Validate uses of self and super

[djrenren]

This change follows on the validation of the crate keyword in paths. It verifies the following things:

super:

• May only be preceded by other super segments
• If in a UseItem then all semantically preceding paths also consist only of super

self

• May only be the start of a path

Just a note, a couple times while working on this I found myself really wanting a Visitor of some sort so that I could traverse descendants while skipping sub-trees that are unimportant. Iterators don't really work for this, so as you can see I reached for recursion. Considering paths are generally small a fancy debounced visitor probably isn't important but figured I'd say something in case we had something like this lying around and I wasn't using it.

[matklad]

Yeah, I somehow don't really like visitors. Like, you are supposed to use visitors in compilers, but, every time I did one, I reverted back to traversal and matching.

I think in this case we might want to preserve the caller-driven API of descendants, but allow some control over traversal. Not sure exactly how the API should look like, but I imagine something like this might be possible:

let traversal = Traversal::new(root);
let mut cntl = traversal.cntl();

for node in traversal.iter() {
    if cond {
        cntl.skip_children()
    }
}

the cntl think would hold a &Cell<bool>, which will be stored inside Traversal and checked on next. Not sure if it's a good idea or too complex, but I'd try this API over a visitor.

bors r+

[djrenren]

Yeah I really like the iterator-style as well. I'd be happy if I never had to write a rustc style visitor again. I'll see if I can hack something like this up because it would be so nice.

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript

[matklad]

The source for the current traversal is here: https://github.com/rust-analyzer/rowan/blob/40fe55c450c88f4da7cf315bf027bec3250a7352/src/cursor.rs#L389-L412

We should lift the traversal to rowan eventually, but it makes sense to start experimenting with it here