Fix case where we were erroneously offering to convert a dictionary to use a collection expression. (#75897)

Author: CyrusNajmabadi

src/Analyzers/CSharp/Analyzers/UseCollectionInitializer/CSharpUseCollectionInitializerDiagnosticAnalyzer.cs

@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
 using System.Threading;
 using Microsoft.CodeAnalysis.CSharp.Extensions;
 using Microsoft.CodeAnalysis.CSharp.LanguageService;
@@ -10,10 +13,13 @@
 using Microsoft.CodeAnalysis.CSharp.UseCollectionExpression;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.LanguageService;
+using Microsoft.CodeAnalysis.UseCollectionExpression;
 using Microsoft.CodeAnalysis.UseCollectionInitializer;
 
 namespace Microsoft.CodeAnalysis.CSharp.UseCollectionInitializer;
 
+using static SyntaxFactory;
+
 [DiagnosticAnalyzer(LanguageNames.CSharp)]
 internal sealed class CSharpUseCollectionInitializerDiagnosticAnalyzer :
     AbstractUseCollectionInitializerDiagnosticAnalyzer<
@@ -40,13 +46,39 @@ protected override bool AreCollectionInitializersSupported(Compilation compilati
     protected override bool AreCollectionExpressionsSupported(Compilation compilation)
         => compilation.LanguageVersion().SupportsCollectionExpressions();
 
-    protected override bool CanUseCollectionExpression(SemanticModel semanticModel, BaseObjectCreationExpressionSyntax objectCreationExpression, INamedTypeSymbol? expressionType, bool allowSemanticsChange, CancellationToken cancellationToken, out bool changesSemantics)
+    protected override bool CanUseCollectionExpression(
+        SemanticModel semanticModel,
+        BaseObjectCreationExpressionSyntax objectCreationExpression,
+        INamedTypeSymbol? expressionType,
+        ImmutableArray<CollectionMatch<SyntaxNode>> preMatches,
+        bool allowSemanticsChange,
+        CancellationToken cancellationToken,
+        out bool changesSemantics)
     {
         // Synthesize the final collection expression we would replace this object-creation with.  That will allow us to
         // determine if we end up calling the right overload in cases of overloaded methods.
-        var replacement = UseCollectionExpressionHelpers.CreateReplacementCollectionExpressionForAnalysis(objectCreationExpression.Initializer);
+        var replacement = CollectionExpression(SeparatedList(
+            GetMatchElements(preMatches).Concat(GetInitializerElements(objectCreationExpression.Initializer))));
 
         return UseCollectionExpressionHelpers.CanReplaceWithCollectionExpression(
             semanticModel, objectCreationExpression, replacement, expressionType, isSingletonInstance: false, allowSemanticsChange, skipVerificationForReplacedNode: true, cancellationToken, out changesSemantics);
+
+        static IEnumerable<CollectionElementSyntax> GetMatchElements(ImmutableArray<CollectionMatch<SyntaxNode>> preMatches)
+        {
+            foreach (var match in preMatches)
+            {
+                if (match.Node is ExpressionSyntax expression)
+                    yield return match.UseSpread ? SpreadElement(expression) : ExpressionElement(expression);
+            }
+        }
+
+        static IEnumerable<CollectionElementSyntax> GetInitializerElements(InitializerExpressionSyntax? initializer)
+        {
+            if (initializer != null)
+            {
+                foreach (var expression in initializer.Expressions)
+                    yield return ExpressionElement(expression);
+            }
+        }
     }
 }

src/Analyzers/CSharp/Tests/UseCollectionInitializer/UseCollectionInitializerTests_CollectionExpression.cs

@@ -5866,4 +5866,26 @@ public class Class2 { }
             ReferenceAssemblies = ReferenceAssemblies.Net.Net80,
         }.RunAsync();
     }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/75894")]
+    public async Task TestNotOnDictionaryConstructor()
+    {
+        await new VerifyCS.Test
+        {
+            TestCode = """
+                using System.Collections.Generic;
+
+                class C
+                {
+                    void Main()
+                    {
+                        Dictionary<string, string> a = null;
+                        Dictionary<string, string> d = new(a);
+                    }
+                }
+                """,
+            LanguageVersion = LanguageVersion.CSharp13,
+            ReferenceAssemblies = ReferenceAssemblies.Net.Net90,
+        }.RunAsync();
+    }
 }

src/Analyzers/Core/Analyzers/UseCollectionInitializer/AbstractUseCollectionInitializerDiagnosticAnalyzer.cs

@@ -80,7 +80,13 @@ protected AbstractUseCollectionInitializerDiagnosticAnalyzer()
     protected abstract bool AreCollectionInitializersSupported(Compilation compilation);
     protected abstract bool AreCollectionExpressionsSupported(Compilation compilation);
     protected abstract bool CanUseCollectionExpression(
-        SemanticModel semanticModel, TObjectCreationExpressionSyntax objectCreationExpression, INamedTypeSymbol? expressionType, bool allowSemanticsChange, CancellationToken cancellationToken, out bool changesSemantics);
+        SemanticModel semanticModel,
+        TObjectCreationExpressionSyntax objectCreationExpression,
+        INamedTypeSymbol? expressionType,
+        ImmutableArray<CollectionMatch<SyntaxNode>> preMatches,
+        bool allowSemanticsChange,
+        CancellationToken cancellationToken,
+        out bool changesSemantics);
 
     protected abstract TAnalyzer GetAnalyzer();
 
@@ -218,18 +224,18 @@ private void AnalyzeNode(
             if (!this.AreCollectionExpressionsSupported(context.Compilation))
                 return null;
 
-            var (_, matches) = analyzer.Analyze(semanticModel, syntaxFacts, objectCreationExpression, analyzeForCollectionExpression: true, cancellationToken);
+            var (preMatches, postMatches) = analyzer.Analyze(semanticModel, syntaxFacts, objectCreationExpression, analyzeForCollectionExpression: true, cancellationToken);
 
             // If analysis failed, we can't change this, no matter what.
-            if (matches.IsDefault)
+            if (preMatches.IsDefault || postMatches.IsDefault)
                 return null;
 
             // Check if it would actually be legal to use a collection expression here though.
             var allowSemanticsChange = preferExpressionOption.Value == CollectionExpressionPreference.WhenTypesLooselyMatch;
-            if (!CanUseCollectionExpression(semanticModel, objectCreationExpression, expressionType, allowSemanticsChange, cancellationToken, out var changesSemantics))
+            if (!CanUseCollectionExpression(semanticModel, objectCreationExpression, expressionType, preMatches, allowSemanticsChange, cancellationToken, out var changesSemantics))
                 return null;
 
-            return (matches, shouldUseCollectionExpression: true, changesSemantics);
+            return (preMatches.Concat(postMatches), shouldUseCollectionExpression: true, changesSemantics);
         }
     }
 

src/Analyzers/VisualBasic/Analyzers/UseCollectionInitializer/VisualBasicUseCollectionInitializerDiagnosticAnalyzer.vb

@@ -2,9 +2,11 @@
 ' The .NET Foundation licenses this file to you under the MIT license.
 ' See the LICENSE file in the project root for more information.
 
+Imports System.Collections.Immutable
 Imports System.Threading
 Imports Microsoft.CodeAnalysis.Diagnostics
 Imports Microsoft.CodeAnalysis.LanguageService
+Imports Microsoft.CodeAnalysis.UseCollectionExpression
 Imports Microsoft.CodeAnalysis.UseCollectionInitializer
 Imports Microsoft.CodeAnalysis.VisualBasic.LanguageService
 Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
@@ -38,7 +40,14 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.UseCollectionInitializer
             Return False
         End Function
 
-        Protected Overrides Function CanUseCollectionExpression(semanticModel As SemanticModel, objectCreationExpression As ObjectCreationExpressionSyntax, expressionType As INamedTypeSymbol, allowSemanticsChange As Boolean, cancellationToken As CancellationToken, ByRef changesSemantics As Boolean) As Boolean
+        Protected Overrides Function CanUseCollectionExpression(
+                semanticModel As SemanticModel,
+                objectCreationExpression As ObjectCreationExpressionSyntax,
+                expressionType As INamedTypeSymbol,
+                matches As ImmutableArray(Of CollectionMatch(Of SyntaxNode)),
+                allowSemanticsChange As Boolean,
+                cancellationToken As CancellationToken,
+                ByRef changesSemantics As Boolean) As Boolean
             Throw ExceptionUtilities.Unreachable()
         End Function
     End Class