9206: minor: Speed up fst items lookup during completions r=SomeoneToIgnore a=SomeoneToIgnore
Part of https://github.com/rust-analyzer/rust-analyzer/issues/7542
A number of profile calls added for `import_on_the_fly` contents.
Before:
<img width="606" alt="Screenshot 2021-06-11 at 00 19 13" src="https://user-images.githubusercontent.com/
2690773/
121598998-
22321e80-ca4b-11eb-9a3d-
dc9cb2936705.png">
After:
<img width="859" alt="Screenshot 2021-06-11 at 00 19 27" src="https://user-images.githubusercontent.com/
2690773/
121599022-
2a8a5980-ca4b-11eb-82b6-
13ab0ed56d58.png">
As a result, low hanging fruit was spotted: crazy amount of `fst_path` calls. Reducing that won ~200ms in the `import_on_the_fly @ sel` case in the `integrated_completion_benchmark`:
<img width="861" alt="Screenshot 2021-06-11 at 00 19 38" src="https://user-images.githubusercontent.com/
2690773/
121599277-
7d641100-ca4b-11eb-8667-
53206994de27.png">
I'm not sure how to proceed with the remaining `???` marks in such methods as `collect_import_map` though: there's nothing but library calls in cycles, but maybe I'll come up with something later.
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>