Feature Request: saturating_add(Duration) for SystemTime / Instant

[woody77]

Both SystemTime and Instant have checked_add(Duration), but no saturating_add(Duration).

This seems like an omission, as when doing math operations on time, a developer has to choose between:

1. allowing for the (hopefully very remote) potential for overflows
2. determining what to do with the None case of the checked_add()
3. letting it checked_add(long_duration).unwrap() panic after an overflow

Given that the SystemTime and Instant both opaquely wrap a platform-dependent structure, it's not clear what "far future" values are valid on a given platform (e.g. issue #44394), and what values a developer could correctly use with checked_add(long_duration).unwrap_or(...?).

[woody77]

However, I just discovered that impl Add<Duration> for SystemTime panics on overflow, as it internally is

self.checked_add(duration).expect("overflow when adding duration......

https://doc.rust-lang.org/src/std/time.rs.html#534

[tarcieri]

Came across this issue and was also thinking that if there were a SystemTime::MAX, it could be implemented (even in end-user code) as .checked_add(duration).unwrap_or(SystemTime::MAX).