Auto merge of #111807 - erikdesjardins:noalias, r=oli-obk

[rustc_ty_utils] Treat `drop_in_place`'s *mut argument like &mut when adding LLVM attributes

This resurrects PR #103614, which has sat idle for a while.

This could probably use a new perf run, since we're on a new LLVM version now.

r? `@oli-obk`
cc `@RalfJung`

---

LLVM can make use of the `noalias` parameter attribute on the parameter to `drop_in_place` in areas like argument promotion. Because the Rust compiler fully controls the code for `drop_in_place`, it can soundly deduce parameter attributes on it.

In #103957, Miri was changed to retag `drop_in_place`'s argument as if it was `&mut`, matching this change.

Author: bors

core/src/ptr/mod.rs

@@ -441,19 +441,25 @@ mod mut_ptr;
 ///
 /// * `to_drop` must be [valid] for both reads and writes.
 ///
-/// * `to_drop` must be properly aligned.
+/// * `to_drop` must be properly aligned, even if `T` has size 0.
 ///
-/// * The value `to_drop` points to must be valid for dropping, which may mean it must uphold
-///   additional invariants - this is type-dependent.
+/// * `to_drop` must be nonnull, even if `T` has size 0.
+///
+/// * The value `to_drop` points to must be valid for dropping, which may mean
+///   it must uphold additional invariants. These invariants depend on the type
+///   of the value being dropped. For instance, when dropping a Box, the box's
+///   pointer to the heap must be valid.
+///
+/// * While `drop_in_place` is executing, the only way to access parts of
+///   `to_drop` is through the `&mut self` references supplied to the
+///   `Drop::drop` methods that `drop_in_place` invokes.
 ///
 /// Additionally, if `T` is not [`Copy`], using the pointed-to value after
 /// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
 /// foo` counts as a use because it will cause the value to be dropped
 /// again. [`write()`] can be used to overwrite data without causing it to be
 /// dropped.
 ///
-/// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
-///
 /// [valid]: self#safety
 ///
 /// # Examples