Failed to automatically apply fixes to a procedural macro crate

[dtolnay]

Cargo.toml

[package]
name = "procedural"
version = "0.0.1"
authors = ["David Tolnay <dtolnay@gmail.com>"]
edition = "2018"

[lib]
proc-macro = true

src/lib.rs

extern crate proc_macro;
use self::proc_macro::TokenStream;

#[proc_macro]
pub fn demo(_input: TokenStream) -> TokenStream {
    TokenStream::new()
}

---

Running cargo fix --edition-idioms I get:

warning: failed to automatically apply fixes suggested by rustc to crate `procedural`

after fixes were automatically applied the compiler reported errors within these files:

  * src/lib.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see 
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/cargo/issues
quoting the full output of this command we'd be very appreciative!

warning: `extern crate` is not idiomatic in the new edition
 --> src/lib.rs:1:1
  |
1 | extern crate proc_macro;
  | ^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
  |
  = note: `-W unused-extern-crates` implied by `-W rust-2018-idioms`

warning: `extern crate` is not idiomatic in the new edition
 --> src/lib.rs:1:1
  |
1 | extern crate proc_macro;
  | ^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
  |
  = note: `-W unused-extern-crates` implied by `-W rust-2018-idioms`

[ehuss]

There have been a few other reports of people having problems with the extern prelude changes. See also rust-lang/rust#54381 rust-lang/rust#53964. The original PR mentioned something about cargo growing support for --extern crate_name, but I haven't been able to find any detail about a plan on how this is supposed to work.

[alexcrichton]

@eddyb sorry I may be confused, but it looks like rust-lang/rust#54116 was merged with no recourse for Cargo? Cargo can't pass --extern proc_macro because it's an unstable flag, and the edition is supposed to be in theory stable on beta?

[alexcrichton]

The fix for this issue is that Cargo isn't passing --extern proc_macro, which in turn requires -Z unstable-options, which in turn means that this is simply a bug in the edition that will require more work than a patch to Cargo. We'll wait to see what the pings on the various issues come up with.

[alexcrichton]

@eddyb mentions on IRC that the intention was for the lint to not fire on extern crate proc_macro;, basically continuing to consider that annotation "idiomatic for now". @eddyb will work on a patch to rustc to skip linting about extern crate proc_macro (specifically just that one crate) and that will in turn fix this issue.

[eddyb]

I believe we're tracking this as rust-lang/rust#54381, right?

[alexcrichton]

@eddyb I think both can be fixed in the same way, yeah, so I'd be fine tracking this there too

[ehuss]

This appears to be fixed in the latest nightly via rust-lang/rust#54650.