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.)
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