Remove Cache when non-existing file or file outside of workspace gets closed

[Booksbaum]

Currently caches for all files are kept forever (inside State).

That's ok, but in certain situations unnecessary and we can safely remove caches:

• When a non-existing file gets closed (-> deleted file or untitled file (never existed in filesystem))
• When a file outside the current workspace gets closed

It's nice to slightly reduce memory footprint for long sessions, but also fixes following cases:

• VSCode shows old diags for a new untitled file with same name (even when language not set to F# in new untitled file...) -> clearing cache (or more precise: clearing diags) removes old diags

  • To reproduce:
    • In VSCode create new untitled document (Ctrl+N) (-> Untitled-1 if it's the only untitled doc)
    • set language to F# (-> F# script file)
    • write let foo = bar -> error "The value or constructor 'bar' is not defined."
    • close untitled document
    • create new untitled document (-> should have same name (Untitled-1))
    • Still shows error from old document

• Iterating over all projects and their source files in State contains closed non-existing files (like untitled files) and external files -> not something to consider when doing something in FSAC, but are currently inside State.

  • Also an issue for tests: Same server, every testCase creates and untitled files (and closes it afterwards) -> State of 2nd testCase contains file & project for 1st testCase
  • I don't think that's currently an actual issue because nothing iterates over all files in all projects.
    But I'm looking into "Find all references" -> Sometimes want to find in all files -> iterate over all projects & their files. But this contains obsolete files too

This PR removes non-existing files and files outside workspace when they get closed (textDocument/didClose Notification) -> removed from State & diags get cleared for that file.

Some issues:

• I think file might get reintroduced into State when a textDocument/didChange notification is still processed when textDocument/DidClose comes in
• I think: A file outside of current workspace & outside of project might still be kept if file was previously part of project but removed from it during the current session.
  • Reason: Changes (at least file removals) inside fsproj aren't adopted by FSAC -> inside State the file is still part of a project and requires a restart to revaluate.

[baronfel]

Great idea, and nice implementation. I agree with both of your caveats, but I don't think either is severe enough to prevent taking this,