Allow _ in async use! _ pattern (lift FS1228 restriction)

docs/release-notes/.FSharp.Compiler.Service/9.0.200.md

@@ -19,6 +19,7 @@
 * Fix a race condition in file book keeping in the compiler service ([#18008](https://github.com/dotnet/fsharp/pull/18008))
 * Fix trimming '%' characters when lowering interpolated string to a concat call [PR #18123](https://github.com/dotnet/fsharp/pull/18123)
 * Completion: fix qualified completion in sequence expressions [PR #18111](https://github.com/dotnet/fsharp/pull/18111)
+* Allow `_` in `use! _` pattern (lift FS1228 restriction) ([PR #18189](https://github.com/dotnet/fsharp/pull/18189))
 * Symbols: try to use ValReprInfoForDisplay in Mfv.CurriedParameterGroups ([PR #18124](https://github.com/dotnet/fsharp/pull/18124))
 * Shim/file system: fix leaks of the shim [PR #18144](https://github.com/dotnet/fsharp/pull/18144)
 

docs/release-notes/.Language/preview.md

@@ -8,5 +8,6 @@
 ### Fixed
 
 * Warn on uppercase identifiers in patterns. ([PR #15816](https://github.com/dotnet/fsharp/pull/15816))
+* Allow `_` in `use! _` pattern (lift FS1228 restriction) ([PR #18189](https://github.com/dotnet/fsharp/pull/18189))
 
 ### Changed

src/Compiler/Checking/Expressions/CheckComputationExpressions.fs

@@ -1843,7 +1843,7 @@ let rec TryTranslateComputationExpression
                         ceenv.builderTy
                 )
             then
-                error (Error(FSComp.SR.tcRequireBuilderMethod ("Using"), mBind))
+                error (Error(FSComp.SR.tcRequireBuilderMethod "Using", mBind))
 
             Some(
                 translatedCtxt (mkSynCall "Using" mBind [ rhsExpr; consumeExpr ] ceenv.builderValName)
@@ -1900,105 +1900,55 @@ let rec TryTranslateComputationExpression
             bindDebugPoint = spBind
             isUse = true
             isFromSource = isFromSource
-            pat = SynPat.Named(ident = SynIdent(id, _); isThisVal = false) as pat
-            rhs = rhsExpr
-            andBangs = []
-            body = innerComp
-            trivia = { LetOrUseBangKeyword = mBind })
-        | SynExpr.LetOrUseBang(
-            bindDebugPoint = spBind
-            isUse = true
-            isFromSource = isFromSource
-            pat = SynPat.LongIdent(longDotId = SynLongIdent(id = [ id ])) as pat
+            pat = pat
             rhs = rhsExpr
-            andBangs = []
+            andBangs = andBangs
             body = innerComp
             trivia = { LetOrUseBangKeyword = mBind }) ->
-
-            if ceenv.isQuery then
-                error (Error(FSComp.SR.tcBindMayNotBeUsedInQueries (), mBind))
-
-            if
-                isNil (
-                    TryFindIntrinsicOrExtensionMethInfo
-                        ResultCollectionSettings.AtMostOneResult
-                        cenv
-                        ceenv.env
-                        mBind
-                        ceenv.ad
-                        "Using"
-                        ceenv.builderTy
-                )
-            then
-                error (Error(FSComp.SR.tcRequireBuilderMethod ("Using"), mBind))
-
-            if
-                isNil (
-                    TryFindIntrinsicOrExtensionMethInfo
-                        ResultCollectionSettings.AtMostOneResult
+            match pat with
+            | SynPat.Named(ident = SynIdent(id, _); isThisVal = false)
+            | SynPat.LongIdent(longDotId = SynLongIdent(id = [ id ])) when andBangs.IsEmpty ->
+                TranslateComputationExpressionLetOrUseBang
+                    cenv
+                    ceenv
+                    pat
+                    (Some id)
+                    innerComp
+                    rhsExpr
+                    translatedCtxt
+                    spBind
+                    isFromSource
+                    mBind
+            | SynPat.Wild _ when cenv.g.langVersion.SupportsFeature(LanguageFeature.UseBangBindingValueDiscard) ->
+                if not (isNil andBangs) then
+                    let m =
+                        match andBangs with
+                        | [] -> comp.Range
+                        | h :: _ -> h.Trivia.AndBangKeyword
+
+                    error (Error(FSComp.SR.tcInvalidUseBangBindingNoAndBangs (), m))
+                else
+                    TranslateComputationExpressionLetOrUseBang
                         cenv
-                        ceenv.env
-                        mBind
-                        ceenv.ad
-                        "Bind"
-                        ceenv.builderTy
-                )
-            then
-                error (Error(FSComp.SR.tcRequireBuilderMethod ("Bind"), mBind))
-
-            let bindExpr =
-                let consumeExpr =
-                    SynExpr.MatchLambda(
-                        false,
-                        mBind,
-                        [
-                            SynMatchClause(
-                                pat,
-                                None,
-                                TranslateComputationExpressionNoQueryOps ceenv innerComp,
-                                innerComp.Range,
-                                DebugPointAtTarget.Yes,
-                                SynMatchClauseTrivia.Zero
-                            )
-                        ],
-                        DebugPointAtBinding.NoneAtInvisible,
-                        mBind
-                    )
-
-                let consumeExpr =
-                    mkSynCall "Using" mBind [ SynExpr.Ident id; consumeExpr ] ceenv.builderValName
-
-                let consumeExpr =
-                    SynExpr.MatchLambda(
-                        false,
-                        mBind,
-                        [
-                            SynMatchClause(pat, None, consumeExpr, id.idRange, DebugPointAtTarget.No, SynMatchClauseTrivia.Zero)
-                        ],
-                        DebugPointAtBinding.NoneAtInvisible,
+                        ceenv
+                        pat
+                        None
+                        innerComp
+                        rhsExpr
+                        translatedCtxt
+                        spBind
+                        isFromSource
                         mBind
-                    )
-
-                let rhsExpr =
-                    mkSourceExprConditional isFromSource rhsExpr ceenv.sourceMethInfo ceenv.builderValName
-
-                mkSynCall "Bind" mBind [ rhsExpr; consumeExpr ] ceenv.builderValName
-                |> addBindDebugPoint spBind
-
-            Some(translatedCtxt bindExpr)
-
-        // 'use! pat = e1 ... in e2' where 'pat' is not a simple name -> error
-        | SynExpr.LetOrUseBang(isUse = true; andBangs = andBangs; trivia = { LetOrUseBangKeyword = mBind }) ->
-            if isNil andBangs then
-                error (Error(FSComp.SR.tcInvalidUseBangBinding (), mBind))
-            else
-                let m =
-                    match andBangs with
-                    | [] -> comp.Range
-                    | h :: _ -> h.Trivia.AndBangKeyword
-
-                error (Error(FSComp.SR.tcInvalidUseBangBindingNoAndBangs (), m))
+            | _ ->
+                if isNil andBangs then
+                    error (Error(FSComp.SR.tcInvalidUseBangBinding (), mBind))
+                else
+                    let m =
+                        match andBangs with
+                        | [] -> comp.Range
+                        | h :: _ -> h.Trivia.AndBangKeyword
 
+                    error (Error(FSComp.SR.tcInvalidUseBangBindingNoAndBangs (), m))
         // 'let! pat1 = expr1 and! pat2 = expr2 in ...' -->
         //     build.BindN(expr1, expr2, ...)
         // or
@@ -2469,6 +2419,93 @@ let rec TryTranslateComputationExpression
 
         | _ -> None
 
+and TranslateComputationExpressionLetOrUseBang
+    cenv
+    ceenv
+    pat
+    (id: Ident option)
+    (innerComp: SynExpr)
+    rhsExpr
+    translatedCtxt
+    spBind
+    isFromSource
+    mBind
+    =
+    if ceenv.isQuery then
+        error (Error(FSComp.SR.tcBindMayNotBeUsedInQueries (), mBind))
+
+    if
+        isNil (
+            TryFindIntrinsicOrExtensionMethInfo
+                ResultCollectionSettings.AtMostOneResult
+                cenv
+                ceenv.env
+                mBind
+                ceenv.ad
+                "Using"
+                ceenv.builderTy
+        )
+    then
+        error (Error(FSComp.SR.tcRequireBuilderMethod "Using", mBind))
+
+    if
+        isNil (
+            TryFindIntrinsicOrExtensionMethInfo
+                ResultCollectionSettings.AtMostOneResult
+                cenv
+                ceenv.env
+                mBind
+                ceenv.ad
+                "Bind"
+                ceenv.builderTy
+        )
+    then
+        error (Error(FSComp.SR.tcRequireBuilderMethod "Bind", mBind))
+
+    let consumeExpr =
+        let consumeExpr =
+            SynExpr.MatchLambda(
+                false,
+                mBind,
+                [
+                    SynMatchClause(
+                        pat,
+                        None,
+                        TranslateComputationExpressionNoQueryOps ceenv innerComp,
+                        innerComp.Range,
+                        DebugPointAtTarget.Yes,
+                        SynMatchClauseTrivia.Zero
+                    )
+                ],
+                DebugPointAtBinding.NoneAtInvisible,
+                mBind
+            )
+
+        match id with
+        | Some id ->
+            let consumeExpr =
+                mkSynCall "Using" mBind [ SynExpr.Ident id; consumeExpr ] ceenv.builderValName
+
+            SynExpr.MatchLambda(
+                false,
+                mBind,
+                [
+                    SynMatchClause(pat, None, consumeExpr, id.idRange, DebugPointAtTarget.No, SynMatchClauseTrivia.Zero)
+                ],
+                DebugPointAtBinding.NoneAtInvisible,
+                mBind
+            )
+        | None -> consumeExpr
+
+    let bindExpr =
+        let rhsExpr =
+            mkSourceExprConditional isFromSource rhsExpr ceenv.sourceMethInfo ceenv.builderValName
+
+        mkSynCall "Bind" mBind [ rhsExpr; consumeExpr ] ceenv.builderValName
+        |> addBindDebugPoint spBind
+
+    Some(translatedCtxt bindExpr)
+
 and ConsumeCustomOpClauses
     (ceenv: ComputationExpressionContext<'a>)
     (comp: SynExpr)

src/Compiler/FSComp.txt

@@ -1791,3 +1791,4 @@ featureDontWarnOnUppercaseIdentifiersInBindingPatterns,"Don't warn on uppercase
 3873,chkDeprecatePlacesWhereSeqCanBeOmitted,"This construct is deprecated. Sequence expressions should be of the form 'seq {{ ... }}'"
 featureDeprecatePlacesWhereSeqCanBeOmitted,"Deprecate places where 'seq' can be omitted"
 featureSupportValueOptionsAsOptionalParameters,"Support ValueOption as valid type for optional member parameters"
+featureUseBangBindingValueDiscard,"Use 'use!' binding for value discard"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -98,6 +98,7 @@ type LanguageFeature =
     | UseTypeSubsumptionCache
     | DeprecatePlacesWhereSeqCanBeOmitted
     | SupportValueOptionsAsOptionalParameters
+    | UseBangBindingValueDiscard
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -227,6 +228,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.DontWarnOnUppercaseIdentifiersInBindingPatterns, previewVersion
                 LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted, previewVersion
                 LanguageFeature.SupportValueOptionsAsOptionalParameters, previewVersion
+                LanguageFeature.UseBangBindingValueDiscard, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -388,6 +390,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.UseTypeSubsumptionCache -> FSComp.SR.featureUseTypeSubsumptionCache ()
         | LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted -> FSComp.SR.featureDeprecatePlacesWhereSeqCanBeOmitted ()
         | LanguageFeature.SupportValueOptionsAsOptionalParameters -> FSComp.SR.featureSupportValueOptionsAsOptionalParameters ()
+        | LanguageFeature.UseBangBindingValueDiscard -> FSComp.SR.featureUseBangBindingValueDiscard ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -89,6 +89,7 @@ type LanguageFeature =
     | UseTypeSubsumptionCache
     | DeprecatePlacesWhereSeqCanBeOmitted
     | SupportValueOptionsAsOptionalParameters
+    | UseBangBindingValueDiscard
 
 /// LanguageVersion management
 type LanguageVersion =