#[must_use] does not warn for unused must_use tuple components

[nagisa]

#[must_use]
struct MU;

fn a() -> (u8, MU) {
    (0, MU)
}

pub fn main() {
    a(); // does not warn
    a().1; // warns
    a().0; // does not warn
}

I believe it should warn in all three cases.

[steveklabnik]

Triage: no change

[Mark-Simulacrum]

Closing in favor of #39524.