Avoid a collection and iteration on empty passes

Just some mini optimization I saw in the wild. This way, we avoid a `collect` and `map` on an empty `passes`. Honestly, I don't even think this is big enough of a change to make a benchmark, but I'd still like to see results.

Based on [this book](https://nnethercote.github.io/perf-book/iterators.html#collect-and-extend)

…iaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#120584 (For a rigid projection, recursively look at the self type's item bounds to fix the `associated_type_bounds` feature)
 - rust-lang#120589 (std::thread::available_parallelism merging linux/android/freebsd version)
 - rust-lang#120596 ([rustdoc] Correctly generate path for non-local items in source code pages)
 - rust-lang#120629 (Move some test files)
 - rust-lang#120846 (Update jobserver-rs to 0.1.28)
 - rust-lang#120850 (ast_lowering: Fix regression in `use ::{}` imports.)
 - rust-lang#120853 (Avoid a collection and iteration on empty passes)

Failed merges:

 - rust-lang#120549 (modify alias-relate to also normalize ambiguous opaques)

r? `@ghost`
`@rustbot` modify labels: rollup

…iaskrgr

Rollup of 6 pull requests

Successful merges:

 - rust-lang#120584 (For a rigid projection, recursively look at the self type's item bounds to fix the `associated_type_bounds` feature)
 - rust-lang#120596 ([rustdoc] Correctly generate path for non-local items in source code pages)
 - rust-lang#120629 (Move some test files)
 - rust-lang#120846 (Update jobserver-rs to 0.1.28)
 - rust-lang#120850 (ast_lowering: Fix regression in `use ::{}` imports.)
 - rust-lang#120853 (Avoid a collection and iteration on empty passes)

r? `@ghost`
`@rustbot` modify labels: rollup