Update documentation for from_raw_parts

Fixes #19269.
collections: dramatically speed up Vec::reserve with magic

Somehow llvm is able to optimize this version of Vec::reserve
into dramatically faster than the old version. In micro-benchmarks
this was 2-10 times faster. It also shaved 14 minutes off of
rust's compile times.

Closes #19281.
auto merge of #19306 : steveklabnik/rust/gh19269, r=nikomatsakis,brson

Fixes #19269.

/cc @thestinger @mahkoh @mitsuhiko
auto merge of #19574 : erickt/rust/vec-reserve, r=alexcrichton

(I don't understand why this works, and so I don't quite trust this yet. I'm pushing it up to see if anyone else can replicate this performance increase)

Somehow llvm is able to optimize this version of Vec::reserve into dramatically faster than the old version. In micro-benchmarks this was 2-10 times faster. It also reduce my Rust compile time from 41 minutes to 27 minutes.

Closes #19281.
Link regions in `ref` bindings from fn arguments.
auto merge of #19456 : nikomatsakis/rust/reborrow-closure-arg, r=pnkfelix

Otherwise region inference can fail when closure arguments include `ref` bindings. Test case included in the PR.