Fix outgoing link to snappy in the FFI guide

Google have migrated snappy to GitHub.
Add a comment missing from 5aff0e7

For some reason, I had this comment in my local tree but not in the
branch I sent with the PR.
Remove libworkcache

This was only ever used by rustpkg and is very unmaintained.

[breaking-change]
auto merge of #14555 : tomjakubowski/rust/fix-snappy-link, r=alexcrichton

Google have migrated snappy to GitHub.
auto merge of #14556 : sfackler/rust/kill-workcache, r=alexcrichton

This was only ever used by rustpkg and is very unmaintained.

[breaking-change]
Remove unnecessary allocations / clones during method lookup

By dropping the intermediate vector that holds the relevant candidates
including duplicates and directly building the vector that has the
duplicates removed we can eliminate quite a few allocations. This
reduces the times for type checking by 5-10% (measured with libstd,
libsyntax and librustc).
auto merge of #14557 : zwarich/rust/missing-comment, r=alexcrichton

For some reason, I had this comment in my local tree but not in the
branch I sent with the PR.
auto merge of #14563 : dotdash/rust/clone_kill, r=huonw

By dropping the intermediate vector that holds the relevant candidates
including duplicates and directly building the vector that has the
duplicates removed we can eliminate quite a few allocations. This
reduces the times for type checking by 5-10% (measured with libstd,
libsyntax and librustc).
Fix broken markup in query_to_str documentation
auto merge of #14565 : fhartwig/rust/master, r=sfackler
Make the match arm type mismatch message point to the arm's span

Fixes #11319