Wrong formatting of negative zero when sign is requested

[nagisa]

use std::mem;
fn main() { 
    let neg_zero: f32 = unsafe { mem::transmute(0x8000_0000u32) };
    assert_eq!(format!("{:+}", neg_zero), "-0");
}

Fails with “assertion failed: (left == right) (left: "+0", right: "-0")”

playpen

[hanna-kruppe]

Note that {:?+} produces the desired result. So this is consistent with the behavior agreed upon in #20596: Display treats negative zero as positive, and Debug differentiates them. In the light of that, it's not clear to me why the + format code should make a difference. It would be more confusing if {} and {:+} disagreed about the sign (since absence of a sign indicates positive).

IMHO this is "wrong" if and only if RFC PR 1074 is accepted. Whether Display should distinguish negative zero should be duked out there.

Also, by the way: You can produce a negative zero with -0.0, no need for the transmute.

[bstrie]

Triage: still relevant.

[workingjubilee]

@rustbot modify labels: +A-floating-point, +A-fmt