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.

2 files changed, 42 insertions, 1 deletions

diff --git a/tests/codegen/drop-in-place-noalias.rs b/tests/codegen/drop-in-place-noalias.rs
new file mode 100644
index 00000000000..725e6fc048d
--- /dev/null
+++ b/tests/codegen/drop-in-place-noalias.rs
@@ -0,0 +1,38 @@
+// compile-flags: -O -C no-prepopulate-passes
+
+// Tests that the compiler can apply `noalias` and other &mut attributes to `drop_in_place`.
+// Note that non-Unpin types should not get `noalias`, matching &mut behavior.
+
+#![crate_type="lib"]
+
+use std::marker::PhantomPinned;
+
+// CHECK: define internal void @{{.*}}core{{.*}}ptr{{.*}}drop_in_place{{.*}}StructUnpin{{.*}}({{.*\*|ptr}} noalias noundef align 4 dereferenceable(12) %{{.+}})
+
+// CHECK: define internal void @{{.*}}core{{.*}}ptr{{.*}}drop_in_place{{.*}}StructNotUnpin{{.*}}({{.*\*|ptr}} noundef nonnull align 4 %{{.+}})
+
+pub struct StructUnpin {
+    a: i32,
+    b: i32,
+    c: i32,
+}
+
+impl Drop for StructUnpin {
+    fn drop(&mut self) {}
+}
+
+pub struct StructNotUnpin {
+    a: i32,
+    b: i32,
+    c: i32,
+    p: PhantomPinned,
+}
+
+impl Drop for StructNotUnpin {
+    fn drop(&mut self) {}
+}
+
+pub unsafe fn main(x: StructUnpin, y: StructNotUnpin) {
+    drop(x);
+    drop(y);
+}
diff --git a/tests/codegen/noalias-box-off.rs b/tests/codegen/noalias-box-off.rs
index afd17c7c160..c82c53b2a48 100644
--- a/tests/codegen/noalias-box-off.rs
+++ b/tests/codegen/noalias-box-off.rs
@@ -4,5 +4,8 @@
 
 // CHECK-LABEL: @box_should_not_have_noalias_if_disabled(
 // CHECK-NOT: noalias
+// CHECK-SAME: %foo)
 #[no_mangle]
-pub fn box_should_not_have_noalias_if_disabled(_b: Box<u8>) {}
+pub fn box_should_not_have_noalias_if_disabled(foo: Box<u8>) {
+    drop(foo);
+}