Merge #989

Merge #989

989: Implement naive version of fill_struct_fields assist r=matklad a=yanchith

Fixes #964

This implements the `fill_struct_fields` assist. Currently only works for named struct fields, but not for tuple structs, because we seem to be missing a `TupleStructLit` (akin to `StructLit`, but for tuple structs). I am happy to implement `TupleStructLit` parsing given some guidance (provided it's really missing) and make the assist work for tuple structs as well. Could do so either in this PR, or another one 🙂

Sorry if I missed something important, this is my first PR for Rust Analyzer.

Btw is there any way to run the assists in emacs?

UPDATE: I just realized that parsing `TupleStructLit` would be quite difficult as it it really similar, if not identical to a function call...

Co-authored-by: yanchith <yanchi.toth@gmail.com>