Clean up tests and add documentation for GATs related stuff
This is a follow-up PR for #13494.
- addresses https://github.com/rust-lang/rust-analyzer/pull/13494#discussion_r1006774897
- documents the ordering constraint on `Binders` and `Substitution` (which is not really follow-up for the previous PR, but it was introduced to support GATs and I strongly feel it's worth it)
feat: type inference for generic associated types
This PR implements type inference for generic associated types. Basically, this PR lowers generic arguments for associated types in valid places and creates `Substitution`s for them.
I focused on the inference for correct Rust programs, so there are cases where we *accidentally* manage to infer things that are actually invalid (which would then be reported by flycheck so I deem them non-fatal). See the following tests and FIXME notes on them: `gats_with_dyn`, `gats_with_impl_trait`.
The added tests are rather arbitrary. Let me know if there are cases I'm missing or I should add.
Closes#9673
Switch to upstream `positionEncoding`
Closes#13481
This drops support for the custom extension, but that's probably fine.
Draft because it's not tested yet.
fix: Test all generic args for trait when finding matching impl
Addresses https://github.com/rust-lang/rust-analyzer/pull/13463#issuecomment-1287816680
When finding matching impl for a trait method, we've been testing the unifiability of self type. However, there can be multiple impl of a trait for the same type with different generic arguments for the trait. This patch takes it into account and tests the unifiability of all type arguments for the trait (the first being the self type) thus enables rust-analyzer to find the correct impl even in such cases.
