#[rustfmt::skip] requires proc_macro_path_invoc on nightly

[gnzlbg]

My crate uses rust_2018_preview (don't know how relevant this is).

Using tool_attributes I get an error:

error[E0658]: paths of length greater than one in macro invocations are currently unstable (see issue #38356)
  --> src/sealed.rs:28:3
   |
28 | #[rustfmt::skip]
   |   ^^^^^^^^^^^^^
   |
   = help: add #![feature(proc_macro_path_invoc)] to the crate attributes to enable

And if I enable proc_macro_path_invoc I get:

error[E0433]: failed to resolve. Use of undeclared type or module `rustfmt`
  --> src/sealed.rs:28:3
   |
28 | #[rustfmt::skip]
   |   ^^^^^^^ Use of undeclared type or module `rustfmt`

[gnzlbg]

MWE, the following code works unless I uncomment the 2018 preview feature (playground):

// #![feature(rust_2018_preview)]  // Uncomment for ERRORs
#![feature(tool_attributes)]
#![allow(dead_code)]

#[rustfmt::skip]
fn foo(){}
fn main() {}

[gnzlbg]

cc @nrc

[gnzlbg]

Workaround: replace #[rustfmt::skip] with #[cfg_attr(rustfmt, rustfmt_skip)]

[topecongiro]

I think this is an issue due to libsyntax, not rustfmt, as the MWE above does not even compile when the first line is uncommented.

[nrc]

This is a bug in rustc, I'll follow-up tomorrow

[gnzlbg]

Let me know if I should open a bug in rustc or you want to take it from here.

[nrc]

Reported in rust-lang/rust#51940

[nrc]

Should be fixed in rustc now.