Enable setting the TypeMap entrypoint assembly via a RuntimeHostConfigurationOption

[jtschuster]

Implements the alternative to #121138 mentioned in #121138 (comment)

Enables the TypeMap to be used in Library output types. Projects can set the following RuntimeHostConfigurationOption to enable this behavior. If it exists, this option takes priority over the entrypoint assembly.

<ItemGroup>
    <RuntimeHostConfigurationOption Include="System.Runtime.InteropServices.TypeMappingEntryAssembly"
        Value="$(TypeMapEntryAssembly)"
        Condition="$(TypeMapEntryAssembly) != ''" />
</ItemGroup>

[Copilot]

Pull Request Overview

This PR implements a new feature that enables the TypeMap entrypoint assembly to be configured via a RuntimeHostConfigurationOption, allowing TypeMap to be used with Library output types instead of only executable output types. The change provides a way to specify which assembly should be scanned for TypeMap attributes when the entry point assembly is not appropriate (e.g., in library scenarios).

Key changes:

• Added support for System.Runtime.InteropServices.TypeMappingEntryAssembly configuration option across the CoreCLR compiler, trimmer, and runtime
• Modified the --feature flag in illink to accept string values in addition to boolean values
• Added new tests to validate TypeMap entry assembly configuration works correctly

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/TypeMapLazyDictionary.cs	Modified runtime to check AppContext for TypeMappingEntryAssembly override before falling back to entry assembly
src/coreclr/tools/aot/ILCompiler/Program.cs	Added runtime knob parsing to extract TypeMappingEntryAssembly and use it for TypeMap metadata
src/tools/illink/src/linker/Linker/Driver.cs	Modified --feature flag to accept and store string values alongside boolean values
src/tools/illink/src/linker/Linker/LinkContext.cs	Added Features dictionary to store string-valued features
src/tools/illink/src/linker/Linker.Steps/MarkStep.cs	Added logic to check Features for TypeMappingEntryAssembly override
src/coreclr/tools/aot/ILCompiler.Trimming.Tests/TestCasesRunner/TrimmingDriver.cs	Added comment explaining test behavior with entry assembly
src/tests/Interop/TypeMap/*.cs	Added new runtime test files validating TypeMappingEntryAssembly configuration
src/tools/illink/test/Mono.Linker.Tests.Cases/Reflection/*.cs	Added trimmer test cases and updated existing tests for renamed method

[MichalStrehovsky]

Looks good otherwise!

[AaronRobinsonMSFT]

This is odd. The as operator implies the returned type could be a non RuntimeAssembly. I don't believe that is true. This is a confusing cast. It should just be (RuntimeAssembly).

[jtschuster]

Ah, I forgot (RuntimeAssembly)null! returns null instead of throwing. Will update to make that clear.