Turn duration consts into associated consts

[ghost]

As suggested in #57391 (comment), I'm moving Duration constants (SECOND, MILLISECOND and so on; currently behind unstable duration_constants feature) into the impl Duration block.

cc @frewsxcv @SimonSapin

[rust-highfive]

r? @withoutboats

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[oli-obk]

O_o we have zero tests using these?

Can you add a bunch of tests (maybe just as doctests on the constants showing a usage example)?

r? @oli-obk

[ghost]

@oli-obk Seems so 😅 I've added a few doctests on the constants now.

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:05fba768:start=1550696384926645461,finish=1550696486158318672,duration=101231673211
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---
travis_time:start:test_debuginfo
Check compiletest suite=debuginfo mode=debuginfo-both (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:16:00] 
[01:16:00] running 119 tests
[01:16:25] .iiiii...i.....i..i...i..i.i..i.ii...i.....i..i....i..........iiii..........i...ii...i.......ii.i.i. 100/119
[01:16:29] i......iii.i.....ii
[01:16:29] 
[01:16:29]  finished in 28.512
[01:16:29] travis_fold:end:test_debuginfo

---
[01:30:53] .................................................................................................... 1900/2269
[01:31:07] .................................................................................................... 2000/2269
[01:31:23] .................................................................................................... 2100/2269
[01:31:40] .................................................................................i.................. 2200/2269
[01:31:49] ........................................FF.FF........................
[01:31:49] 
[01:31:49] ---- time.rs - time::Duration::MICROSECOND (line 90) stdout ----
[01:31:49] ---- time.rs - time::Duration::MICROSECOND (line 90) stdout ----
[01:31:49] error[E0658]: use of unstable library feature 'duration_constants' (see issue #57391)
[01:31:49]  --> time.rs:93:12
[01:31:49]   |
[01:31:49] 6 | assert_eq!(Duration::MICROSECOND, Duration::from_micros(1));
[01:31:49]   |
[01:31:49]   = help: add #![feature(duration_constants)] to the crate attributes to enable
[01:31:49] 
[01:31:49] thread 'time.rs - time::Duration::MICROSECOND (line 90)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
[01:31:49] thread 'time.rs - time::Duration::MICROSECOND (line 90)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
[01:31:49] note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
[01:31:49] 
[01:31:49] ---- time.rs - time::Duration::MILLISECOND (line 78) stdout ----
[01:31:49] error[E0658]: use of unstable library feature 'duration_constants' (see issue #57391)
[01:31:49]  --> time.rs:81:12
[01:31:49]   |
[01:31:49] 6 | assert_eq!(Duration::MILLISECOND, Duration::from_millis(1));
[01:31:49]   |
[01:31:49]   = help: add #![feature(duration_constants)] to the crate attributes to enable
[01:31:49] 
[01:31:49] thread 'time.rs - time::Duration::MILLISECOND (line 78)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
[01:31:49] thread 'time.rs - time::Duration::MILLISECOND (line 78)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
[01:31:49] 
[01:31:49] ---- time.rs - time::Duration::NANOSECOND (line 102) stdout ----
[01:31:49] error[E0658]: use of unstable library feature 'duration_constants' (see issue #57391)
[01:31:49]  --> time.rs:105:12
[01:31:49]   |
[01:31:49] 6 | assert_eq!(Duration::NANOSECOND, Duration::from_nanos(1));
[01:31:49]   |
[01:31:49]   = help: add #![feature(duration_constants)] to the crate attributes to enable
[01:31:49] 
[01:31:49] thread 'time.rs - time::Duration::NANOSECOND (line 102)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
[01:31:49] thread 'time.rs - time::Duration::NANOSECOND (line 102)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
[01:31:49] 
[01:31:49] ---- time.rs - time::Duration::SECOND (line 66) stdout ----
[01:31:49] error[E0658]: use of unstable library feature 'duration_constants' (see issue #57391)
[01:31:49]  --> time.rs:69:12
[01:31:49]   |
[01:31:49] 6 | assert_eq!(Duration::SECOND, Duration::from_secs(1));
[01:31:49]   |
[01:31:49]   = help: add #![feature(duration_constants)] to the crate attributes to enable
[01:31:49] 
[01:31:49] thread 'time.rs - time::Duration::SECOND (line 66)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
---
[01:31:49] 
[01:31:49] error: test failed, to rerun pass '--doc'
[01:31:49] 
[01:31:49] 
[01:31:49] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "core" "--" "--quiet"
[01:31:49] 
[01:31:49] 
[01:31:49] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:31:49] Build completed unsuccessfully in 0:27:01
[01:31:49] Build completed unsuccessfully in 0:27:01
[01:31:49] make: *** [check] Error 1
[01:31:49] Makefile:48: recipe for target 'check' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:2607f246
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Feb 20 22:33:25 UTC 2019
---
travis_time:end:0919f182:start=1550702007407071609,finish=1550702007411498413,duration=4426804
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:14deaea9
$ ln -s . checkout && for CORE 

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[oli-obk]

👍

@bors r+

[bors]

📌 Commit c6d24cd has been approved by oli-obk

[Centril]

@bors rollup