Fix exception case in compiler resolver (#75000)

* Fix exception case in compiler resolver

This fixes how we deal with exceptions in the compiler assembly resolver.

* better exception semantics

src/Compilers/Core/CodeAnalysisTest/CompilerAnalyzerAssemblyResolverTests.cs

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#if NET
+using System.Runtime.Loader;
+using System.Reflection;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.UnitTests;
+
+public sealed class CompilerAnalyzerAssemblyResolverTests
+{
+    [Fact]
+    public void ExceptionReturnsNull()
+    {
+        var context = new AssemblyLoadContext(nameof(ExceptionReturnsNull), isCollectible: true);
+        var resolver = new AnalyzerAssemblyLoader.CompilerAnalyzerAssemblyResolver(context);
+        var name = new AssemblyName("NotARealAssembly");
+        Assert.Null(resolver.ResolveAssembly(name));
+        context.Unload();
+    }
+}
+#endif

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

@@ -227,7 +227,19 @@ internal sealed class CompilerAnalyzerAssemblyResolver(AssemblyLoadContext compi
         {
             private readonly AssemblyLoadContext _compilerAlc = compilerContext;
 
-            public Assembly? ResolveAssembly(AssemblyName assemblyName) => _compilerAlc.LoadFromAssemblyName(assemblyName);
+            public Assembly? ResolveAssembly(AssemblyName assemblyName)
+            {
+                try
+                {
+                    return _compilerAlc.LoadFromAssemblyName(assemblyName);
+                }
+                catch
+                {
+                    // The LoadFromAssemblyName method will throw if the assembly cannot be found. Need
+                    // to catch this exception and return null to satisfy the interface contract.
+                    return null;
+                }
+            }
         }
     }
 }

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

@@ -9,6 +9,7 @@
 using System.IO;
 using System.Linq;
 using System.Reflection;
+using Microsoft.CodeAnalysis.ErrorReporting;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis
@@ -415,7 +416,7 @@ internal string GetRealAnalyzerLoadPath(string originalFullPath)
                             return resolvedAssembly;
                         }
                     }
-                    catch
+                    catch (Exception ex) when (FatalError.ReportAndCatch(ex, ErrorSeverity.Diagnostic))
                     {
                         // Ignore if the external resolver throws
                     }