Simplify --exclude syntax and make it more useful

[jyn514]

I think we should have a holistic conversation about what excludes should look like. The current syntax is currently quite baroque and AFAIK @pietroalbini's CI scripts are the only ones using most of the features. Slapping git-style ignores on top seems like it will make things more confusing, not less.

That said, I agree the behavior is confusing and there's room for improvement; I'd love to get rid of the doc:: prefixes somehow and make the various paths unambiguous.

See https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/excludes for the discussion; this issue is just a placeholder so it doesn't get lost.

Originally posted by @jyn514 in #102706 (review)

[jyn514]

That said, I agree the behavior is confusing and there's room for improvement; I'd love to get rid of the doc:: prefixes somehow and make the various paths unambiguous.

I wonder if we could do this by making it so that --exclude only applies to the subcommand that was run; so for example x test --exclude library/std would behave the same as x test --exclude test::library/std today. I don't think there's a use case for x test --exclude doc::library/std, that will cause tests to fail anyway.

[pietroalbini]

I don't think there's a use case for x test --exclude doc::library/std, that will cause tests to fail anyway.

So, I just checked and in Ferrocene we don't use --exclude test::library/std (or any kind:: prefix) anymore after a CI refactor (linkchecker runs in a separate job than tests now), so I don't think I'm a blocker for this anymore.

The behavior you mentioned would still fix #91794, and I don't see any extra reasoning in #91965 that would require a doc:: prefix when running tests.

[jyn514]

Mentoring instructions:

1. Change is_excluded to always return false if step_description.kind != builder.config.kind:

   rust/src/bootstrap/builder.rs

   Line 301 in 3905517

   fn is_excluded(&self, builder: &Builder<'_>, pathset: &PathSet) -> bool {

2. Remove TaskPath and replace all uses with Path directly:

   rust/src/bootstrap/builder.rs

   Lines 139 to 142 in 3905517

   	#[derive(Clone, PartialOrd, Ord, PartialEq, Eq)]
   	pub struct TaskPath {
   	pub path: PathBuf,
   	pub kind: Option<Kind>,

3. Verify that x test --stage 1 --exclude library/core linkchecker fails before and passes afterwards, like in Linkchecker tool reports a huge number of broken links #91794 (comment).