Allow collection expressions with CreateCompilation

src/Compilers/CSharp/Test/Emit2/Semantics/CollectionExpressionTests.cs

@@ -418,7 +418,7 @@ static void Main()
                     static T F<T>(T t) => t;
                 }
                 """;
-            var comp = CreateCompilation(new[] { source, s_collectionExtensions });
+            var comp = CreateCompilation([source, s_collectionExtensions]);
             comp.VerifyEmitDiagnostics(
                 // 0.cs(5,9): error CS0411: The type arguments for method 'Program.F<T>(T)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
                 //         F([1, 2]).Report();
@@ -467,7 +467,7 @@ static void Main()
                     }
                 }
                 """;
-            var comp = CreateCompilation(new[] { source, s_collectionExtensions });
+            var comp = CreateCompilation([source, s_collectionExtensions]);
             comp.VerifyEmitDiagnostics(
                 // 0.cs(6,25): error CS9174: Cannot initialize type 'IEnumerable' with a collection expression because the type is not constructible.
                 //         IEnumerable a = [1];
@@ -503,7 +503,7 @@ static void Main()
                 }
                 """;
             CompileAndVerify(
-                new[] { source, s_collectionExtensions },
+                [source, s_collectionExtensions],
                 expectedOutput: "(System.Int32[]) [], (System.Collections.Generic.List<System.Int32>) [], (System.Collections.Generic.List<System.Int32>) [], (System.Int32[]) [], (System.Int32[]) [], ");
         }
 
@@ -530,7 +530,7 @@ static void Main()
                 }
                 """;
             CompileAndVerify(
-                new[] { source, s_collectionExtensions },
+                [source, s_collectionExtensions],
                 expectedOutput: "(<>z__ReadOnlyArray<System.Int32>) [1], (System.Collections.Generic.List<System.Int32>) [2], (System.Collections.Generic.List<System.Int32>) [3], (<>z__ReadOnlyArray<System.Int32>) [4], (<>z__ReadOnlyArray<System.Int32>) [5], ");
         }
 

src/Compilers/CSharp/Test/Semantic/Semantics/InterceptorsTests.cs

@@ -4635,7 +4635,7 @@ class C
         var pathMap = ImmutableArray.Create(new KeyValuePair<string, string>(pathPrefix, @"\_/"));
 
         var comp = CreateCompilation(
-            new[] { (source, path), s_attributesSource },
+            [(source, path), s_attributesSource],
             parseOptions: RegularWithInterceptors,
             options: TestOptions.DebugExe.WithSourceReferenceResolver(
                 new SourceFileResolver(ImmutableArray<string>.Empty, null, pathMap)));

src/Compilers/Test/Core/CollectionBuilderAttribute.cs

@@ -0,0 +1,23 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#if !NET8_0_OR_GREATER
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface)]
+    public sealed class CollectionBuilderAttribute : Attribute
+    {
+        public CollectionBuilderAttribute(Type builderType, string methodName)
+        {
+            BuilderType = builderType;
+            MethodName = methodName;
+        }
+
+        public Type BuilderType { get; }
+        public string MethodName { get; }
+    }
+}
+
+#endif

src/Compilers/Test/Utilities/CSharp/CSharpTestSource.cs

@@ -5,10 +5,10 @@
 #nullable disable
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
-using System.IO;
 using System.Linq;
 using System.Text;
 using Microsoft.CodeAnalysis.Text;
@@ -19,7 +19,8 @@ namespace Microsoft.CodeAnalysis.CSharp.Test.Utilities
     /// Represents the source code used for a C# test. Allows us to have single helpers that enable all the different ways
     /// we typically provide source in testing.
     /// </summary>
-    public readonly struct CSharpTestSource
+    [System.Runtime.CompilerServices.CollectionBuilder(typeof(CSharpTestSourceBuilder), nameof(CSharpTestSourceBuilder.Create))]
+    public readonly struct CSharpTestSource : IEnumerable<CSharpTestSource>
     {
         public static CSharpTestSource None => new CSharpTestSource(null);
 
@@ -83,5 +84,17 @@ public SyntaxTree[] GetSyntaxTrees(CSharpParseOptions parseOptions, string sourc
         public static implicit operator CSharpTestSource(List<SyntaxTree> source) => new CSharpTestSource(source.ToArray());
         public static implicit operator CSharpTestSource(ImmutableArray<SyntaxTree> source) => new CSharpTestSource(source.ToArray());
         public static implicit operator CSharpTestSource(CSharpTestSource[] source) => new CSharpTestSource(source);
+
+        // Dummy IEnumerable support to satisfy the collection expression and CollectionBuilder requirements
+        IEnumerator<CSharpTestSource> IEnumerable<CSharpTestSource>.GetEnumerator() => throw new NotImplementedException();
+        IEnumerator IEnumerable.GetEnumerator() => throw new NotImplementedException();
+
+        internal class CSharpTestSourceBuilder
+        {
+            public static CSharpTestSource Create(ReadOnlySpan<CSharpTestSource> source)
+            {
+                return source.ToArray();
+            }
+        }
     }
 }