Document for-loop in tutorial section on loops
 Explain container iteration in the loop tutorial

 As suggested by @pnkfelix in #12161, this extends the examples by a for-loop
 that iterates over a string as an example for container iteration.
Adds support for working with URL Paths

It is sometimes useful to parse just the path portion of a URL (path,
query string and fragment) rather than the entire URL.
Remove Freeze bounds from sync::MutexArc

With Rc no longer trying to statically prevent cycles (and thus no
longer using the Freeze bound), it seems appropriate to remove that
restriction from MutexArc as well.
Update tasks guide: SharedChan as been removed

The code examples are up to date, but the surrounding explanations are not.
std: iteratize str::is_utf16 & add tests.

Most of the tests are randomly generated with Python 3 and rely on it's
UTF-16be encoder/decoder being correct.
std: convert str::from_utf16 to an external iterator.

Fixes #12316.
str: provide lossy UTF-16 support.

This replaces the iterator with one that handles lone surrogates
gracefully and uses that to implement `from_utf16_lossy` which replaces
invalid `u16`s with U+FFFD.
std: decode even numbered non-BMP planes in the UTF-16 decoder.

Fixes #12318.
std: make str::from_utf16 return an Option.

The rest of the codebase is moving toward avoiding `fail!` so we do it
here too!
Remove the compile-fail test that's now obsolete
backticks for syntax elements
Spellcheck library docs.
Fix a segfault in the rustuv tests

The details can be found in the comments I added to the test, but the gist of it
is that capturing output injects rescheduling a green task on failure, which
wasn't desired for the test in question.

cc #12340
Test ordering of glue_drop and glue_takew in self-re-assignment. Closes #3290.
Made fail_bounds_check more careful with strings.

Fixes GH #11976.
Update compiler-rt

Closes #12356
auto merge of #12342 : ehsanul/rust/remove-shared-chan-tasks-guide, r=alexcrichton

The code examples are up to date, but the surrounding explanations are not.
auto merge of #12345 : huonw/rust/speeling, r=cmr
mk: Fix the regexp of SHOW_DOCS

The tag marks were missing, and `make tips` didn't work.