Allow ParseFileInProject be called from outside Reactor

[vasily-kirichenko]

Currently (very fast) parsing jobs have to wait in Reactor queue for (slow) type check jobs. This results with a situation when you open a file in project and it's started to check (and all files above it, if it's the first time a file in the project is touched), blocking any ParseFileInProject until it's finished. It means no structure guidelines, no nav bar for long time (I know Rider guys are also struggling with it). As I understand, parsing is performed under reactor because it gets IncrementalBulider for project to get TcConfig, project source file list and dependency list from it, see https://github.com/Microsoft/visualfsharp/blob/master/src/fsharp/vs/service.fs#L2278-L2285

Is it possible allow parsing outside Reactor?

[dsyme]

@vasily-kirichenko Yes and no.

For ParseOneFile it should be possible to remove the CompilationThread requirement, see this comment:

          // This function requires the compilation thread because we install error handlers, whose callbacks must
          // be invoked on the compilation thread, no other reason known to date.
          // We should check whether those are "real" reasons - we could for example make collecting errors thread safe.
          RequireCompilationThread ctok 

The error loggers can actually be installed on any thread and contrary to what the comment says . AFAIK the same logger is not being used in multiple parsing calls, so it should be safe.

However prior to calling ParseOneFile we need to do this:

            let! builderOpt,creationErrors,decrement = getOrCreateBuilderAndKeepAlive (ctok, options, userOpName)

which does require the compilation thread. This is basically creating a project builder just to get the TcConfig configuration (conditional defines and any other settings relevant to parsing). Roslyn separates out compilation options relevant to parsing - the F# compiler implementation currently does not.

[dsyme]

Note that I believe #3063 would address many of these "long wait for language service features to become active on startup" problems. Perhaps you can try it out.