Tracking Issue for const `SystemTime` methods

[clarfonthey]

Feature gate: #![feature(const_system_time)]

This is a tracking issue for the constification of various SystemTime methods.

Public API

The following methods are made const:

• SystemTime::duration_since
• SystemTime::checked_add
• SystemTime::checked_sub
• SystemTimeError::duration

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• Implementation: Constify SystemTime methods #144519
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[ChrisDenton]

How could these usefully be const? The operations themselves are const enough but getting a SystemTime in the first place can only happen at runtime.

[ChrisDenton]

Oh, I guess the const methods might be useful to get an absolute time since the epoch.

[clarfonthey]

Yeah, I explicitly clarify this in the PR (which I hadn't yet posted when you saw this, although now have). Essentially, you're right about that for Instant, but SystemTime specifically lets you operate on the epoch, and it could be useful to have a constant time that isn't the epoch, but some other anchor.

Apologies for the weird delay, but I wanted to post the tracking issue so I could put the number in the code for the PR.