Fix warnings when #[pin_project] used within macro_rules! macros

[taiki-e]

When #[pin_project] attribute used within macro_rules! macros, the generated code receives warnings.

#![warn(missing_debug_implementations)]

#[pin_project::pin_project] // <- no warning
#[derive(Debug)]
pub struct A {
    #[pin]
    _f: (),
}

macro_rules! mac {
    () => {
        #[pin_project::pin_project] //~WARN type does not implement `Debug`
        #[derive(Debug)]
        pub struct B {
            #[pin]
            _f: (),
        }
    };
}

mac!();

playground

Refs: rust-lang/rust#77973

[Aaron1011]

The missing_debug_implementations warning seems like a rustc bug.

[taiki-e]

@Aaron1011 I agree. I'll file an issue on rust repo.

[taiki-e]

Filed rust-lang/rust#77973

[Aaron1011]

Actually, it looks like the warning is actually being emitted for the generated projection struct (__B<'pin>) both with and without the wrapping macro_rules!. Due to the span at pin-project uses, the lint appears to be for the main struct.

However, it looks like the lint normally get suppresed here: https://github.com/rust-lang/rust/blob/7f587168102498a488abf608a86c7fdfa62fb7bb/compiler/rustc_middle/src/lint.rs#L245-L260, due to it being emitted from the external pin-project macro.

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci