.section_mut(section).flags =
+                SectionFlags::Elf { sh_flags: elf::SHF_EXCLUDE as u64 };
         }
         _ => {}
     };
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 9a76c05e4f6..f0c73d0c2f3 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -2041,11 +2041,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 if let ValuePairs::Types(ty::error::ExpectedFound { expected, found }) =
                     trace.values
                 {
-                    // If a tuple of length one was expected and the found expression has
-                    // parentheses around it, perhaps the user meant to write `(expr,)` to
-                    // build a tuple (issue #86100)
                     match (expected.kind(), found.kind()) {
                         (ty::Tuple(_), ty::Tuple(_)) => {}
+                        // If a tuple of length one was expected and the found expression has
+                        // parentheses around it, perhaps the user meant to write `(expr,)` to
+                        // build a tuple (issue #86100)
                         (ty::Tuple(_), _) if expected.tuple_fields().count() == 1 => {
                             if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
                                 if let Some(code) =
@@ -2060,6 +2060,41 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 }
                             }
                         }
+                        // If a character was expected and the found expression is a string literal
+                        // containing a single character, perhaps the user meant to write `'c'` to
+                        // specify a character literal (issue #92479)
+                        (ty::Char, ty::Ref(_, r, _)) if r.is_str() => {
+                            if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
+                                if let Some(code) =
+                                    code.strip_prefix('"').and_then(|s| s.strip_suffix('"'))
+                                {
+                                    if code.chars().nth(1).is_none() {
+                                        err.span_suggestion(
+                                            span,
+                                            "if you meant to write a `char` literal, use single quotes",
+                                            format!("'{}'", code),
+                                            Applicability::MachineApplicable,
+                                        );
+                                    }
+                                }
+                            }
+                        }
+                        // If a string was expected and the found expression is a character literal,
+                        // perhaps the user meant to write `"s"` to specify a string literal.
+                        (ty::Ref(_, r, _), ty::Char) if r.is_str() => {
+                            if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
+                                if let Some(code) =
+                                    code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
+                                {
+                                    err.span_suggestion(
+                                        span,
+                                        "if you meant to write a `str` literal, use double quotes",
+                                        format!("\"{}\"", code),
+                                        Applicability::MachineApplicable,
+                                    );
+                                }
+                            }
+                        }
                         _ => {}
                     }
                 }
diff --git a/compiler/rustc_middle/Cargo.toml b/compiler/rustc_middle/Cargo.toml
index f8d3fb6c48d..a9db8469384 100644
--- a/compiler/rustc_middle/Cargo.toml
+++ b/compiler/rustc_middle/Cargo.toml
@@ -10,7 +10,7 @@ doctest = false
 rustc_arena = { path = "../rustc_arena" }
 bitflags = "1.2.1"
 either = "1.5.0"
-gsgdt = "0.1.3"
+gsgdt = "0.1.2"
 tracing = "0.1"
 rustc-rayon = "0.3.1"
 rustc-rayon-core = "0.3.1"
diff --git a/compiler/rustc_middle/src/mir/generic_graph.rs b/compiler/rustc_middle/src/mir/generic_graph.rs
index 44d0ce935df..770b52a4d4b 100644
--- a/compiler/rustc_middle/src/mir/generic_graph.rs
+++ b/compiler/rustc_middle/src/mir/generic_graph.rs
@@ -55,7 +55,7 @@ fn bb_to_graph_node(block: BasicBlock, body: &Body<'_>, dark_mode: bool) -> Node
     data.terminator().kind.fmt_head(&mut terminator_head).unwrap();
     stmts.push(terminator_head);
 
-    Node::from_list(stmts, label, title, style)
+    Node::new(stmts, label, title, style)
 }
 
 // Must match `[0-9A-Za-z_]*`. This does not appear in the rendered graph, so
diff --git a/library/alloc/src/alloc.rs b/library/alloc/src/alloc.rs
index 66ef92558d8..d075658f51a 100644
--- a/library/alloc/src/alloc.rs
+++ b/library/alloc/src/alloc.rs
@@ -323,17 +323,21 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 
 #[cfg_attr(not(test), lang = "box_free")]
 #[inline]
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
 // This signature has to be the same as `Box`, otherwise an ICE will happen.
 // When an additional parameter to `Box` is added (like `A: Allocator`), this has to be added here as
 // well.
 // For example if `Box` is changed to  `struct Box<T: ?Sized, A: Allocator>(Unique<T>, A)`,
 // this function has to be changed to `fn box_free<T: ?Sized, A: Allocator>(Unique<T>, A)` as well.
-pub(crate) unsafe fn box_free<T: ?Sized, A: Allocator>(ptr: Unique<T>, alloc: A) {
+pub(crate) const unsafe fn box_free<T: ?Sized, A: ~const Allocator + ~const Drop>(
+    ptr: Unique<T>,
+    alloc: A,
+) {
     unsafe {
         let size = size_of_val(ptr.as_ref());
         let align = min_align_of_val(ptr.as_ref());
         let layout = Layout::from_size_align_unchecked(size, align);
-        alloc.deallocate(ptr.cast().into(), layout)
+        alloc.deallocate(From::from(ptr.cast()), layout)
     }
 }
 
@@ -361,13 +365,22 @@ extern "Rust" {
 /// [`set_alloc_error_hook`]: ../../std/alloc/fn.set_alloc_error_hook.html
 /// [`take_alloc_error_hook`]: ../../std/alloc/fn.take_alloc_error_hook.html
 #[stable(feature = "global_alloc", since = "1.28.0")]
+#[rustc_const_unstable(feature = "const_alloc_error", issue = "92523")]
 #[cfg(all(not(no_global_oom_handling), not(test)))]
 #[rustc_allocator_nounwind]
 #[cold]
-pub fn handle_alloc_error(layout: Layout) -> ! {
-    unsafe {
-        __rust_alloc_error_handler(layout.size(), layout.align());
+pub const fn handle_alloc_error(layout: Layout) -> ! {
+    const fn ct_error(_: Layout) -> ! {
+        panic!("allocation failed");
     }
+
+    fn rt_error(layout: Layout) -> ! {
+        unsafe {
+            __rust_alloc_error_handler(layout.size(), layout.align());
+        }
+    }
+
+    unsafe { core::intrinsics::const_eval_select((layout,), ct_error, rt_error) }
 }
 
 // For alloc test `std::alloc::handle_alloc_error` can be used directly.
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index ab41f5646e5..aa7344ba405 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -346,9 +346,13 @@ impl<T, A: Allocator> Box<T, A> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[must_use]
     #[inline]
-    pub fn new_in(x: T, alloc: A) -> Self {
+    pub const fn new_in(x: T, alloc: A) -> Self
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let mut boxed = Self::new_uninit_in(alloc);
         unsafe {
             boxed.as_mut_ptr().write(x);
@@ -372,8 +376,13 @@ impl<T, A: Allocator> Box<T, A> {
     /// # Ok::<(), std::alloc::AllocError>(())
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError> {
+    pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
+    where
+        T: ~const Drop,
+        A: ~const Allocator + ~const Drop,
+    {
         let mut boxed = Self::try_new_uninit_in(alloc)?;
         unsafe {
             boxed.as_mut_ptr().write(x);
@@ -402,10 +411,14 @@ impl<T, A: Allocator> Box<T, A> {
     /// assert_eq!(*five, 5)
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[cfg(not(no_global_oom_handling))]
     #[must_use]
     // #[unstable(feature = "new_uninit", issue = "63291")]
-    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
+    pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
         // That would make code size bigger.
@@ -439,7 +452,11 @@ impl<T, A: Allocator> Box<T, A> {
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
-    pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         let ptr = alloc.allocate(layout)?.cast();
         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
@@ -466,10 +483,14 @@ impl<T, A: Allocator> Box<T, A> {
     ///
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[cfg(not(no_global_oom_handling))]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     #[must_use]
-    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
+    pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
         // That would make code size bigger.
@@ -503,7 +524,11 @@ impl<T, A: Allocator> Box<T, A> {
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
-    pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         let ptr = alloc.allocate_zeroed(layout)?.cast();
         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
@@ -513,20 +538,22 @@ impl<T, A: Allocator> Box<T, A> {
     /// `x` will be pinned in memory and unable to be moved.
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[must_use]
     #[inline(always)]
-    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
+    pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
     where
-        A: 'static,
+        A: 'static + ~const Allocator + ~const Drop,
     {
-        Self::new_in(x, alloc).into()
+        Self::into_pin(Self::new_in(x, alloc))
     }
 
     /// Converts a `Box<T>` into a `Box<[T]>`
     ///
     /// This conversion does not allocate on the heap and happens in place.
     #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
-    pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
         let (raw, alloc) = Box::into_raw_with_allocator(boxed);
         unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
     }
@@ -543,8 +570,12 @@ impl<T, A: Allocator> Box<T, A> {
     /// assert_eq!(Box::into_inner(c), 5);
     /// ```
     #[unstable(feature = "box_into_inner", issue = "80437")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn into_inner(boxed: Self) -> T {
+    pub const fn into_inner(boxed: Self) -> T
+    where
+        Self: ~const Drop,
+    {
         *boxed
     }
 }
@@ -758,8 +789,9 @@ impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
     /// assert_eq!(*five, 5)
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub unsafe fn assume_init(self) -> Box<T, A> {
+    pub const unsafe fn assume_init(self) -> Box<T, A> {
         let (raw, alloc) = Box::into_raw_with_allocator(self);
         unsafe { Box::from_raw_in(raw as *mut T, alloc) }
     }
@@ -792,8 +824,9 @@ impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
     /// }
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
+    pub const fn write(mut boxed: Self, value: T) -> Box<T, A> {
         unsafe {
             (*boxed).write(value);
             boxed.assume_init()
@@ -938,8 +971,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// [memory layout]: self#memory-layout
     /// [`Layout`]: crate::Layout
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
+    pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
         Box(unsafe { Unique::new_unchecked(raw) }, alloc)
     }
 
@@ -1035,8 +1069,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     ///
     /// [memory layout]: self#memory-layout
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
+    pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
         let (leaked, alloc) = Box::into_unique(b);
         (leaked.as_ptr(), alloc)
     }
@@ -1046,9 +1081,10 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
         issue = "none",
         reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
     )]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
     #[doc(hidden)]
-    pub fn into_unique(b: Self) -> (Unique<T>, A) {
+    pub const fn into_unique(b: Self) -> (Unique<T>, A) {
         // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
         // raw pointer for the type system. Turning it directly into a raw pointer would not be
         // recognized as "releasing" the unique pointer to permit aliased raw accesses,
@@ -1064,8 +1100,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
     /// is so that there is no conflict with a method on the inner type.
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn allocator(b: &Self) -> &A {
+    pub const fn allocator(b: &Self) -> &A {
         &b.1
     }
 
@@ -1105,8 +1142,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// assert_eq!(*static_ref, [4, 2, 3]);
     /// ```
     #[stable(feature = "box_leak", since = "1.26.0")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn leak<'a>(b: Self) -> &'a mut T
+    pub const fn leak<'a>(b: Self) -> &'a mut T
     where
         A: 'a,
     {
@@ -1119,7 +1157,8 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     ///
     /// This is also available via [`From`].
     #[unstable(feature = "box_into_pin", issue = "62370")]
-    pub fn into_pin(boxed: Self) -> Pin<Self>
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn into_pin(boxed: Self) -> Pin<Self>
     where
         A: 'static,
     {
@@ -1131,7 +1170,8 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> const Drop for Box<T, A> {
     fn drop(&mut self) {
         // FIXME: Do nothing, drop is currently performed by compiler.
     }
@@ -1341,7 +1381,8 @@ impl<T> From<T> for Box<T> {
 }
 
 #[stable(feature = "pin", since = "1.33.0")]
-impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const From<Box<T, A>> for Pin<Box<T, A>>
 where
     A: 'static,
 {
@@ -1720,7 +1761,8 @@ impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const Deref for Box<T, A> {
     type Target = T;
 
     fn deref(&self) -> &T {
@@ -1729,7 +1771,8 @@ impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const DerefMut for Box<T, A> {
     fn deref_mut(&mut self) -> &mut T {
         &mut **self
     }
@@ -1908,7 +1951,8 @@ impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
  *  could have a method to project a Pin<T> from it.
  */
 #[stable(feature = "pin", since = "1.33.0")]
-impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static {}
 
 #[unstable(feature = "generator_trait", issue = "43122")]
 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index 600862c4224..7e663fab16a 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -96,9 +96,20 @@
 #![feature(array_windows)]
 #![feature(async_stream)]
 #![feature(coerce_unsized)]
+#![cfg_attr(not(no_global_oom_handling), feature(const_alloc_error))]
+#![feature(const_box)]
 #![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
 #![feature(const_cow_is_borrowed)]
+#![feature(const_convert)]
+#![feature(const_size_of_val)]
+#![feature(const_align_of_val)]
+#![feature(const_ptr_read)]
+#![feature(const_maybe_uninit_write)]
+#![feature(const_maybe_uninit_as_mut_ptr)]
+#![feature(const_refs_to_cell)]
 #![feature(core_intrinsics)]
+#![feature(const_eval_select)]
+#![feature(const_pin)]
 #![feature(dispatch_from_dyn)]
 #![feature(exact_size_is_empty)]
 #![feature(extend_one)]
@@ -134,8 +145,13 @@
 #![feature(box_syntax)]
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
+#![feature(const_deref)]
 #![feature(const_fn_trait_bound)]
+#![feature(const_mut_refs)]
+#![feature(const_ptr_write)]
+#![feature(const_precise_live_drops)]
 #![feature(const_trait_impl)]
+#![feature(const_try)]
 #![cfg_attr(bootstrap, feature(destructuring_assignment))]
 #![feature(dropck_eyepatch)]
 #![feature(exclusive_range_pattern)]
diff --git a/library/alloc/tests/boxed.rs b/library/alloc/tests/boxed.rs
index bfe66b2687e..0d7acfed8c6 100644
--- a/library/alloc/tests/boxed.rs
+++ b/library/alloc/tests/boxed.rs
@@ -1,6 +1,7 @@
-use std::cell::Cell;
-use std::mem::MaybeUninit;
-use std::ptr::NonNull;
+use core::alloc::{AllocError, Allocator, Layout};
+use core::cell::Cell;
+use core::mem::MaybeUninit;
+use core::ptr::NonNull;
 
 #[test]
 fn uninitialized_zero_size_box() {
@@ -57,3 +58,110 @@ fn box_deref_lval() {
     x.set(1000);
     assert_eq!(x.get(), 1000);
 }
+
+pub struct ConstAllocator;
+
+unsafe impl const Allocator for ConstAllocator {
+    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+        match layout.size() {
+            0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
+            _ => unsafe {
+                let ptr = core::intrinsics::const_allocate(layout.size(), layout.align());
+                Ok(NonNull::new_unchecked(ptr as *mut [u8; 0] as *mut [u8]))
+            },
+        }
+    }
+
+    unsafe fn deallocate(&self, _ptr: NonNull<u8>, layout: Layout) {
+        match layout.size() {
+            0 => { /* do nothing */ }
+            _ => { /* do nothing too */ }
+        }
+    }
+
+    fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+        let ptr = self.allocate(layout)?;
+        if layout.size() > 0 {
+            unsafe {
+                ptr.as_mut_ptr().write_bytes(0, layout.size());
+            }
+        }
+        Ok(ptr)
+    }
+
+    unsafe fn grow(
+        &self,
+        ptr: NonNull<u8>,
+        old_layout: Layout,
+        new_layout: Layout,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        debug_assert!(
+            new_layout.size() >= old_layout.size(),
+            "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
+        );
+
+        let new_ptr = self.allocate(new_layout)?;
+        if new_layout.size() > 0 {
+            new_ptr.as_mut_ptr().copy_from_nonoverlapping(ptr.as_ptr(), old_layout.size());
+            self.deallocate(ptr, old_layout);
+        }
+        Ok(new_ptr)
+    }
+
+    unsafe fn grow_zeroed(
+        &self,
+        ptr: NonNull<u8>,
+        old_layout: Layout,
+        new_layout: Layout,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        let new_ptr = self.grow(ptr, old_layout, new_layout)?;
+        if new_layout.size() > 0 {
+            let old_size = old_layout.size();
+            let new_size = new_layout.size();
+            let raw_ptr = new_ptr.as_mut_ptr();
+            raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
+        }
+        Ok(new_ptr)
+    }
+
+    unsafe fn shrink(
+        &self,
+        ptr: NonNull<u8>,
+        old_layout: Layout,
+        new_layout: Layout,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        debug_assert!(
+            new_layout.size() <= old_layout.size(),
+            "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
+        );
+
+        let new_ptr = self.allocate(new_layout)?;
+        if new_layout.size() > 0 {
+            new_ptr.as_mut_ptr().copy_from_nonoverlapping(ptr.as_ptr(), new_layout.size());
+            self.deallocate(ptr, old_layout);
+        }
+        Ok(new_ptr)
+    }
+
+    fn by_ref(&self) -> &Self
+    where
+        Self: Sized,
+    {
+        self
+    }
+}
+
+#[test]
+fn const_box() {
+    const VALUE: u32 = {
+        let mut boxed = Box::new_in(1u32, ConstAllocator);
+        assert!(*boxed == 1);
+
+        *boxed = 42;
+        assert!(*boxed == 42);
+
+        *boxed
+    };
+
+    assert!(VALUE == 42);
+}
diff --git a/library/alloc/tests/lib.rs b/library/alloc/tests/lib.rs
index 68e48348b07..eec24a5c3f7 100644
--- a/library/alloc/tests/lib.rs
+++ b/library/alloc/tests/lib.rs
@@ -1,8 +1,19 @@
 #![feature(allocator_api)]
+#![feature(alloc_layout_extra)]
 #![feature(assert_matches)]
 #![feature(box_syntax)]
 #![feature(cow_is_borrowed)]
+#![feature(const_box)]
+#![feature(const_convert)]
 #![feature(const_cow_is_borrowed)]
+#![feature(const_heap)]
+#![feature(const_intrinsic_copy)]
+#![feature(const_mut_refs)]
+#![feature(const_nonnull_slice_from_raw_parts)]
+#![feature(const_ptr_offset)]
+#![feature(const_ptr_write)]
+#![feature(const_try)]
+#![feature(core_intrinsics)]
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
 #![feature(new_uninit)]
@@ -26,6 +37,7 @@
 #![feature(const_default_impls)]
 #![feature(const_trait_impl)]
 #![feature(const_str_from_utf8)]
+#![feature(nonnull_slice_from_raw_parts)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};
diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index d9389892c0c..d8f6c85e428 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -866,7 +866,6 @@ pub(crate) mod builtin {
                   language use and is subject to change"
     )]
     #[allow_internal_unstable(fmt_internals)]
-    #[doc(hidden)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! format_args_nl {
@@ -1428,6 +1427,10 @@ pub(crate) mod builtin {
     }
 
     /// Attribute macro used to apply derive macros.
+    ///
+    /// See [the reference] for more info.
+    ///
+    /// [the reference]: ../../../reference/attributes/derive.html
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_builtin_macro]
     pub macro derive($item:item) {
@@ -1435,6 +1438,10 @@ pub(crate) mod builtin {
     }
 
     /// Attribute macro applied to a function to turn it into a unit test.
+    ///
+    /// See [the reference] for more info.
+    ///
+    /// [the reference]: ../../../reference/attributes/testing.html#the-test-attribute
     #[stable(feature = "rust1", since = "1.0.0")]
     #[allow_internal_unstable(test, rustc_attrs)]
     #[rustc_builtin_macro]
@@ -1469,7 +1476,7 @@ pub(crate) mod builtin {
 
     /// Attribute macro applied to a static to register it as a global allocator.
     ///
-    /// See also [`std::alloc::GlobalAlloc`](../std/alloc/trait.GlobalAlloc.html).
+    /// See also [`std::alloc::GlobalAlloc`](../../../std/alloc/trait.GlobalAlloc.html).
     #[stable(feature = "global_allocator", since = "1.28.0")]
     #[allow_internal_unstable(rustc_attrs)]
     #[rustc_builtin_macro]
@@ -1507,6 +1514,7 @@ pub(crate) mod builtin {
         since = "1.52.0",
         reason = "rustc-serialize is deprecated and no longer supported"
     )]
+    #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
     pub macro RustcDecodable($item:item) {
         /* compiler built-in */
     }
@@ -1519,6 +1527,7 @@ pub(crate) mod builtin {
         since = "1.52.0",
         reason = "rustc-serialize is deprecated and no longer supported"
     )]
+    #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
     pub macro RustcEncodable($item:item) {
         /* compiler built-in */
     }
diff --git a/library/core/src/prelude/v1.rs b/library/core/src/prelude/v1.rs
index 54f498d1dc1..0fb8846288b 100644
--- a/library/core/src/prelude/v1.rs
+++ b/library/core/src/prelude/v1.rs
@@ -69,23 +69,21 @@ pub use crate::{
 #[doc(no_inline)]
 pub use crate::concat_bytes;
 
+// Do not `doc(inline)` these `doc(hidden)` items.
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[allow(deprecated, deprecated_in_future)]
-#[doc(no_inline)]
-pub use crate::macros::builtin::{
-    bench, global_allocator, test, test_case, RustcDecodable, RustcEncodable,
-};
+#[allow(deprecated)]
+pub use crate::macros::builtin::{RustcDecodable, RustcEncodable};
 
+// Do not `doc(no_inline)` so that they become doc items on their own
+// (no public module for them to be re-exported from).
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[doc(no_inline)]
-pub use crate::macros::builtin::derive;
+pub use crate::macros::builtin::{bench, derive, global_allocator, test, test_case};
 
 #[unstable(
     feature = "cfg_accessible",
     issue = "64797",
     reason = "`cfg_accessible` is not fully implemented"
 )]
-#[doc(no_inline)]
 pub use crate::macros::builtin::cfg_accessible;
 
 #[unstable(
@@ -93,5 +91,4 @@ pub use crate::macros::builtin::cfg_accessible;
     issue = "82679",
     reason = "`cfg_eval` is a recently implemented feature"
 )]
-#[doc(no_inline)]
 pub use crate::macros::builtin::cfg_eval;
diff --git a/library/std/src/prelude/v1.rs b/library/std/src/prelude/v1.rs
index 743dd51333d..b52bcdfca9e 100644
--- a/library/std/src/prelude/v1.rs
+++ b/library/std/src/prelude/v1.rs
@@ -54,33 +54,30 @@ pub use core::prelude::v1::{
 #[doc(no_inline)]
 pub use core::prelude::v1::concat_bytes;
 
-// FIXME: Attribute and internal derive macros are not documented because for them rustdoc generates
-// dead links which fail link checker testing.
+// Do not `doc(inline)` these `doc(hidden)` items.
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[allow(deprecated, deprecated_in_future)]
-#[doc(hidden)]
-pub use core::prelude::v1::{
-    bench, global_allocator, test, test_case, RustcDecodable, RustcEncodable,
-};
+#[allow(deprecated)]
+pub use core::prelude::v1::{RustcDecodable, RustcEncodable};
 
+// Do not `doc(no_inline)` so that they become doc items on their own
+// (no public module for them to be re-exported from).
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[doc(hidden)]
-pub use core::prelude::v1::derive;
+pub use core::prelude::v1::{bench, derive, global_allocator, test, test_case};
 
+// Do not `doc(no_inline)` either.
 #[unstable(
     feature = "cfg_accessible",
     issue = "64797",
     reason = "`cfg_accessible` is not fully implemented"
 )]
-#[doc(hidden)]
 pub use core::prelude::v1::cfg_accessible;
 
+// Do not `doc(no_inline)` either.
 #[unstable(
     feature = "cfg_eval",
     issue = "82679",
     reason = "`cfg_eval` is a recently implemented feature"
 )]
-#[doc(hidden)]
 pub use core::prelude::v1::cfg_eval;
 
 // The file so far is equivalent to src/libcore/prelude/v1.rs,
diff --git a/library/std/src/sync/mpsc/mod.rs b/library/std/src/sync/mpsc/mod.rs
index 2cf678ef69b..2e54321e127 100644
--- a/library/std/src/sync/mpsc/mod.rs
+++ b/library/std/src/sync/mpsc/mod.rs
@@ -429,12 +429,13 @@ pub struct TryIter<'a, T: 'a> {
 }
 
 /// An owning iterator over messages on a [`Receiver`],
-/// created by **Receiver::into_iter**.
+/// created by [`into_iter`].
 ///
 /// This iterator will block whenever [`next`]
 /// is called, waiting for a new message, and [`None`] will be
 /// returned if the corresponding channel has hung up.
 ///
+/// [`into_iter`]: Receiver::into_iter
 /// [`next`]: Iterator::next
 ///
 /// # Examples
diff --git a/library/std/src/sys/windows/stdio.rs b/library/std/src/sys/windows/stdio.rs
index eb0925b3fda..684b8e3155e 100644
--- a/library/std/src/sys/windows/stdio.rs
+++ b/library/std/src/sys/windows/stdio.rs
@@ -15,7 +15,9 @@ use core::str::utf8_char_width;
 // the value over time (such as if a process calls `SetStdHandle` while it's running). See #40490.
 pub struct Stdin {
     surrogate: u16,
+    incomplete_utf8: IncompleteUtf8,
 }
+
 pub struct Stdout {
     incomplete_utf8: IncompleteUtf8,
 }
@@ -29,6 +31,25 @@ struct IncompleteUtf8 {
     len: u8,
 }
 
+impl IncompleteUtf8 {
+    // Implemented for use in Stdin::read.
+    fn read(&mut self, buf: &mut [u8]) -> usize {
+        // Write to buffer until the buffer is full or we run out of bytes.
+        let to_write = cmp::min(buf.len(), self.len as usize);
+        buf[..to_write].copy_from_slice(&self.bytes[..to_write]);
+
+        // Rotate the remaining bytes if not enough remaining space in buffer.
+        if usize::from(self.len) > buf.len() {
+            self.bytes.copy_within(to_write.., 0);
+            self.len -= to_write as u8;
+        } else {
+            self.len = 0;
+        }
+
+        to_write
+    }
+}
+
 // Apparently Windows doesn't handle large reads on stdin or writes to stdout/stderr well (see
 // #13304 for details).
 //
@@ -205,7 +226,7 @@ fn write_u16s(handle: c::HANDLE, data: &[u16]) -> io::Result<usize> {
 
 impl Stdin {
     pub const fn new() -> Stdin {
-        Stdin { surrogate: 0 }
+        Stdin { surrogate: 0, incomplete_utf8: IncompleteUtf8::new() }
     }
 }
 
@@ -221,24 +242,39 @@ impl io::Read for Stdin {
             }
         }
 
-        if buf.len() == 0 {
-            return Ok(0);
-        } else if buf.len() < 4 {
-            return Err(io::Error::new_const(
-                io::ErrorKind::InvalidInput,
-                &"Windows stdin in console mode does not support a buffer too small to \
-                 guarantee holding one arbitrary UTF-8 character (4 bytes)",
-            ));
+        // If there are bytes in the incomplete utf-8, start with those.
+        // (No-op if there is nothing in the buffer.)
+        let mut bytes_copied = self.incomplete_utf8.read(buf);
+
+        if bytes_copied == buf.len() {
+            return Ok(bytes_copied);
+        } else if buf.len() - bytes_copied < 4 {
+            // Not enough space to get a UTF-8 byte. We will use the incomplete UTF8.
+            let mut utf16_buf = [0u16; 1];
+            // Read one u16 character.
+            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(&utf16_buf[..read], &mut self.incomplete_utf8.bytes)?;
+
+            // Read in the bytes from incomplete_utf8 until the buffer is full.
+            self.incomplete_utf8.len = read_bytes as u8;
+            // No-op if no bytes.
+            bytes_copied += self.incomplete_utf8.read(&mut buf[bytes_copied..]);
+            Ok(bytes_copied)
+        } else {
+            let mut utf16_buf = [0u16; MAX_BUFFER_SIZE / 2];
+            // In the worst case, a UTF-8 string can take 3 bytes for every `u16` of a UTF-16. So
+            // we can read at most a third of `buf.len()` chars and uphold the guarantee no data gets
+            // lost.
+            let amount = cmp::min(buf.len() / 3, utf16_buf.len());
+            let read =
+                read_u16s_fixup_surrogates(handle, &mut utf16_buf, amount, &mut self.surrogate)?;
+
+            match utf16_to_utf8(&utf16_buf[..read], buf) {
+                Ok(value) => return Ok(bytes_copied + value),
+                Err(e) => return Err(e),
+            }
         }
-
-        let mut utf16_buf = [0u16; MAX_BUFFER_SIZE / 2];
-        // In the worst case, a UTF-8 string can take 3 bytes for every `u16` of a UTF-16. So
-        // we can read at most a third of `buf.len()` chars and uphold the guarantee no data gets
-        // lost.
-        let amount = cmp::min(buf.len() / 3, utf16_buf.len());
-        let read = read_u16s_fixup_surrogates(handle, &mut utf16_buf, amount, &mut self.surrogate)?;
-
-        utf16_to_utf8(&utf16_buf[..read], buf)
     }
 }
 
diff --git a/src/test/ui/inference/char-as-str-multi.rs b/src/test/ui/inference/char-as-str-multi.rs
new file mode 100644
index 00000000000..21bbc6f20b2
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-multi.rs
@@ -0,0 +1,6 @@
+// When a MULTI-character string literal is used where a char should be,
+// DO NOT suggest changing to single quotes.
+
+fn main() {
+    let _: char = "foo"; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/char-as-str-multi.stderr b/src/test/ui/inference/char-as-str-multi.stderr
new file mode 100644
index 00000000000..c3ba17a5579
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-multi.stderr
@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/char-as-str-multi.rs:5:19
+   |
+LL |     let _: char = "foo";
+   |            ----   ^^^^^ expected `char`, found `&str`
+   |            |
+   |            expected due to this
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/inference/char-as-str-single.fixed b/src/test/ui/inference/char-as-str-single.fixed
new file mode 100644
index 00000000000..e401492a830
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-single.fixed
@@ -0,0 +1,11 @@
+// When a SINGLE-character string literal is used where a char should be,
+// suggest changing to single quotes.
+
+// Testing both single-byte and multi-byte characters, as we should handle both.
+
+// run-rustfix
+
+fn main() {
+    let _: char = 'a'; //~ ERROR mismatched types
+    let _: char = '人'; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/char-as-str-single.rs b/src/test/ui/inference/char-as-str-single.rs
new file mode 100644
index 00000000000..4f23cea5354
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-single.rs
@@ -0,0 +1,11 @@
+// When a SINGLE-character string literal is used where a char should be,
+// suggest changing to single quotes.
+
+// Testing both single-byte and multi-byte characters, as we should handle both.
+
+// run-rustfix
+
+fn main() {
+    let _: char = "a"; //~ ERROR mismatched types
+    let _: char = "人"; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/char-as-str-single.stderr b/src/test/ui/inference/char-as-str-single.stderr
new file mode 100644
index 00000000000..29075c15414
--- /dev/null
+++ b/src/test/ui/inference/char-as-str-single.stderr
@@ -0,0 +1,29 @@
+error[E0308]: mismatched types
+  --> $DIR/char-as-str-single.rs:9:19
+   |
+LL |     let _: char = "a";
+   |            ----   ^^^ expected `char`, found `&str`
+   |            |
+   |            expected due to this
+   |
+help: if you meant to write a `char` literal, use single quotes
+   |
+LL |     let _: char = 'a';
+   |                   ~~~
+
+error[E0308]: mismatched types
+  --> $DIR/char-as-str-single.rs:10:19
+   |
+LL |     let _: char = "人";
+   |            ----   ^^^^ expected `char`, found `&str`
+   |            |
+   |            expected due to this
+   |
+help: if you meant to write a `char` literal, use single quotes
+   |
+LL |     let _: char = '人';
+   |                   ~~~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/inference/str-as-char.fixed b/src/test/ui/inference/str-as-char.fixed
new file mode 100644
index 00000000000..09f3dec5a17
--- /dev/null
+++ b/src/test/ui/inference/str-as-char.fixed
@@ -0,0 +1,8 @@
+// When a char literal is used where a str should be,
+// suggest changing to double quotes.
+
+// run-rustfix
+
+fn main() {
+    let _: &str = "a"; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/str-as-char.rs b/src/test/ui/inference/str-as-char.rs
new file mode 100644
index 00000000000..7092a612442
--- /dev/null
+++ b/src/test/ui/inference/str-as-char.rs
@@ -0,0 +1,8 @@
+// When a char literal is used where a str should be,
+// suggest changing to double quotes.
+
+// run-rustfix
+
+fn main() {
+    let _: &str = 'a'; //~ ERROR mismatched types
+}
diff --git a/src/test/ui/inference/str-as-char.stderr b/src/test/ui/inference/str-as-char.stderr
new file mode 100644
index 00000000000..ebbe7c80f77
--- /dev/null
+++ b/src/test/ui/inference/str-as-char.stderr
@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/str-as-char.rs:7:19
+   |
+LL |     let _: &str = 'a';
+   |            ----   ^^^ expected `&str`, found `char`
+   |            |
+   |            expected due to this
+   |
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     let _: &str = "a";
+   |                   ~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/issues/issue-23589.stderr b/src/test/ui/issues/issue-23589.stderr
index d126e1bf0b5..e065e17c280 100644
--- a/src/test/ui/issues/issue-23589.stderr
+++ b/src/test/ui/issues/issue-23589.stderr
@@ -12,6 +12,11 @@ error[E0308]: mismatched types
    |
 LL |     let v: Vec(&str) = vec!['1', '2'];
    |                             ^^^ expected `&str`, found `char`
+   |
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     let v: Vec(&str) = vec!["1", '2'];
+   |                             ~~~
 
 error: aborting due to 2 previous errors