Tracking issue for SystemTime::UNIX_EPOCH

[sfackler]

Added in #49501

[sfackler]

@rfcbot fcp merge

[rfcbot]

Team member @sfackler has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Kimundi
• @SimonSapin
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[sfackler]

For background, this is currently a stable constant in std::time, and this issue tracks it as an associated constant of SystemTime, which is way more discoverable.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

[losvedir]

For others that found themselves on this issue from a compilation error on rust 1.27, I fixed it by changing:

use std::time::SystemTime;
SystemTime::now().duration_since(SystemTime::UNIX_EPOCH);

to

use std::time::{SystemTime, UNIX_EPOCH};
SystemTime::now().duration_since(UNIX_EPOCH);

[SimonSapin]

@losvedir Was the error message about the feature being unstable? The module-level constant is older, but the associated constant on the type was stabilized in 1.28 which is currently in beta.

[losvedir]

Yes, the error I got was:

error[E0658]: use of unstable library feature 'assoc_unix_epoch' (see issue #49502)
   --> src/main.rs:127:38
    |
127 |     SystemTime::now().duration_since(SystemTime::UNIX_EPOCH).expect("unix time can't be calculated").as_secs() as u32
    |                                      ^^^^^^^^^^^^^^^^^^^^^^

I think I found the SystemTime::UNIX_EPOCH approach on stack overflow. I'm a brand new rustacean (?) and it wasn't immediately obvious to me from this issue (linked in the compilation error) or the docs that there was a soon-to-be-deprecated (I imagine) approach that works on stable currently.

[SimonSapin]

I don’t know that we’re gonna deprecate the module-level constant. It’s sometimes slightly less convenient, but not "harmful" enough that we should actively push existing users to migrate.