Auto merge of #71496 - Dylan-DPC:rollup-gwxejmk, r=Dylan-DPC

Rollup of 6 pull requests

Successful merges:

 - #70845 (Make the `structural_match` error diagnostic for const generics clearer)
 - #71063 (Document unsafety in core::{option, hash})
 - #71068 (Stabilize UNICODE_VERSION (feature unicode_version))
 - #71426 (fix error code in E0751.md)
 - #71459 (Add leading 0x to offset in Debug fmt of Pointer)
 - #71492 (Document unsafety in core::{panicking, alloc::layout, hint, iter::adapters::zip})

Failed merges:

r? @ghost

9 files changed, 63 insertions, 27 deletions

diff --git a/src/libcore/alloc/layout.rs b/src/libcore/alloc/layout.rs
index 6200cd24266..a09c2387d0d 100644
--- a/src/libcore/alloc/layout.rs
+++ b/src/libcore/alloc/layout.rs
@@ -1,5 +1,3 @@
-// ignore-tidy-undocumented-unsafe
-
 use crate::cmp;
 use crate::fmt;
 use crate::mem;
@@ -77,6 +75,8 @@ impl Layout {
             return Err(LayoutErr { private: () });
         }
 
+        // SAFETY: the conditions for `from_size_align_unchecked` have been
+        // checked above.
         unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }
     }
 
@@ -115,7 +115,7 @@ impl Layout {
     #[inline]
     pub const fn new<T>() -> Self {
         let (size, align) = size_align::<T>();
-        // Note that the align is guaranteed by rustc to be a power of two and
+        // SAFETY: the align is guaranteed by Rust to be a power of two and
         // the size+align combo is guaranteed to fit in our address space. As a
         // result use the unchecked constructor here to avoid inserting code
         // that panics if it isn't optimized well enough.
@@ -129,8 +129,8 @@ impl Layout {
     #[inline]
     pub fn for_value<T: ?Sized>(t: &T) -> Self {
         let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
-        // See rationale in `new` for why this is using an unsafe variant below
         debug_assert!(Layout::from_size_align(size, align).is_ok());
+        // SAFETY: see rationale in `new` for why this is using an unsafe variant below
         unsafe { Layout::from_size_align_unchecked(size, align) }
     }
 
@@ -143,7 +143,7 @@ impl Layout {
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
-        // align is non-zero and a power of two
+        // SAFETY: align is guaranteed to be non-zero
         unsafe { NonNull::new_unchecked(self.align() as *mut u8) }
     }
 
@@ -249,11 +249,9 @@ impl Layout {
         let padded_size = self.size() + self.padding_needed_for(self.align());
         let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;
 
-        unsafe {
-            // self.align is already known to be valid and alloc_size has been
-            // padded already.
-            Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
-        }
+        // SAFETY: self.align is already known to be valid and alloc_size has been
+        // padded already.
+        unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) }
     }
 
     /// Creates a layout describing the record for `self` followed by
diff --git a/src/libcore/char/mod.rs b/src/libcore/char/mod.rs
index 715ad8992ad..d82a482e0f1 100644
--- a/src/libcore/char/mod.rs
+++ b/src/libcore/char/mod.rs
@@ -34,9 +34,7 @@ pub use self::convert::ParseCharError;
 pub use self::convert::{from_digit, from_u32};
 #[stable(feature = "decode_utf16", since = "1.9.0")]
 pub use self::decode::{decode_utf16, DecodeUtf16, DecodeUtf16Error};
-
-// unstable re-exports
-#[unstable(feature = "unicode_version", issue = "49726")]
+#[stable(feature = "unicode_version", since = "1.45.0")]
 pub use crate::unicode::UNICODE_VERSION;
 
 use crate::fmt::{self, Write};
diff --git a/src/libcore/hash/mod.rs b/src/libcore/hash/mod.rs
index 2a7fa58dd30..d80101753cb 100644
--- a/src/libcore/hash/mod.rs
+++ b/src/libcore/hash/mod.rs
@@ -79,8 +79,6 @@
 //! }
 //! ```
 
-// ignore-tidy-undocumented-unsafe
-
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::fmt;
@@ -572,6 +570,10 @@ mod impls {
                 fn hash_slice<H: Hasher>(data: &[$ty], state: &mut H) {
                     let newlen = data.len() * mem::size_of::<$ty>();
                     let ptr = data.as_ptr() as *const u8;
+                    // SAFETY: `ptr` is valid and aligned, as this macro is only used
+                    // for numeric primitives which have no padding. The new slice only
+                    // spans across `data` and is never mutated, and its total size is the
+                    // same as the original `data` so it can't be over `isize::MAX`.
                     state.write(unsafe { slice::from_raw_parts(ptr, newlen) })
                 }
             }
@@ -691,6 +693,11 @@ mod impls {
                 state.write_usize(*self as *const () as usize);
             } else {
                 // Fat pointer
+                // SAFETY: we are accessing the memory occupied by `self`
+                // which is guaranteed to be valid.
+                // This assumes a fat pointer can be represented by a `(usize, usize)`,
+                // which is safe to do in `std` because it is shipped and kept in sync
+                // with the implementation of fat pointers in `rustc`.
                 let (a, b) = unsafe { *(self as *const Self as *const (usize, usize)) };
                 state.write_usize(a);
                 state.write_usize(b);
@@ -706,6 +713,11 @@ mod impls {
                 state.write_usize(*self as *const () as usize);
             } else {
                 // Fat pointer
+                // SAFETY: we are accessing the memory occupied by `self`
+                // which is guaranteed to be valid.
+                // This assumes a fat pointer can be represented by a `(usize, usize)`,
+                // which is safe to do in `std` because it is shipped and kept in sync
+                // with the implementation of fat pointers in `rustc`.
                 let (a, b) = unsafe { *(self as *const Self as *const (usize, usize)) };
                 state.write_usize(a);
                 state.write_usize(b);
diff --git a/src/libcore/hash/sip.rs b/src/libcore/hash/sip.rs
index adfbe243512..ac058609f45 100644
--- a/src/libcore/hash/sip.rs
+++ b/src/libcore/hash/sip.rs
@@ -1,7 +1,5 @@
 //! An implementation of SipHash.
 
-// ignore-tidy-undocumented-unsafe
-
 #![allow(deprecated)] // the types in this module are deprecated
 
 use crate::cmp;
@@ -265,6 +263,7 @@ impl<S: Sip> super::Hasher for Hasher<S> {
 
         if self.ntail != 0 {
             needed = 8 - self.ntail;
+            // SAFETY: `cmp::min(length, needed)` is guaranteed to not be over `length`
             self.tail |= unsafe { u8to64_le(msg, 0, cmp::min(length, needed)) } << (8 * self.ntail);
             if length < needed {
                 self.ntail += length;
@@ -279,10 +278,13 @@ impl<S: Sip> super::Hasher for Hasher<S> {
 
         // Buffered tail is now flushed, process new input.
         let len = length - needed;
-        let left = len & 0x7;
+        let left = len & 0x7; // len % 8
 
         let mut i = needed;
         while i < len - left {
+            // SAFETY: because `len - left` is the biggest multiple of 8 under
+            // `len`, and because `i` starts at `needed` where `len` is `length - needed`,
+            // `i + 8` is guaranteed to be less than or equal to `length`.
             let mi = unsafe { load_int_le!(msg, i, u64) };
 
             self.state.v3 ^= mi;
@@ -292,6 +294,9 @@ impl<S: Sip> super::Hasher for Hasher<S> {
             i += 8;
         }
 
+        // SAFETY: `i` is now `needed + len.div_euclid(8) * 8`,
+        // so `i + left` = `needed + len` = `length`, which is by
+        // definition equal to `msg.len()`.
         self.tail = unsafe { u8to64_le(msg, i, left) };
         self.ntail = left;
     }
diff --git a/src/libcore/hint.rs b/src/libcore/hint.rs
index d406b3ce6ef..0d794de5fe8 100644
--- a/src/libcore/hint.rs
+++ b/src/libcore/hint.rs
@@ -2,8 +2,6 @@
 
 //! Hints to compiler that affects how code should be emitted or optimized.
 
-// ignore-tidy-undocumented-unsafe
-
 use crate::intrinsics;
 
 /// Informs the compiler that this point in the code is not reachable, enabling
@@ -68,11 +66,13 @@ pub fn spin_loop() {
     {
         #[cfg(target_arch = "x86")]
         {
+            // SAFETY: the `cfg` attr ensures that we only execute this on x86 targets.
             unsafe { crate::arch::x86::_mm_pause() };
         }
 
         #[cfg(target_arch = "x86_64")]
         {
+            // SAFETY: the `cfg` attr ensures that we only execute this on x86_64 targets.
             unsafe { crate::arch::x86_64::_mm_pause() };
         }
     }
@@ -81,10 +81,13 @@ pub fn spin_loop() {
     {
         #[cfg(target_arch = "aarch64")]
         {
+            // SAFETY: the `cfg` attr ensures that we only execute this on aarch64 targets.
             unsafe { crate::arch::aarch64::__yield() };
         }
         #[cfg(target_arch = "arm")]
         {
+            // SAFETY: the `cfg` attr ensures that we only execute this on arm targets
+            // with support for the v6 feature.
             unsafe { crate::arch::arm::__yield() };
         }
     }
@@ -112,6 +115,8 @@ pub fn black_box<T>(dummy: T) -> T {
     // this. LLVM's interpretation of inline assembly is that it's, well, a black
     // box. This isn't the greatest implementation since it probably deoptimizes
     // more than we want, but it's so far good enough.
+
+    // SAFETY: the inline assembly is a no-op.
     unsafe {
         llvm_asm!("" : : "r"(&dummy));
         dummy
diff --git a/src/libcore/iter/adapters/zip.rs b/src/libcore/iter/adapters/zip.rs
index b13e12e2e86..e83d36a580f 100644
--- a/src/libcore/iter/adapters/zip.rs
+++ b/src/libcore/iter/adapters/zip.rs
@@ -1,5 +1,3 @@
-// ignore-tidy-undocumented-unsafe
-
 use crate::cmp;
 
 use super::super::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator, TrustedLen};
@@ -176,9 +174,11 @@ where
         if self.index < self.len {
             let i = self.index;
             self.index += 1;
+            // SAFETY: `i` is smaller than `self.len`, thus smaller than `self.a.len()` and `self.b.len()`
             unsafe { Some((self.a.get_unchecked(i), self.b.get_unchecked(i))) }
         } else if A::may_have_side_effect() && self.index < self.a.len() {
             // match the base implementation's potential side effects
+            // SAFETY: we just checked that `self.index` < `self.a.len()`
             unsafe {
                 self.a.get_unchecked(self.index);
             }
@@ -203,11 +203,15 @@ where
             let i = self.index;
             self.index += 1;
             if A::may_have_side_effect() {
+                // SAFETY: the usage of `cmp::min` to calculate `delta`
+                // ensures that `end` is smaller than or equal to `self.len`,
+                // so `i` is also smaller than `self.len`.
                 unsafe {
                     self.a.get_unchecked(i);
                 }
             }
             if B::may_have_side_effect() {
+                // SAFETY: same as above.
                 unsafe {
                     self.b.get_unchecked(i);
                 }
@@ -243,6 +247,8 @@ where
         if self.index < self.len {
             self.len -= 1;
             let i = self.len;
+            // SAFETY: `i` is smaller than the previous value of `self.len`,
+            // which is also smaller than or equal to `self.a.len()` and `self.b.len()`
             unsafe { Some((self.a.get_unchecked(i), self.b.get_unchecked(i))) }
         } else {
             None
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index 3aab8b1b333..63a5277100f 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -133,8 +133,6 @@
 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
 //! [`i32`]: ../../std/primitive.i32.html
 
-// ignore-tidy-undocumented-unsafe
-
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
@@ -301,6 +299,8 @@ impl<T> Option<T> {
     #[inline]
     #[stable(feature = "pin", since = "1.33.0")]
     pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
+        // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
+        // which is pinned.
         unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
     }
 
@@ -310,6 +310,8 @@ impl<T> Option<T> {
     #[inline]
     #[stable(feature = "pin", since = "1.33.0")]
     pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
+        // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
+        // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
         unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
     }
 
@@ -858,6 +860,8 @@ impl<T> Option<T> {
 
         match *self {
             Some(ref mut v) => v,
+            // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
+            // variant in the code above.
             None => unsafe { hint::unreachable_unchecked() },
         }
     }
diff --git a/src/libcore/panicking.rs b/src/libcore/panicking.rs
index 3587f3f0ebf..1e4209fd26b 100644
--- a/src/libcore/panicking.rs
+++ b/src/libcore/panicking.rs
@@ -19,8 +19,6 @@
 //! necessary lang items for the compiler. All panics are funneled through this
 //! one function. The actual symbol is declared through the `#[panic_handler]` attribute.
 
-// ignore-tidy-undocumented-unsafe
-
 #![allow(dead_code, missing_docs)]
 #![unstable(
     feature = "core_panic",
@@ -41,6 +39,7 @@ use crate::panic::{Location, PanicInfo};
 #[lang = "panic"] // needed by codegen for panic on overflow and other `Assert` MIR terminators
 pub fn panic(expr: &str) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
+        // SAFETY: the `abort` intrinsic has no requirements to be called.
         unsafe { super::intrinsics::abort() }
     }
 
@@ -63,6 +62,7 @@ pub fn panic(expr: &str) -> ! {
 #[lang = "panic_bounds_check"] // needed by codegen for panic on OOB array/slice access
 fn panic_bounds_check(index: usize, len: usize) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
+        // SAFETY: the `abort` intrinsic has no requirements to be called.
         unsafe { super::intrinsics::abort() }
     }
 
@@ -77,6 +77,7 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
 #[lang = "panic_bounds_check"] // needed by codegen for panic on OOB array/slice access
 fn panic_bounds_check(location: &Location<'_>, index: usize, len: usize) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
+        // SAFETY: the `abort` intrinsic has no requirements to be called.
         unsafe { super::intrinsics::abort() }
     }
 
@@ -93,6 +94,7 @@ fn panic_bounds_check(location: &Location<'_>, index: usize, len: usize) -> ! {
 #[cfg_attr(not(bootstrap), track_caller)]
 pub fn panic_fmt(fmt: fmt::Arguments<'_>, #[cfg(bootstrap)] location: &Location<'_>) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
+        // SAFETY: the `abort` intrinsic has no requirements to be called.
         unsafe { super::intrinsics::abort() }
     }
 
@@ -108,5 +110,6 @@ pub fn panic_fmt(fmt: fmt::Arguments<'_>, #[cfg(bootstrap)] location: &Location<
     #[cfg(not(bootstrap))]
     let pi = PanicInfo::internal_constructor(Some(&fmt), Location::caller());
 
+    // SAFETY: `panic_impl` is defined in safe Rust code and thus is safe to call.
     unsafe { panic_impl(&pi) }
 }
diff --git a/src/libcore/unicode/mod.rs b/src/libcore/unicode/mod.rs
index 3952ae4482e..28c07f77170 100644
--- a/src/libcore/unicode/mod.rs
+++ b/src/libcore/unicode/mod.rs
@@ -7,9 +7,14 @@ mod unicode_data;
 /// The version of [Unicode](http://www.unicode.org/) that the Unicode parts of
 /// `char` and `str` methods are based on.
 ///
+/// New versions of Unicode are released regularly and subsequently all methods
+/// in the standard library depending on Unicode are updated. Therefore the
+/// behavior of some `char` and `str` methods and the value of this constant
+/// changes over time. This is *not* considered to be a breaking change.
+///
 /// The version numbering scheme is explained in
 /// [Unicode 11.0 or later, Section 3.1 Versions of the Unicode Standard](https://www.unicode.org/versions/Unicode11.0.0/ch03.pdf#page=4).
-#[unstable(feature = "unicode_version", issue = "49726")]
+#[stable(feature = "unicode_version", since = "1.45.0")]
 pub const UNICODE_VERSION: (u8, u8, u8) = unicode_data::UNICODE_VERSION;
 
 // For use in liballoc, not re-exported in libstd.