Tweak SlicePartialEq to allow MIR-inlining the compare_bytes call

library/core/src/slice/cmp.rs

@@ -4,6 +4,7 @@ use super::{from_raw_parts, memchr};
 use crate::ascii;
 use crate::cmp::{self, BytewiseEq, Ordering};
 use crate::intrinsics::compare_bytes;
+use crate::mem::SizedTypeProperties;
 use crate::num::NonZero;
 use crate::ops::ControlFlow;
 
@@ -15,7 +16,14 @@ where
 {
     #[inline]
     fn eq(&self, other: &[U]) -> bool {
-        SlicePartialEq::equal(self, other)
+        let len = self.len();
+        if len == other.len() {
+            // SAFETY: Just checked that they're the same length, and the pointers
+            // come from references-to-slices so they're guaranteed readable.
+            unsafe { SlicePartialEq::equal_same_length(self.as_ptr(), other.as_ptr(), len) }
+        } else {
+            false
+        }
     }
 }
 
@@ -95,12 +103,14 @@ impl<T: PartialOrd> PartialOrd for [T] {
 // intermediate trait for specialization of slice's PartialEq
 #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
 const trait SlicePartialEq<B> {
-    fn equal(&self, other: &[B]) -> bool;
+    /// # Safety
+    /// `lhs` and `rhs` are both readable for `len` elements
+    unsafe fn equal_same_length(lhs: *const Self, rhs: *const B, len: usize) -> bool;
 }
 
 // Generic slice equality
 #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
-impl<A, B> const SlicePartialEq<B> for [A]
+impl<A, B> const SlicePartialEq<B> for A
 where
     A: [const] PartialEq<B>,
 {
@@ -109,19 +119,15 @@ where
     // such as in `<str as PartialEq>::eq`.
     // The codegen backend can still inline it later if needed.
     #[rustc_no_mir_inline]
-    default fn equal(&self, other: &[B]) -> bool {
-        if self.len() != other.len() {
-            return false;
-        }
-
+    default unsafe fn equal_same_length(lhs: *const Self, rhs: *const B, len: usize) -> bool {
         // Implemented as explicit indexing rather
         // than zipped iterators for performance reasons.
         // See PR https://github.com/rust-lang/rust/pull/116846
-        // FIXME(const_hack): make this a `for idx in 0..self.len()` loop.
+        // FIXME(const_hack): make this a `for idx in 0..len` loop.
         let mut idx = 0;
-        while idx < self.len() {
-            // bound checks are optimized away
-            if self[idx] != other[idx] {
+        while idx < len {
+            // SAFETY: idx < len, so both are in-bounds and readable
+            if unsafe { *lhs.add(idx) != *rhs.add(idx) } {
                 return false;
             }
             idx += 1;
@@ -134,30 +140,18 @@ where
 // When each element can be compared byte-wise, we can compare all the bytes
 // from the whole size in one call to the intrinsics.
 #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
-impl<A, B> const SlicePartialEq<B> for [A]
+impl<A, B> const SlicePartialEq<B> for A
 where
     A: [const] BytewiseEq<B>,
 {
-    // This is usually a pretty good backend inlining candidate because the
-    // intrinsic tends to just be `memcmp`.  However, as of 2025-12 letting
-    // MIR inline this makes reuse worse because it means that, for example,
-    // `String::eq` doesn't inline, whereas by keeping this from inling all
-    // the wrappers until the call to this disappear.  If the heuristics have
-    // changed and this is no longer fruitful, though, please do remove it.
-    // In the mean time, it's fine to not inline it in MIR because the backend
-    // will still inline it if it things it's important to do so.
-    #[rustc_no_mir_inline]
     #[inline]
-    fn equal(&self, other: &[B]) -> bool {
-        if self.len() != other.len() {
-            return false;
-        }
-
-        // SAFETY: `self` and `other` are references and are thus guaranteed to be valid.
-        // The two slices have been checked to have the same size above.
+    unsafe fn equal_same_length(lhs: *const Self, rhs: *const B, len: usize) -> bool {
+        // SAFETY: by our precondition, `lhs` and `rhs` are guaranteed to be valid
+        // for reading `len` values, which also means the size is guaranteed
+        // not to overflow because it exists in memory;
         unsafe {
-            let size = size_of_val(self);
-            compare_bytes(self.as_ptr() as *const u8, other.as_ptr() as *const u8, size) == 0
+            let size = crate::intrinsics::unchecked_mul(len, Self::SIZE);
+            compare_bytes(lhs as _, rhs as _, size) == 0
         }
     }
 }