Feature request: Find all assignments / mutable borrows

[jplatte]

In addition to "find all references", it would sometimes be very useful to find all places where a certain variable or struct field is either written to directly, or mutably borrowed.

[matklad]

More generally, occurences should be classified into following buckets:

• creation of new instances
• modification of existing instances
• reading
• doc comments

LSP doesn't currently have a nice way to present this in UI though. For structs, we have a horrible hack that seraching for { or ( narrows the search down to just constructors. I wonder if we can generalize that to writes somehow...

I guess for variables we can allow triggering search on the mut keyword, but that doesn't work for fields.

[matklad]

Some references to code:

• https://github.com/rust-analyzer/rust-analyzer/blob/59c758d0cb86122ecc926f2cb89cff72fbc71749/crates/ide_db/src/search.rs#L61
• https://github.com/rust-analyzer/rust-analyzer/blob/59c758d0cb86122ecc926f2cb89cff72fbc71749/crates/ide/src/references.rs#L172

Note that the second function shouldnt' probably exist -- instead, the relevant info shoudl just be present in ReferenceAccess (which is missnamed, let me fix that...)

[matklad]

cc #10426