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make Instant::{duration_since, elapsed, sub} saturating and remove workarounds #89926

Merged
merged 4 commits into from
Feb 13, 2022
Merged

make Instant::{duration_since, elapsed, sub} saturating and remove workarounds #89926

merged 4 commits into from
Feb 13, 2022
+57 −267

Commits on Feb 13, 2022

  1. make Instant::{duration_since, elapsed, sub} saturating and remove wo… 

    …rkarounds

This removes all mutex/atomics based workarounds for non-monotonic clocks and makes the previously panicking methods saturating instead.

Effectively this moves the monotonization from `Instant` construction to the comparisons.

This has some observable effects, especially on platforms without monotonic clocks:

* Incorrectly ordered Instant comparisons no longer panic. This may hide some programming errors until someone actually looks at the resulting `Duration`
* `checked_duration_since` will now return `None` in more cases. Previously it only happened when one compared instants obtained in the wrong order or
  manually created ones. Now it also does on backslides.

The upside is reduced complexity and lower overhead of `Instant::now`.
    @the8472
    the8472 committed Feb 13, 2022
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  2. Add caveat about the monotonicity guarantee by linking to the later s… 

    …ection
    @the8472
    the8472 committed Feb 13, 2022
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  3. Add panic docs describing old, current and possible future behavior

    @the8472
    the8472 committed Feb 13, 2022
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  4. Capitalize "Rust" 

    Co-authored-by: Mark Rousskov <mark.simulacrum@gmail.com>
    @joshtriplett @Mark-Simulacrum @the8472
    2 people authored and the8472 committed Feb 13, 2022
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