Optimize format string parsing

[vasily-kirichenko]

This reduces completion time from 6 to 3 seconds on ProvidedTypes.fs in https://github.com/fsprojects/FSharp.TypeProviders.SDK/tree/master/tests project.

Before

After

[dsyme]

I'm sceptical.. :) This code will re-allcoate an unevaluated lazy value on every invocation (since property evaluation is on-demand in F#). From a quick code review I don't see how this might make things faster? And looking at the numbers above it seems like all the numbers have been cut be 1/3 and the proportion of time looks about the same

However I do agree that we should do the lines computation only once.

FWIW I don't like having Lazy<_> in APIs for this kind of reason, as it becomes harder to reason about lifetime. In the code, just make the property

abstract FormatStringCheckContext : FormatStringCheckContext option

and put the on-demand computation on the implementing side.

[vasily-kirichenko]

@dsyme wow, thanks, I’m stupid :( I’ll store the lazy in a field.

[vasily-kirichenko]

The property syntax in F# is too similar to let val declaration, with totally different semantics :(

[vasily-kirichenko]

Fixed the lazy usage, but the completion and everything on that file is awfully slow

[vasily-kirichenko]

The above gif is a very happy case. There are ones when completion after asms. appears in 13 seconds.

[dsyme]

Fixed the lazy usage, but the completion and everything on that file is awfully slow

Yes, it's a big horrible file. When #4579 gets fixed we should be able to ship the TPSDK as a DLL (and split that thing into multiple files).

[dsyme]

Minor nit: no parens needed (though they don't hurt)

[KevinRansom]

Thanks for this.