`Duration` `Debug` for 2.5s rounds inconsistently for `duration` and `duration.as_secs_f64()`

[tczajka]

use std::time::Duration;

fn main() {
    let a = Duration::new(2, 500000000);
    println!("{:.0?}", a);
    println!("{:.0?}s", a.as_secs_f64());
    println!("{:.0?}", Duration::MAX);
}

I expected this to print (round to even):

2s
2s
18446744073709551616s

Instead, this prints:

3s
2s
0s

[fmease]

Triage: Current output:

3s
2s
18446744073709551616s

[Enselic]

Triage: The Duration::MAX case was fixed with regression tests in #102484.

The inconsistency between

   println!("{:.0?}", a);
   println!("{:.0?}s", a.as_secs_f64());

still remains however.

[josh-kaplan]

@rustbot claim

If no one is working this, it looks like this might be a good first issue to get started with contributing to Rust. I have a PR open with a proposed fix: #147499

[josh-kaplan]

It looks like the original intent of the implementation was not to roundTiesToEven. Since Duration doesn't actually store the inner values as f64, it seems plausible to me that there is an inconsistency between rounding in Duration Debug formatting and f64 Debug formatting.