unused_parens false positive on cfg_attr(..., derive(..., ...))

[behnam]

UNIC builds started failing today on nightly with these errors:

error: unnecessary parentheses around function argument
  --> unic/bidi/src/level.rs:34:49
   |
34 | #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
   |                                                 ^^^^^^^^^^^
   |
note: lint level defined here
  --> unic/bidi/src/lib.rs:14:20
   |
14 | #![deny(bad_style, unused)]
   |                    ^^^^^^
   = note: #[deny(unused_parens)] implied by #[deny(unused)]
help: remove these parentheses
   |
34 | #[cfg_attr(feature = "serde", derive(Serialize, match <u8 as _serde::Deserialize>::deserialize(__e) {
35 |      ::result::Result::Ok(val) => val,
36 |      ::result::Result::Err(err) => {
37 |          return ::result::Result::Err(::convert::From::from(err))
38 |      }
39 |  }))]
   |
error: unnecessary parentheses around function argument
  --> unic/bidi/src/level.rs:34:49
   |
34 | #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
   |                                                 ^^^^^^^^^^^ help: remove these parentheses
error: aborting due to 2 previous errors

(from https://travis-ci.org/behnam/rust-unic/jobs/351099208)

Problems:

• the cfg_attr lines do not have any unused parentheses, so it's a false positive;
• the location the error message is pointing to is misleading, as there's no parentheses in the span it's pointing to; and
• there are two different error reports here, both pointing to the same symptom, but with different looks.

[zackmdavis]

Possibly-relevant context: the unnecessary-parens lint was extended to arguments in #46980, and neutered for arguments in nested macros in #47896. But if this started failing today, something relevant must have changed elsewhere to reveal this bug.

[ExpHP]

Or it could be a change in serde_derive. This recent commit looks interesting, but it actually removes parentheses instead of adding them. What happens if you try serde on git master?

BTW, I tried building unic locally, and was unable to reproduce this with unic@9466e05e, rustc nightly-2018-03-07, and any of serde_derive versions 1.0.28–1.0.30 (the latest).

[nikomatsakis]

Hello from the @rust-lang/compiler triage meeting.

@benham can you verify if you are able to reproduce? If so, maybe give a specific commit and the output from rustc -vV? It'd be great if we can isolate down what has changed.

[nikomatsakis]

triage: P-high

(But no assignment until we can reproduce.)

[behnam]

Thanks, @nikomatsakis, for the follow up!

Looks at the UNIC build history, apparently it was only failing for one day, on nightly, and got fixed. The version for that nightly build was rustc 1.26.0-nightly (2789b067d 2018-03-06).

I reported the issue right away to help with triage. Since it hasn't been repro'ed for the past 9 days, please feel free to close it.

[nikomatsakis]

Thanks!