The tracking issue for this feature is #43302.

The fn_must_use feature allows functions and methods to be annotated with #[must_use], indicating that the unused_must_use lint should require their return values to be used (similarly to how types annotated with must_use, most notably Result, are linted if not used).

#![feature(fn_must_use)]

#[must_use]
fn double(x: i32) -> i32 {
    2 * x
}

fn main() {
    double(4); // warning: unused return value of `double` which must be used

    let _ = double(4); // (no warning)
}