CoercePointee doesn't tightly enforce the restriction to #[repr(transparent)]-only wrappers, if other proc-macros are involved

[steffahn]

Define a proc macro that strips the #[repr(transparent)].

#[proc_macro_attribute]
pub fn untransparent(_: TokenStream, item: TokenStream) -> TokenStream {
    let mut i = item.into_iter();
    drop(i.next());
    drop(i.next());
    i.collect()
}

use it

#![feature(derive_coerce_pointee)]
use std::marker::CoercePointee;
use …:untransparent;

#[derive(CoercePointee)]
#[untransparent]
#[repr(transparent)]
struct Foo<T: ?Sized>(Box<T>);

---

compiles successfully, expands to

#![feature(prelude_import)]
#![feature(derive_coerce_pointee)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use std::marker::CoercePointee;
use …::untransparent;
struct Foo<T: ?Sized>(Box<T>);
#[automatically_derived]
impl<
    T: ?Sized + ::core::marker::Unsize<__S>,
    __S: ?Sized,
> ::core::ops::DispatchFromDyn<Foo<__S>> for Foo<T> {}
#[automatically_derived]
impl<
    T: ?Sized + ::core::marker::Unsize<__S>,
    __S: ?Sized,
> ::core::ops::CoerceUnsized<Foo<__S>> for Foo<T> {}

As far as I understand the RFC and intent, the intended stabilization should only stabilize allowing these (CoerceUnsized & DispatchFromDyn impls) to be generated for #[repr(transparent)] types.

@rustbot label F-derive_coerce_pointee