Tail recursion warning and attribute, take 2

src/Compiler/FSComp.txt

@@ -1575,6 +1575,7 @@ 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."
@@ -1699,4 +1700,5 @@ 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 'TailCall' 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/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