Merge #3120

3120: Support trait auto import r=matklad a=SomeoneToIgnore

Unfortunately, for real cases it does not work as spectacular as in the tests.

The main reason for that is type inference:
* The callee type [here](https://github.com/rust-analyzer/rust-analyzer/blob/master/crates/ra_hir_ty/src/method_resolution.rs#L369) is unknown for many cases
* The trait solution [here](https://github.com/rust-analyzer/rust-analyzer/blob/master/crates/ra_hir_ty/src/method_resolution.rs#L399) is also often ambiguous

That results in trait candidates being rejected, and some real cases not supported.
Example: no imports for `String::from_str("test")`

Co-authored-by: Kirill Bulatov <[email protected]>

Author: bors[bot]