`unused_unit` triggers when the type annotation is needed to compile

[kristoff3r]

Summary

When running the code snippet from the 2024 edition guide it triggers the unused_unit lint, even though removing the type annotation will make the code stop compiling.

Encountered in bevyengine/bevy#18778

Lint Name

unused_unit

Reproducer

trait Unit {}
impl Unit for () {}

fn run<R: Unit>(f: impl FnOnce() -> R) {
    f();
}


fn main() {
    run(|| -> () { todo!() });
}

Output

warning: unneeded unit return type
  --> src/main.rs:12:11
   |
12 |     run(|| -> () { todo!() });
   |           ^^^^^^ help: remove the `-> ()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit
   = note: `#[warn(clippy::unused_unit)]` on by default

Version

Additional Labels

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[profetia]

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