Improve perf in generator cache cases

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.Helpers.cs

@@ -74,7 +74,6 @@ private static StaticCompilationTagHelperFeature GetStaticTagHelperFeature(Compi
             // the tagHelperFeature will have its Engine property set as part of adding it to the engine, which is used later when doing the actual discovery
             var discoveryProjectEngine = RazorProjectEngine.Create(RazorConfiguration.Default, new VirtualRazorProjectFileSystem(), b =>
             {
-                b.Features.Add(new DefaultMetadataReferenceFeature { References = compilation.References.ToImmutableArray() });
                 b.Features.Add(tagHelperFeature);
                 b.Features.Add(new DefaultTagHelperDescriptorProvider());
 

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.cs

@@ -145,7 +145,39 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                     var ((compilationA, razorSourceGeneratorOptionsA), hasRazorFilesA) = a;
                     var ((compilationB, razorSourceGeneratorOptionsB), hasRazorFilesB) = b;
 
-                    if (!compilationA.References.SequenceEqual(compilationB.References))
+                    // When using the generator cache in the compiler it's possible to encounter metadata references that are different instances
+                    // but ultimately represent the same underlying assembly. We compare the module version ids to determine if the references are the same
+                    if (!compilationA.References.SequenceEqual(compilationB.References, new LambdaComparer<MetadataReference>((old, @new) => 
+                    {
+                        if (ReferenceEquals(old, @new))
+                        {
+                            return true;
+                        }
+
+                        if (old is null || @new is null)
+                        {
+                            return false;
+                        }
+
+                        var oldSymbol = compilationA.GetAssemblyOrModuleSymbol(old);
+                        var newSymbol = compilationB.GetAssemblyOrModuleSymbol(@new);
+
+                        if (SymbolEqualityComparer.Default.Equals(oldSymbol, newSymbol))
+                        {
+                            return true;
+                        }
+
+                        if (oldSymbol is IAssemblySymbol oldAssembly && newSymbol is IAssemblySymbol newAssembly)
+                        {
+                            var oldModuleMVIDs = oldAssembly.Modules.Select(GetMVID);
+                            var newModuleMVIDs = newAssembly.Modules.Select(GetMVID);
+                            return oldModuleMVIDs.SequenceEqual(newModuleMVIDs);
+
+                            static Guid GetMVID(IModuleSymbol m) => m.GetMetadata()?.GetModuleVersionId() ?? Guid.Empty;
+                        }
+
+                        return false;
+                    })))
                     {
                         return false;
                     }

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/SourceGeneratorProjectItem.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.IO;
+using System.Linq;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.CodeAnalysis;
 
@@ -47,7 +48,21 @@ public override Stream Read()
             => throw new NotSupportedException("This API should not be invoked. We should instead be relying on " +
                 "the RazorSourceDocument associated with this item instead.");
 
-        public bool Equals(SourceGeneratorProjectItem? other) => other is not null && AdditionalText == other.AdditionalText;
+        public bool Equals(SourceGeneratorProjectItem? other)
+        {
+            if (ReferenceEquals(AdditionalText, other?.AdditionalText))
+            {
+                return true;
+            }
+
+            // In the compiler server when the generator driver cache is enabled the
+            // additional files are always different instances even if their content is the same.
+            // It's technically possible for these hashes to collide, but other things would
+            // also break in those cases, so for now we're okay with this.
+            var thisHash = AdditionalText.GetText()?.GetContentHash() ?? [];
+            var otherHash = other?.AdditionalText.GetText()?.GetContentHash() ?? [];
+            return thisHash.SequenceEqual(otherHash);
+        }
 
         public override int GetHashCode() => AdditionalText.GetHashCode();
 

src/Compiler/test/Microsoft.NET.Sdk.Razor.SourceGenerators.Tests/RazorSourceGeneratorTests.cs

@@ -3175,5 +3175,93 @@ public class LambdaGenerator(Action<IncrementalGeneratorInitializationContext> a
         {
             public void Initialize(IncrementalGeneratorInitializationContext context) => action(context);
         }
+
+        [Fact]
+        public async Task IncrementalCompilation_NothingRuns_When_AdditionalFiles_HaveSameContent()
+        {
+            // Arrange
+            using var eventListener = new RazorEventListener();
+            var project = CreateTestProject(new()
+            {
+                ["Pages/Index.razor"] = "<h1>Hello world</h1>",
+                ["Pages/Counter.razor"] = "<h1>Counter</h1>",
+            });
+            var compilation = await project.GetCompilationAsync();
+            var (driver, additionalTexts) = await GetDriverWithAdditionalTextAsync(project);
+
+            var result = RunGenerator(compilation!, ref driver);
+            Assert.Empty(result.Diagnostics);
+            Assert.Equal(2, result.GeneratedSources.Length);
+
+            eventListener.Events.Clear();
+
+            result = RunGenerator(compilation!, ref driver)
+                        .VerifyOutputsMatch(result);
+
+            Assert.Empty(result.Diagnostics);
+            Assert.Equal(2, result.GeneratedSources.Length);
+            Assert.Empty(eventListener.Events);
+
+            project = project.RemoveAdditionalDocument(project.AdditionalDocumentIds[1])
+                             .AddAdditionalDocument("Counter.razor", SourceText.From("<h1>Counter</h1>", Encoding.UTF8))
+                             .Project;
+
+            compilation = await project.GetCompilationAsync();
+
+            result = RunGenerator(compilation!, ref driver)
+                        .VerifyOutputsMatch(result);
+
+            Assert.Empty(result.Diagnostics);
+            Assert.Equal(2, result.GeneratedSources.Length);
+
+            Assert.Empty(eventListener.Events);
+        }
+
+        [Fact]
+        public async Task IncrementalCompilation_OnlyCompilationRuns_When_MetadataReferences_SameAssembly()
+        {
+            // Arrange
+            using var eventListener = new RazorEventListener();
+            var project = CreateTestProject(new()
+            {
+                ["Pages/Index.razor"] = "<h1>Hello world</h1>",
+                ["Pages/Counter.razor"] = "<h1>Counter</h1>",
+            });
+            var compilation = await project.GetCompilationAsync();
+            var (driver, additionalTexts) = await GetDriverWithAdditionalTextAsync(project);
+
+            var result = RunGenerator(compilation!, ref driver);
+
+            Assert.Empty(result.Diagnostics);
+            Assert.Equal(2, result.GeneratedSources.Length);
+
+            eventListener.Events.Clear();
+
+            result = RunGenerator(compilation!, ref driver)
+                .VerifyOutputsMatch(result);
+
+            Assert.Empty(result.Diagnostics);
+            Assert.Equal(2, result.GeneratedSources.Length);
+            Assert.Empty(eventListener.Events);
+
+            var reference = (PortableExecutableReference) project.MetadataReferences[^1];
+
+            project = project.RemoveMetadataReference(reference)
+                             .AddMetadataReference(MetadataReference.CreateFromFile(reference.FilePath!));
+
+            compilation = await project.GetCompilationAsync();
+
+            result = RunGenerator(compilation!, ref driver)
+                .VerifyOutputsMatch(result);
+
+            Assert.Empty(result.Diagnostics);
+            Assert.Equal(2, result.GeneratedSources.Length);
+
+            // reference causes the compilation to change so we re-run tag helper discovery there
+            // but we didn't re-check the actual reference itself
+            Assert.Collection(eventListener.Events,
+               e => Assert.Equal("DiscoverTagHelpersFromCompilationStart", e.EventName),
+               e => Assert.Equal("DiscoverTagHelpersFromCompilationStop", e.EventName));
+        }
     }
 }