Tail recursion warning and attribute, analysis after optimization

src/Compiler/Checking/TailCallChecks.fsi

@@ -0,0 +1,13 @@
+module internal FSharp.Compiler.TailCallChecks
+
+open FSharp.Compiler
+open FSharp.Compiler.TcGlobals
+open FSharp.Compiler.TypedTree
+
+/// Perform the TailCall analysis on the optimized TAST for a file.
+/// The TAST is traversed analogously to the PostInferenceChecks phase.
+/// For functions that are annotated with the [<TailCall>] attribute, a warning is emmitted if they are called in a
+/// non-tailrecursive manner in the recursive scope of the function.
+/// The ModuleOrNamespaceContents aren't mutated in any way by performing this check.
+val CheckImplFile:
+    g: TcGlobals * amap: Import.ImportMap * reportErrors: bool * implFileContents: ModuleOrNamespaceContents -> unit

src/Compiler/Driver/fsc.fs

@@ -40,6 +40,7 @@ open FSharp.Compiler.CreateILModule
 open FSharp.Compiler.DependencyManager
 open FSharp.Compiler.Diagnostics
 open FSharp.Compiler.DiagnosticsLogger
+open FSharp.Compiler.Features
 open FSharp.Compiler.IlxGen
 open FSharp.Compiler.InfoReader
 open FSharp.Compiler.IO
@@ -878,6 +879,14 @@ let main3
 
         optimizedImpls, EncodeOptimizationData(tcGlobals, tcConfig, outfile, exportRemapping, (generatedCcu, optimizationData), false)
 
+    if tcGlobals.langVersion.SupportsFeature LanguageFeature.WarningWhenTailRecAttributeButNonTailRecUsage then
+        match optimizedImpls with
+        | CheckedAssemblyAfterOptimization checkedImplFileAfterOptimizations ->
+            ReportTime tcConfig ("TailCall Checks")
+
+            for f in checkedImplFileAfterOptimizations do
+                TailCallChecks.CheckImplFile(tcGlobals, tcImports.GetImportMap(), true, f.ImplFile.Contents)
+
     let refAssemblySignatureHash =
         match tcConfig.emitMetadataAssembly with
         | MetadataAssemblyGeneration.None -> None

src/Compiler/FSComp.txt

@@ -1576,6 +1576,7 @@ featureExtendedStringInterpolation,"Extended string interpolation similar to C#
 featureWarningWhenMultipleRecdTypeChoice,"Raises warnings when multiple record type matches were found during name resolution because of overlapping field names."
 featureImprovedImpliedArgumentNames,"Improved implied argument names"
 featureStrictIndentation,"Raises errors on incorrect indentation, allows better recovery and analysis during editing"
+featureChkNotTailRecursive,"Raises warnings if a member or function has the 'TailCall' attribute, but is not being used in a tail recursive way."
 3353,fsiInvalidDirective,"Invalid directive '#%s %s'"
 3354,tcNotAFunctionButIndexerNamedIndexingNotYetEnabled,"This value supports indexing, e.g. '%s.[index]'. The syntax '%s[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."
 3354,tcNotAFunctionButIndexerIndexingNotYetEnabled,"This expression supports indexing, e.g. 'expr.[index]'. The syntax 'expr[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."
@@ -1700,4 +1701,5 @@ featureInformationalObjInferenceDiagnostic,"Diagnostic 3559 (warn when obj infer
 3566,tcMultipleRecdTypeChoice,"Multiple type matches were found:\n%s\nThe type '%s' was used. Due to the overlapping field names\n%s\nconsider using type annotations or change the order of open statements."
 3567,parsMissingMemberBody,"Expecting member body"
 3568,parsMissingKeyword,"Missing keyword '%s'"
+3569,chkNotTailRecursive,"The member or function '%s' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way."
 3577,tcOverrideUsesMultipleArgumentsInsteadOfTuple,"This override takes a tuple instead of multiple arguments. Try to add an additional layer of parentheses at the method definition (e.g. 'member _.Foo((x, y))'), or remove parentheses at the abstract method declaration (e.g. 'abstract member Foo: 'a * 'b -> 'c')."

src/Compiler/FSharp.Compiler.Service.fsproj

@@ -337,6 +337,8 @@
     <Compile Include="Checking\QuotationTranslator.fs" />
     <Compile Include="Checking\PostInferenceChecks.fsi" />
     <Compile Include="Checking\PostInferenceChecks.fs" />
+    <Compile Include="Checking\TailCallChecks.fsi" />
+    <Compile Include="Checking\TailCallChecks.fs" />
     <Compile Include="Checking\CheckBasics.fsi" />
     <Compile Include="Checking\CheckBasics.fs" />
     <Compile Include="Checking\CheckRecordSyntaxHelpers.fsi" />

src/Compiler/Facilities/LanguageFeatures.fs

@@ -71,6 +71,7 @@ type LanguageFeature =
     | WarningWhenMultipleRecdTypeChoice
     | ImprovedImpliedArgumentNames
     | DiagnosticForObjInference
+    | WarningWhenTailRecAttributeButNonTailRecUsage
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -165,6 +166,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.ImprovedImpliedArgumentNames, previewVersion
                 LanguageFeature.DiagnosticForObjInference, previewVersion
                 LanguageFeature.StrictIndentation, previewVersion
+                LanguageFeature.WarningWhenTailRecAttributeButNonTailRecUsage, previewVersion
 
             ]
 
@@ -291,6 +293,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.ImprovedImpliedArgumentNames -> FSComp.SR.featureImprovedImpliedArgumentNames ()
         | LanguageFeature.DiagnosticForObjInference -> FSComp.SR.featureInformationalObjInferenceDiagnostic ()
         | LanguageFeature.StrictIndentation -> FSComp.SR.featureStrictIndentation ()
+        | LanguageFeature.WarningWhenTailRecAttributeButNonTailRecUsage -> FSComp.SR.featureChkNotTailRecursive ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -61,6 +61,7 @@ type LanguageFeature =
     | WarningWhenMultipleRecdTypeChoice
     | ImprovedImpliedArgumentNames
     | DiagnosticForObjInference
+    | WarningWhenTailRecAttributeButNonTailRecUsage
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/TypedTree/TcGlobals.fs

@@ -1509,6 +1509,7 @@ type TcGlobals(
   member val attrib_CompilerFeatureRequiredAttribute       = findSysAttrib "System.Runtime.CompilerServices.CompilerFeatureRequiredAttribute"
   member val attrib_SetsRequiredMembersAttribute           = findSysAttrib "System.Diagnostics.CodeAnalysis.SetsRequiredMembersAttribute"
   member val attrib_RequiredMemberAttribute                = findSysAttrib "System.Runtime.CompilerServices.RequiredMemberAttribute"
+  member val attrib_TailCallAttribute                      = mk_MFCore_attrib "TailCallAttribute"
 
   member g.improveType tcref tinst = improveTy tcref tinst