Improve memory usage of libsyntax
Replaces some usage of `.to_string()` with `.into_string()`

Signed-off-by: Peter Atashian <retep998@gmail.com>
doc: that felt like it needed a stronger break than what comma provides

Remove trailing whitespace while at it
Fix issue #17216

The cleanup scope of for loop bindings should translated when
popped to ensure resources are not leaked.
Add regression test for issue #17216
Display the doc-block toggle on everything again.

This needs a clone otherwise each successive insertion detaches `toggle`
from the previous position.

Fixes #17125.
Fix two typos
Align with _mut conventions

As per [RFC
52](https://github.com/rust-lang/rfcs/blob/master/active/0052-ownership-variants.md),
use `_mut` suffixes to mark mutable variants, and `into_iter` for moving
iterators.

[breaking-change]
auto merge of #17213 : mo/rust/fix_typos, r=aturon
Fallout from renaming
auto merge of #17268 : aturon/rust/mut-conventions, r=alexcrichton

As per [RFC 52](https://github.com/rust-lang/rfcs/blob/master/active/0052-ownership-variants.md), use `_mut` suffixes to mark mutable variants, and `into_iter` for moving iterators. Additional details and motivation in the RFC.

Note that the iterator *type* names are not changed by this RFC; those are awaiting a separate RFC for standardization.

Closes #13660
Closes #16810

[breaking-change]
auto merge of #17223 : retep998/rust/into_string, r=huonw

Replaces some usage of `.to_string()` with `.into_string()`
auto merge of #17227 : tshepang/rust/stronger-break, r=aturon

Remove trailing whitespace while at it
move most of front to libsyntax
move std_inject to libsyntax
move feature_gate to libsyntax
rebasing fixes
auto merge of #17160 : nick29581/rust/front, r=pcwalton

r?
auto merge of #17246 : bkoropoff/rust/issue-17216, r=pnkfelix

Trans the cleanup scope of for loop bindings so we don't leak resources.  Regression test included.

Closes #17216
auto merge of #17247 : huonw/rust/toggle-clone, r=alexcrichton

This needs a clone otherwise each successive insertion detaches `toggle`
from the previous position.

Fixes #17125.
syntax: use an index in CodeMap instead of Gc for ExpnInfo.