[WIP] RFC-1331 Expression-scoped opens

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -3,6 +3,7 @@
 * Add support for `when 'T : Enum` library-only static optimization constraint. ([PR #18546](https://github.com/dotnet/fsharp/pull/18546))
 * Add support for tail calls in computation expressions ([PR #18804](https://github.com/dotnet/fsharp/pull/18804))
 * Add `--typecheck-only` flag support for F# Interactive (FSI) scripts to type-check without execution. ([Issue #18686](https://github.com/dotnet/fsharp/issues/18686))
+* Allow open declarations in expression scope. ([Suggestion](https://github.com/fsharp/fslang-suggestions/issues/96), [PR #18814](https://github.com/dotnet/fsharp/pull/18814))
 
 ### Fixed
 

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

@@ -11,6 +11,7 @@
 * Allow `let!`, `use!`, `and!` type annotations without requiring parentheses (([PR #18508](https://github.com/dotnet/fsharp/pull/18508) and [PR #18682](https://github.com/dotnet/fsharp/pull/18682)))
 * Exception names are now validated for illegal characters using the same mechanism as types/modules/namespaces ([Issue #18763](https://github.com/dotnet/fsharp/issues/18763))
 * Support tail calls in computation expressions ([PR #18804](https://github.com/dotnet/fsharp/pull/18804))
+* Allow open declarations in expression scope. ([Suggestion](https://github.com/fsharp/fslang-suggestions/issues/96), [PR #18814](https://github.com/dotnet/fsharp/pull/18814))
 
 ### Fixed
 

src/Compiler/Checking/CheckBasics.fs

@@ -379,3 +379,93 @@ type TcFileState =
         }
 
     override _.ToString() = "<cenv>"
+
+open FSharp.Compiler.AttributeChecking
+open FSharp.Compiler.Syntax.PrettyNaming
+open FSharp.Compiler.Text.Range
+open FSharp.Compiler.TypedTreeBasics
+
+let CheckNamespaceModuleOrTypeName (g: TcGlobals) (id: Ident) = 
+    // type names '[]' etc. are used in fslib
+    if not g.compilingFSharpCore && id.idText.IndexOfAny IllegalCharactersInTypeAndNamespaceNames <> -1 then 
+        errorR(Error(FSComp.SR.tcInvalidNamespaceModuleTypeUnionName(), id.idRange))
+
+/// Adjust the TcEnv to account for opening the set of modules or namespaces implied by an `open` declaration
+let OpenModuleOrNamespaceRefs tcSink g amap scopem root env mvvs openDeclaration =
+    let env =
+        if isNil mvvs then env else
+        { env with eNameResEnv = AddModuleOrNamespaceRefsContentsToNameEnv g amap env.eAccessRights scopem root env.eNameResEnv mvvs }
+    CallEnvSink tcSink (scopem, env.NameEnv, env.eAccessRights)
+    CallOpenDeclarationSink tcSink openDeclaration
+    env
+
+//-------------------------------------------------------------------------
+// Bind 'open' declarations
+//------------------------------------------------------------------------- 
+
+let TcOpenLidAndPermitAutoResolve tcSink (env: TcEnv) amap (longId : Ident list) =
+    let ad = env.AccessRights
+    match longId with
+    | [] -> []
+    | id :: rest ->
+        let m = longId |> List.map (fun id -> id.idRange) |> List.reduce unionRanges
+        match ResolveLongIdentAsModuleOrNamespace tcSink amap m true OpenQualified env.NameEnv ad id rest true ShouldNotifySink.Yes with 
+        | Result res -> res
+        | Exception err ->
+            errorR(err); []
+
+let TcOpenModuleOrNamespaceDecl tcSink g amap scopem env (longId, m) = 
+    match TcOpenLidAndPermitAutoResolve tcSink env amap longId with
+    | [] -> env, []
+    | modrefs ->
+
+    // validate opened namespace names
+    for id in longId do
+        if id.idText <> MangledGlobalName then
+            CheckNamespaceModuleOrTypeName g id
+
+    let IsPartiallyQualifiedNamespace (modref: ModuleOrNamespaceRef) = 
+        let (CompPath(_, _, p)) = modref.CompilationPath 
+        // Bug FSharp 1.0 3274: FSI paths don't count when determining this warning
+        let p = 
+            match p with 
+            | [] -> []
+            | (h, _) :: t -> if h.StartsWithOrdinal FsiDynamicModulePrefix then t else p
+
+        // See https://fslang.uservoice.com/forums/245727-f-language/suggestions/6107641-make-microsoft-prefix-optional-when-using-core-f
+        let isFSharpCoreSpecialCase =
+            match ccuOfTyconRef modref with 
+            | None -> false
+            | Some ccu -> 
+                ccuEq ccu g.fslibCcu &&
+                // Check if we're using a reference one string shorter than what we expect.
+                //
+                // "p" is the fully qualified path _containing_ the thing we're opening, e.g. "Microsoft.FSharp" when opening "Microsoft.FSharp.Data"
+                // "longId" is the text being used, e.g. "FSharp.Data"
+                //    Length of thing being opened = p.Length + 1
+                //    Length of reference = longId.Length
+                // So the reference is a "shortened" reference if (p.Length + 1) - 1 = longId.Length
+                (p.Length + 1) - 1 = longId.Length && 
+                fst p[0] = "Microsoft" 
+
+        modref.IsNamespace && 
+        p.Length >= longId.Length &&
+        not isFSharpCoreSpecialCase
+        // Allow "open Foo" for "Microsoft.Foo" from FSharp.Core
+
+    modrefs |> List.iter (fun (_, modref, _) ->
+       if modref.IsModule && HasFSharpAttribute g g.attrib_RequireQualifiedAccessAttribute modref.Attribs then 
+           errorR(Error(FSComp.SR.tcModuleRequiresQualifiedAccess(fullDisplayTextOfModRef modref), m)))
+
+    // Bug FSharp 1.0 3133: 'open Lexing'. Skip this warning if we successfully resolved to at least a module name
+    if not (modrefs |> List.exists (fun (_, modref, _) -> modref.IsModule && not (HasFSharpAttribute g g.attrib_RequireQualifiedAccessAttribute modref.Attribs))) then
+        modrefs |> List.iter (fun (_, modref, _) ->
+            if IsPartiallyQualifiedNamespace modref then 
+                 errorR(Error(FSComp.SR.tcOpenUsedWithPartiallyQualifiedPath(fullDisplayTextOfModRef modref), m)))
+
+    let modrefs = List.map p23 modrefs
+    modrefs |> List.iter (fun modref -> CheckEntityAttributes g modref m |> CommitOperationResult)        
+
+    let openDecl = OpenDeclaration.Create (SynOpenDeclTarget.ModuleOrNamespace (SynLongIdent(longId, [], []), m), modrefs, [], scopem, false)
+    let env = OpenModuleOrNamespaceRefs tcSink g amap scopem false env modrefs openDecl
+    env, [openDecl]

src/Compiler/Checking/CheckBasics.fsi

@@ -358,3 +358,12 @@ type TcFileState =
                 -> range * Expr * TType * SynExpr
                 -> Expr * UnscopedTyparEnv) ->
             TcFileState
+
+val TcOpenModuleOrNamespaceDecl:
+    tcSink: TcResultsSink ->
+    g: TcGlobals ->
+    amap: Import.ImportMap ->
+    scopem: range ->
+    env: TcEnv ->
+    longId: LongIdent * m: range ->
+        TcEnv * OpenDeclaration list

src/Compiler/Checking/CheckDeclarations.fs

@@ -294,14 +294,6 @@ let OpenModuleOrNamespaceRefs tcSink g amap scopem root env mvvs openDeclaration
     CallOpenDeclarationSink tcSink openDeclaration
     env
 
-/// Adjust the TcEnv to account for opening a type implied by an `open type` declaration
-let OpenTypeContent tcSink g amap scopem env (ty: TType) openDeclaration =
-    let env =
-        { env with eNameResEnv = AddTypeContentsToNameEnv g amap env.eAccessRights scopem env.eNameResEnv ty }
-    CallEnvSink tcSink (scopem, env.NameEnv, env.eAccessRights)
-    CallOpenDeclarationSink tcSink openDeclaration
-    env
-
 /// Adjust the TcEnv to account for a new root Ccu being available, e.g. a referenced assembly
 let AddRootModuleOrNamespaceRefs g amap m env modrefs =
     if isNil modrefs then env else
@@ -681,99 +673,9 @@ let TcTyconMemberSpecs cenv env containerInfo declKind tpenv augSpfn =
 // Bind 'open' declarations
 //------------------------------------------------------------------------- 
 
-let TcOpenLidAndPermitAutoResolve tcSink (env: TcEnv) amap (longId : Ident list) =
-    let ad = env.AccessRights
-    match longId with
-    | [] -> []
-    | id :: rest ->
-        let m = longId |> List.map (fun id -> id.idRange) |> List.reduce unionRanges
-        match ResolveLongIdentAsModuleOrNamespace tcSink amap m true OpenQualified env.NameEnv ad id rest true ShouldNotifySink.Yes with 
-        | Result res -> res
-        | Exception err ->
-            errorR(err); []
-
 let TcOpenModuleOrNamespaceDecl tcSink g amap scopem env (longId, m) = 
-    match TcOpenLidAndPermitAutoResolve tcSink env amap longId with
-    | [] -> env, []
-    | modrefs ->
-
-    // validate opened namespace names
-    for id in longId do
-        if id.idText <> MangledGlobalName then
-            CheckNamespaceModuleOrTypeName g id
-
-    let IsPartiallyQualifiedNamespace (modref: ModuleOrNamespaceRef) = 
-        let (CompPath(_, _, p)) = modref.CompilationPath 
-        // Bug FSharp 1.0 3274: FSI paths don't count when determining this warning
-        let p = 
-            match p with 
-            | [] -> []
-            | (h, _) :: t -> if h.StartsWithOrdinal FsiDynamicModulePrefix then t else p
-
-        // See https://fslang.uservoice.com/forums/245727-f-language/suggestions/6107641-make-microsoft-prefix-optional-when-using-core-f
-        let isFSharpCoreSpecialCase =
-            match ccuOfTyconRef modref with 
-            | None -> false
-            | Some ccu -> 
-                ccuEq ccu g.fslibCcu &&
-                // Check if we're using a reference one string shorter than what we expect.
-                //
-                // "p" is the fully qualified path _containing_ the thing we're opening, e.g. "Microsoft.FSharp" when opening "Microsoft.FSharp.Data"
-                // "longId" is the text being used, e.g. "FSharp.Data"
-                //    Length of thing being opened = p.Length + 1
-                //    Length of reference = longId.Length
-                // So the reference is a "shortened" reference if (p.Length + 1) - 1 = longId.Length
-                (p.Length + 1) - 1 = longId.Length && 
-                fst p[0] = "Microsoft" 
-
-        modref.IsNamespace && 
-        p.Length >= longId.Length &&
-        not isFSharpCoreSpecialCase
-        // Allow "open Foo" for "Microsoft.Foo" from FSharp.Core
-
-    modrefs |> List.iter (fun (_, modref, _) ->
-       if modref.IsModule && HasFSharpAttribute g g.attrib_RequireQualifiedAccessAttribute modref.Attribs then 
-           errorR(Error(FSComp.SR.tcModuleRequiresQualifiedAccess(fullDisplayTextOfModRef modref), m)))
-
-    // Bug FSharp 1.0 3133: 'open Lexing'. Skip this warning if we successfully resolved to at least a module name
-    if not (modrefs |> List.exists (fun (_, modref, _) -> modref.IsModule && not (HasFSharpAttribute g g.attrib_RequireQualifiedAccessAttribute modref.Attribs))) then
-        modrefs |> List.iter (fun (_, modref, _) ->
-            if IsPartiallyQualifiedNamespace modref then 
-                 errorR(Error(FSComp.SR.tcOpenUsedWithPartiallyQualifiedPath(fullDisplayTextOfModRef modref), m)))
-
-    let modrefs = List.map p23 modrefs
-    modrefs |> List.iter (fun modref -> CheckEntityAttributes g modref m |> CommitOperationResult)        
-
-    let openDecl = OpenDeclaration.Create (SynOpenDeclTarget.ModuleOrNamespace (SynLongIdent(longId, [], []), m), modrefs, [], scopem, false)
-    let env = OpenModuleOrNamespaceRefs tcSink g amap scopem false env modrefs openDecl
-    env, [openDecl]
-
-let TcOpenTypeDecl (cenv: cenv) mOpenDecl scopem env (synType: SynType, m) =
-    let g = cenv.g
-
-    checkLanguageFeatureError g.langVersion LanguageFeature.OpenTypeDeclaration mOpenDecl
-
-    let ty, _tpenv = TcType cenv NoNewTypars CheckCxs ItemOccurrence.Open WarnOnIWSAM.Yes env emptyUnscopedTyparEnv synType
-
-    if not (isAppTy g ty) then
-        errorR(Error(FSComp.SR.tcNamedTypeRequired("open type"), m))
-
-    if isByrefTy g ty then
-        errorR(Error(FSComp.SR.tcIllegalByrefsInOpenTypeDeclaration(), m))
-
-    let openDecl = OpenDeclaration.Create (SynOpenDeclTarget.Type (synType, m), [], [ty], scopem, false)
-    let env = OpenTypeContent cenv.tcSink g cenv.amap scopem env ty openDecl
-    env, [openDecl]
-
-let TcOpenDecl (cenv: cenv) mOpenDecl scopem env target = 
-    let g = cenv.g
-    match target with
-    | SynOpenDeclTarget.ModuleOrNamespace (longId, m) ->
-        TcOpenModuleOrNamespaceDecl cenv.tcSink g cenv.amap scopem env (longId.LongIdent, m)
-
-    | SynOpenDeclTarget.Type (synType, m) ->
-        TcOpenTypeDecl cenv mOpenDecl scopem env (synType, m)
-
+    CheckBasics.TcOpenModuleOrNamespaceDecl tcSink g amap scopem env (longId, m)
+
 let MakeSafeInitField (cenv: cenv) env m isStatic = 
     let id =
         // Ensure that we have an g.CompilerGlobalState

src/Compiler/Checking/Expressions/CheckComputationExpressions.fs

@@ -2242,6 +2242,10 @@ let rec TryTranslateComputationExpression
 
             Some(translatedCtxt yieldOrReturnCall)
 
+        | SynExpr.Open(target, mOpen, m, body) ->
+            let body = TranslateComputationExpressionNoQueryOps ceenv body
+            Some(translatedCtxt (SynExpr.Open(target, mOpen, m, body)))
+
         | _ -> None
 
 and ConsumeCustomOpClauses

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -4052,6 +4052,14 @@ type ImplicitlyBoundTyparsAllowed =
     | NewTyparsOK
     | NoNewTypars
 
+/// Adjust the TcEnv to account for opening a type implied by an `open type` declaration
+let OpenTypeContent tcSink g amap scopem env (ty: TType) openDeclaration =
+    let env =
+        { env with eNameResEnv = AddTypeContentsToNameEnv g amap env.eAccessRights scopem env.eNameResEnv ty }
+    CallEnvSink tcSink (scopem, env.NameEnv, env.eAccessRights)
+    CallOpenDeclarationSink tcSink openDeclaration
+    env
+
 //-------------------------------------------------------------------------
 // Checking types and type constraints
 //-------------------------------------------------------------------------
@@ -6083,6 +6091,11 @@ and TcExprUndelayed (cenv: cenv) (overallTy: OverallTy) env tpenv (synExpr: SynE
     | SynExpr.IndexRange (range=m) ->
         error(Error(FSComp.SR.tcInvalidIndexerExpression(), m))
 
+    | SynExpr.Open (target, mOpen, _m, body) ->
+        checkLanguageFeatureAndRecover g.langVersion LanguageFeature.OpensInExpressionScope mOpen
+        let env, _openDecls = TcOpenDecl cenv mOpen body.Range env target
+        TcExprThatCanBeCtorBody cenv overallTy env tpenv body
+
 and TcExprMatch (cenv: cenv) overallTy env tpenv synInputExpr spMatch synClauses =
     let inputExpr, inputTy, tpenv =
         let env = { env with eIsControlFlow = false }
@@ -9205,6 +9218,7 @@ and TcImplicitOpItemThen (cenv: cenv) overallTy env id sln tpenv mItem delayed =
         | SynExpr.TraitCall _
         | SynExpr.IndexFromEnd _
         | SynExpr.IndexRange _
+        | SynExpr.Open _
             -> false
 
     // Propagate the known application structure into function types
@@ -12840,6 +12854,32 @@ and TcLetrecBindings overridesOK (cenv: cenv) env tpenv (binds, bindsm, scopem)
     let envbody = AddLocalVals g cenv.tcSink scopem prelimRecValues env
     binds, envbody, tpenv
 
+and TcOpenTypeDecl (cenv: cenv) mOpenDecl scopem env (synType: SynType, m) =
+    let g = cenv.g
+
+    checkLanguageFeatureError g.langVersion LanguageFeature.OpenTypeDeclaration mOpenDecl
+
+    let ty, _tpenv = TcType cenv NoNewTypars CheckCxs ItemOccurrence.Open WarnOnIWSAM.Yes env emptyUnscopedTyparEnv synType
+
+    if not (isAppTy g ty) then
+        errorR(Error(FSComp.SR.tcNamedTypeRequired("open type"), m))
+
+    if isByrefTy g ty then
+        errorR(Error(FSComp.SR.tcIllegalByrefsInOpenTypeDeclaration(), m))
+
+    let openDecl = OpenDeclaration.Create (SynOpenDeclTarget.Type (synType, m), [], [ty], scopem, false)
+    let env = OpenTypeContent cenv.tcSink g cenv.amap scopem env ty openDecl
+    env, [openDecl]
+
+and TcOpenDecl (cenv: cenv) mOpenDecl scopem env target = 
+    let g = cenv.g
+    match target with
+    | SynOpenDeclTarget.ModuleOrNamespace (longId, m) ->
+        TcOpenModuleOrNamespaceDecl cenv.tcSink g cenv.amap scopem env (longId.LongIdent, m)
+
+    | SynOpenDeclTarget.Type (synType, m) ->
+        TcOpenTypeDecl cenv mOpenDecl scopem env (synType, m)
+
 //-------------------------------------------------------------------------
 // Bind specifications of values
 //-------------------------------------------------------------------------

src/Compiler/Checking/Expressions/CheckExpressions.fsi

@@ -883,6 +883,14 @@ val TcRuntimeTypeTest:
     srcTy: TType ->
         unit
 
+val TcOpenDecl:
+    cenv: TcFileState ->
+    mOpenDecl: range ->
+    scopem: range ->
+    env: TcEnv ->
+    target: SynOpenDeclTarget ->
+        TcEnv * OpenDeclaration list
+
 /// Allow the inference of structness from the known type, e.g.
 ///    let (x: struct (int * int)) = (3,4)
 val UnifyTupleTypeAndInferCharacteristics:

src/Compiler/Driver/GraphChecking/FileContentMapping.fs

@@ -574,6 +574,7 @@ let visitSynExpr (e: SynExpr) : FileContentEntry list =
         | SynExpr.Dynamic(funcExpr, _, argExpr, _) ->
             let continuations = List.map visit [ funcExpr; argExpr ]
             Continuation.concatenate continuations continuation
+        | SynExpr.Open(body = body) -> visit body continuation
 
     visit e id
 

src/Compiler/FSComp.txt

@@ -1809,3 +1809,4 @@ featureAllowLetOrUseBangTypeAnnotationWithoutParens,"Allow let! and use! type an
 3877,lexLineDirectiveMappingIsNotUnique,"The file '%s' was also pointed to in a line directive in '%s'. Proper warn directive application may not be possible."
 3878,tcAttributeIsNotValidForUnionCaseWithFields,"This attribute is not valid for use on union cases with fields."
 featureReturnFromFinal,"Support for ReturnFromFinal/YieldFromFinal in computation expressions to enable tailcall optimization when available on the builder."
+featureOpensInExpressionScope,"'open' declarations in expression scopes"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -104,6 +104,7 @@ type LanguageFeature =
     | ScopedNowarn
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
+    | OpensInExpressionScope
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -243,6 +244,7 @@ type LanguageVersion(versionText) =
 
                 // F# preview (still preview in 10.0)
                 LanguageFeature.FromEndSlicing, previewVersion // Unfinished features --- needs work
+                LanguageFeature.OpensInExpressionScope, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -412,6 +414,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.ScopedNowarn -> FSComp.SR.featureScopedNowarn ()
         | LanguageFeature.AllowTypedLetUseAndBang -> FSComp.SR.featureAllowLetOrUseBangTypeAnnotationWithoutParens ()
         | LanguageFeature.ReturnFromFinal -> FSComp.SR.featureReturnFromFinal ()
+        | LanguageFeature.OpensInExpressionScope -> FSComp.SR.featureOpensInExpressionScope ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -95,6 +95,7 @@ type LanguageFeature =
     | ScopedNowarn
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
+    | OpensInExpressionScope
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/Service/FSharpParseFileResults.fs

@@ -771,6 +771,7 @@ type FSharpParseFileResults(diagnostics: FSharpDiagnostic[], input: ParsedInput,
                                 yield! walkExpr true eAndBang
 
                             yield! walkExpr true bodyExpr
+                        | SynExpr.Open(body = bodyExpr) -> yield! walkExpr true bodyExpr
                 ]
 
             // Process a class declaration or F# type declaration

src/Compiler/Service/ServiceParseTreeWalk.fs

@@ -423,7 +423,8 @@ module SyntaxTraversal =
                 | SynExpr.TypeApp(expr = synExpr)
                 | SynExpr.DotLambda(expr = synExpr)
                 | SynExpr.Quote(quotedExpr = synExpr)
-                | SynExpr.Paren(expr = synExpr) -> traverseSynExpr synExpr
+                | SynExpr.Paren(expr = synExpr)
+                | SynExpr.Open(body = synExpr) -> traverseSynExpr synExpr
 
                 | SynExpr.InterpolatedString(contents = parts) ->
                     [