Rollup merge of #120107 - shepmaster:dead-code-repr-transparent, r=Nilstrieb

dead_code treats #[repr(transparent)] the same as #[repr(C)]

In #92972 we enabled linting on unused fields in tuple structs. In #118297 that lint was enabled by default. That exposed issues like #119659, where the fields of a struct marked `#[repr(transparent)]` were reported by the `dead_code` lint. The language team [decided](rust-lang/rust#119659 (comment)) that the lint should treat `repr(transparent)` the same as `#[repr(C)]`.

Fixes #119659

Author: matthiaskrgr

tests/fail/issue-miri-1112.rs

@@ -1,7 +1,7 @@
 trait Empty {}
 
 #[repr(transparent)]
-pub struct FunnyPointer(#[allow(dead_code)] dyn Empty);
+pub struct FunnyPointer(dyn Empty);
 
 #[repr(C)]
 pub struct Meta {

tests/fail/unaligned_pointers/drop_in_place.rs

@@ -1,7 +1,7 @@
 //@compile-flags: -Cdebug-assertions=no
 
 #[repr(transparent)]
-struct HasDrop(#[allow(dead_code)] u8);
+struct HasDrop(u8);
 
 impl Drop for HasDrop {
     fn drop(&mut self) {}