Update reference safety requirements

Per rust-lang#116677 (comment), the language as written promises too much. This PR relaxes the language to be consistent with current semantics. If and when rust-lang#117945 is implemented, we can revert to the old language.

While we're here, we also require that references be non-null.

cc ``@RalfJung``

…ape-examples-ordering, r=jieyouxu

Migrate rustdoc scrape examples ordering

Part of rust-lang#121876.

This one adds a lot of utility methods/functions. To prevent having too much changes at once, I didn't make the existing rmake tests use these yet but I'll send a follow-up so they all use it.

r? `@jieyouxu`

…r=compiler-errors

Fix some minor issues from the ui-test auto-porting

I'm not sure if these count as false positives, because well, starting a comment with `// incremental` was probably a valid compiletest directive.

But anyway, these tests directives became clearly goofy and now with the better syntax we can straighten things out.

r? jieyouxu

…errors

solve: all "non-structural" logging to trace

This enables us to start with `RUSTC_LOG=rustc_trait_selection::solve=debug` to figure out *where* something went wrong, to then separately use `trace` to get to the details.

r? ``@compiler-errors``