compiler: Forward attributes to eii-expanded macros

[Enselic]

Since #150592 is quite complicated to reason about I figured it would be good to split it up in smaller pieces that are easier to digest. Here is the attribute fix in isolation.

The Problem

With this eii in library/std/src/io/mod.rs:

/// Foo
#[eii(on_broken_pipe)]
#[unstable(feature = "on_broken_pipe", issue = "150588")]
pub fn on_broken_pipe() -> OnBrokenPipe {
    OnBrokenPipe::BackwardsCompatible
}

you currently get this compilation error:

error: attribute macro has missing stability attribute
    --> library/std/src/io/mod.rs:2269:1
     |
2269 | #[eii(on_broken_pipe)]
     | ^^^^^^^^^^^^^^^^^^^^--
     | |
     | in this attribute macro expansion
     |
    ::: library/core/src/macros/mod.rs:1899:5
     |
1899 |     pub macro eii($item:item) {
     |     ------------- in this expansion of `#[eii]`

because with MAGIC_EXTRA_RUSTFLAGS=-Zunpretty=expanded ./x build library/std we can see that a pub item in the expanded code is indeed missing that attribute:

const _: () =
    {
        #[on_broken_pipe]
        fn on_broken_pipe() -> OnBrokenPipe {
            OnBrokenPipe::BackwardsCompatible
        }
    };
unsafe extern "Rust" {
    /// Foo
    #[unstable(feature = "on_broken_pipe", issue = "150588")]
    #[rustc_eii_extern_item]
    pub safe fn on_broken_pipe()
    -> OnBrokenPipe;
}
#[rustc_builtin_macro(eii_shared_macro)]
#[eii_extern_target(on_broken_pipe)]
pub macro on_broken_pipe { () => {} }

The Solution

With the fix, that error goes away because we get this expanded code instead:

const _: () =
    {
        #[on_broken_pipe]
        fn on_broken_pipe() -> OnBrokenPipe {
            OnBrokenPipe::BackwardsCompatible
        }
    };
unsafe extern "Rust" {
    /// Foo
    #[unstable(feature = "on_broken_pipe", issue = "150588")]
    #[rustc_eii_extern_item]
    pub safe fn on_broken_pipe()
    -> OnBrokenPipe;
}
/// Foo
#[unstable(feature = "on_broken_pipe", issue = "150588")]
#[rustc_builtin_macro(eii_shared_macro)]
#[eii_extern_target(on_broken_pipe)]
pub macro on_broken_pipe { () => {} }

Note that we also need to forward the docs, otherwise get get (fatal) warnings like these:

warning: missing documentation for an attribute macro
    --> library/std/src/io/mod.rs:2269:1

r? @jdonszelmann

Tracking issues:

• Tracking Issue for externally implementable items #125418
• Tracking Issue for Externally Implementable Item #[std::io::on_broken_pipe] #150588

What about a test?

#150591 will prevent regressions once it lands since it does not build without this fix. I think it is overkill to add a temporary eii to std before that.

[jdonszelmann]

@bors r+ rollup

[rust-bors]

📌 Commit a0df7b2 has been approved by jdonszelmann

It is now in the queue for this repository.