Heisenwarning `unused_braces` with `tracing`

[Cerber-Ursi]

Code

#[tracing::instrument]
fn foo() -> u8 { 42 }

#[tracing::instrument]
fn bar() -> u8 { 
    42
}

Current output

warning: unnecessary braces around block return value
 --> src/lib.rs:2:16
  |
2 | fn foo() -> u8 { 42 }
  |                ^^  ^^
  |
  = note: `#[warn(unused_braces)]` on by default
help: remove these braces
  |
2 - fn foo() -> u8 { 42 }
2 + fn foo() -> u8 42
  |

Desired output

No response

Rationale and extra context

I'd expect both foo and bar to either trigger the warning or not, since they differ only in formatting (in fact, bar is just a rustfmt-ed foo). Looking just at the suggestion, it seems that not warning is better (since this suggestion is not applicable), but this might be a bug in tracing and not in rustc.

Other cases

No response

Anything else?

Initially found on URLO.

[workingjubilee]

The lint does not fire in this case:

fn foo() -> u8 { 42 }

fn bar() -> u8 { 
    42
}

[p-kraszewski]

Yes, but it does with #[tracing::instrument], which expands to the same code - and this expanded code somehow behaves differently in rustc for single- and multi-line function.

More discussion is here: https://users.rust-lang.org/t/probably-invalid-compiler-hint/100687/1 (I am the thread creator).

My compiler-internals-fu is weak, but @Cerber-Ursi has some interesting hypotheses.

[workingjubilee]

Peculiarly, the lint stops firing with this:

#[tracing::instrument]
fn foo() -> u8 { (); 42 }

But does fire on this:

#[tracing::instrument]
fn foo() -> u8 { dbg!(42) }

Thus I believe this is a bug in rustc, not in tracing.

[Cerber-Ursi]

Possibly-connected problem, in a sense that the code shown after expanding macros is not the same as the code used for compiling (therefore making debug problematic) - https://github.com/dtolnay/cargo-expand/issues/201/

[shepmaster]

I opened https://github.com/tokio-rs/tracing/issues/2830 about this and then we realized it's likely a rustc issue.

The braces warning is whitespace-sensitive:

fn warns_about_braces() -> u8 {
    { 1 }
}

fn does_not_warn_about_braces() -> u8 {
    { 
        1
    }
}

[shepmaster]

May be a duplicate of #88104 / #73068 / #70814