C# collection expression support for F# lists & sets

VisualFSharp.sln

@@ -193,6 +193,8 @@ Project("{6EC3EE1D-3C4E-46DD-8F32-0CC8E7565705}") = "MicroPerf", "tests\benchmar
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MicroPerfCSharp", "tests\benchmarks\CompiledCodeBenchmarks\MicroPerf\CS\MicroPerfCSharp.csproj", "{9F9DD315-37DA-4413-928E-1CFC6924B64F}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "FSharp.Core.UnitTests.CSharp", "tests\FSharp.Core.UnitTests.CSharp\FSharp.Core.UnitTests.CSharp.csproj", "{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -1019,6 +1021,18 @@ Global
 		{9F9DD315-37DA-4413-928E-1CFC6924B64F}.Release|Any CPU.Build.0 = Release|Any CPU
 		{9F9DD315-37DA-4413-928E-1CFC6924B64F}.Release|x86.ActiveCfg = Release|Any CPU
 		{9F9DD315-37DA-4413-928E-1CFC6924B64F}.Release|x86.Build.0 = Release|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Debug|x86.Build.0 = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Proto|Any CPU.ActiveCfg = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Proto|Any CPU.Build.0 = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Proto|x86.ActiveCfg = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Proto|x86.Build.0 = Debug|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Release|Any CPU.Build.0 = Release|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Release|x86.ActiveCfg = Release|Any CPU
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -1099,6 +1113,7 @@ Global
 		{6734FC6F-B5F3-45E1-9A72-720378BB49C9} = {DFB6ADD7-3149-43D9-AFA0-FC4A818B472B}
 		{601CD5C1-EAFA-4AE3-8FB9-F667B5728213} = {DFB6ADD7-3149-43D9-AFA0-FC4A818B472B}
 		{9F9DD315-37DA-4413-928E-1CFC6924B64F} = {DFB6ADD7-3149-43D9-AFA0-FC4A818B472B}
+		{BC444300-EE7A-4AEB-96AA-760DCC5BFD45} = {CFE3259A-2D30-4EB0-80D5-E8B5F3D01449}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {48EDBBBE-C8EE-4E3C-8B19-97184A487B37}

src/FSharp.Core/prim-types.fs

@@ -566,6 +566,9 @@ namespace Microsoft.FSharp.Core
 
     open BasicInlinedOperations
 
+    // This exists solely so that it can be used in the CollectionBuilderAttribute on List<'T> in prim-types.fsi.
+    module TypeOfUtils =
+        let inline typeof<'T> = typeof<'T>
 
     module TupleUtils =
 
@@ -4069,6 +4072,28 @@ namespace Microsoft.FSharp.Core
 
     and 'T voption = ValueOption<'T>
 
+#if !NET8_0_OR_GREATER
+namespace System.Runtime.CompilerServices
+    open System
+    open System.ComponentModel
+    open Microsoft.FSharp.Core
+
+    [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden=true)>]
+    [<Sealed>]
+    [<AttributeUsage(AttributeTargets.Class ||| AttributeTargets.Struct ||| AttributeTargets.Interface, Inherited = false)>]
+    type CollectionBuilderAttribute (builderType: Type, methodName: string) =
+        inherit Attribute ()
+        member _.BuilderType = builderType
+        member _.MethodName = methodName
+
+    [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden=true)>]
+    [<EditorBrowsable(EditorBrowsableState.Never)>]
+    [<Sealed>]
+    [<AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)>]
+    type ScopedRefAttribute () =
+        inherit Attribute ()
+#endif
+
 namespace Microsoft.FSharp.Collections
 
     //-------------------------------------------------------------------------
@@ -4086,6 +4111,9 @@ namespace Microsoft.FSharp.Collections
     open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicFunctions
     open Microsoft.FSharp.Core.BasicInlinedOperations
 
+#if NETSTANDARD2_1_OR_GREATER
+    [<System.Runtime.CompilerServices.CollectionBuilder(typeof<List>, "Create")>]
+#endif
     [<DefaultAugmentation(false)>]
     [<DebuggerTypeProxyAttribute(typedefof<ListDebugView<_>>)>]
     [<DebuggerDisplay("{DebugDisplay,nq}")>]
@@ -4111,6 +4139,19 @@ namespace Microsoft.FSharp.Collections
 
     and 'T list = List<'T>
 
+#if NETSTANDARD2_1_OR_GREATER
+    and [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden=true);
+          Sealed;
+          AbstractClass;
+          CompiledName("FSharpList")>] List =
+        [<CompilerMessage("This method is for compiler use and should not be used directly", 1204, IsHidden=true)>]
+        static member Create([<System.Runtime.CompilerServices.ScopedRef>] items: System.ReadOnlySpan<'T>) =
+            let mutable list : 'T list = []
+            for i = items.Length - 1 downto 0 do
+                list <- items[i] :: list
+            list
+#endif
+
     //-------------------------------------------------------------------------
     // List (debug view)
     //-------------------------------------------------------------------------

src/FSharp.Core/prim-types.fsi

@@ -1238,6 +1238,10 @@ namespace Microsoft.FSharp.Core
     /// <category>ByRef and Pointer Types</category>
     type outref<'T> = byref<'T, ByRefKinds.Out>
 
+    // This exists solely so that it can be used in the CollectionBuilderAttribute on List<'T> below.
+    module internal TypeOfUtils =
+        val inline typeof<'T>: Type
+
     /// <summary>Language primitives associated with the F# language</summary>
     ///
     /// <category index="9">Language Primitives</category>
@@ -2578,12 +2582,52 @@ namespace Microsoft.FSharp.Core
       /// Represents an Error or a Failure. The code failed with a value of 'TError representing what went wrong.
       | Error of ErrorValue:'TError
 
+#if !NET8_0_OR_GREATER
+namespace System.Runtime.CompilerServices
+    open System
+    open System.ComponentModel
+    open Microsoft.FSharp.Core
+
+    [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden=true)>]
+    [<Sealed>]
+    [<AttributeUsage(AttributeTargets.Class ||| AttributeTargets.Struct ||| AttributeTargets.Interface, Inherited = false)>]
+    type CollectionBuilderAttribute =
+        inherit Attribute
+
+        /// <summary>Initialize the attribute to refer to the <paramref name="methodName"/> method on the <paramref name="builderType"/> type.</summary>
+        /// <param name="builderType">The type of the builder to use to construct the collection.</param>
+        /// <param name="methodName">The name of the method on the builder to use to construct the collection.</param>
+        /// <remarks>
+        /// <paramref name="methodName"/> must refer to a static method that accepts a single parameter of
+        /// type <see cref="T:System.ReadOnlySpan`1"/> and returns an instance of the collection being built containing
+        /// a copy of the data from that span. In future releases of .NET, additional patterns may be supported.
+        /// </remarks>
+        new: builderType: Type * methodName: string -> CollectionBuilderAttribute
+
+        /// <summary>Gets the type of the builder to use to construct the collection.</summary>
+        member BuilderType: Type
+
+        /// <summary>Gets the name of the method on the builder to use to construct the collection.</summary>
+        /// <remarks>This should match the metadata name of the target method. For example, this might be ".ctor" if targeting the type's constructor.</remarks>
+        member MethodName: string
+
+    [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden=true)>]
+    [<EditorBrowsable(EditorBrowsableState.Never)>]
+    [<Sealed>]
+    [<AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)>]
+    type ScopedRefAttribute =
+        inherit Attribute
+
+        new: unit -> ScopedRefAttribute
+#endif
+
 namespace Microsoft.FSharp.Collections
 
     open System
     open System.Collections
     open System.Collections.Generic
     open Microsoft.FSharp.Core
+    open Microsoft.FSharp.Core.TypeOfUtils
 
     /// <summary>The type of immutable singly-linked lists.</summary>
     ///
@@ -2593,6 +2637,9 @@ namespace Microsoft.FSharp.Collections
     /// </remarks>
     ///
     /// <exclude />
+#if NETSTANDARD2_1_OR_GREATER
+    [<System.Runtime.CompilerServices.CollectionBuilder(typeof<List>, "Create")>]
+#endif
     [<DefaultAugmentation(false)>]
     [<StructuralEquality; StructuralComparison>]
     [<CompiledName("FSharpList`1")>]
@@ -2646,7 +2693,7 @@ namespace Microsoft.FSharp.Collections
         ///
         /// <returns>The list with head appended to the front of tail.</returns>
         static member Cons: head: 'T * tail: 'T list -> 'T list
-        
+
         interface IEnumerable<'T>
         interface IEnumerable
         interface IReadOnlyCollection<'T>
@@ -2664,6 +2711,21 @@ namespace Microsoft.FSharp.Collections
     /// </remarks>
     and 'T list = List<'T>
 
+#if NETSTANDARD2_1_OR_GREATER
+    /// <summary>Contains methods for compiler use related to lists.</summary>
+    and [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden=true);
+          Sealed;
+          AbstractClass;
+          CompiledName("FSharpList")>] List =
+        /// <summary>Creates a list with the specified items.</summary>
+        ///
+        /// <param name="items">The items to store in the list.</param>
+        ///
+        /// <returns>A list containing the specified items.</returns>
+        [<CompilerMessage("This method is for compiler use and should not be used directly", 1204, IsHidden=true)>]
+        static member Create: [<System.Runtime.CompilerServices.ScopedRef>] items: System.ReadOnlySpan<'T> -> 'T list
+#endif
+
     /// <summary>An abbreviation for the CLI type <see cref="T:System.Collections.Generic.List`1"/></summary>
     type ResizeArray<'T> = System.Collections.Generic.List<'T>
 

src/FSharp.Core/set.fs

@@ -697,6 +697,9 @@ module internal SetTree =
     let ofArray comparer l =
         Array.fold (fun acc k -> add comparer k acc) empty l
 
+#if NETSTANDARD2_1_OR_GREATER
+[<System.Runtime.CompilerServices.CollectionBuilder(typeof<Set>, "Create")>]
+#endif
 [<Sealed>]
 [<CompiledName("FSharpSet`1")>]
 [<DebuggerTypeProxy(typedefof<SetDebugView<_>>)>]
@@ -1023,6 +1026,22 @@ type Set<[<EqualityConditionalOn>] 'T when 'T: comparison>(comparer: IComparer<'
                 .Append("; ... ]")
                 .ToString()
 
+#if NETSTANDARD2_1_OR_GREATER
+and [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden = true);
+      Sealed;
+      AbstractClass;
+      CompiledName("FSharpSet")>] Set =
+    [<CompilerMessage("This method is for compiler use and should not be used directly", 1204, IsHidden = true)>]
+    static member Create([<System.Runtime.CompilerServices.ScopedRef>] items: System.ReadOnlySpan<'T>) =
+        let comparer = LanguagePrimitives.FastGenericComparer<'T>
+        let mutable acc = SetTree.empty
+
+        for item in items do
+            acc <- SetTree.add LanguagePrimitives.FastGenericComparer<'T> item acc
+
+        Set(comparer, acc)
+#endif
+
 and [<Sealed>] SetDebugView<'T when 'T: comparison>(v: Set<'T>) =
 
     [<DebuggerBrowsable(DebuggerBrowsableState.RootHidden)>]

src/FSharp.Core/set.fsi

@@ -13,6 +13,9 @@ open Microsoft.FSharp.Collections
 /// <remarks>See the <see cref="T:Microsoft.FSharp.Collections.SetModule"/> module for further operations on sets.
 ///
 /// All members of this class are thread-safe and may be used concurrently from multiple threads.</remarks>
+#if NETSTANDARD2_1_OR_GREATER
+[<System.Runtime.CompilerServices.CollectionBuilder(typeof<Set>, "Create")>]
+#endif
 [<Sealed>]
 [<CompiledName("FSharpSet`1")>]
 type Set<[<EqualityConditionalOn>] 'T when 'T: comparison> =
@@ -233,6 +236,15 @@ type Set<[<EqualityConditionalOn>] 'T when 'T: comparison> =
     interface IReadOnlyCollection<'T>
     override Equals: obj -> bool
 
+#if NETSTANDARD2_1_OR_GREATER
+and [<CompilerMessage("This type is for compiler use and should not be used directly", 1204, IsHidden = true);
+      Sealed;
+      AbstractClass;
+      CompiledName("FSharpSet")>] Set =
+    [<CompilerMessage("This method is for compiler use and should not be used directly", 1204, IsHidden = true)>]
+    static member Create: [<System.Runtime.CompilerServices.ScopedRef>] items: System.ReadOnlySpan<'T> -> Set<'T>
+#endif
+
 namespace Microsoft.FSharp.Collections
 
 open System

tests/FSharp.Core.UnitTests.CSharp/FSharp.Core.UnitTests.CSharp.csproj

@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFrameworks>$(FSharpNetCoreProductTargetFramework);net472</TargetFrameworks>
+    <TargetFrameworks Condition="'$(OS)' == 'Unix'">$(FSharpNetCoreProductTargetFramework)</TargetFrameworks>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>12.0</LangVersion>
+    <UnitTestType>xunit</UnitTestType>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="$(FSharpSourcesRoot)\FSharp.Core\FSharp.Core.fsproj" />
+    <ProjectReference Include="$(FSharpSourcesRoot)\..\tests\FSharp.Test.Utilities\FSharp.Test.Utilities.fsproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+</Project>