Warn about the TailCall attribute on non-rec functions or let-bound values

src/Compiler/Checking/TailCallChecks.fs

@@ -727,6 +727,23 @@ and CheckBindings cenv binds =
         CheckBinding cenv false PermitByRefExpr.Yes bind
 
 let CheckModuleBinding cenv (isRec: bool) (TBind _ as bind) =
+
+    // warn for non-rec functions which have the attribute
+    if
+        cenv.reportErrors
+        && cenv.g.langVersion.SupportsFeature LanguageFeature.WarningWhenTailCallAttrOnNonRec
+    then
+        let isNotAFunction =
+            match bind.Var.ValReprInfo with
+            | Some info -> info.HasNoArgs
+            | _ -> false
+
+        if
+            (not isRec || isNotAFunction)
+            && HasFSharpAttribute cenv.g cenv.g.attrib_TailCallAttribute bind.Var.Attribs
+        then
+            warning (Error(FSComp.SR.chkTailCallAttrOnNonRec (), bind.Var.Range))
+
     // Check that a let binding to the result of a rec expression is not inside the rec expression
     // see test ``Warn for invalid tailcalls in seq expression because of bind`` for an example
     // see test ``Warn successfully for rec call in binding`` for an example

src/Compiler/FSComp.txt

@@ -1590,6 +1590,7 @@ featureExtendedFixedBindings,"extended fixed bindings for byref and GetPinnableR
 featurePreferStringGetPinnableReference,"prefer String.GetPinnableReference in fixed bindings"
 featurePreferExtensionMethodOverPlainProperty,"prefer extension method over plain property"
 featureWarningIndexedPropertiesGetSetSameType,"Indexed properties getter and setter must have the same type"
+featureChkTailCallAttrOnNonRec,"Raises warnings if the 'TailCall' attribute is used on non-recursive functions."
 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."
 3355,tcNotAnIndexerNamedIndexingNotYetEnabled,"The value '%s' is not a function and does not support index notation."
@@ -1731,4 +1732,5 @@ featureUnmanagedConstraintCsharpInterop,"Interop between C#'s and F#'s unmanaged
 3584,tcDotLambdaAtNotSupportedExpression,"Shorthand lambda syntax is only supported for atomic expressions, such as method, property, field or indexer on the implied '_' argument. For example: 'let f = _.Length'."
 3855,tcNoStaticMemberFoundForOverride,"No static abstract member was found that corresponds to this override"
 3859,tcNoStaticPropertyFoundForOverride,"No static abstract property was found that corresponds to this override"
-3860,chkStaticMembersOnObjectExpressions,"Object expressions cannot implement interfaces with static abstract members or declare static members."
+3860,chkStaticMembersOnObjectExpressions,"Object expressions cannot implement interfaces with static abstract members or declare static members."
+3861,chkTailCallAttrOnNonRec,"The TailCall attribute should only be applied to recursive functions."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -81,6 +81,7 @@ type LanguageFeature =
     | PreferStringGetPinnableReference
     | PreferExtensionMethodOverPlainProperty
     | WarningIndexedPropertiesGetSetSameType
+    | WarningWhenTailCallAttrOnNonRec
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -189,6 +190,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.UnmanagedConstraintCsharpInterop, previewVersion
                 LanguageFeature.PreferExtensionMethodOverPlainProperty, previewVersion
                 LanguageFeature.WarningIndexedPropertiesGetSetSameType, previewVersion
+                LanguageFeature.WarningWhenTailCallAttrOnNonRec, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -327,6 +329,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.PreferStringGetPinnableReference -> FSComp.SR.featurePreferStringGetPinnableReference ()
         | LanguageFeature.PreferExtensionMethodOverPlainProperty -> FSComp.SR.featurePreferExtensionMethodOverPlainProperty ()
         | LanguageFeature.WarningIndexedPropertiesGetSetSameType -> FSComp.SR.featureWarningIndexedPropertiesGetSetSameType ()
+        | LanguageFeature.WarningWhenTailCallAttrOnNonRec -> FSComp.SR.featureChkTailCallAttrOnNonRec ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -72,6 +72,7 @@ type LanguageFeature =
     /// RFC-1137
     | PreferExtensionMethodOverPlainProperty
     | WarningIndexedPropertiesGetSetSameType
+    | WarningWhenTailCallAttrOnNonRec
 
 /// LanguageVersion management
 type LanguageVersion =