Stop emitting UbChecks on every Vec→Slice

library/alloc/src/vec/mod.rs

@@ -1747,7 +1747,11 @@ impl<T, A: Allocator> Vec<T, A> {
         // * We only construct `&mut` references to `self.buf` through `&mut self` methods; borrow-
         //   check ensures that it is not possible to mutably alias `self.buf` within the
         //   returned lifetime.
-        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
+        unsafe {
+            // normally this would use `slice::from_raw_parts`, but it's
+            // instantiated often enough that avoiding the UB check is worth it
+            &*core::intrinsics::aggregate_raw_ptr::<*const [T], _, _>(self.as_ptr(), self.len)
+        }
     }
 
     /// Extracts a mutable slice of the entire vector.
@@ -1779,7 +1783,11 @@ impl<T, A: Allocator> Vec<T, A> {
         // * We only construct references to `self.buf` through `&self` and `&mut self` methods;
         //   borrow-check ensures that it is not possible to construct a reference to `self.buf`
         //   within the returned lifetime.
-        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
+        unsafe {
+            // normally this would use `slice::from_raw_parts_mut`, but it's
+            // instantiated often enough that avoiding the UB check is worth it
+            &mut *core::intrinsics::aggregate_raw_ptr::<*mut [T], _, _>(self.as_mut_ptr(), self.len)
+        }
     }
 
     /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer

library/core/src/slice/cmp.rs

@@ -13,13 +13,10 @@ impl<T, U> const PartialEq<[U]> for [T]
 where
     T: [const] PartialEq<U>,
 {
+    #[inline]
     fn eq(&self, other: &[U]) -> bool {
         SlicePartialEq::equal(self, other)
     }
-
-    fn ne(&self, other: &[U]) -> bool {
-        SlicePartialEq::not_equal(self, other)
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -99,10 +96,6 @@ impl<T: PartialOrd> PartialOrd for [T] {
 #[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
 const trait SlicePartialEq<B> {
     fn equal(&self, other: &[B]) -> bool;
-
-    fn not_equal(&self, other: &[B]) -> bool {
-        !self.equal(other)
-    }
 }
 
 // Generic slice equality
@@ -111,6 +104,11 @@ impl<A, B> const SlicePartialEq<B> for [A]
 where
     A: [const] PartialEq<B>,
 {
+    // It's not worth trying to inline the loops underneath here *in MIR*,
+    // and preventing it encourages more useful inlining upstream,
+    // 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;
@@ -140,6 +138,16 @@ 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;

tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-abort.diff

@@ -26,12 +26,12 @@
                   scope 4 {
                       debug _x => _8;
                   }
-                  scope 18 (inlined foo) {
+                  scope 19 (inlined foo) {
                       let mut _27: *const [()];
                   }
               }
-              scope 16 (inlined slice_from_raw_parts::<()>) {
-                  scope 17 (inlined std::ptr::from_raw_parts::<[()], ()>) {
+              scope 17 (inlined slice_from_raw_parts::<()>) {
+                  scope 18 (inlined std::ptr::from_raw_parts::<[()], ()>) {
                   }
               }
           }
@@ -49,19 +49,21 @@
               scope 7 {
                   let _21: std::ptr::NonNull<[u8]>;
                   scope 8 {
-                      scope 11 (inlined NonNull::<[u8]>::as_mut_ptr) {
-                          scope 12 (inlined NonNull::<[u8]>::as_non_null_ptr) {
-                              scope 13 (inlined NonNull::<[u8]>::cast::<u8>) {
+                      scope 12 (inlined NonNull::<[u8]>::as_mut_ptr) {
+                          scope 13 (inlined NonNull::<[u8]>::as_non_null_ptr) {
+                              scope 14 (inlined NonNull::<[u8]>::cast::<u8>) {
                                   let mut _25: *mut [u8];
-                                  scope 14 (inlined NonNull::<[u8]>::as_ptr) {
+                                  scope 15 (inlined NonNull::<[u8]>::as_ptr) {
                                   }
                               }
                           }
-                          scope 15 (inlined NonNull::<u8>::as_ptr) {
+                          scope 16 (inlined NonNull::<u8>::as_ptr) {
                           }
                       }
                   }
                   scope 10 (inlined <std::alloc::Global as Allocator>::allocate) {
+                      scope 11 (inlined std::alloc::Global::alloc_impl) {
+                      }
                   }
               }
               scope 9 (inlined #[track_caller] Layout::from_size_align_unchecked) {
@@ -192,8 +194,8 @@
 +         _18 = const Layout {{ size: 0_usize, align: std::ptr::Alignment(std::ptr::alignment::AlignmentEnum::_Align1Shl0) }};
           StorageDead(_24);
           StorageLive(_19);
--         _19 = std::alloc::Global::alloc_impl(const alloc::alloc::exchange_malloc::promoted[0], copy _18, const false) -> [return: bb7, unwind unreachable];
-+         _19 = std::alloc::Global::alloc_impl(const alloc::alloc::exchange_malloc::promoted[0], const Layout {{ size: 0_usize, align: std::ptr::Alignment(std::ptr::alignment::AlignmentEnum::_Align1Shl0) }}, const false) -> [return: bb7, unwind unreachable];
+-         _19 = std::alloc::Global::alloc_impl_runtime(copy _18, const false) -> [return: bb7, unwind unreachable];
++         _19 = std::alloc::Global::alloc_impl_runtime(const Layout {{ size: 0_usize, align: std::ptr::Alignment(std::ptr::alignment::AlignmentEnum::_Align1Shl0) }}, const false) -> [return: bb7, unwind unreachable];
       }
 
       bb7: {