Do not warn unused variables inside extern declarations (dotnet#2032)

* Try to reproduce dotnet#485

* Do not warn unused variables inside extern declarations

Fix dotnet#485.

Author: dungpa

src/fsharp/PostInferenceChecks.fs

@@ -140,7 +140,9 @@ type env =
       /// Are we in a quotation?
       quote : bool 
       /// Are we under [<ReflectedDefinition>]?
-      reflect : bool } 
+      reflect : bool
+      /// Are we in an extern declaration?
+      external : bool } 
 
 let BindTypar env (tp:Typar) = 
     { env with 
@@ -178,11 +180,12 @@ type cenv =
       mutable usesQuotations : bool
       mutable entryPointGiven:bool  }
 
-let BindVal cenv (v:Val) = 
+let BindVal cenv env (v:Val) = 
     //printfn "binding %s..." v.DisplayName
     let alreadyDone = cenv.boundVals.ContainsKey v.Stamp
     cenv.boundVals.[v.Stamp] <- 1
-    if not alreadyDone &&
+    if not env.external &&
+       not alreadyDone &&
        cenv.reportErrors && 
        not v.HasBeenReferenced && 
        not v.IsCompiledAsTopLevel && 
@@ -195,7 +198,7 @@ let BindVal cenv (v:Val) =
         | _ -> 
             warning (Error(FSComp.SR.chkUnusedValue v.DisplayName, v.Range))
 
-let BindVals cenv vs = List.iter (BindVal cenv) vs
+let BindVals cenv env vs = List.iter (BindVal cenv env) vs
 
 //--------------------------------------------------------------------------
 // approx walk of type
@@ -529,7 +532,7 @@ and CheckExpr (cenv:cenv) (env:env) expr (context:ByrefContext) =
 
     | Expr.Let (bind,body,_,_) ->  
         CheckBinding cenv env false bind  
-        BindVal cenv bind.Var
+        BindVal cenv env bind.Var
         CheckExpr cenv env body context
 
     | Expr.Const (_,m,ty) -> 
@@ -700,7 +703,7 @@ and CheckExpr (cenv:cenv) (env:env) expr (context:ByrefContext) =
         CheckDecisionTreeTargets cenv env targets context
 
     | Expr.LetRec (binds,e,_,_) ->  
-        BindVals cenv (valsOfBinds binds)
+        BindVals cenv env (valsOfBinds binds)
         CheckBindings cenv env binds
         CheckExprNoByrefs cenv env e
 
@@ -943,7 +946,7 @@ and CheckLambdas isTop (memInfo: ValMemberInfo option) cenv env inlined topValIn
             restArgs |> List.iter (fun arg -> if isByrefTy cenv.g arg.Type then arg.SetHasBeenReferenced())
 
         syntacticArgs |> List.iter (CheckValSpec cenv env)
-        syntacticArgs |> List.iter (BindVal cenv)
+        syntacticArgs |> List.iter (BindVal cenv env)
 
         // Trigger a test hook
         match memInfo with 
@@ -1014,7 +1017,7 @@ and CheckDecisionTreeTargets cenv env targets context =
     targets |> Array.iter (CheckDecisionTreeTarget cenv env context) 
 
 and CheckDecisionTreeTarget cenv env context (TTarget(vs,e,_)) = 
-    BindVals cenv vs 
+    BindVals cenv env vs 
     vs |> List.iter (CheckValSpec cenv env)
     CheckExpr cenv env e context 
 
@@ -1136,6 +1139,8 @@ and CheckBinding cenv env alwaysCheckNoReraise (TBind(v,bindRhs,_) as bind) =
     let isTop = Option.isSome bind.Var.ValReprInfo
     //printfn "visiting %s..." v.DisplayName
 
+    let env = { env with external = env.external || cenv.g.attrib_DllImportAttribute |> Option.exists (fun attr -> HasFSharpAttribute cenv.g attr v.Attribs) }
+
     // Check that active patterns don't have free type variables in their result
     match TryGetActivePatternInfo (mkLocalValRef v) with 
     | Some _apinfo when _apinfo.ActiveTags.Length > 1 -> 
@@ -1669,13 +1674,13 @@ and CheckDefnInModule cenv env x =
     match x with 
     | TMDefRec(isRec,tycons,mspecs,m) -> 
         CheckNothingAfterEntryPoint cenv m
-        if isRec then BindVals cenv (allValsOfModDef x |> Seq.toList)
+        if isRec then BindVals cenv env (allValsOfModDef x |> Seq.toList)
         CheckEntityDefns cenv env tycons
         List.iter (CheckModuleSpec cenv env) mspecs
     | TMDefLet(bind,m)  -> 
         CheckNothingAfterEntryPoint cenv m
         CheckModuleBinding cenv env bind 
-        BindVal cenv bind.Var
+        BindVal cenv env bind.Var
     | TMDefDo(e,m)  -> 
         CheckNothingAfterEntryPoint cenv m
         CheckNoReraise cenv None e
@@ -1686,7 +1691,7 @@ and CheckDefnInModule cenv env x =
 and CheckModuleSpec cenv env x =
     match x with 
     | ModuleOrNamespaceBinding.Binding bind ->
-        BindVals cenv (valsOfBinds [bind])
+        BindVals cenv env (valsOfBinds [bind])
         CheckModuleBinding cenv env bind
     | ModuleOrNamespaceBinding.Module (mspec, rhs) ->
         CheckEntityDefn cenv env mspec
@@ -1726,7 +1731,8 @@ let CheckTopImpl (g,amap,reportErrors,infoReader,internalsVisibleToPaths,viewCcu
           boundTyparNames=[]
           argVals = ValMap.Empty
           boundTypars= TyparMap.Empty
-          reflect=false }
+          reflect=false
+          external=false }
 
     CheckModuleExpr cenv env mexpr
     CheckAttribs cenv env extraAttribs

tests/fsharpqa/Source/Warnings/DontWarnExternalFunctionAsUnused.fs

@@ -0,0 +1,19 @@
+//<Expects status="success" ></Expects>
+open System
+open System.Runtime.InteropServices
+
+module Test =
+
+    [<DllImport("shell32.dll", CharSet=CharSet.Auto)>]
+    extern int32 ExtractIconEx(string szFileName, int nIconIndex,IntPtr[] phiconLarge, IntPtr[] phiconSmall,uint32 nIcons)
+
+    [<DllImport("user32.dll", EntryPoint="DestroyIcon", SetLastError=true)>]
+    extern int DestroyIcon(IntPtr hIcon)
+
+[<EntryPoint>]
+let main _argv = 
+    let _ = Test.DestroyIcon IntPtr.Zero
+
+    let _ = Test.ExtractIconEx("", 0, [| |], [| |], 0u)
+
+    0

tests/fsharpqa/Source/Warnings/env.lst

@@ -20,6 +20,7 @@
 	SOURCE=AccessOfTypeAbbreviation4.fs # AccessOfTypeAbbreviation4.fs
 	SOURCE=AccessOfTypeAbbreviation5.fs # AccessOfTypeAbbreviation5.fs
 	SOURCE=AccessOfTypeAbbreviation6.fs # AccessOfTypeAbbreviation6.fs
+	SOURCE=DontWarnExternalFunctionAsUnused.fs SCFLAGS="--warnon:1182 --warnaserror+" # DontWarnExternalFunctionAsUnused.fs
 	SOURCE=SuggestTypesInModule.fs # SuggestTypesInModule.fs
 	SOURCE=SuggestGenericType.fs # SuggestGenericType.fs
 	SOURCE=SuggestTypeParameters.fs # SuggestTypeParameters.fs