Fixed uninitialized mutable locals inside loops (#6899)

* Fixed uninitialized mutable locals inside loops

* Update ForInDoMutableRegressionTest.fs

src/fsharp/IlxGen.fs

@@ -826,8 +826,15 @@ and IlxGenEnv =
 
       /// Are we under the scope of a try, catch or finally? If so we can't tailcall. SEH = structured exception handling
       withinSEH: bool
+
+      /// Are we inside of a recursive let binding, while loop, or a for loop?
+      isInLoop: bool
     }
 
+let SetIsInLoop isInLoop eenv =
+    if eenv.isInLoop = isInLoop then eenv
+    else { eenv with isInLoop = isInLoop }
+
 let ReplaceTyenv tyenv (eenv: IlxGenEnv) = {eenv with tyenv = tyenv }
 
 let EnvForTypars tps eenv = {eenv with tyenv = TypeReprEnv.ForTypars tps }
@@ -3369,6 +3376,7 @@ and GenTryFinally cenv cgbuf eenv (bodyExpr, handlerExpr, m, resty, spTry, spFin
 //--------------------------------------------------------------------------
 
 and GenForLoop cenv cgbuf eenv (spFor, v, e1, dir, e2, loopBody, m) sequel =
+    let eenv = SetIsInLoop true eenv
     let g = cenv.g
 
     // The JIT/NGen eliminate array-bounds checks for C# loops of form:
@@ -3459,6 +3467,7 @@ and GenForLoop cenv cgbuf eenv (spFor, v, e1, dir, e2, loopBody, m) sequel =
 //--------------------------------------------------------------------------
 
 and GenWhileLoop cenv cgbuf eenv (spWhile, e1, e2, m) sequel =
+    let eenv = SetIsInLoop true eenv
     let finish = CG.GenerateDelayMark cgbuf "while_finish"
     let startTest = CG.GenerateMark cgbuf "startTest"
 
@@ -5083,6 +5092,7 @@ and GenLetRecFixup cenv cgbuf eenv (ilxCloSpec: IlxClosureSpec, e, ilField: ILFi
 
 /// Generate letrec bindings
 and GenLetRecBindings cenv (cgbuf: CodeGenBuffer) eenv (allBinds: Bindings, m) =
+    let eenv = SetIsInLoop true eenv
     // Fix up recursion for non-toplevel recursive bindings
     let bindsPossiblyRequiringFixup =
         allBinds |> List.filter (fun b ->
@@ -5324,8 +5334,8 @@ and GenBindingAfterSequencePoint cenv cgbuf eenv sp (TBind(vspec, rhsExpr, _)) s
     | _ ->
         let storage = StorageForVal cenv.g m vspec eenv
         match storage, rhsExpr with
-        // locals are zero-init, no need to initialize them
-        | Local (_, realloc, _), Expr.Const (Const.Zero, _, _) when not realloc ->
+        // locals are zero-init, no need to initialize them, except if you are in a loop and the local is mutable.
+        | Local (_, realloc, _), Expr.Const (Const.Zero, _, _) when not realloc && not (eenv.isInLoop && vspec.IsMutable) ->
             CommitStartScope cgbuf startScopeMarkOpt
         | _ ->
             GenBindingRhs cenv cgbuf eenv SPSuppress vspec rhsExpr
@@ -7463,7 +7473,8 @@ let GetEmptyIlxGenEnv (ilg: ILGlobals) ccu =
       liveLocals=IntMap.empty()
       innerVals = []
       sigToImplRemapInfo = [] (* "module remap info" *)
-      withinSEH = false }
+      withinSEH = false
+      isInLoop = false }
 
 type IlxGenResults =
     { ilTypeDefs: ILTypeDef list

tests/fsharp/Compiler/Regressions/ForInDoMutableRegressionTest.fs

@@ -0,0 +1,73 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.UnitTests
+
+open System
+open NUnit.Framework
+
+[<TestFixture()>]
+module ForInDoMutableRegressionTest =
+
+    /// This test is to ensure we initialize locals inside loops.
+    [<Test>]
+    let Script_ForInDoMutableRegressionTest() =
+        let script = 
+            """
+open System.Collections.Generic
+
+let bug() = 
+    for a in [1;2;3;4] do
+      let mutable x = null
+      if x = null then
+        x <- HashSet<int>()
+      x.Add a |> ignore
+      let expected = [a]
+      let actual = List.ofSeq x
+      if expected <> actual then
+        failwith "Bug"
+
+let not_a_bug() = 
+  for a in [1;2;3;4] do
+    let x = ref null
+    if (!x) = null then
+      x := HashSet<int>()
+    (!x).Add a |> ignore
+    let expected = [a]
+    let actual = List.ofSeq (!x)
+    if expected <> actual then
+      failwith "Bug"
+
+let rec test_rec xs =
+    let mutable x = null
+    match xs with
+    | [] -> ()
+    | a :: xs ->
+        if x = null then
+          x <- HashSet<int>()
+        x.Add a |> ignore
+        let expected = [a]
+        let actual = List.ofSeq x
+        if expected <> actual then
+          failwith "Bug"
+        test_rec xs
+
+let test_for_loop () =
+    let xs = [|1;2;3;4|]
+    for i = 0 to xs.Length - 1 do
+      let a = xs.[i]
+      let mutable x = null
+      if x = null then
+        x <- HashSet<int>()
+      x.Add a |> ignore
+      let expected = [a]
+      let actual = List.ofSeq x
+      if expected <> actual then
+        failwith "Bug"
+
+bug ()
+not_a_bug ()
+test_rec [1;2;3;4]
+test_for_loop ()
+            """
+
+        CompilerAssert.RunScript script []

tests/fsharp/FSharpSuite.Tests.fsproj

@@ -37,6 +37,7 @@
     <Compile Include="Compiler\Language\SpanOptimizationTests.fs" />
     <Compile Include="Compiler\Language\SpanTests.fs" />
     <Compile Include="Compiler\Language\StringConcatOptimizationTests.fs" />
+    <Compile Include="Compiler\Regressions\ForInDoMutableRegressionTest.fs" />
     <Content Include="packages.config" />
     <None Include="app.config" />
     <None Include="update.base.line.with.actuals.fsx" />