Fix AssemblyChecker dependency resolution

eng/Subsets.props

@@ -515,6 +515,8 @@
   </ItemGroup>
 
   <ItemGroup Condition="$(_subset.Contains('+clr.toolstests+'))">
+    <ProjectToBuild Include="$(CoreClrProjectRoot)tools\AssemblyChecker.Tests\AssemblyChecker.Tests.csproj"
+      Test="true" Category="clr" Condition="'$(DotNetBuildSourceOnly)' != 'true'"/>
     <ProjectToBuild Include="$(CoreClrProjectRoot)tools\aot\ILCompiler.DependencyAnalysisFramework.Tests\ILCompiler.DependencyAnalysisFramework.Tests.csproj"
       Test="true" Category="clr" Condition="'$(DotNetBuildSourceOnly)' != 'true'"/>
     <ProjectToBuild Include="$(CoreClrProjectRoot)tools\aot\ILCompiler.TypeSystem.Tests\ILCompiler.TypeSystem.Tests.csproj"

src/coreclr/tools/AssemblyChecker.Tests/AssemblyChecker.Tests.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <AssemblyName>AssemblyChecker.Tests</AssemblyName>
+    <Configurations>Debug;Release;Checked</Configurations>
+    <EnableDefaultCompileItems>false</EnableDefaultCompileItems>
+    <TargetFramework>$(NetCoreAppToolCurrent)</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="../AssemblyChecker/AssemblyChecker.csproj" />
+    <ProjectReference Include="TestAssets/AssemblyWithExternalAttribute/AssemblyWithExternalAttribute.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="AssemblyInspectorTests.cs" />
+  </ItemGroup>
+</Project>

src/coreclr/tools/AssemblyChecker.Tests/AssemblyInspectorTests.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using AssemblyChecker.Tests.AssemblyWithExternalAttribute;
+using Xunit;
+
+namespace AssemblyChecker.Tests;
+
+public class AssemblyInspectorTests
+{
+    [Fact]
+    public void IsDebugResolvesAssemblyLevelAttributeFromAdjacentAssembly()
+    {
+        Assert.False(AssemblyInspector.IsDebug(typeof(Marker).Assembly.Location));
+    }
+}

src/coreclr/tools/AssemblyChecker.Tests/TestAssets/AssemblyWithExternalAttribute/AssemblyWithExternalAttribute.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <DebugSymbols>false</DebugSymbols>
+    <DebugType>none</DebugType>
+    <EnableDefaultCompileItems>false</EnableDefaultCompileItems>
+    <Optimize>true</Optimize>
+    <OutputType>Library</OutputType>
+    <SkipTestRun>true</SkipTestRun>
+    <TargetFramework>$(NetCoreAppToolCurrent)</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="../ExternalAttribute/ExternalAttribute.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="Marker.cs" />
+  </ItemGroup>
+</Project>

src/coreclr/tools/AssemblyChecker.Tests/TestAssets/AssemblyWithExternalAttribute/Marker.cs

@@ -0,0 +1,12 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using AssemblyChecker.Tests.ExternalAttribute;
+
+[assembly: ReferencedAssembly]
+
+namespace AssemblyChecker.Tests.AssemblyWithExternalAttribute;
+
+public sealed class Marker
+{
+}

src/coreclr/tools/AssemblyChecker.Tests/TestAssets/ExternalAttribute/ExternalAttribute.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <EnableDefaultCompileItems>false</EnableDefaultCompileItems>
+    <OutputType>Library</OutputType>
+    <SkipTestRun>true</SkipTestRun>
+    <TargetFramework>$(NetCoreAppToolCurrent)</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="ReferencedAssemblyAttribute.cs" />
+  </ItemGroup>
+</Project>

src/coreclr/tools/AssemblyChecker.Tests/TestAssets/ExternalAttribute/ReferencedAssemblyAttribute.cs

@@ -0,0 +1,11 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+
+namespace AssemblyChecker.Tests.ExternalAttribute;
+
+[AttributeUsage(AttributeTargets.Assembly)]
+public sealed class ReferencedAssemblyAttribute : Attribute
+{
+}

src/coreclr/tools/AssemblyChecker/AssemblyChecker.csproj

@@ -15,6 +15,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <InternalsVisibleTo Include="AssemblyChecker.Tests" />
     <PackageReference Include="System.Reflection.MetadataLoadContext" Version="$(SystemReflectionMetadataLoadContextPackageVersion)" />
   </ItemGroup>
 </Project>

src/coreclr/tools/AssemblyChecker/AssemblyInspector.cs

@@ -3,17 +3,19 @@
 
 using System.Diagnostics;
 using System.Reflection;
+using System.Runtime.InteropServices;
 
 namespace AssemblyChecker
 {
     internal static class AssemblyInspector
     {
-        private static readonly RuntimeAssemblyResolver s_resolver = new();
-
         internal static bool IsDebug(string path)
         {
             string assemblyPath = Path.GetFullPath(path);
-            using MetadataLoadContext loadContext = new(s_resolver);
+            string? assemblyDirectory = Path.GetDirectoryName(assemblyPath);
+            Debug.Assert(assemblyDirectory is not null);
+
+            using MetadataLoadContext loadContext = new(new AssemblyResolver(assemblyDirectory));
             Assembly assembly = loadContext.LoadFromAssemblyPath(assemblyPath);
             foreach (CustomAttributeData attribute in assembly.GetCustomAttributesData())
             {
@@ -56,10 +58,10 @@ internal static bool IsDebug(string path)
             return false;
         }
 
-        private sealed class RuntimeAssemblyResolver : MetadataAssemblyResolver
+        private sealed class AssemblyResolver(string assemblyDirectory) : MetadataAssemblyResolver
         {
-            private readonly string _runtimeDirectory = Path.GetDirectoryName(typeof(object).Assembly.Location)
-                ?? throw new InvalidOperationException("The runtime assembly directory is not available.");
+            private readonly string _assemblyDirectory = assemblyDirectory;
+            private readonly string _runtimeDirectory = RuntimeEnvironment.GetRuntimeDirectory();
 
             public override Assembly? Resolve(MetadataLoadContext context, AssemblyName assemblyName)
             {
@@ -68,7 +70,13 @@ private sealed class RuntimeAssemblyResolver : MetadataAssemblyResolver
                     return null;
                 }
 
-                string assemblyPath = Path.Combine(_runtimeDirectory, name + ".dll");
+                string fileName = name + ".dll";
+                string assemblyPath = Path.Combine(_assemblyDirectory, fileName);
+                if (!File.Exists(assemblyPath))
+                {
+                    assemblyPath = Path.Combine(_runtimeDirectory, fileName);
+                }
+
                 return File.Exists(assemblyPath) ? context.LoadFromAssemblyPath(assemblyPath) : null;
             }
         }