4227: Report invalid, nested, multi-segment crate-paths r=matklad a=djrenren
There was a bug in the previous path-validating code that didn't detect multi-segment paths that started with `crate`.
```rust
// Successfully reported
use foo::{crate};
// BUG: was not being reported
use foo::{crate::bar};
```
This was due to my confusion about path-associativity. That is, the path with no qualifier is the innermost path, not the outermost. I've updated the code with a lot of comments to explain what's going on.
This bug was discovered when I found an erroneous `ok` test which I reported here:
https://github.com/rust-analyzer/rust-analyzer/issues/4226
This test now fails and has been modified, hopefully in the spirit of the original test, to be correct. Sorry about submitting the bug in the first place!
Co-authored-by: John Renner <john@jrenner.net>
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