Introduce FEATURE_ASSEMBLYLOADCONTEXT

```sh-session
$ rep -r "#if !FEATURE_ASSEMBLY_LOADFROM" "#if FEATURE_ASSEMBLYLOADCONTEXT" *.cs
1679 files processed. 0 files changed. 0 replacements.
$ rep -r "#if FEATURE_ASSEMBLY_LOADFROM" "#if !FEATURE_ASSEMBLYLOADCONTEXT" *.cs
1679 files processed. 7 files changed. 29 replacements.
```

And manual move/rename of the definition.

src/Build.UnitTests/BackEnd/TaskExecutionHost_Tests.cs

@@ -1157,7 +1157,7 @@ private void InitializeHost(bool throwOnExecute)
             ProjectInstance project = CreateTestProject();
 
             TypeLoader typeLoader = new TypeLoader(IsTaskFactoryClass);
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             AssemblyLoadInfo loadInfo = AssemblyLoadInfo.Create(Assembly.GetAssembly(typeof(TaskBuilderTestTask.TaskBuilderTestTaskFactory)).FullName, null);
 #else
             AssemblyLoadInfo loadInfo = AssemblyLoadInfo.Create(typeof(TaskBuilderTestTask.TaskBuilderTestTaskFactory).GetTypeInfo().FullName, null);

src/Build.UnitTests/BackEnd/TaskHostConfiguration_Tests.cs

@@ -126,7 +126,7 @@ public void ConstructorWithNullLocation()
            );
         }
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         /// <summary>
         /// Test that an exception is thrown when the path to the task assembly is empty
         /// </summary>
@@ -296,7 +296,7 @@ public void TestTranslationWithNullDictionary()
             TaskHostConfiguration deserializedConfig = packet as TaskHostConfiguration;
 
             Assert.Equal(config.TaskName, deserializedConfig.TaskName);
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             Assert.Equal(config.TaskLocation, config.TaskLocation);
 #endif
             Assert.Null(deserializedConfig.TaskParameters);
@@ -336,7 +336,7 @@ public void TestTranslationWithEmptyDictionary()
             TaskHostConfiguration deserializedConfig = packet as TaskHostConfiguration;
 
             Assert.Equal(config.TaskName, deserializedConfig.TaskName);
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             Assert.Equal(config.TaskLocation, config.TaskLocation);
 #endif
             Assert.NotNull(deserializedConfig.TaskParameters);
@@ -375,7 +375,7 @@ public void TestTranslationWithValueTypesInDictionary()
             TaskHostConfiguration deserializedConfig = packet as TaskHostConfiguration;
 
             Assert.Equal(config.TaskName, deserializedConfig.TaskName);
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             Assert.Equal(config.TaskLocation, config.TaskLocation);
 #endif
             Assert.NotNull(deserializedConfig.TaskParameters);
@@ -420,7 +420,7 @@ public void TestTranslationWithITaskItemInDictionary()
             TaskHostConfiguration deserializedConfig = packet as TaskHostConfiguration;
 
             Assert.Equal(config.TaskName, deserializedConfig.TaskName);
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             Assert.Equal(config.TaskLocation, config.TaskLocation);
 #endif
             Assert.NotNull(deserializedConfig.TaskParameters);
@@ -464,7 +464,7 @@ public void TestTranslationWithITaskItemArrayInDictionary()
             TaskHostConfiguration deserializedConfig = packet as TaskHostConfiguration;
 
             Assert.Equal(config.TaskName, deserializedConfig.TaskName);
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             Assert.Equal(config.TaskLocation, config.TaskLocation);
 #endif
             Assert.NotNull(deserializedConfig.TaskParameters);

src/Build/BackEnd/Components/SdkResolution/SdkResolverLoader.cs

@@ -16,7 +16,7 @@ namespace Microsoft.Build.BackEnd.SdkResolution
 {
     internal class SdkResolverLoader
     {
-#if !FEATURE_ASSEMBLY_LOADFROM
+#if FEATURE_ASSEMBLYLOADCONTEXT
         private readonly CoreClrAssemblyLoader _loader = new CoreClrAssemblyLoader();
 #endif
 
@@ -135,7 +135,7 @@ protected virtual IEnumerable<Type> GetResolverTypes(Assembly assembly)
 
         protected virtual Assembly LoadResolverAssembly(string resolverPath, LoggingContext loggingContext, ElementLocation location)
         {
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             return Assembly.LoadFrom(resolverPath);
 #else
             return _loader.LoadFromPath(resolverPath);

src/Directory.BeforeCommon.targets

@@ -24,7 +24,6 @@
     <FeatureAppDomain>true</FeatureAppDomain>
     <DefineConstants>$(DefineConstants);FEATURE_APPDOMAIN_UNHANDLED_EXCEPTION</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_ASPNET_COMPILER</DefineConstants>
-    <DefineConstants>$(DefineConstants);FEATURE_ASSEMBLY_LOADFROM</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_ASSEMBLY_LOCATION</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_ASSEMBLY_GETENTRYASSEMBLY</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_ASSEMBLYNAME_CLONE</DefineConstants>
@@ -120,6 +119,7 @@
 
   <PropertyGroup Condition="'$(NetCoreBuild)'=='true'">
     <CompilerToolsDir>$([System.IO.Path]::Combine($(ToolPackagesDir)Microsoft.Net.Compilers, $(CompilerToolsVersion), "tools"))$([System.IO.Path]::DirectorySeparatorChar)</CompilerToolsDir>
+    <DefineConstants>$(DefineConstants);FEATURE_ASSEMBLYLOADCONTEXT</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_PROCESSSTARTINFO_ENVIRONMENT</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_RUNTIMEINFORMATION</DefineConstants>
     <DefineConstants>$(DefineConstants);USE_MSBUILD_DLL_EXTN</DefineConstants>

src/Shared/AssemblyNameExtension.cs

@@ -11,7 +11,7 @@
 using System.Configuration.Assemblies;
 using System.Runtime.Serialization;
 using System.IO;
-#if !FEATURE_ASSEMBLY_LOADFROM
+#if FEATURE_ASSEMBLYLOADCONTEXT
 using System.Reflection.PortableExecutable;
 using System.Reflection.Metadata;
 #endif
@@ -183,7 +183,7 @@ private AssemblyNameExtension(SerializationInfo info, StreamingContext context)
         internal static AssemblyNameExtension GetAssemblyNameEx(string path)
         {
             AssemblyName assemblyName = null;
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             try
             {
                 assemblyName = AssemblyName.GetAssemblyName(path);

src/Shared/TaskEngineAssemblyResolver.cs

@@ -7,7 +7,7 @@
 using System.Diagnostics;
 using System.Globalization;
 
-#if !FEATURE_ASSEMBLY_LOADFROM
+#if FEATURE_ASSEMBLYLOADCONTEXT
 using System.Runtime.Loader;
 #endif
 using Microsoft.Build.Shared;

src/Shared/TypeLoader.cs

@@ -18,7 +18,7 @@ namespace Microsoft.Build.Shared
     /// </summary>
     internal class TypeLoader
     {
-#if !FEATURE_ASSEMBLY_LOADFROM
+#if FEATURE_ASSEMBLYLOADCONTEXT
         /// <summary>
         /// AssemblyContextLoader used to load DLLs outside of msbuild.exe directory
         /// </summary>
@@ -40,7 +40,7 @@ internal class TypeLoader
         /// </summary>
         private Func<Type, object, bool> _isDesiredType;
 
-#if !FEATURE_ASSEMBLY_LOADFROM
+#if FEATURE_ASSEMBLYLOADCONTEXT
         static TypeLoader()
         {
             s_coreClrAssemblyLoader = new CoreClrAssemblyLoader();
@@ -155,7 +155,7 @@ private static Assembly LoadAssembly(AssemblyLoadInfo assemblyLoadInfo)
                 }
                 else
                 {
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
                     loadedAssembly = Assembly.UnsafeLoadFrom(assemblyLoadInfo.AssemblyFile);
 #else
                     loadedAssembly = s_coreClrAssemblyLoader.LoadFromPath(assemblyLoadInfo.AssemblyFile);

src/Tasks/AssemblyDependency/AssemblyInformation.cs

@@ -14,7 +14,7 @@
 
 using Microsoft.Build.Shared;
 using Microsoft.Build.Shared.FileSystem;
-#if !FEATURE_ASSEMBLY_LOADFROM || MONO
+#if FEATURE_ASSEMBLYLOADCONTEXT || MONO
 using System.Reflection.PortableExecutable;
 using System.Reflection.Metadata;
 #endif
@@ -30,7 +30,7 @@ internal class AssemblyInformation : DisposableBase
     {
         private AssemblyNameExtension[] _assemblyDependencies;
         private string[] _assemblyFiles;
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         private readonly IMetaDataDispenser _metadataDispenser;
         private readonly IMetaDataAssemblyImport _assemblyImport;
         private static Guid s_importerGuid = new Guid(((GuidAttribute)Attribute.GetCustomAttribute(typeof(IMetaDataImport), typeof(GuidAttribute), false)).Value);
@@ -39,14 +39,14 @@ internal class AssemblyInformation : DisposableBase
         private readonly string _sourceFile;
         private FrameworkName _frameworkName;
 
-#if !FEATURE_ASSEMBLY_LOADFROM || MONO
+#if FEATURE_ASSEMBLYLOADCONTEXT || MONO
         private bool _metadataRead;
 #endif
 
-#if FEATURE_ASSEMBLY_LOADFROM && !MONO
+#if !FEATURE_ASSEMBLYLOADCONTEXT && !MONO
         private static string s_targetFrameworkAttribute = "System.Runtime.Versioning.TargetFrameworkAttribute";
 #endif
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         // Borrowed from genman.
         private const int GENMAN_STRING_BUF_SIZE = 1024;
         private const int GENMAN_LOCALE_BUF_SIZE = 64;
@@ -56,7 +56,7 @@ static AssemblyInformation()
         {
             AppDomain.CurrentDomain.ReflectionOnlyAssemblyResolve += ReflectionOnlyAssemblyResolve;
         }
-#endif // FEATURE_ASSEMBLY_LOADFROM
+#endif // !FEATURE_ASSEMBLYLOADCONTEXT
 
         /// <summary>
         /// Construct an instance for a source file.
@@ -68,7 +68,7 @@ internal AssemblyInformation(string sourceFile)
             ErrorUtilities.VerifyThrowArgumentNull(sourceFile, nameof(sourceFile));
             _sourceFile = sourceFile;
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             if (NativeMethodsShared.IsWindows)
             {
                 // Create the metadata dispenser and open scope on the source file.
@@ -82,7 +82,7 @@ internal AssemblyInformation(string sourceFile)
 #endif
         }
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         private static Assembly ReflectionOnlyAssemblyResolve(object sender, ResolveEventArgs args)
         {
             string[] nameParts = args.Name.Split(MSBuildConstants.CommaChar);
@@ -274,7 +274,7 @@ private FrameworkName GetFrameworkName()
 // Assembly.GetCustomAttributes* for an attribute which belongs
 // to an assembly that mono cannot find, causes a crash!
 // Instead, opt for using PEReader and friends to get that info
-#if FEATURE_ASSEMBLY_LOADFROM && !MONO
+#if !FEATURE_ASSEMBLYLOADCONTEXT && !MONO
             if (!NativeMethodsShared.IsWindows)
             {
                 if (String.Equals(Environment.GetEnvironmentVariable("MONO29679"), "1", StringComparison.OrdinalIgnoreCase))
@@ -345,7 +345,7 @@ private FrameworkName GetFrameworkName()
 #endif
         }
 
-#if !FEATURE_ASSEMBLY_LOADFROM || MONO
+#if FEATURE_ASSEMBLYLOADCONTEXT || MONO
         /// <summary>
         /// Read everything from the assembly in a single stream.
         /// </summary>
@@ -454,7 +454,7 @@ private static AssemblyName GetAssemblyName(MetadataReader metadataReader, Assem
 
 // Enabling this for MONO, because it's required by GetFrameworkName.
 // More details are in the comment for that method
-#if !FEATURE_ASSEMBLY_LOADFROM || MONO
+#if FEATURE_ASSEMBLYLOADCONTEXT || MONO
         //  This method copied from DNX source: https://github.com/aspnet/dnx/blob/e0726f769aead073af2d8cd9db47b89e1745d574/src/Microsoft.Dnx.Tooling/Utils/LockFileUtils.cs#L385
         //  System.Reflection.Metadata 1.1 is expected to have an API that helps with this.
         /// <summary>
@@ -514,7 +514,7 @@ private static List<string> GetFixedStringArguments(MetadataReader reader, Custo
         }
 #endif
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         /// <summary>
         /// Release interface pointers on Dispose(). 
         /// </summary>
@@ -586,7 +586,7 @@ internal static string GetRuntimeVersion(string path)
         /// <returns>The array of assembly dependencies.</returns>
         private AssemblyNameExtension[] ImportAssemblyDependencies()
         {
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             var asmRefs = new List<AssemblyNameExtension>();
 
             if (!NativeMethodsShared.IsWindows)
@@ -683,7 +683,7 @@ private AssemblyNameExtension[] ImportAssemblyDependencies()
         /// <returns>The extra files of assembly dependencies.</returns>
         private string[] ImportFiles()
         {
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             var files = new List<string>();
             IntPtr fileEnum = IntPtr.Zero;
             var fileTokens = new UInt32[GENMAN_ENUM_TOKEN_BUF_SIZE];
@@ -726,7 +726,7 @@ private string[] ImportFiles()
 #endif
         }
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         /// <summary>
         /// Allocate assembly metadata structure buffer.
         /// </summary>

src/Tasks/GenerateResource.cs

@@ -2280,7 +2280,7 @@ internal string StronglyTypedClassName
         /// </summary>
         private bool _stronglyTypedClassIsPublic;
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         /// <summary>
         /// Class that gets called by the ResxResourceReader to resolve references
         /// to assemblies within the .RESX.
@@ -2428,7 +2428,7 @@ internal void Run(
             _portableLibraryCacheInfo = new List<ResGenDependencies.PortableLibraryFile>();
             _usePreserializedResources = usePreserializedResources;
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
             // If references were passed in, we will have to give the ResxResourceReader an object
             // by which it can resolve types that are referenced from within the .RESX.
             if ((_assemblyFiles != null) && (_assemblyFiles.Length > 0))
@@ -2439,7 +2439,7 @@ internal void Run(
 
             try
             {
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
                 // Install assembly resolution event handler.
                 _eventHandler = new ResolveEventHandler(ResolveAssembly);
                 AppDomain.CurrentDomain.AssemblyResolve += _eventHandler;
@@ -2461,15 +2461,15 @@ internal void Run(
             }
             finally
             {
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
                 // Remove the event handler.
                 AppDomain.CurrentDomain.AssemblyResolve -= _eventHandler;
                 _eventHandler = null;
 #endif
             }
         }
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         /// <summary>
         /// Callback to resolve assembly names to assemblies.
         /// </summary>
@@ -2987,7 +2987,7 @@ private void ReadResources(String filename, bool shouldUseSourcePath, String out
 
             if (format == Format.Assembly) // Multiple input .resources files within one assembly
             {
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
                 ReadAssemblyResources(filename, outFileOrDir);
 #else
                 throw new InputFormatNotSupportedException("Reading resources from Assembly not supported on .NET Core MSBuild");
@@ -3080,7 +3080,7 @@ private void AddResourcesUsingMinimalCoreResxParsing(string filename, ReaderInfo
             }
         }
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
         /// <summary>
         /// Reads resources from an assembly.
         /// </summary>
@@ -3978,7 +3978,7 @@ internal int LinePosition
 #endregion // Code from ResGen.EXE
     }
 
-#if FEATURE_ASSEMBLY_LOADFROM
+#if !FEATURE_ASSEMBLYLOADCONTEXT
     /// <summary>
     /// This implemention of ITypeResolutionService is passed into the ResxResourceReader
     /// class, which calls back into the methods on this class in order to resolve types