Rust-analyzer: Adding a `use` should have higher priority than renaming to similar

[cjhopman]

When there's some use of a symbol SomeTrait that's from some other module and there's a similar symbol in scope (like HasSomeTrait), the option to add a use for SomeTrait should appear in vscode's action list thing as the first item, above renaming to the similar symbol.

[matklad]

Interesting problem! This happens because we have, effectively, three sources of light bulbs:

• our assists (which are correctly sorted by the target element)
• out diagnostics (which are not really sorted)
• diagnostics from rustc (which are not sorted as well)

And we just append diagnositcs before all other assists here:

https://github.com/rust-analyzer/rust-analyzer/blob/ebd1309c9a3ac0e6b0cee197f30a962d5263e727/crates/rust-analyzer/src/main_loop/handlers.rs#L704-L722

I think a better behavior would be to use this sort order:

• our diagnostics (b/c we know they are high priority)
• out assists
• rustc diagnostics.

[bnjjj]

I would be happy to work on this topic this weekend. Do you have any code example to be sure I understand the issue.