Compiler service: parallel parsing.

Is your feature request related to a problem? Please describe.

IDE interactions in F# projects are slow and they make development experience subpar.

Describe the solution you’d like

One way to speed them up is to allow all files in a project to be parsed in parallel, when type-checking of multiple files/whole project is about to be performed.

Describe alternatives you’ve considered

Introducing a parsing cache of configurable size, that applies to all endpoints correctly would limit the slowdown caused by slow, sequential parsing. I raised https://github.com/dotnet/fsharp/issues/14848 for that. However:

1. this doesn’t apply to first-time parsing after IDE load
2. parsing cache can get full. I do think that a cache large-enough to cover all source files in the solution is likely to have a small-enough memory footprint to be acceptable for most users - so hopefully this isn’t very relevant.
3. Every time a project changes, all parsing results get invalidated.

Additional context

• Parallel parsing already happens in compilation. Doing the same in the service will be more difficult, as the IncrementalBuilder has the idea of sequential type-checking+parsing very much embedded in it (see the BoundModel type).
• A discussion I raised about parallel operations for code analysis

[majocha]

@safesparrow I have something that works already and shows some modest gains in benchmarks. It feels OK in the IDE, although I yet have to install it and use for a while.

[majocha]

1. looks like one IB is constructed per project, it already gets a complete file list that resides in InitialState (does not change). Parsing happens at the type check phase later.
2. I think it is safe in the current model where cached ParsedInputs lifetime is connected with IB lifetime. How often are IBs destroyed? That’s a question.
3. We can’t really cancel a single parsing task at all, from what I understand. I think by the time you unload a project, switch branches etc. there will be no running tasks. What’s to look for is the initial impact, opening first file and waiting for typecheck.