Macro-generated macro can no longer be called

[dtolnay]

The following crate no longer compiles as of the most recent nightly. The relevant commit range is f7af19c...bdfd698. Mentioning @petrochenkov @matthewjasper because #63535 sounds relevant – is this an intentional change?

Cargo.toml

[package]
name = "repro"
version = "0.0.0"
edition = "2018"
publish = false

[lib]
proc-macro = true

src/lib.rs

extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_derive(repro)]
pub fn d(_input: TokenStream) -> TokenStream {
    "macro_rules! m {()=>{}}".parse().unwrap()
}

src/main.rs

#[derive(repro::repro)]
pub struct S;

m!();

fn main() {}

---

$ cargo +stable check
   Compiling repro v0.0.0
    Finished dev [unoptimized + debuginfo] target(s) in 0.18s

$ cargo +nightly-2019-08-16 check
   Compiling repro v0.0.0
    Finished dev [unoptimized + debuginfo] target(s) in 0.18s

$ cargo +nightly-2019-08-17 check
   Compiling repro v0.0.0
error: cannot find macro `m!` in this scope
 --> src/main.rs:4:1
  |
4 | m!();
  | ^
  |
  = help: have you added the `#[macro_use]` on the module/import?

[petrochenkov]

I'll investigate.

[petrochenkov]

Fixed in #63667.