Investigate trimming of static interface methods when the interface is not used

[vitek-karas]

Currently linker will keep the interface if there's a method which implements one of the static interface methods on it and that implementation method is kept. This is unlike non-static interface methods where the same situation still allows the linker to remove the interface.

Sample:

void Main()
{
    TestClass.Method (); // Use the implementation method only
}

[Kept] // ??? Why
interface IStaticIFace
{
    [Kept] // ??? Why
    static abstract Method();
}

[Kept] // This is correct
class TestClass : IStaticIFace
{
    [Kept] // This is correct
    public static abstract Method() {};
}

[vitek-karas]

I debugged this a little. The main difference is that non-static methods implement the interface method implicitly - there's no override in the IL metadata. Static interface method implementation do have an explicit override in the metadata. Currently linker if method has overrides marks those overrides (and interfaces they belong to) without any additional logic.

This needs a bit of investigation:

• Is it OK to not keep the interface/method in this case? Will Cecil do the right thing with metadata if linker removed them?
• How to implement such behavior - linker would explicitly ignore override in the IL metadata - this might need careful handling