Do not use collectible AssemblyLoadContexts in AnalyzerAssemblyLoader. (#79990)

We ship several analyzers and source-generators in the SDK that are
added to most projects. We want to ship these as Ready2Run so that we
reduce JIT time. However, when an assembly is loaded into a collectible
AssemblyLoadContext it prevents any of the R2R logic from being used.

Author: JoeRobich

src/Compilers/Core/CodeAnalysisTest/AnalyzerAssemblyLoaderTests.cs

@@ -23,9 +23,7 @@
 using Basic.Reference.Assemblies;
 using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
 using Microsoft.CodeAnalysis.Emit;
-using Microsoft.CodeAnalysis.UnitTests.Diagnostics;
 using System.Diagnostics;
-using System.ComponentModel;
 
 #if NET
 using Roslyn.Test.Utilities.CoreClr;
@@ -1653,7 +1651,7 @@ public void AssemblyLoadingInNonDefaultContext_AnalyzerReferencesSystemCollectio
             // Load the compiler assembly and a modified version of S.C.I into the compiler load context. We
             // expect the analyzer will use the bogus S.C.I in the compiler context instead of the one 
             // in the host context.
-            var alc = new AssemblyLoadContext(nameof(AssemblyResolver_FirstOneWins), isCollectible: true);
+            var alc = new AssemblyLoadContext(nameof(AssemblyResolver_FirstOneWins), isCollectible: false);
             _ = alc.LoadFromAssemblyPath(TestFixture.UserSystemCollectionsImmutable);
             _ = alc.LoadFromAssemblyPath(typeof(AnalyzerAssemblyLoader).GetTypeInfo().Assembly.Location);
             var loader = kind switch
@@ -1677,8 +1675,33 @@ public void AssemblyLoadingInNonDefaultContext_AnalyzerReferencesSystemCollectio
 
                 Assert.Equal("42", sb.ToString());
             });
+        }
 
-            alc.Unload();
+        [Theory]
+        [CombinatorialData]
+        public void AssemblyLoading_DoesNotUseCollectibleALCs(AnalyzerTestKind kind)
+        {
+            // This validation is critical to our VS / CLI performance. We ship several analyzers and source-generators in the
+            // SDK (NetAnalyzers & Razor generators) that are added to most projects. We want to ship these as Ready2Run so that
+            // we reduce JIT time. However, when an assembly is loaded into a collectible AssemblyLoadContext it prevents any of
+            // the R2R logic from being used.
+
+            Run(kind, static (AnalyzerAssemblyLoader loader, AssemblyLoadTestFixture testFixture) =>
+            {
+                loader.AddDependencyLocation(testFixture.Delta1);
+                loader.AddDependencyLocation(testFixture.Gamma);
+
+                Assembly gamma = loader.LoadFromPath(testFixture.Gamma);
+                Assert.NotNull(gamma);
+
+                var contexts = loader.GetDirectoryLoadContextsSnapshot();
+                Assert.NotEmpty(contexts);
+
+                foreach (var context in contexts)
+                {
+                    Assert.False(context.IsCollectible, "AnalyzerAssemblyLoader should not use collectible assembly load contexts.");
+                }
+            });
         }
 #endif
 

src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerAssemblyLoader.Core.cs

@@ -165,29 +165,10 @@ internal DirectoryLoadContext[] GetDirectoryLoadContextsSnapshot()
 
         private partial void DisposeWorker()
         {
-            var contexts = ArrayBuilder<DirectoryLoadContext>.GetInstance();
             lock (_guard)
             {
-                foreach (var (_, context) in _loadContextByDirectory)
-                    contexts.Add(context);
-
                 _loadContextByDirectory.Clear();
             }
-
-            foreach (var context in contexts)
-            {
-                try
-                {
-                    context.Unload();
-                    CodeAnalysisEventSource.Log.DisposeAssemblyLoadContext(context.Directory, context.ToString());
-                }
-                catch (Exception ex) when (FatalError.ReportAndCatch(ex, ErrorSeverity.Critical))
-                {
-                    CodeAnalysisEventSource.Log.DisposeAssemblyLoadContextException(context.Directory, ex.ToString(), context.ToString());
-                }
-            }
-
-            contexts.Free();
         }
 
         internal sealed class DirectoryLoadContext : AssemblyLoadContext
@@ -196,7 +177,7 @@ internal sealed class DirectoryLoadContext : AssemblyLoadContext
             private readonly AnalyzerAssemblyLoader _loader;
 
             public DirectoryLoadContext(string directory, AnalyzerAssemblyLoader loader)
-                : base(isCollectible: true)
+                : base(isCollectible: false)
             {
                 Directory = directory;
                 _loader = loader;