From e7cad62257e5b0d59ca18515a522e90924177330 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 2 Jan 2023 15:11:19 +0100
Subject: [PATCH] do not add noalias in return position

---
 compiler/rustc_ty_utils/src/abi.rs     | 6 ++++--
 src/test/codegen/function-arguments.rs | 2 +-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs
index d644cbccea11b..73d2d278f93f3 100644
--- a/compiler/rustc_ty_utils/src/abi.rs
+++ b/compiler/rustc_ty_utils/src/abi.rs
@@ -273,9 +273,11 @@ fn adjust_for_rust_scalar<'tcx>(
                 | PointerKind::UniqueBorrowed
                 | PointerKind::UniqueBorrowedPinned => false,
                 PointerKind::UniqueOwned => noalias_for_box,
-                PointerKind::Frozen => !is_return,
+                PointerKind::Frozen => true,
             };
-            if no_alias {
+            // We can never add `noalias` in return position; that LLVM attribute has some very surprising semantics
+            // (see <https://github.com/rust-lang/unsafe-code-guidelines/issues/385#issuecomment-1368055745>).
+            if no_alias && !is_return {
                 attrs.set(ArgAttribute::NoAlias);
             }
 
diff --git a/src/test/codegen/function-arguments.rs b/src/test/codegen/function-arguments.rs
index 44fee952307f2..0f9e90f6ba779 100644
--- a/src/test/codegen/function-arguments.rs
+++ b/src/test/codegen/function-arguments.rs
@@ -145,7 +145,7 @@ pub fn raw_struct(_: *const S) {
 
 // `Box` can get deallocated during execution of the function, so it should
 // not get `dereferenceable`.
-// CHECK: noalias noundef nonnull align 4 {{i32\*|ptr}} @_box({{i32\*|ptr}} noalias noundef nonnull align 4 %x)
+// CHECK: noundef nonnull align 4 {{i32\*|ptr}} @_box({{i32\*|ptr}} noalias noundef nonnull align 4 %x)
 #[no_mangle]
 pub fn _box(x: Box<i32>) -> Box<i32> {
   x