Description
Proposal
Problem statement
When measuring time in nanoseconds, u64 can "only" represent just under 600 years, so we often reach for u128. In particular, even Duration::as_nanos returns a u128. However, currently there is no convenient way to convert a u128 back to a Duration, since Duration::from_nanos takes a u64.
Motivating examples or use cases
Miri hand-rolls a version of this conversion here; @joshtriplett also mentioned wanting it a couple of times.
Solution sketch
impl Duration {
/// Creates a new Duration from the specified number of nanoseconds.
///
/// # Panics
///
/// Panics if the given number of nanoseconds overflows the Duration size.
pub fn from_nanos_u128(nanos: u128) -> Duration;
}Alternatives
Instead of panicking on overflow, we could have a "checked" version or so dome sort of saturation. Panicking is consistent with the (unstable) from_hours etc; the stable Duration::new(secs, nanos) can also panic.
Links and related work
What happens now?
This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.
Possible responses
The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):
- We think this problem seems worth solving, and the standard library might be the right place to solve it.
- We think that this probably doesn't belong in the standard library.
Second, if there's a concrete solution:
- We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
- We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.