[WIP] Support --typecheck-only for fsi run (just typecheck, no execution) (#18687)

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -1,6 +1,7 @@
 ### Added
 * Add opt-in warning attribute not valid for union case with fields [PR #18532](https://github.com/dotnet/fsharp/pull/18532))
 * Add support for `when 'T : Enum` library-only static optimization constraint. ([PR #18546](https://github.com/dotnet/fsharp/pull/18546))
+* Add `--typecheck-only` flag support for F# Interactive (FSI) scripts to type-check without execution. ([Issue #18686](https://github.com/dotnet/fsharp/issues/18686))
 
 ### Fixed
 

src/Compiler/Driver/CompilerOptions.fs

@@ -1308,6 +1308,13 @@ let advancedFlagsFsi tcConfigB =
             None,
             Some(FSComp.SR.optsClearResultsCache ())
         )
+        CompilerOption(
+            "typecheck-only",
+            tagNone,
+            OptionUnit(fun () -> tcConfigB.typeCheckOnly <- true),
+            None,
+            Some(FSComp.SR.optsTypecheckOnly ())
+        )
     ]
 
 let advancedFlagsFsc tcConfigB =

src/Compiler/FSComp.txt

@@ -902,6 +902,7 @@ optsVersion,"Display compiler version banner and exit"
 optsResponseFile,"Read response file for more options"
 optsCodepage,"Specify the codepage used to read source files"
 optsClearResultsCache,"Clear the package manager results cache"
+optsTypecheckOnly,"Perform type checking only, do not execute code"
 optsUtf8output,"Output messages in UTF-8 encoding"
 optsFullpaths,"Output messages with fully qualified paths"
 optsLib,"Specify a directory for the include path which is used to resolve source files and assemblies (Short form: -I)"

src/Compiler/Interactive/fsi.fs

@@ -2217,6 +2217,11 @@ type internal FsiDynamicCompiler
                     inputs
                 ))
 
+        // typeCheckOnly either reports all errors found so far or exits with 0 - it stops processing the script
+        if tcConfig.typeCheckOnly then
+            diagnosticsLogger.AbortOnError(fsiConsoleOutput)
+            raise StopProcessing
+
         let codegenResults, optEnv, fragName =
             ProcessTypedImpl(
                 diagnosticsLogger,
@@ -4763,6 +4768,7 @@ type FsiEvaluationSession
         let userRes =
             match res with
             | Choice1Of2 r -> Choice1Of2 r
+            | Choice2Of2 None when errorInfos.Length = 0 && tcConfigB.typeCheckOnly -> Choice1Of2 None
             | Choice2Of2 None ->
                 Choice2Of2(FsiCompilationException(FSIstrings.SR.fsiOperationCouldNotBeCompleted (), Some errorInfos) :> exn)
             | Choice2Of2(Some userExn) -> Choice2Of2 userExn