#![feature(proc_macro)] breaks some custom derive attributes

[alexcrichton]

Originally reported as rust-lang-nursery/rustfix#126 this is reduced to:

// foo.rs
#![crate_type = "proc-macro"]

extern crate proc_macro;

use proc_macro::TokenStream;

#[proc_macro_derive(Foo)]
pub fn foo(a: TokenStream) -> TokenStream {
    "".parse().unwrap()
}

#[proc_macro_derive(Bar, attributes(custom))]
pub fn bar(a: TokenStream) -> TokenStream {
    "".parse().unwrap()
}

// bar.rs
#![cfg_attr(broken, feature(proc_macro))]
#![crate_type = "lib"]

#[macro_use]
extern crate foo;

#[derive(Foo, Bar, Debug)]
#[custom = "test"]
pub enum A{
    B,
    C,
}

compiles with:

$ rustc +nightly foo.rs
$ rustc +nightly bar.rs -L .
$ rustc +nightly bar.rs -L . --cfg broken
error: cannot find attribute macro `custom` in this scope
 --> bar.rs:8:3
  |
8 | #[custom = "test"]
  |   ^^^^^^

error: aborting due to previous error

[alexcrichton]

cc @petrochenkov, I'm not really sure what this classifies under, it seems like maybe #50911 although #![feature(use_extern_macros)] doesn't break it, only #![feature(proc_macro)] does. After #52081 though it may be broken by use_extern_macros (I'm not 100% sure where this is happening)

[petrochenkov]

I'm not really sure what this classifies under, it seems like maybe #50911 although #![feature(use_extern_macros)] doesn't break it

This falls under "attribute resolution is mess" and "some things are gated by proc_macro while they should be gated by macro modularization (use_extern_macros)".

[cramertj]

I hit this error last week when trying to update a crate that used failure's derive macros. I found that useing the macro would give me the attribute error, but #[macro_use]ing the crate worked.

[alexcrichton]

@cramertj I think that may be a separate bug, can you file a different issue?

I believe I've fixed this issue in #52230