Resolve generic descriptor method signatures in ILC (dotnet#128123)

NativeAOT could not resolve XML descriptor method signatures that
reference generic parameters, e.g. `System.Void Method(T)`, while ILLink
accepted the same descriptor.

- **Descriptor matching**
  - Formats method signatures with method/type generic context.
- Resolves signature variables back to their generic parameter names for
XML comparison.

- **ILTrim coverage and parity**
- Re-enables existing ILTrim coverage for descriptor-rooted methods by
removing expected-failure entries that now pass.
- Removes the test-specific `--disable-opt unreachablebodies test`
workaround.
- Adds an ILTrim `IsWorthConvertingToThrow` approximation so empty
method bodies composed of an optional `nop` followed by `ret` are
preserved as-is, matching ILLink behavior closely enough for these
tests.
- Uses that heuristic when deciding whether to rewrite unreachable
method bodies.

- **ILCompiler test coverage**
- Updates the shared LinkXml test expectation so NativeAOT/ILCompiler
now expects the generic descriptor-rooted method to be kept.

Example descriptor now handled consistently:

```xml
<type fullname="MyGenericClass`1">
  <method signature="System.Void GenericMethod(T)" />
</type>
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: MichalStrehovsky <13110571+MichalStrehovsky@users.noreply.github.com>

Author: Copilot

src/coreclr/tools/Common/Compiler/ProcessLinkerXmlBase.cs

@@ -530,7 +530,8 @@ protected static string GetAttribute(XPathNavigator nav, string attribute)
         public static string GetMethodSignature(MethodDesc meth, bool includeGenericParameters)
         {
             StringBuilder sb = new StringBuilder();
-            CecilTypeNameFormatter.Instance.AppendName(sb, meth.Signature.ReturnType);
+            var formatter = new CecilTypeNameFormatter(meth);
+            formatter.AppendName(sb, meth.Signature.ReturnType);
             sb.Append(' ');
             sb.Append(meth.GetName());
             if (includeGenericParameters && meth.HasInstantiation)
@@ -545,7 +546,7 @@ public static string GetMethodSignature(MethodDesc meth, bool includeGenericPara
                 if (i > 0)
                     sb.Append(',');
 
-                CecilTypeNameFormatter.Instance.AppendName(sb, meth.Signature[i]);
+                formatter.AppendName(sb, meth.Signature[i]);
             }
 
             sb.Append(')');
@@ -572,7 +573,14 @@ protected void LogWarning(XPathNavigator position, DiagnosticId id, params strin
 
         private sealed class CecilTypeNameFormatter : TypeNameFormatter
         {
-            public static readonly CecilTypeNameFormatter Instance = new CecilTypeNameFormatter();
+            public static readonly CecilTypeNameFormatter Instance = new CecilTypeNameFormatter(null);
+
+            private readonly MethodDesc? _method;
+
+            public CecilTypeNameFormatter(MethodDesc? method)
+            {
+                _method = method;
+            }
 
             public override void AppendName(StringBuilder sb, ArrayType type)
             {
@@ -620,9 +628,21 @@ public override void AppendName(StringBuilder sb, GenericParameterDesc type)
             }
             public override void AppendName(StringBuilder sb, SignatureMethodVariable type)
             {
+                if (_method is not null &&
+                    type.Index < _method.Instantiation.Length &&
+                    _method.Instantiation[type.Index] is GenericParameterDesc genericParameter)
+                {
+                    sb.Append(genericParameter.Name);
+                }
             }
             public override void AppendName(StringBuilder sb, SignatureTypeVariable type)
             {
+                if (_method is not null &&
+                    type.Index < _method.OwningType.Instantiation.Length &&
+                    _method.OwningType.Instantiation[type.Index] is GenericParameterDesc genericParameter)
+                {
+                    sb.Append(genericParameter.Name);
+                }
             }
             protected override void AppendNameForInstantiatedType(StringBuilder sb, DefType type)
             {

src/coreclr/tools/ILTrim.Core/DependencyAnalysis/TokenBased/MethodDefinitionNode.cs

@@ -7,9 +7,12 @@
 using System.Reflection.Metadata;
 using System.Reflection.Metadata.Ecma335;
 
+using Internal.IL;
 using Internal.TypeSystem;
 using Internal.TypeSystem.Ecma;
 
+using Mono.Linker;
+
 namespace ILCompiler.DependencyAnalysis
 {
     /// <summary>
@@ -172,6 +175,8 @@ protected override EntityHandle WriteInternal(ModuleWritingContext writeContext)
             EcmaType ecmaType = (EcmaType)_module.GetObject(methodDef.GetDeclaringType());
             MethodBodyNode bodyNode = writeContext.Factory.MethodBody(_module, Handle);
             int bodyOffset = bodyNode.Marked
+                || !writeContext.Factory.Settings.Optimizations.IsEnabled(CodeOptimizations.UnreachableBodies, _module.Assembly.GetName().Name)
+                || !IsWorthConvertingToThrow(methodDef)
                 ? bodyNode.Write(writeContext)
                 : writeContext.WriteUnreachableMethodBody(Handle, _module);
 
@@ -201,6 +206,31 @@ protected override EntityHandle WriteInternal(ModuleWritingContext writeContext)
             return outputHandle;
         }
 
+        private bool IsWorthConvertingToThrow(MethodDefinition methodDef)
+        {
+            // Some bodies are cheaper size-wise to preserve as-is than to convert to a throw.
+            const int EmptyBodyLength = 0; // No IL body.
+            const int RetOnlyBodyLength = 1; // ret
+            const int NopRetBodyLength = 2; // nop; ret
+
+            int rva = methodDef.RelativeVirtualAddress;
+            if (rva == 0)
+                return false;
+
+            BlobReader ilReader = _module.PEReader.GetMethodBody(rva).GetILReader();
+            int ilLength = ilReader.Length;
+            if (ilLength == EmptyBodyLength)
+                return false;
+
+            if (ilLength == RetOnlyBodyLength)
+                return ilReader.ReadByte() != (byte)ILOpcode.ret;
+
+            if (ilLength == NopRetBodyLength)
+                return ilReader.ReadByte() != (byte)ILOpcode.nop || ilReader.ReadByte() != (byte)ILOpcode.ret;
+
+            return true;
+        }
+
         public override string ToString()
         {
             // TODO: would be nice to have a common formatter we can call into that also includes owning type

src/coreclr/tools/ILTrim.Tests/ILTrimExpectedFailures.txt

@@ -119,7 +119,6 @@ DataFlow.GenericParameterDataFlowMarking
 DataFlow.GenericParameterWarningLocation
 DataFlow.InterfaceImplementedThroughBaseValidation
 DataFlow.IReflectDataflow
-DataFlow.LocalDataFlowKeptMembers
 DataFlow.MemberTypes
 DataFlow.MemberTypesAllOnCopyAssembly
 DataFlow.MethodParametersDataFlow
@@ -174,9 +173,7 @@ Generics.VariantCasting
 Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.CanDisableOverrideRemoval
 Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.OverrideOfAbstractIsKept
 Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.OverrideOfAbstractIsKeptNonEmpty
-Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.OverrideOfVirtualCanBeRemoved
 Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.OverrideOfVirtualCanBeRemoved2
-Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.OverrideOfVirtualCanBeRemoved3
 Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.OverrideThatAlsoFulfilsInterface
 Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.PreservesOverriddenMethodOverrideOfUsedVirtualStillRemoved
 Inheritance.AbstractClasses.NoKeptCtor.OverrideRemoval.PreservesOverriddenMethodOverrideOfUsedVirtualStillRemoved2
@@ -229,7 +226,6 @@ Inheritance.Interfaces.OnReferenceType.NoKeptCtor.LocalDowncastDoesNotCuaseOther
 Inheritance.Interfaces.OnReferenceType.NoKeptCtor.PreserveDependencyPreservesInterfaceMethod
 Inheritance.Interfaces.OnReferenceType.NoKeptCtor.UnusedTypeHasExplicitInterfaceMethodPreservedViaXml
 Inheritance.Interfaces.OnReferenceType.NoKeptCtor.UnusedTypeHasExplicitInterfacePropertyPreservedViaXml
-Inheritance.Interfaces.OnReferenceType.NoKeptCtor.UnusedTypeHasInterfaceMethodPreservedViaXml
 Inheritance.Interfaces.OnReferenceType.NoKeptCtor.UnusedTypeWithPreserveMethodsAndInterfaceTypeMarked
 Inheritance.Interfaces.OnReferenceType.NoKeptCtorButInterfaceNeeded.ArrayWithIndexAssignedToReturnValue
 Inheritance.Interfaces.OnReferenceType.NoKeptCtorButInterfaceNeeded.FieldDowncastedToInterface
@@ -324,10 +320,7 @@ LinkXml.EmbeddedLinkXmlPreservesAdditionalAssemblyWithOverriddenMethod
 LinkXml.LinkXmlErrorCases
 LinkXml.UnusedFieldPreservedByLinkXmlIsKept
 LinkXml.UnusedInterfaceTypeOnTypeWithPreserveAllIsKept
-LinkXml.UnusedMethodPreservedByLinkXmlIsKept
 LinkXml.UnusedNonRequiredTypeIsRemoved
-LinkXml.UnusedTypeWithPreserveFieldsHasMethodsRemoved
-LinkXml.UnusedTypeWithPreserveNothingAndPreserveMembers
 LinkXml.UsedNonRequiredExportedTypeIsKept
 LinkXml.UsedNonRequiredExportedTypeIsKeptWhenRooted
 LinkXml.UsedNonRequiredTypeIsKeptWithSingleMethod
@@ -370,17 +363,13 @@ Reflection.AssemblyImportedViaReflectionWithDerivedType
 Reflection.CoreLibMessages
 Reflection.EventHanderTypeGetInvokeMethod
 Reflection.EventsUsedViaReflection
-Reflection.ExpressionCallString
 Reflection.IsAssignableFrom
 Reflection.MethodsUsedViaReflection
 Reflection.MethodUsedViaReflection
 Reflection.ObjectGetType
 Reflection.ObjectGetTypeLibraryMode
 Reflection.ParametersUsedViaReflection
-Reflection.PropertiesUsedViaReflection
-Reflection.PropertyUsedViaReflection
 Reflection.RunClassConstructor
-Reflection.RuntimeReflectionExtensionsCalls
 Reflection.TypeHierarchyLibraryModeSuppressions
 Reflection.TypeHierarchyReflectionWarnings
 Reflection.TypeMap
@@ -392,7 +381,6 @@ RequiresCapability.ReflectionAccessFromCompilerGeneratedCode
 RequiresCapability.RequiresAccessedThrough
 RequiresCapability.RequiresAttributeMismatch
 RequiresCapability.RequiresCapabilityFromCopiedAssembly
-RequiresCapability.RequiresCapabilityReflectionAnalysisEnabled
 RequiresCapability.RequiresExcludeStatics
 RequiresCapability.RequiresInCompilerGeneratedCode
 RequiresCapability.RequiresInCompilerGeneratedCodeRelease
@@ -506,7 +494,6 @@ UnreachableBody.ExplicitInstructionCheck
 UnreachableBody.InterfaceMethod
 UnreachableBody.LinkedOtherIncludedLibrary
 UnreachableBody.LinkedOtherIncludedLibraryNoInstanceCtor
-UnreachableBody.NotWorthConvertingEmpty
 UnreachableBody.NotWorthConvertingReturnDouble
 UnreachableBody.NotWorthConvertingReturnFalse
 UnreachableBody.NotWorthConvertingReturnFloat

src/tools/illink/test/Mono.Linker.Tests.Cases/LinkXml/UnusedMethodPreservedByLinkXmlIsKept.cs

@@ -38,9 +38,7 @@ private void NotPreservedMethod()
             {
             }
 
-            // NativeAOT fails to translate Cecil generic methods
-            // https://github.com/dotnet/runtime/issues/80462
-            [Kept(By = Tool.Trimmer)]
+            [Kept]
             private void PreservedMethod5<T>(T arg)
             {
             }