BindingFlagSupport source is duplicated between CoreLib and System.Reflection.MetadataLoadContext

[ericstj]

In working on #116080 I noticed that nativeaot Corelib has a copy of the same code that's in MetadataLoadContext

runtime/src/coreclr/nativeaot/System.Private.CoreLib/src/System.Private.CoreLib.csproj

Lines 380 to 393 in fcd197c

	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\ConstructorPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\EventPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\FieldPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\MemberPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\MemberTypeIndex.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\MethodPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\NameFilter.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\NameFilter.NativeFormat.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\NestedTypePolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\PropertyPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\QueriedMemberList.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\QueryResult.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\QueryResult.Enumerator.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\Shared.cs" />

runtime/src/libraries/System.Reflection.MetadataLoadContext/src/System.Reflection.MetadataLoadContext.csproj

Lines 28 to 41 in fcd197c

	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\ConstructorPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\EventPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\FieldPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\MemberPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\MemberTypeIndex.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\MethodPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\NameFilter.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\NameFilter.Ecma.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\NestedTypePolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\PropertyPolicies.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\QueriedMemberList.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\QueryResult.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\QueryResult.Enumerator.cs" />
	<Compile Include="System\Reflection\Runtime\BindingFlagSupport\Shared.cs" />

We should diff this to see if it's diverged and reconcile if it has, then deduplicate and place in common.

cc @jkotas @MichalStrehovsky

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-reflection
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-reflection
See info in area-owners.md if you want to be subscribed.

[MichalStrehovsky]

This is System.Reflection. MetadataLoadContext forked the native AOT reflection stack, it should not be the responsibility of the AOT team to reconcile.

[jkotas]

Also, there are two more logical copies of this code: in regular CoreCLR reflection implementation and Mono reflection implementation. Ideally, all these 4 copies would be 80+% shared code.

[ericstj]

Fair enough. Originally I assigned to nativeaot because it introduced the duplication when it was introduced 3 years after MLC. I see now that this came from Ati's work in https://github.com/dotnet/corert/commits/master/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/BindingFlagSupport. I guess this nativeaot code had already diverged when it was brought over so it wasn't deduplicated at that time.

Honestly I don't care who does it, I was just trying to guess at who would care the most about it. This seemed like low-hanging fruit since the code is already very similar.

@jkotas - there's a pretty significant layering difference between CoreCLR reflection right now - that's built at a different layer of metadata API. What's your thinking about which direction that should move / how to handle the different metadata sources?

Here's the current diff of these two copies -- https://gist.github.com/ericstj/702ff654d2d902df37431357f4d0a219/revisions