Allow _ in use bindings

src/fsharp/CheckExpressions.fs

@@ -9770,7 +9770,14 @@ and TcLetBinding cenv isUse env containerInfo declKind tpenv (synBinds, synBinds
 
                 let tmp, _ = mkCompGenLocal m "patternInput" (generalizedTypars +-> tauTy)
 
-                if isUse || isFixed then
+                if isUse then
+                    let isDiscarded = match checkedPat with TPat_wild _ -> true | _ -> false
+                    if not isDiscarded then
+                        errorR(Error(FSComp.SR.tcInvalidUseBinding(), m))
+                    else
+                        ErrorLogger.checkLanguageFeatureError cenv.g.langVersion Features.LanguageFeature.UseBindingValueDiscard checkedPat.Range
+
+                elif isFixed then
                     errorR(Error(FSComp.SR.tcInvalidUseBinding(), m))
 
                 // If the overall declaration is declaring statics or a module value, then force the patternInputTmp to also
@@ -9798,6 +9805,8 @@ and TcLetBinding cenv isUse env containerInfo declKind tpenv (synBinds, synBinds
         // Add the dispose of any "use x = ..." to bodyExpr
         let mkCleanup (bodyExpr, bodyExprTy) =
             if isUse && not isFixed then
+                let isDiscarded = match checkedPat2 with TPat_wild _ -> true | _ -> false
+                let allValsDefinedByPattern = if isDiscarded then [patternInputTmp] else allValsDefinedByPattern
                 (allValsDefinedByPattern, (bodyExpr, bodyExprTy)) ||> List.foldBack (fun v (bodyExpr, bodyExprTy) ->
                     AddCxTypeMustSubsumeType ContextInfo.NoContext denv cenv.css v.Range NoTrace cenv.g.system_IDisposable_ty v.Type
                     let cleanupE = BuildDisposableCleanup cenv env m v

src/fsharp/FSComp.txt

@@ -1231,6 +1231,7 @@ invalidFullNameForProvidedType,"invalid full name for provided type"
 featureOverloadsForCustomOperations,"overloads for custom operations"
 featureExpandedMeasurables,"more types support units of measure"
 featurePrintfBinaryFormat,"binary formatting for integers"
+featureDiscardUseValue,"discard pattern in use binding"
 3090,tcIfThenElseMayNotBeUsedWithinQueries,"An if/then/else expression may not be used within queries. Consider using either an if/then expression, or use a sequence expression instead."
 3091,ilxgenUnexpectedArgumentToMethodHandleOfDuringCodegen,"Invalid argument to 'methodhandleof' during codegen"
 3092,etProvidedTypeReferenceMissingArgument,"A reference to a provided type was missing a value for the static parameter '%s'. You may need to recompile one or more referenced assemblies."

src/fsharp/LanguageFeatures.fs

@@ -37,6 +37,7 @@ type LanguageFeature =
     | OverloadsForCustomOperations
     | ExpandedMeasurables
     | PrintfBinaryFormat
+    | UseBindingValueDiscard
 
 /// LanguageVersion management
 type LanguageVersion (specifiedVersionAsString) =
@@ -79,6 +80,7 @@ type LanguageVersion (specifiedVersionAsString) =
             LanguageFeature.ExpandedMeasurables, previewVersion
             LanguageFeature.FromEndSlicing, previewVersion
             LanguageFeature.PrintfBinaryFormat, previewVersion
+            LanguageFeature.UseBindingValueDiscard, previewVersion
         ]
 
     let specified =
@@ -153,6 +155,7 @@ type LanguageVersion (specifiedVersionAsString) =
         | LanguageFeature.OverloadsForCustomOperations -> FSComp.SR.featureOverloadsForCustomOperations()
         | LanguageFeature.ExpandedMeasurables -> FSComp.SR.featureExpandedMeasurables()
         | LanguageFeature.PrintfBinaryFormat -> FSComp.SR.featurePrintfBinaryFormat()
+        | LanguageFeature.UseBindingValueDiscard -> FSComp.SR.featureDiscardUseValue()
 
     /// Get a version string associated with the given feature.
     member _.GetFeatureVersionString feature =

src/fsharp/LanguageFeatures.fsi

@@ -25,6 +25,7 @@ type LanguageFeature =
     | OverloadsForCustomOperations
     | ExpandedMeasurables
     | PrintfBinaryFormat
+    | UseBindingValueDiscard
 
 /// LanguageVersion management
 type LanguageVersion =

src/fsharp/xlf/FSComp.txt.cs.xlf

@@ -97,6 +97,11 @@
         <target state="translated">využití člena výchozího rozhraní</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">literál float32 bez tečky</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.de.xlf

@@ -97,6 +97,11 @@
         <target state="translated">standardmäßige Schnittstellenmembernutzung</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">punktloses float32-Literal</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.es.xlf

@@ -97,6 +97,11 @@
         <target state="translated">consumo de miembros de interfaz predeterminados</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">literal float32 sin punto</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.fr.xlf

@@ -97,6 +97,11 @@
         <target state="translated">consommation par défaut des membres d'interface</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">littéral float32 sans point</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.it.xlf

@@ -97,6 +97,11 @@
         <target state="translated">utilizzo predefinito dei membri di interfaccia</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">valore letterale float32 senza punti</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.ja.xlf

@@ -97,6 +97,11 @@
         <target state="translated">既定のインターフェイス メンバーの消費</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">ドットなしの float32 リテラル</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.ko.xlf

@@ -97,6 +97,11 @@
         <target state="translated">기본 인터페이스 멤버 사용</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">점이 없는 float32 리터럴</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.pl.xlf

@@ -97,6 +97,11 @@
         <target state="translated">domyślne użycie składowej interfejsu</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">bezkropkowy literał float32</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.pt-BR.xlf

@@ -97,6 +97,11 @@
         <target state="translated">consumo de membro da interface padrão</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">literal float32 sem ponto</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.ru.xlf

@@ -97,6 +97,11 @@
         <target state="translated">использование элемента интерфейса по умолчанию</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">литерал float32 без точки</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.tr.xlf

@@ -97,6 +97,11 @@
         <target state="translated">varsayılan arabirim üyesi tüketimi</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">noktasız float32 sabit değeri</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.zh-Hans.xlf

@@ -97,6 +97,11 @@
         <target state="translated">默认接口成员消耗</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">无点 float32 文本</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

src/fsharp/xlf/FSComp.txt.zh-Hant.xlf

@@ -97,6 +97,11 @@
         <target state="translated">預設介面成員使用</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureDiscardUseValue">
+        <source>discard pattern in use binding</source>
+        <target state="new">discard pattern in use binding</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDotlessFloat32Literal">
         <source>dotless float32 literal</source>
         <target state="translated">無點號的 float32 常值</target>
@@ -7659,4 +7664,4 @@
       </trans-unit>
     </body>
   </file>
-</xliff>
+</xliff>

tests/FSharp.Compiler.ComponentTests/Conformance/DeclarationElements/LetBindings/UseBindings.fs

@@ -0,0 +1,62 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.ComponentTests.Conformance.DeclarationElements.LetBindings
+
+open Xunit
+open FSharp.Test.Utilities.Compiler
+open FSharp.Test.Utilities.Xunit.Attributes
+
+module UseBindings =
+
+    [<Theory; Directory(__SOURCE_DIRECTORY__ + "/../../../resources/tests/Conformance/DeclarationElements/LetBindings", Includes=[|"UseBindingDiscard01.fs"|])>]
+    let ``UseBindings - UseBindingDiscard01.fs - Compiles`` compilation =
+        compilation
+        |> asFsx
+        |> withOptions ["--langversion:preview"]
+        |> compile
+        |> shouldSucceed
+        |> ignore
+
+    [<Theory; Directory(__SOURCE_DIRECTORY__ + "/../../../resources/tests/Conformance/DeclarationElements/LetBindings", Includes=[|"UseBindingDiscard01.fs"|])>]
+    let ``UseBindings - UseBindingDiscard01.fs - Bad LangVersion`` compilation =
+        compilation
+        |> asFsx
+        |> withOptions ["--langversion:5.0"]
+        |> compile
+        |> shouldFail
+        |> withErrorCode 3350
+        |> withDiagnosticMessageMatches "Feature 'discard pattern in use binding' is not available.*"
+
+    [<Fact>]
+    let ``Dispose called for discarded value of use binding`` () =
+        Fsx """
+type private Disposable() =
+    [<DefaultValue>] static val mutable private disposedTimes: int
+    [<DefaultValue>] static val mutable private constructedTimes: int
+
+    do Disposable.constructedTimes <- Disposable.constructedTimes + 1
+
+    static member DisposeCallCount() = Disposable.disposedTimes
+    static member ConsturctorCallCount() = Disposable.constructedTimes
+
+    interface System.IDisposable with
+        member _.Dispose() =
+            Disposable.disposedTimes <- Disposable.disposedTimes + 1
+
+let _scope =
+    use _ = new Disposable()
+    ()
+
+let disposeCalls = Disposable.DisposeCallCount()
+if disposeCalls <> 1 then
+    failwith "was not disposed or disposed too many times"
+
+let ctorCalls = Disposable.ConsturctorCallCount()
+if ctorCalls <> 1 then
+    failwithf "unexpected constructor call count: %i" ctorCalls
+
+        """
+        |> asExe
+        |> withLangVersionPreview
+        |> compileAndRun
+        |> shouldSucceed

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -66,6 +66,7 @@
     <Compile Include="Conformance\BasicTypeAndModuleDefinitions\RecordTypes.fs" />
     <Compile Include="Conformance\BasicTypeAndModuleDefinitions\UnionTypes.fs" />
     <Compile Include="Conformance\DeclarationElements\LetBindings\TypeFunctions.fs" />
+    <Compile Include="Conformance\DeclarationElements\LetBindings\UseBindings.fs" />
     <Compile Include="Conformance\Expressions\ApplicationExpressions\BasicApplication.fs" />
     <Compile Include="Conformance\Expressions\ControlFlowExpressions\PatternMatching.fs" />
     <Compile Include="Conformance\Expressions\ControlFlowExpressions\SequenceIteration.fs" />

tests/FSharp.Compiler.ComponentTests/resources/tests/Conformance/DeclarationElements/LetBindings/UseBindingDiscard01.fs

@@ -0,0 +1,6 @@
+// #DeclarationElements #LetBindings 
+//<Expects status="success"></Expects>
+
+let answer =
+    use _ = new System.IO.MemoryStream()
+    42