[release/7.0] Don't generate a nearly empty file

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/IncrementalGeneratorInitializationContextExtensions.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Linq;
 using System.Text;
 using Microsoft.CodeAnalysis;
@@ -27,26 +28,31 @@ public static void RegisterDiagnostics(this IncrementalGeneratorInitializationCo
         public static void RegisterConcatenatedSyntaxOutputs<TNode>(this IncrementalGeneratorInitializationContext context, IncrementalValuesProvider<TNode> nodes, string fileName)
             where TNode : SyntaxNode
         {
-            IncrementalValueProvider<string> generatedMethods = nodes
+            IncrementalValueProvider<ImmutableArray<string>> generatedMethods = nodes
                 .Select(
                     static (node, ct) => node.NormalizeWhitespace().ToFullString())
-                .Collect()
-                .Select(static (generatedSources, ct) =>
+                .Collect();
+
+            context.RegisterSourceOutput(generatedMethods,
+                (context, generatedSources) =>
                 {
+                    // Don't generate a file if we don't have to, to avoid the extra IDE overhead once we have generated
+                    // files in play.
+                    if (generatedSources.IsEmpty)
+                    {
+                        return;
+                    }
+
                     StringBuilder source = new();
                     // Mark in source that the file is auto-generated.
                     source.AppendLine("// <auto-generated/>");
                     foreach (string generated in generatedSources)
                     {
                         source.AppendLine(generated);
                     }
-                    return source.ToString();
-                });
 
-            context.RegisterSourceOutput(generatedMethods,
-                (context, source) =>
-                {
-                    context.AddSource(fileName, source);
+                    // Once https://github.com/dotnet/roslyn/issues/61326 is resolved, we can avoid the ToString() here.
+                    context.AddSource(fileName, source.ToString());
                 });
         }
     }

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Compiles.cs

@@ -641,33 +641,30 @@ public async Task ValidateSnippetsWithMultipleSources(string id, string[] source
             TestUtils.AssertPostSourceGeneratorCompilation(newComp);
         }
 
-        public static IEnumerable<object[]> CodeSnippetsToCompileToValidateAllowUnsafeBlocks()
+        public static IEnumerable<object[]> CodeSnippetsToVerifyNoTreesProduced()
         {
-            yield return new object[] { ID(), CodeSnippets.TrivialClassDeclarations, TestTargetFramework.Net, true };
-
-            {
-                string source = @"
+            string source = @"
 using System.Runtime.InteropServices;
 public class Basic { }
 ";
-                yield return new object[] { ID(), source, TestTargetFramework.Standard, false };
-                yield return new object[] { ID(), source, TestTargetFramework.Framework, false };
-                yield return new object[] { ID(), source, TestTargetFramework.Net, false };
-            }
+            yield return new object[] { ID(), source, TestTargetFramework.Standard };
+            yield return new object[] { ID(), source, TestTargetFramework.Framework };
+            yield return new object[] { ID(), source, TestTargetFramework.Net };
         }
 
         [Theory]
-        [MemberData(nameof(CodeSnippetsToCompileToValidateAllowUnsafeBlocks))]
-        public async Task ValidateRequireAllowUnsafeBlocksDiagnosticNoTrigger(string id, string source, TestTargetFramework framework, bool allowUnsafe)
+        [MemberData(nameof(CodeSnippetsToVerifyNoTreesProduced))]
+        public async Task ValidateNoGeneratedOuptutForNoImport(string id, string source, TestTargetFramework framework)
         {
             TestUtils.Use(id);
-            Compilation comp = await TestUtils.CreateCompilation(source, framework, allowUnsafe: allowUnsafe);
+            Compilation comp = await TestUtils.CreateCompilation(source, framework, allowUnsafe: false);
             TestUtils.AssertPreSourceGeneratorCompilation(comp);
 
             var newComp = TestUtils.RunGenerators(comp, out var generatorDiags, new Microsoft.Interop.LibraryImportGenerator());
             Assert.Empty(generatorDiags);
 
-            TestUtils.AssertPostSourceGeneratorCompilation(newComp);
+            // Assert we didn't generate any syntax trees, even empty ones
+            Assert.Same(comp, newComp);
         }
     }
 }