[Tooling Optimization] Skip background type-checking on implementation files if signature files exist (dotnet#10199)

* Initial work to enable FSI optimizations in tooling

* Remove extra type-check function

* Not working but initial work to lazily eval

* some work

* simplify

* simplify again

* Using Async

* work

* work

* builds

* almost there

* Minor change

* minor refactor

* working, but not quite optimized

* Fix dependency files

* Refactor types

* Added 'enableLazyTypeChecking'.

* Enable lazy type checking in VS

* Added documentation

* Added SyntaxTreeAccumulator

* prevent parsing impl files if possible

* Trying to fix tests

* Refactor

* Throw if enablePartialChecking and keepAssemblyContents are both enabled

* minor comment update

* minor refactor

* Trying to pass tests

* Only getting assembly data

* Remove unnecessary change

* Added some documentation

* Minor comment update

* Renamed quickCheck to skipImplIfSigExists

* Minor update

* Feedback

Author: TIHan

src/fsharp/ParseAndCheckInputs.fs

@@ -576,7 +576,7 @@ type TcState =
 
     member x.NextStateAfterIncrementalFragment tcEnvAtEndOfLastInput = 
         { x with tcsTcSigEnv = tcEnvAtEndOfLastInput
-                 tcsTcImplEnv = tcEnvAtEndOfLastInput } 
+                 tcsTcImplEnv = tcEnvAtEndOfLastInput }
 
 
 /// Create the initial type checking state for compiling an assembly
@@ -621,7 +621,7 @@ let GetInitialTcState(m, ccuName, tcConfig: TcConfig, tcGlobals, tcImports: TcIm
       tcsCcuSig = Construct.NewEmptyModuleOrNamespaceType Namespace }
 
 /// Typecheck a single file (or interactive entry into F# Interactive)
-let TypeCheckOneInputEventually (checkForErrors, tcConfig: TcConfig, tcImports: TcImports, tcGlobals, prefixPathOpt, tcSink, tcState: TcState, inp: ParsedInput) =
+let TypeCheckOneInputEventually (checkForErrors, tcConfig: TcConfig, tcImports: TcImports, tcGlobals, prefixPathOpt, tcSink, tcState: TcState, inp: ParsedInput, skipImplIfSigExists: bool) =
 
     eventually {
         try 
@@ -686,11 +686,21 @@ let TypeCheckOneInputEventually (checkForErrors, tcConfig: TcConfig, tcImports:
               let conditionalDefines =
                   if tcConfig.noConditionalErasure then None else Some (tcConfig.conditionalCompilationDefines)
 
+              let hadSig = rootSigOpt.IsSome
+
               // Typecheck the implementation file 
-              let! topAttrs, implFile, _implFileHiddenType, tcEnvAtEnd, createsGeneratedProvidedTypes = 
-                  TypeCheckOneImplFile (tcGlobals, tcState.tcsNiceNameGen, amap, tcState.tcsCcu, checkForErrors, conditionalDefines, tcSink, tcConfig.internalTestSpanStackReferring) tcImplEnv rootSigOpt file
+              let typeCheckOne = 
+                  if skipImplIfSigExists && hadSig then
+                    let dummyExpr = ModuleOrNamespaceExprWithSig.ModuleOrNamespaceExprWithSig(rootSigOpt.Value, ModuleOrNamespaceExpr.TMDefs [], range.Zero)
+                    let dummyImplFile = TypedImplFile.TImplFile(qualNameOfFile, [], dummyExpr, false, false, StampMap [])
+
+                    (EmptyTopAttrs, dummyImplFile, Unchecked.defaultof<_>, tcImplEnv, false)
+                    |> Eventually.Done
+                  else
+                    TypeCheckOneImplFile (tcGlobals, tcState.tcsNiceNameGen, amap, tcState.tcsCcu, checkForErrors, conditionalDefines, tcSink, tcConfig.internalTestSpanStackReferring) tcImplEnv rootSigOpt file
+
+              let! topAttrs, implFile, _implFileHiddenType, tcEnvAtEnd, createsGeneratedProvidedTypes = typeCheckOne
 
-              let hadSig = rootSigOpt.IsSome
               let implFileSigType = SigTypeOfImplFile implFile
 
               let rootImpls = Zset.add qualNameOfFile tcState.tcsRootImpls
@@ -741,7 +751,7 @@ let TypeCheckOneInput (ctok, checkForErrors, tcConfig, tcImports, tcGlobals, pre
     // 'use' ensures that the warning handler is restored at the end
     use unwindEL = PushErrorLoggerPhaseUntilUnwind(fun oldLogger -> GetErrorLoggerFilteringByScopedPragmas(false, GetScopedPragmasForInput inp, oldLogger) )
     use unwindBP = PushThreadBuildPhaseUntilUnwind BuildPhase.TypeCheck
-    TypeCheckOneInputEventually (checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt, TcResultsSink.NoSink, tcState, inp) 
+    TypeCheckOneInputEventually (checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt, TcResultsSink.NoSink, tcState, inp, false) 
         |> Eventually.force ctok
 
 /// Finish checking multiple files (or one interactive entry into F# Interactive)
@@ -756,7 +766,7 @@ let TypeCheckMultipleInputsFinish(results, tcState: TcState) =
 let TypeCheckOneInputAndFinishEventually(checkForErrors, tcConfig: TcConfig, tcImports, tcGlobals, prefixPathOpt, tcSink, tcState, input) =
     eventually {
         Logger.LogBlockStart LogCompilerFunctionId.CompileOps_TypeCheckOneInputAndFinishEventually
-        let! results, tcState = TypeCheckOneInputEventually(checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt, tcSink, tcState, input)
+        let! results, tcState = TypeCheckOneInputEventually(checkForErrors, tcConfig, tcImports, tcGlobals, prefixPathOpt, tcSink, tcState, input, false)
         let result = TypeCheckMultipleInputsFinish([results], tcState)
         Logger.LogBlockStop LogCompilerFunctionId.CompileOps_TypeCheckOneInputAndFinishEventually
         return result
@@ -779,4 +789,3 @@ let TypeCheckClosedInputSet (ctok, checkForErrors, tcConfig, tcImports, tcGlobal
     let (tcEnvAtEndOfLastFile, topAttrs, implFiles, _), tcState = TypeCheckMultipleInputsFinish(results, tcState)
     let tcState, declaredImpls = TypeCheckClosedInputSetFinish (implFiles, tcState)
     tcState, topAttrs, declaredImpls, tcEnvAtEndOfLastFile
-

src/fsharp/ParseAndCheckInputs.fsi

@@ -87,7 +87,8 @@ val TypeCheckOneInputEventually :
     LongIdent option *
     NameResolution.TcResultsSink *
     TcState *
-    ParsedInput  
+    ParsedInput *
+    skipImplIfSigExists: bool
       -> Eventually<(TcEnv * TopAttribs * TypedImplFile option * ModuleOrNamespaceType) * TcState>
 
 /// Finish the checking of multiple inputs