Improve comments

Author: Noratrieb

compiler/rustc_ty_utils/src/abi.rs

@@ -219,7 +219,6 @@ fn adjust_for_rust_scalar<'tcx>(
         return;
     }
 
-    // Scalars which have invalid values cannot be undef.
     if !scalar.is_uninit_valid() {
         attrs.set(ArgAttribute::NoUndef);
     }
@@ -246,11 +245,6 @@ fn adjust_for_rust_scalar<'tcx>(
                 PointerKind::SharedMutable | PointerKind::UniqueOwned => Size::ZERO,
             };
 
-            // `Box`, `&T`, and `&mut T` cannot be undef.
-            // Note that this only applies to the value of the pointer itself;
-            // this attribute doesn't make it UB for the pointed-to data to be undef.
-            attrs.set(ArgAttribute::NoUndef);
-
             // The aliasing rules for `Box<T>` are still not decided, but currently we emit
             // `noalias` for it. This can be turned off using an unstable flag.
             // See https://github.com/rust-lang/unsafe-code-guidelines/issues/326

tests/codegen/loads.rs

@@ -50,7 +50,7 @@ pub fn load_scalar_pair<'a>(x: &(&'a i32, &'a Align16)) -> (&'a i32, &'a Align16
 // CHECK-LABEL: @load_raw_pointer
 #[no_mangle]
 pub fn load_raw_pointer<'a>(x: &*const i32) -> *const i32 {
-    // loaded raw pointer should not have !nonnull, !align, or !noundef metadata
+    // loaded raw pointer should not have !nonnull or !align metadata
     // CHECK: load {{i32\*|ptr}}, {{i32\*\*|ptr}} %x, align [[PTR_ALIGNMENT]], !noundef !2{{$}}
     *x
 }