Auto merge of #79804 - tmiasko:improper-ctypes-no-niche, r=pnkfelix

Types with a hidden niche are not known to be non-null

Fixes #79787.

1 files changed, 7 insertions, 1 deletions

diff --git a/compiler/rustc_lint/src/types.rs b/compiler/rustc_lint/src/types.rs
index 784b36c2837..af0f5565572 100644
--- a/compiler/rustc_lint/src/types.rs
+++ b/compiler/rustc_lint/src/types.rs
@@ -672,7 +672,7 @@ pub fn transparent_newtype_field<'a, 'tcx>(
 }
 
 /// Is type known to be non-null?
-crate fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKind) -> bool {
+fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKind) -> bool {
     let tcx = cx.tcx;
     match ty.kind() {
         ty::FnPtr(_) => true,
@@ -685,6 +685,12 @@ crate fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: C
                 return true;
             }
 
+            // Types with a `#[repr(no_niche)]` attribute have their niche hidden.
+            // The attribute is used by the UnsafeCell for example (the only use so far).
+            if def.repr.hide_niche() {
+                return false;
+            }
+
             for variant in &def.variants {
                 if let Some(field) = transparent_newtype_field(cx.tcx, variant) {
                     if ty_is_known_nonnull(cx, field.ty(tcx, substs), mode) {