Disallow #[must_use] for functions with no return value

[leonardo-m]

This code:

#[must_use]
fn foo() {}
fn main() {
    foo();
}

Gives (rustc 1.31.0-nightly 2c2e2c5 2018-10-12):

warning: unused return value of `foo` which must be used
 --> ...\test.rs:4:5
  |
4 |     foo();
  |     ^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default

But what I expected is no warning on the foo call, and an error (or warning) for adding #[must_use] on a function without return value.

In (rare) code like this, where the unit (empty tuple) is returned in an explicit way, I expect no errors nor warnings:

#[must_use]
fn foo() -> () {}
fn main() {
    let _ = foo();
}

[varkor]

It was decided #[must_use] should act just as on any function in this case: #54828. The "explicit return syntax" is semantically no different from the implicit return type and making that different would be misleading.

[leonardo-m]

I am not sure that decision is the best, but OK, I close this down.