Merge from rustc

Author: saethlin

alloc/src/rc.rs

@@ -1738,11 +1738,11 @@ impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
 
     /// Inequality for two `Rc`s.
     ///
-    /// Two `Rc`s are unequal if their inner values are unequal.
+    /// Two `Rc`s are not equal if their inner values are not equal.
     ///
     /// If `T` also implements `Eq` (implying reflexivity of equality),
     /// two `Rc`s that point to the same allocation are
-    /// never unequal.
+    /// always equal.
     ///
     /// # Examples
     ///

alloc/src/sync.rs

@@ -2475,10 +2475,10 @@ impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
 
     /// Inequality for two `Arc`s.
     ///
-    /// Two `Arc`s are unequal if their inner values are unequal.
+    /// Two `Arc`s are not equal if their inner values are not equal.
     ///
     /// If `T` also implements `Eq` (implying reflexivity of equality),
-    /// two `Arc`s that point to the same value are never unequal.
+    /// two `Arc`s that point to the same value are always equal.
     ///
     /// # Examples
     ///

core/src/fmt/mod.rs

@@ -392,8 +392,31 @@ enum FlagV1 {
 }
 
 impl<'a> Arguments<'a> {
+    #[doc(hidden)]
+    #[inline]
+    #[unstable(feature = "fmt_internals", issue = "none")]
+    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
+    pub const fn new_const(pieces: &'a [&'static str]) -> Self {
+        if pieces.len() > 1 {
+            panic!("invalid args");
+        }
+        Arguments { pieces, fmt: None, args: &[] }
+    }
+
     /// When using the format_args!() macro, this function is used to generate the
     /// Arguments structure.
+    #[cfg(not(bootstrap))]
+    #[doc(hidden)]
+    #[inline]
+    #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
+    pub fn new_v1(pieces: &'a [&'static str], args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
+        if pieces.len() < args.len() || pieces.len() > args.len() + 1 {
+            panic!("invalid args");
+        }
+        Arguments { pieces, fmt: None, args }
+    }
+
+    #[cfg(bootstrap)]
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
@@ -417,8 +440,7 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
-    pub const fn new_v1_formatted(
+    pub fn new_v1_formatted(
         pieces: &'a [&'static str],
         args: &'a [ArgumentV1<'a>],
         fmt: &'a [rt::v1::Argument],

core/src/hash/mod.rs

@@ -158,7 +158,7 @@ mod sip;
 ///
 /// Implementations of `hash` should ensure that the data they
 /// pass to the `Hasher` are prefix-free. That is,
-/// unequal values should cause two different sequences of values to be written,
+/// values which are not equal should cause two different sequences of values to be written,
 /// and neither of the two sequences should be a prefix of the other.
 ///
 /// For example, the standard implementation of [`Hash` for `&str`][impl] passes an extra

core/src/intrinsics.rs

@@ -2020,6 +2020,16 @@ extern "rust-intrinsic" {
     #[rustc_safe_intrinsic]
     pub fn saturating_sub<T: Copy>(a: T, b: T) -> T;
 
+    /// This is an implementation detail of [`crate::ptr::read`] and should
+    /// not be used anywhere else.  See its comments for why this exists.
+    ///
+    /// This intrinsic can *only* be called where the argument is a local without
+    /// projections (`read_via_copy(p)`, not `read_via_copy(*p)`) so that it
+    /// trivially obeys runtime-MIR rules about derefs in operands.
+    #[cfg(not(bootstrap))]
+    #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
+    pub fn read_via_copy<T>(p: *const T) -> T;
+
     /// Returns the value of the discriminant for the variant in 'v';
     /// if `T` has no discriminant, returns `0`.
     ///

core/src/iter/traits/iterator.rs

@@ -3721,7 +3721,7 @@ pub trait Iterator {
         }
     }
 
-    /// Determines if the elements of this [`Iterator`] are unequal to those of
+    /// Determines if the elements of this [`Iterator`] are not equal to those of
     /// another.
     ///
     /// # Examples

core/src/panicking.rs

@@ -111,7 +111,7 @@ pub const fn panic(expr: &'static str) -> ! {
     // truncation and padding (even though none is used here). Using
     // Arguments::new_v1 may allow the compiler to omit Formatter::pad from the
     // output binary, saving up to a few kilobytes.
-    panic_fmt(fmt::Arguments::new_v1(&[expr], &[]));
+    panic_fmt(fmt::Arguments::new_const(&[expr]));
 }
 
 /// Like `panic`, but without unwinding and track_caller to reduce the impact on codesize.
@@ -120,7 +120,7 @@ pub const fn panic(expr: &'static str) -> ! {
 #[lang = "panic_nounwind"] // needed by codegen for non-unwinding panics
 #[rustc_nounwind]
 pub fn panic_nounwind(expr: &'static str) -> ! {
-    panic_nounwind_fmt(fmt::Arguments::new_v1(&[expr], &[]));
+    panic_nounwind_fmt(fmt::Arguments::new_const(&[expr]));
 }
 
 #[inline]

core/src/primitive_docs.rs

@@ -1110,7 +1110,7 @@ impl<T: Copy> Copy for (T,) {
 /// - [NaN (not a number)](#associatedconstant.NAN): this value results from
 ///   calculations like `(-1.0).sqrt()`. NaN has some potentially unexpected
 ///   behavior:
-///   - It is unequal to any float, including itself! This is the reason `f32`
+///   - It is not equal to any float, including itself! This is the reason `f32`
 ///     doesn't implement the `Eq` trait.
 ///   - It is also neither smaller nor greater than any float, making it
 ///     impossible to sort by the default comparison operation, which is the

core/src/ptr/mod.rs

@@ -1135,27 +1135,58 @@ pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
 #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 pub const unsafe fn read<T>(src: *const T) -> T {
-    // We are calling the intrinsics directly to avoid function calls in the generated code
-    // as `intrinsics::copy_nonoverlapping` is a wrapper function.
-    extern "rust-intrinsic" {
-        #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
-        fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
-    }
+    // It would be semantically correct to implement this via `copy_nonoverlapping`
+    // and `MaybeUninit`, as was done before PR #109035. Calling `assume_init`
+    // provides enough information to know that this is a typed operation.
 
-    let mut tmp = MaybeUninit::<T>::uninit();
-    // SAFETY: the caller must guarantee that `src` is valid for reads.
-    // `src` cannot overlap `tmp` because `tmp` was just allocated on
-    // the stack as a separate allocated object.
+    // However, as of March 2023 the compiler was not capable of taking advantage
+    // of that information.  Thus the implementation here switched to an intrinsic,
+    // which lowers to `_0 = *src` in MIR, to address a few issues:
     //
-    // Also, since we just wrote a valid value into `tmp`, it is guaranteed
-    // to be properly initialized.
+    // - Using `MaybeUninit::assume_init` after a `copy_nonoverlapping` was not
+    //   turning the untyped copy into a typed load. As such, the generated
+    //   `load` in LLVM didn't get various metadata, such as `!range` (#73258),
+    //   `!nonnull`, and `!noundef`, resulting in poorer optimization.
+    // - Going through the extra local resulted in multiple extra copies, even
+    //   in optimized MIR.  (Ignoring StorageLive/Dead, the intrinsic is one
+    //   MIR statement, while the previous implementation was eight.)  LLVM
+    //   could sometimes optimize them away, but because `read` is at the core
+    //   of so many things, not having them in the first place improves what we
+    //   hand off to the backend.  For example, `mem::replace::<Big>` previously
+    //   emitted 4 `alloca` and 6 `memcpy`s, but is now 1 `alloc` and 3 `memcpy`s.
+    // - In general, this approach keeps us from getting any more bugs (like
+    //   #106369) that boil down to "`read(p)` is worse than `*p`", as this
+    //   makes them look identical to the backend (or other MIR consumers).
+    //
+    // Future enhancements to MIR optimizations might well allow this to return
+    // to the previous implementation, rather than using an intrinsic.
+
+    // SAFETY: the caller must guarantee that `src` is valid for reads.
     unsafe {
         assert_unsafe_precondition!(
             "ptr::read requires that the pointer argument is aligned and non-null",
             [T](src: *const T) => is_aligned_and_not_null(src)
         );
-        copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
-        tmp.assume_init()
+
+        #[cfg(bootstrap)]
+        {
+            // We are calling the intrinsics directly to avoid function calls in the
+            // generated code as `intrinsics::copy_nonoverlapping` is a wrapper function.
+            extern "rust-intrinsic" {
+                #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+                fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
+            }
+
+            // `src` cannot overlap `tmp` because `tmp` was just allocated on
+            // the stack as a separate allocated object.
+            let mut tmp = MaybeUninit::<T>::uninit();
+            copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
+            tmp.assume_init()
+        }
+        #[cfg(not(bootstrap))]
+        {
+            crate::intrinsics::read_via_copy(src)
+        }
     }
 }
 

core/tests/fmt/mod.rs

@@ -22,11 +22,11 @@ fn test_pointer_formats_data_pointer() {
 #[test]
 fn test_estimated_capacity() {
     assert_eq!(format_args!("").estimated_capacity(), 0);
-    assert_eq!(format_args!("{}", "").estimated_capacity(), 0);
+    assert_eq!(format_args!("{}", {""}).estimated_capacity(), 0);
     assert_eq!(format_args!("Hello").estimated_capacity(), 5);
-    assert_eq!(format_args!("Hello, {}!", "").estimated_capacity(), 16);
-    assert_eq!(format_args!("{}, hello!", "World").estimated_capacity(), 0);
-    assert_eq!(format_args!("{}. 16-bytes piece", "World").estimated_capacity(), 32);
+    assert_eq!(format_args!("Hello, {}!", {""}).estimated_capacity(), 16);
+    assert_eq!(format_args!("{}, hello!", {"World"}).estimated_capacity(), 0);
+    assert_eq!(format_args!("{}. 16-bytes piece", {"World"}).estimated_capacity(), 32);
 }
 
 #[test]