4178: Validate the location of `crate` in paths r=matklad a=djrenren
**This solution does not fully handle `use` statements. See below**
This pull requests implements simple validation of usages of the `crate` keyword in `Path`s. Specifically it validates that:
- If a `PathSegment` is starts with the `crate` keyword, it is also the first segment of the `Path`
- All other usages of `crate` in `Path`s are considered errors.
This aligns with `rustc`'s rules. Unlike rustc this implementation does not issue a special error message in the case of `::crate` but it does catch the error.
Furthermore, this change does not cover all error cases. Specifically the following is not caught:
```rust
use foo::{crate}
```
This is because this check is context sensitive. From an AST perspective, `crate` is the root of the `Path`. Only by inspecting the full `UseItem` do we see that it is not in fact the root. This problem becomes worse because `UseTree`s are allowed to be arbitrarily nested:
```rust
use {crate, {{crate, foo::{crate}}}
```
So this is a hard problem to solve without essentially a breadth-first search. In a traditional compiler, I'd say this error is most easily found during the AST -> HIR conversion pass but within rust-analyzer I'm not sure where it belongs.
Under the implementation in this PR, such errors are ignored so we're *more correct* just not *entirely correct*.
Co-authored-by: John Renner <john@jrenner.net>
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