Make str indexing generic on SliceIndex.

[alercah]

Fixes #55603

[rust-highfive]

r? @kennytm

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[kennytm]

Could you revert the formatting or split it into a separate commit? It is hard to review what is actually changed.

[alercah]

My bad; my editor's autoformat must have snuck back on when I wasn't paying attention.

[alercah]

Fixed, with some slight UX improvements too.

[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:0f089337:start=1547502980065685847,finish=1547503093718173008,duration=113652487161
$ 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
---

[00:03:55] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:55] tidy error: /checkout/src/libcore/slice/mod.rs:2383: trailing whitespace
[00:03:55] tidy error: /checkout/src/libcore/slice/mod.rs:2384: line longer than 100 chars
[00:03:55] tidy error: /checkout/src/test/ui/slice/slice-idx.rs:1: trailing whitespace
[00:03:55] tidy error: /checkout/src/test/ui/slice/slice-mut-idx.rs:3: trailing whitespace
[00:03:57] some tidy checks failed
[00:03:57] 
[00:03:57] 
[00:03:57] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:03:57] 
[00:03:57] 
[00:03:57] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:57] Build completed unsuccessfully in 0:00:52
[00:03:57] Build completed unsuccessfully in 0:00:52
[00:03:57] make: *** [tidy] Error 1
[00:03:57] Makefile:69: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:04278efd
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Mon Jan 14 22:02:19 UTC 2019
---
travis_time:end:0d5b7a32:start=1547503340691036326,finish=1547503340695740254,duration=4703928
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:08e67398
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:03f6ba24
travis_time:start:03f6ba24
$ 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:08272f4c
$ 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)

[alercah]

One more round of test failure fixes. @kennytm this should be ready for review.

[bors]

☔ The latest upstream changes (presumably #57747) made this pull request unmergeable. Please resolve the merge conflicts.

[alercah]

Ping for review. @sfackler since you said we could ping. ;)

[sfackler]

@bors r+

Thanks!

[bors]

📌 Commit c7d25a2 has been approved by sfackler