Suggest proper 'Async'-suffixed name for Task-returning symbols (#79518)

Fixes #17989

Author: CyrusNajmabadi

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/DeclarationNameCompletionProviderTests.cs

@@ -380,9 +380,10 @@ async Task<C> $$
             }
             """, "GetCAsync");
 
-    [Fact(Skip = "not yet implemented")]
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/17989")]
     public Task NonAsyncTaskOfT()
         => VerifyItemExistsAsync("""
+            using System.Threading.Tasks;
             public class C
             {
                 Task<C> $$

src/Features/CSharp/Portable/Completion/CompletionProviders/DeclarationName/DeclarationNameCompletionProvider.cs

@@ -73,7 +73,8 @@ public override async Task ProvideCompletionsAsync(CompletionContext completionC
             var sortValue = 0;
             foreach (var recommender in Recommenders)
             {
-                var names = await recommender.Value.ProvideRecommendedNamesAsync(completionContext, document, context, nameInfo, cancellationToken).ConfigureAwait(false);
+                var names = await recommender.Value.ProvideRecommendedNamesAsync(
+                    completionContext, document, context, nameInfo, cancellationToken).ConfigureAwait(false);
 
                 foreach (var (name, glyph) in names)
                 {

src/Features/CSharp/Portable/Completion/CompletionProviders/DeclarationName/DeclarationNameInfo.cs

@@ -198,8 +198,8 @@ private static bool IsPropertyDeclaration(SyntaxToken token, SemanticModel seman
         return result.Type != null;
     }
 
-    private static bool IsMethodDeclaration(SyntaxToken token, SemanticModel semanticModel,
-        CancellationToken cancellationToken, out NameDeclarationInfo result)
+    private static bool IsMethodDeclaration(
+        SyntaxToken token, SemanticModel semanticModel, CancellationToken cancellationToken, out NameDeclarationInfo result)
     {
         result = IsLastTokenOfType<MethodDeclarationSyntax>(
             token,
@@ -220,45 +220,30 @@ private static NameDeclarationInfo IsFollowingTypeOrComma<TSyntaxNode>(
         Func<DeclarationModifiers, ImmutableArray<SymbolKindOrTypeKind>> possibleDeclarationComputer,
         CancellationToken cancellationToken) where TSyntaxNode : SyntaxNode
     {
-        if (!IsPossibleTypeToken(token) && !token.IsKind(SyntaxKind.CommaToken))
-        {
+        var afterComma = token.IsKind(SyntaxKind.CommaToken);
+        if (!IsPossibleTypeToken(token) && !afterComma)
             return default;
-        }
 
         var target = token.GetAncestor<TSyntaxNode>();
         if (target == null)
-        {
             return default;
-        }
 
-        if (token.IsKind(SyntaxKind.CommaToken) && token.Parent != target)
-        {
+        if (afterComma && token.Parent != target)
             return default;
-        }
 
         var typeSyntax = typeSyntaxGetter(target);
         if (typeSyntax == null)
-        {
             return default;
-        }
 
-        if (!token.IsKind(SyntaxKind.CommaToken) && token != typeSyntax.GetLastToken())
-        {
+        if (!afterComma && token != typeSyntax.GetLastToken())
             return default;
-        }
 
         var modifiers = modifierGetter(target);
         if (modifiers == null)
-        {
             return default;
-        }
 
-        return new NameDeclarationInfo(
-            possibleDeclarationComputer(GetDeclarationModifiers(modifiers.Value)),
-            GetAccessibility(modifiers.Value),
-            GetDeclarationModifiers(modifiers.Value),
-            semanticModel.GetTypeInfo(typeSyntax, cancellationToken).Type,
-            semanticModel.GetAliasInfo(typeSyntax, cancellationToken));
+        return ComputeInfo(
+            semanticModel, possibleDeclarationComputer, typeSyntax, modifiers.Value, tryInferAsync: !afterComma, cancellationToken);
     }
 
     private static NameDeclarationInfo IsLastTokenOfType<TSyntaxNode>(
@@ -295,11 +280,36 @@ private static NameDeclarationInfo IsLastTokenOfType<TSyntaxNode>(
 
         var modifiers = modifierGetter(target);
 
+        return ComputeInfo(semanticModel, possibleDeclarationComputer, typeSyntax, modifiers, tryInferAsync: true, cancellationToken);
+    }
+
+    private static NameDeclarationInfo ComputeInfo(
+        SemanticModel semanticModel,
+        Func<DeclarationModifiers, ImmutableArray<SymbolKindOrTypeKind>> possibleDeclarationComputer,
+        SyntaxNode typeSyntax,
+        SyntaxTokenList modifiers,
+        bool tryInferAsync,
+        CancellationToken cancellationToken)
+    {
+        var declarationModifiers = GetDeclarationModifiers(modifiers);
+        var possibleDeclarations = possibleDeclarationComputer(declarationModifiers);
+
+        // Treat a declaration as async if if it is explicitly marked as 'async' or if it is a method that returns a
+        // Task-like type. The latter ensures that even if the user didn't have an explicit 'async' modifier, we still
+        // name the member in an appropriate fashion (e.g. `GetCustomerAsync` for `public Task<Customer> $$`).
+        var type = semanticModel.GetTypeInfo(typeSyntax, cancellationToken).Type;
+        if (!declarationModifiers.IsAsync && tryInferAsync)
+        {
+            var knownTaskTypes = new KnownTaskTypes(semanticModel.Compilation);
+            if (knownTaskTypes.IsTaskLike(type))
+                declarationModifiers = declarationModifiers.WithAsync(true);
+        }
+
         return new NameDeclarationInfo(
-            possibleDeclarationComputer(GetDeclarationModifiers(modifiers)),
+            possibleDeclarations,
             GetAccessibility(modifiers),
-            GetDeclarationModifiers(modifiers),
-            semanticModel.GetTypeInfo(typeSyntax, cancellationToken).Type,
+            declarationModifiers,
+            type,
             semanticModel.GetAliasInfo(typeSyntax, cancellationToken));
     }
 

src/Features/CSharp/Portable/Completion/CompletionProviders/DeclarationName/DeclarationNameRecommender.cs

@@ -215,22 +215,14 @@ private static void GetRecommendedNames(
 
         foreach (var kind in declarationInfo.PossibleSymbolKinds)
         {
-            ProcessRules(rules, firstMatchOnly: true, kind, baseNames, declarationInfo, context, result, semanticFactsService, seenBaseNames, seenUniqueNames, cancellationToken);
-            ProcessRules(supplementaryRules, firstMatchOnly: false, kind, baseNames, declarationInfo, context, result, semanticFactsService, seenBaseNames, seenUniqueNames, cancellationToken);
+            ProcessRules(rules, firstMatchOnly: true, kind);
+            ProcessRules(supplementaryRules, firstMatchOnly: false, kind);
         }
 
-        static void ProcessRules(
+        void ProcessRules(
             ImmutableArray<NamingRule> rules,
             bool firstMatchOnly,
-            SymbolSpecification.SymbolKindOrTypeKind kind,
-            ImmutableArray<ImmutableArray<string>> baseNames,
-            NameDeclarationInfo declarationInfo,
-            CSharpSyntaxContext context,
-            ArrayBuilder<(string, Glyph)> result,
-            ISemanticFactsService semanticFactsService,
-            PooledHashSet<string> seenBaseNames,
-            PooledHashSet<string> seenUniqueNames,
-            CancellationToken cancellationToken)
+            SymbolSpecification.SymbolKindOrTypeKind kind)
         {
             var modifiers = declarationInfo.Modifiers;
             foreach (var rule in rules)

src/Workspaces/SharedUtilitiesAndExtensions/Workspace/Core/Extensions/KnownTypes.cs

@@ -2,6 +2,8 @@
 // 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.Diagnostics.CodeAnalysis;
+
 namespace Microsoft.CodeAnalysis.Shared.Extensions;
 
 internal readonly struct KnownTaskTypes(Compilation compilation)
@@ -17,22 +19,25 @@ internal readonly struct KnownTaskTypes(Compilation compilation)
     public readonly INamedTypeSymbol? IAsyncEnumerableOfTType = compilation.IAsyncEnumerableOfTType();
     public readonly INamedTypeSymbol? IAsyncEnumeratorOfTType = compilation.IAsyncEnumeratorOfTType();
 
-    public bool IsTaskLike(ITypeSymbol returnType)
+    public bool IsTaskLike([NotNullWhen(true)] ITypeSymbol? returnType)
     {
-        if (returnType.Equals(this.TaskType))
-            return true;
+        if (returnType is not null)
+        {
+            if (returnType.Equals(this.TaskType))
+                return true;
 
-        if (returnType.Equals(this.ValueTaskType))
-            return true;
+            if (returnType.Equals(this.ValueTaskType))
+                return true;
 
-        if (returnType.OriginalDefinition.Equals(this.TaskOfTType))
-            return true;
+            if (returnType.OriginalDefinition.Equals(this.TaskOfTType))
+                return true;
 
-        if (returnType.OriginalDefinition.Equals(this.ValueTaskOfTType))
-            return true;
+            if (returnType.OriginalDefinition.Equals(this.ValueTaskOfTType))
+                return true;
 
-        if (returnType.IsErrorType())
-            return returnType.Name is "Task" or "ValueTask";
+            if (returnType.IsErrorType())
+                return returnType.Name is "Task" or "ValueTask";
+        }
 
         return false;
     }