internal: Handle trait alias definitions

[lowr]

Part of #2773

This PR adds a bunch of structs and enum variants for trait aliases. Trait aliases should be handled as an independent item because they are semantically distinct from traits.

I basically started by adding TraitAlias{Id, Loc} to hir_def::item_tree and iterated adding necessary stuffs until compiler stopped complaining what's missing. Let me know if there's still anything I need to add.

I'm opening up this PR for early review and stuff. I'm planning to add tests for IDE functionalities in this PR, but not type-related support, for which I put FIXME notes.

[flodiebold]

Looks good to me 🎉

[lowr]

Update: I began adding test, which quickly made me realize that I forgot to impl ToDef - which wasn't straightforward because that trait pretty much requires one AST type to map to one HIR type whereas (previously) ast::Trait maps to either hir::Trait or hir::TraitAlias. There are 2 ways to solve:

• map ast::Trait to Either<hir::Trait, hir::TraitAlias>
• split ast::TraitAlias from ast::Trait

I've implemented taking the second approach as it seemed simpler, but I'm happy to hear opinions and thoughts.

EDIT: I've experimented with the first approach and I found it somewhat cumbersome to deal with Either every time we get def from ast::Trait. I'm inclined toward my current implementation.

[lowr]

This is now ready for full review. The commits are starting to get cluttered, I can squash relevant commits after review if desired.

Fully working IDE functionalities:

• doc_links
• file_structure
• move_item
• navigation_target
• runnables
• highlight_related

Partially working IDE functionalities (generally working for trait alias itself, but needs type support for full functionalities):

• goto_definition
• goto_type_definition
• hover
• inlay_hints
• moniker
• references
• rename
• signature_help

non-applicable functionalities: all others

[bors]

☔ The latest upstream changes (presumably #14223) made this pull request unmergeable. Please resolve the merge conflicts.

[lowr]

Rebased and organized commits. There's no diff in the final tree modulo now resolved conflict (which was merely one import statement). This PR should be good to go!

[Veykril]

@bors r+

[bors]

📌 Commit f8eac19 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit f8eac19 with merge 6756294...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 6756294 to master...

[lowr]

Might be worth noting explicitly in the changelog that we haven't implemented trait alias in our type system.

FYI, I was going to implement type support for trait alias, but after seeing rust-lang/rust#107614, I'm putting it on hold until types/lang teams decide what to do with it.

[HKalbasi]

@lowr Do you know what is the current state of trait aliases? Did the dust settle?

[lowr]

@HKalbasi I'm not really sure about its current state. The PR I mentioned in my last comment has been merged but without any decisions involved (which is reasonable as it's an unstable feature after all).