rust private

Rollup of 11 pull requests Successful merges: - #51807 (Deprecation of str::slice_unchecked(_mut)) - #52051 (mem::swap the obvious way for types smaller than the SIMD optimization's block size) - #52465 (Add CI test harness for `thumb*` targets. [IRR-2018-embedded]) - #52507 (Reword when `_` couldn't be inferred) - #52508 (Document that Unique::empty() and NonNull::dangling() aren't sentinel values) - #52521 (Fix links in rustdoc book.) - #52581 (Avoid using `#[macro_export]` for documenting builtin macros) - #52582 (Typo) - #52587 (Add missing backtick in UniversalRegions doc comment) - #52594 (Run the error index tool against the sysroot libdir) - #52615 (Added new lines to .gitignore.)