#[tracing::instrument] swallows lints such as dead_code

[andrewhickman]

Bug Report

Version

0.1.25

Platform

Windows 10 64-bit

Crates

• tracing

Description

Functions wrapped in #[tracing::instrument] will not trigger lints such as dead_code. See andrewhickman/fn-error-context#3 for a very similar issue (in a much simpler macro) which was caused by dropping some spans on the macro tokens.

Minimal example to reproduce:

#[tracing::instrument]
fn never_used() {}

fn main() {}

Without the instrument attribute, we get a dead_code lint warning, but with it applied the warning is suppressed.

[dvdsk]

This is particularly confusing if combined with a dead code warning inside the function.

This only seems to happens if the traced function is inside a implementation block.
Like this (playground):

struct Test;
impl Test {
    #[tracing::instrument]
    fn never_used(var: i32) {
        const IS_USED: &str = "I get a dead code warning";

        if IS_USED == "" {
            println!("");
        }
    }
}

fn main() {
    let _test = Test;
}

If one comments out the constant IS_USED you get a undefined variable warning. Given there is no dead_code warning this gets really confusing.

I brought this specific case up with the compiler to see if there is anything they can do about this especially confusing case.