Also emit noundef for #[repr(transparent)] structs/enums containi…

…ng array immediate eligble arguments

compiler/rustc_ty_utils/src/abi.rs

@@ -786,8 +786,19 @@ fn fn_abi_adjust_for_abi<'tcx>(
                 arg.cast_to(Reg { kind: RegKind::Integer, size });
 
                 // Let's see if we can add a `noundef`. This is only legal for arrays, definitely
-                // not for unions.
-                if arg.layout.ty.is_array() {
+                // not for unions. This is also legal for `#[repr(transparent)] struct` or
+                // `#[repr(transparent)] enum` containing array.
+                let is_transparent_array =
+                    if arg.layout.is_transparent::<LayoutCx<'tcx, TyCtxt<'tcx>>>()
+                        && let Some((_, layout)) = arg.layout.non_1zst_field(cx)
+                        && layout.ty.is_array()
+                    {
+                        true
+                    } else {
+                        false
+                    };
+
+                if arg.layout.ty.is_array() || is_transparent_array {
                     // Fixup arg attribute with `noundef`.
                     let PassMode::Cast { ref mut cast, .. } = &mut arg.mode else {
                         bug!("this cannot fail because of the previous cast_to `Reg`");

tests/codegen/array-immediate-param-noundef.rs

@@ -73,3 +73,23 @@ pub fn replace_short_array_u8x3(r: &mut [u8; 3], v: [u8; 3]) -> [u8; 3] {
 pub fn replace_short_array_u8x8(r: &mut [u8; 8], v: [u8; 8]) -> [u8; 8] {
     std::mem::replace(r, v)
 }
+
+#[repr(transparent)]
+pub struct Foo([u8; 4]);
+
+// CHECK: define noundef i32 @replace_repr_transparent_struct_short_array(ptr noalias noundef align 1 dereferenceable(4) %r, i32 noundef %0)
+#[no_mangle]
+pub fn replace_repr_transparent_struct_short_array(r: &mut Foo, v: Foo) -> Foo {
+    std::mem::replace(r, v)
+}
+
+#[repr(transparent)]
+pub enum Bar {
+    Default([u8; 4])
+}
+
+// CHECK: define noundef i32 @replace_repr_transparent_enum_short_array(ptr noalias noundef align 1 dereferenceable(4) %r, i32 noundef %0)
+#[no_mangle]
+pub fn replace_repr_transparent_enum_short_array(r: &mut Bar, v: Bar) -> Bar {
+    std::mem::replace(r, v)
+}