Use FCS nuget package

[alfonsogarciacaro]

@ncave If I'm understanding correctly latest improvements in the F# compiler, FCS already does some similar caching to service_slim so may be it's a good opportunity to use the Nuget package directly and reduce the work of maintaining a fork. Only thing is we still need to maintain/update fcs_fable for the repl and fable-compiler-js... and by "we" I mean "you" ;) What do you think?

[ncave]

@alfonsogarciacaro Looks good to me. It was never a goal in itself to maintain a fork of FCS for .NET, it was only to overcome the transitional period when FCS was released less often and from a downstream repo. For FCS-Fable (repl and fable-compiler-js) I will have to maintain the forks, it's not that much effort, and it's nice to let people play with Fable online.

That said, I need to update the forks soon, do you have a preference which dotnet/fsharp branch to use, or do we just do main and stay on the bleeding edge (as always :) ?

[ncave]

@alfonsogarciacaro What does the performance looks like after switching to fcs-nuget, any difference on large projects?

[alfonsogarciacaro]

I spent some time trying to see if we could parallelize the build or release memory for already compiled files. But this didn't yield positive results. I haven't properly measured compilation as it's now, I just run it once on a big project and the first impression was the speed was more or less the same, but I need to check how watch compilations behave.

[alfonsogarciacaro]

@ncave These are the numbers I got when compiling fable standalone. I triggered the watch compilation by touching illib.fs which is at the top of the file list.

• main

  • First F# compilation: 19457ms
  • Watch F# compilation: 11602ms

• fcs-nuget

  • First F# compilation: 18765ms
  • Watch F# compilation: 11908ms

So the numbers are very similar. Maybe we could improve the watch compilation for files on top of the list by checking only the dirty files instead of the whole project, but that would be a different PR :)

[alfonsogarciacaro]

@ncave You're probably already into it, but I already tried to fix fcs-fable (better to hide whitespace to see the diff, I just added |> Seq.toList wherever the compiler was complaining).

Actually I'm a bit puzzled I had to fix some build errors in fcs-fable because these files weren't touched 🤔

However, when trying to build Fable.Standalone I still get this error:

src\fcs-fable\src\fsharp\TypedTree.fs(1,1): error EXCEPTION: The exception has been reported. This internal exception should now be caught at an error recovery point on the stack. Original message: Exception of type 'FSharp.Compiler.ConstraintSolver+ConstraintSolverError' was thrown.)
Compilation failed

Not sure if it rings a bell. It'd be nice to be able to merge this with fable-standalone working, but we can also wait until fcs-fable is updated to keep it in sync with Fable.Cli.

[ncave]

@alfonsogarciacaro I was waiting for F# 6 and .NET 6 to officially drop before I start on fcs-fable, I guess I'll have to start now that it is released.