Improve the zsh completions.

Currently the ZSH completions are quite old an nearly useless. This
brings them up to be compatible with current rust and makes them far
more useful.

Closes #17305
Case-insensitive environment keys.
test: Un-ignore some GDB pretty printing tests

I've confirmed that these are working on the snapshot builders

Closes #16919
Remove unstable marker from std::rt

The `std::rt` module was marked `unstable` [a while
back](https://github.com/rust-lang/rust/commit/b6d4d117f4c2770649c7ddc2ad9ad4ce4c3b13b1),
and this change was not reverted when we moved to an `experimental`
baseline for `std`.
Fix warning and make code follow the text better
Document that CStrings live in the libc heap

Insists on the fact that the memory is managed by malloc
and not jemalloc
Closes #17067
support LTO against libraries built with codegen-units > 1
stop clamping the DWARF version to 3 on Linux

Closes #13611
auto merge of #17341 : alexcrichton/rust/unignore, r=brson

I've confirmed that these are working on the snapshot builders

Closes #16919
Pacify lint gods.
auto merge of #17349 : aturon/rust/rt-experimental, r=alexcrichton

The `std::rt` module was marked `unstable` [a while back](https://github.com/rust-lang/rust/commit/b6d4d117f4c2770649c7ddc2ad9ad4ce4c3b13b1), and this change was not reverted when we moved to an `experimental` baseline for `std`.
Document that CStrings live in the libc heap

Closes issue #17067 and improves pr #17355
auto merge of #17364 : thestinger/rust/debuginfo, r=pnkfelix

Closes #13611
syntax: use an index in CodeMap instead of Gc for ExpnInfo.
rustc: fix fallout from removing the use of Gc for ExpnInfo.
Fix fallout in tests from removing the use of Gc in ExpnInfo.
rustc: remove DefArg and DefBinding in favor of DefLocal.
rustc: remove BindingMode from DefLocal.
rustc: add a closure depth to DefUpvar.
rustc: move type definitions from middle::freevars to middle::ty.