Fix from_utf8* links

[GuillaumeGomez]

Fixes #55666.

r? @QuietMisdreavus

[rust-highfive]

The job x86_64-gnu-llvm-5.0 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:06cab522:start=1541354273525415759,finish=1541354330566736164,duration=57041320405
$ 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-5.0
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travis_time:start:test_codegen
Check compiletest suite=codegen mode=codegen (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
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[01:08:04]  finished in 3.506
[01:08:04] travis_fold:end:test_codegen

---
travis_time:start:test_debuginfo
Check compiletest suite=debuginfo mode=debuginfo-both (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:08:19] 
[01:08:19] running 118 tests
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[01:08:48]  finished in 29.405
[01:08:48] travis_fold:end:test_debuginfo

---
[01:38:45] travis_fold:end:stage0-linkchecker

[01:38:45] travis_time:end:stage0-linkchecker:start=1541360263285010366,finish=1541360265642009548,duration=2356999182

[01:39:02] std/primitive.str.html:89: broken link - /checkout/obj/build/x86_64-unknown-linux-gnu/std/str/fn.from_utf8.html
[01:39:02] std/primitive.str.html:98: broken link - /checkout/obj/build/x86_64-unknown-linux-gnu/std/str/fn.from_utf8_mut.html
[01:42:22] thread 'main' panicked at 'found some broken links', tools/linkchecker/main.rs:49:9
[01:42:22] 
[01:42:22] 
[01:42:22] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/linkchecker" "/checkout/obj/build/x86_64-unknown-linux-gnu/doc"
[01:42:22] expected success, got: exit code: 101
[01:42:22] expected success, got: exit code: 101
[01:42:22] 
[01:42:22] 
[01:42:22] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:42:22] Build completed unsuccessfully in 0:53:34
[01:42:22] make: *** [check] Error 1
[01:42:22] Makefile:58: recipe for target 'check' failed

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)

[frewsxcv]

[01:39:02] std/primitive.str.html:89: broken link - /checkout/obj/build/x86_64-unknown-linux-gnu/std/str/fn.from_utf8.html
[01:39:02] std/primitive.str.html:98: broken link - /checkout/obj/build/x86_64-unknown-linux-gnu/std/str/fn.from_utf8_mut.html

[GuillaumeGomez]

This code is re-used everywhere... Talk about weird docss.

[QuietMisdreavus]

These links are ones that are impossible to make work with relative links like that. They appear in both std/primitive.str.html and on std/string/struct.String.html because of how String Derefs to str. Does an intra-doc link work?

[GuillaumeGomez]

Apparently not. I was completely unable to make intra links work with this one. We'll need more advanced intra-links support before making this one work.

[frewsxcv]

We'll need more advanced intra-links support before making this one work.

is there a tracking issue for this?

[Mark-Simulacrum]

Sounds like maybe this is currently ~impossible to do because of lack of support in the compiler? As such I'm going to close this but feel free to reopen if a better solutions arises.