Rollup merge of #118383 - shepmaster:unused-tuple-struct-field-cleanup-stdlib, r=m-ou-se

Address unused tuple struct fields in the standard library

library/alloc/src/boxed/thin.rs

@@ -171,6 +171,7 @@ fn with_header(&self) -> &WithHeader<<T as Pointee>::Metadata> {
 /// An opaque representation of `WithHeader<H>` to avoid the
 /// projection invariance of `<T as Pointee>::Metadata`.
 #[repr(transparent)]
+#[allow(unused_tuple_struct_fields)] // Field only used through `WithHeader` type above.
 struct WithOpaqueHeader(NonNull<u8>);
 
 impl WithOpaqueHeader {