Clippy warns twice about unknown lints

[jyn514]

I tried this code:

#[warn(x)]
fn main() {}

I expected to see this happen: Clippy warns once that x is an unknown lint.

Instead, this happened: Clippy warns twice.

warning: unknown lint: `x`
 --> src/main.rs:1:8
  |
1 | #[warn(x)]
  |        ^
  |
  = note: `#[warn(unknown_lints)]` on by default

warning: unknown lint: `x`
 --> src/main.rs:1:8
  |
1 | #[warn(x)]
  |        ^

warning: 2 warnings emitted

Meta

• cargo clippy -V: 0.1.51 (2021-01-16 8a65184)
• rustc -V: 1.49.0

See also rust-lang/rust#80524 (comment) - this looks unrelated to the UNKNOWN_CLIPPY_LINTS pass because it shows up even with that disabled.

cc @flip1995

[flip1995]

This is not a Clippy issue, but a rustc issue in general. This happens when linting attributes. Somehow rustc diagnostics doesn't realize, that this warning was already emitted and emits it again. rustc diagnostics has code that hashes warnings/errors, to make sure that they are not emitted twice, which for some reason fails for attributes (for the first attribute linted).

[jyn514]

@flip1995 this issue doesn't happen with rustdoc:

> rustdoc warn.rs 
warning: unknown lint: `x`
 --> warn.rs:1:8
  |
1 | #[warn(x)]
  |        ^
  |
  = note: `#[warn(unknown_lints)]` on by default

warning: 1 warning emitted

The issue may be caused by rustc, but it's specific to clippy.

[jyn514]

rustc diagnostics has code that hashes warnings/errors, to make sure that they are not emitted twice, which for some reason fails for attributes (for the first attribute linted).

This may be true, but rustdoc still only emits one lint even with -Z deduplicate-diagnostics=no.

[flip1995]

Oh, I see. So Clippy's lint machinery somehow breaks rustc's diagnostics 🤔

[flip1995]

I label it as E-hard, since I already put in many hours trying to fix it about 2 years ago and wasn't even able to find the cause.