Implement DoubleEndedIterator for CaseMappingIter

[KamilaBorowska]

No description provided.

[rust-highfive]

r? @cramertj

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[Centril]

r? @Amanieu

[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:04a321ac:start=1555679515669706898,finish=1555679609158484179,duration=93488777281
$ 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
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
travis_time:start:test_assembly
Check compiletest suite=assembly mode=assembly (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:15:40] 
[01:15:40] running 9 tests
[01:15:40] iiiiiiiii
[01:15:40] 
[01:15:40]  finished in 0.156
[01:15:40] travis_fold:end:test_assembly

---
travis_time:start:test_debuginfo
Check compiletest suite=debuginfo mode=debuginfo-both (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:15:57] 
[01:15:57] running 121 tests
[01:16:23] .iiiii...i.....i..i...i..i.i.i..i.ii...i.....i..i....i..........iiii..........i...ii...i.......ii.i. 100/121
[01:16:28] i.i......iii.i.....ii
[01:16:28] 
[01:16:28]  finished in 31.094
[01:16:28] travis_fold:end:test_debuginfo

---
[01:26:47]    Compiling core v0.0.0 (/checkout/src/libcore)
[01:26:52] error[E0277]: `std::string::String` is not an iterator
[01:26:52]   --> src/libcore/../libcore/tests/char.rs:85:36
[01:26:52]    |
[01:26:52] 85 |         assert!(iter.chars().rev().eq(rev_iter));
[01:26:52]    |                                    ^^ `std::string::String` is not an iterator; try calling `.chars()` or `.bytes()`
[01:26:52]    = help: the trait `std::iter::Iterator` is not implemented for `std::string::String`
[01:26:52]    = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `std::string::String`
[01:26:52] 
[01:26:52] error[E0277]: `std::string::String` is not an iterator
[01:26:52] error[E0277]: `std::string::String` is not an iterator
[01:26:52]    --> src/libcore/../libcore/tests/char.rs:114:36
[01:26:52]     |
[01:26:52] 114 |         assert!(iter.chars().rev().eq(rev_iter));
[01:26:52]     |                                    ^^ `std::string::String` is not an iterator; try calling `.chars()` or `.bytes()`
[01:26:52]     = help: the trait `std::iter::Iterator` is not implemented for `std::string::String`
[01:26:52]     = note: required because of the requirements on the impl of `std::iter::IntoIterator` for `std::string::String`
[01:26:52] 
[01:27:01] error: aborting due to 2 previous errors
[01:27:01] error: aborting due to 2 previous errors
[01:27:01] 
[01:27:01] For more information about this error, try `rustc --explain E0277`.
[01:27:01] error: Could not compile `core`.
[01:27:01] 
[01:27:01] To learn more, run the command again with --verbose.
[01:27:01] 
[01:27:01] 
[01:27:01] 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"
[01:27:01] 
[01:27:01] 
[01:27:01] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:27:01] Build completed unsuccessfully in 0:23:25
[01:27:01] Build completed unsuccessfully in 0:23:25
[01:27:01] make: *** [check] Error 1
[01:27:01] Makefile:48: recipe for target 'check' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0efa8fa0
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Fri Apr 19 14:40:40 UTC 2019
---
travis_time:end:250152c3:start=1555684842286782040,finish=1555684842361534126,duration=74752086
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:073ddaf0
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:3c707b55
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[Amanieu]

@rfcbot fcp merge

[rfcbot]

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

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

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), 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]

Could you give an example of when you'd want to use this?

[clarfonthey]

This was denied before in #38968.

[KamilaBorowska]

There is no real point, it's mostly for consistency, I cannot see the CaseMappingIter ever changing in a way which would cause issues with having a DoubleEndedIterator implementation.

About ExactSizeIterator, it was implemented in #58778 (although ExactSizeIterator is somewhat more useful), which I did implement while considering a potential Clippy lint requiring size_hint for Iterator implementations, and noticing that one didn't have implementation of it while having a reasonable implementation of it. Often it's possible to do better than (0, None) - even something like flat_map could be checking size lower bound hints of iterators it's already processing, and check whether the main iterator is non-empty. Of course, there are exceptions like from_fn which have no useful information for size_hint, but those should be rare.

[sfackler]

Our policy is generally to not take code that no one wants to use. Every new API imposes some cost, and we should at least be getting some value out of it.

[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.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

[Centril]

r? @sfackler

[Centril]

cc @rust-lang/libs Can someone please review this? :)

[alexcrichton]

Looks like a review was provided but since there aren't compelling use cases for it and it's otherwise seen as perhaps a bit too much code for libstd for now, I'm going to close this.

[KamilaBorowska]

For future reference (in case somebody found this pull request), this was implemented in #88858.