Auto merge of #97027 - cuviper:yesalias-refcell, r=thomcc

Use pointers in `cell::{Ref,RefMut}` to avoid `noalias`

When `Ref` and `RefMut` were based on references, they would get LLVM `noalias` attributes that were incorrect, because that alias guarantee is only true until the guard drops. A `&RefCell` on the same value can get a new borrow that aliases the previous guard, possibly leading to miscompilation. Using `NonNull` pointers in `Ref` and `RefCell` avoids `noalias`.

Fixes the library side of #63787, but we still might want to explore language solutions there.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/test/codegen/noalias-refcell.rs b/src/test/codegen/noalias-refcell.rs
new file mode 100644
index 00000000000..dba73937abf
--- /dev/null
+++ b/src/test/codegen/noalias-refcell.rs
@@ -0,0 +1,14 @@
+// compile-flags: -O -C no-prepopulate-passes -Z mutable-noalias=yes
+
+#![crate_type = "lib"]
+
+use std::cell::{Ref, RefCell, RefMut};
+
+// Make sure that none of the arguments get a `noalias` attribute, because
+// the `RefCell` might alias writes after either `Ref`/`RefMut` is dropped.
+
+// CHECK-LABEL: @maybe_aliased(
+// CHECK-NOT: noalias
+// CHECK-SAME: %_refcell
+#[no_mangle]
+pub unsafe fn maybe_aliased(_: Ref<'_, i32>, _: RefMut<'_, i32>, _refcell: &RefCell<i32>) {}