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

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

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -273,6 +273,7 @@
     <Compile Include="Interop\ParamArray.fs" />
     <Compile Include="Interop\Literals.fs" />
     <Compile Include="Scripting\Interactive.fs" />
+    <Compile Include="Scripting\TypeCheckOnlyTests.fs" />
     <Compile Include="TypeChecks\SeqTypeCheckTests.fs" />
     <Compile Include="TypeChecks\CheckDeclarationsTests.fs" />
     <Compile Include="TypeChecks\Graph\Utils.fs" />

tests/FSharp.Compiler.ComponentTests/Scripting/TypeCheckOnlyTests.fs

@@ -0,0 +1,47 @@
+module FSharp.Compiler.ComponentTests.Scripting.TypeCheckOnlyTests
+
+open Xunit
+open FSharp.Test
+open FSharp.Test.Compiler
+
+[<Fact>]
+let ``typecheck-only flag works for valid script``() =
+    Fsx """
+let x = 42
+printfn "This should not execute"
+exit 999 // this would have crashed if really running
+"""
+    |> withOptions ["--typecheck-only"]
+    |> runFsi
+    |> shouldSucceed
+
+[<Fact>]
+let ``typecheck-only flag catches type errors``() =
+    Fsx """
+let x: int = "string"  // Type error
+"""
+    |> withOptions ["--typecheck-only"]
+    |> runFsi
+    |> shouldFail
+    |> withStdErrContains """This expression was expected to have type"""
+
+[<Fact>]
+let ``typecheck-only flag prevents execution side effects``() =
+    Fsx """
+printfn "MyCrazyString"
+let x = 42
+"""
+    |> withOptions ["--typecheck-only"]
+    |> runFsi
+    |> shouldSucceed
+    |> verifyNotInOutput "MyCrazyString"
+
+[<Fact>]
+let ``script executes without typecheck-only flag``() =
+    Fsx """
+let x = 21+21
+"""
+    |> withOptions ["--nologo"]
+    |> runFsi
+    |> shouldSucceed
+    |> verifyOutputContains [|"val x: int = 42"|]

tests/FSharp.Test.Utilities/Compiler.fs

@@ -1471,6 +1471,15 @@ Actual:
                     failwith $"""Output does not match expected:{Environment.NewLine}{item}{Environment.NewLine}Actual:{Environment.NewLine}{actual}{Environment.NewLine}"""
             cResult
 
+    let verifyNotInOutput (expected: string) (cResult: CompilationResult) : CompilationResult =
+        match getOutput cResult with
+        | None -> cResult
+        | Some actual ->
+            if actual.Contains(expected) then
+                failwith $"""Output should not contain:{Environment.NewLine}{expected}{Environment.NewLine}Actual:{Environment.NewLine}{actual}{Environment.NewLine}"""
+            else
+                cResult
+
     type ImportScope = { Kind: ImportDefinitionKind; Name: string }
 
     type PdbVerificationOption =

tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_net9.0.bsl

@@ -21,7 +21,7 @@
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CodeAnalysis.Hosted.CompilerHelpers::fscCompile([FSharp.Compiler.Service]FSharp.Compiler.CodeAnalysis.LegacyReferenceResolver, string, string[])][offset 0x00000082][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CodeAnalysis.Hosted.CompilerHelpers::fscCompile([FSharp.Compiler.Service]FSharp.Compiler.CodeAnalysis.LegacyReferenceResolver, string, string[])][offset 0x0000008B][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+MagicAssemblyResolution::ResolveAssemblyCore([FSharp.Compiler.Service]Internal.Utilities.Library.CompilationThreadToken, [FSharp.Compiler.Service]FSharp.Compiler.Text.Range, [FSharp.Compiler.Service]FSharp.Compiler.CompilerConfig+TcConfigBuilder, [FSharp.Compiler.Service]FSharp.Compiler.CompilerImports+TcImports, [FSharp.Compiler.Service]FSharp.Compiler.Interactive.Shell+FsiDynamicCompiler, [FSharp.Compiler.Service]FSharp.Compiler.Interactive.Shell+FsiConsoleOutput, string)][offset 0x00000015][found Char] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+clo@3494-813::Invoke([S.P.CoreLib]System.Tuple`3<char[],int32,int32>)][offset 0x000001E5][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+clo@3499-813::Invoke([S.P.CoreLib]System.Tuple`3<char[],int32,int32>)][offset 0x000001E5][found Char] Unexpected type on the stack.
 [IL]: Error [UnmanagedPointer]: : FSharp.Compiler.Interactive.Shell+Utilities+pointerToNativeInt@106::Invoke(object)][offset 0x00000007] Unmanaged pointers are not a verifiable type.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$FSharpCheckerResults+dataTipOfReferences@2225::Invoke([FSharp.Core]Microsoft.FSharp.Core.Unit)][offset 0x00000084][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$ServiceLexing+clo@924-515::Invoke([FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`3<FSharp.Compiler.Parser+token,int32,int32>,Microsoft.FSharp.Core.Unit>)][offset 0x00000032][found Char] Unexpected type on the stack.

tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl

@@ -28,7 +28,7 @@
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CodeAnalysis.Hosted.CompilerHelpers::fscCompile([FSharp.Compiler.Service]FSharp.Compiler.CodeAnalysis.LegacyReferenceResolver, string, string[])][offset 0x0000008B][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+FsiStdinSyphon::GetLine(string, int32)][offset 0x00000039][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+MagicAssemblyResolution::ResolveAssemblyCore([FSharp.Compiler.Service]Internal.Utilities.Library.CompilationThreadToken, [FSharp.Compiler.Service]FSharp.Compiler.Text.Range, [FSharp.Compiler.Service]FSharp.Compiler.CompilerConfig+TcConfigBuilder, [FSharp.Compiler.Service]FSharp.Compiler.CompilerImports+TcImports, [FSharp.Compiler.Service]FSharp.Compiler.Interactive.Shell+FsiDynamicCompiler, [FSharp.Compiler.Service]FSharp.Compiler.Interactive.Shell+FsiConsoleOutput, string)][offset 0x00000015][found Char] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+clo@3494-813::Invoke([S.P.CoreLib]System.Tuple`3<char[],int32,int32>)][offset 0x000001E5][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+clo@3499-813::Invoke([S.P.CoreLib]System.Tuple`3<char[],int32,int32>)][offset 0x000001E5][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.Interactive.Shell+FsiInteractionProcessor::CompletionsForPartialLID([FSharp.Compiler.Service]FSharp.Compiler.Interactive.Shell+FsiDynamicCompilerState, string)][offset 0x0000001B][found Char] Unexpected type on the stack.
 [IL]: Error [UnmanagedPointer]: : FSharp.Compiler.Interactive.Shell+Utilities+pointerToNativeInt@106::Invoke(object)][offset 0x00000007] Unmanaged pointers are not a verifiable type.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$FSharpCheckerResults+dataTipOfReferences@2225::Invoke([FSharp.Core]Microsoft.FSharp.Core.Unit)][offset 0x00000084][found Char] Unexpected type on the stack.