metadata Reader::attribute_args enhancements

[ChrisDenton]

I've been playing with reading attribute metadata and wanted to help fix a couple of issues I encountered:

• ComVisibleAttribute has a bool argument, so it's useful to be able to read it. I also added a panic if the blob bool value is not either 0 or 1 (as per the spec). Admittedly this may be overkill but I figured it'd help detect if something has gone wrong somewhere.
• UnmanagedFunctionPointerAttribute requires System.Runtime.InteropServices.CallingConvention and AttributeUsageAttribute requires System.AttributeTargets. Neither of which are part of the default metadata files. I've added the type name to the panic message to make this easier to diagnose but I guess this is a win32metadata issue?

[riverar]

• UnmanagedFunctionPointerAttribute requires System.Runtime.InteropServices.CallingConvention and AttributeUsageAttribute requires System.AttributeTargets. Neither of which are part of the default metadata files. I've added the type name to the panic message to make this easier to diagnose but I guess this is a win32metadata issue?

Don't believe metadata is using these attributes. Are you by any chance using ILSpy? (It automatically/annoyingly loads in unrelated .NET reference assemblies.)

[ChrisDenton]

No, just the metadata reader. A simplified example:

let files = metadata::reader::File::with_default(&[]).unwrap();
let reader = &metadata::reader::Reader::new(&files);
let tree = reader.tree("Windows", &[]).unwrap();

for tree in tree.flatten() {
    for ty in reader.namespace_types(tree.namespace) {
        for attr in reader.type_def_attributes(ty) {
            let name = reader.attribute_name(attr);
            if name == "UnmanagedFunctionPointerAttribute" {
                reader.attribute_args(attr);
            }
        }
    }
}

Panics with the message:

thread 'main' panicked at 'Type not found: System.Runtime.InteropServices.CallingConvention', crates\libs\metadata\src\reader\mod.rs:1597:13

[riverar]

I think I'm having a moment. Are you saying the reader is pulling in default metadata with references to UnmanagedFunctionPointerAttribute or System.Runtime.InteropServices.CallingConvention? I'm not seeing that here, so I think I'm missing something.

[ChrisDenton]

I mean, they do appear to exist in the generated C# as well. E.g. https://github.com/microsoft/win32metadata/search?q=UnmanagedFunctionPointer

[kennykerr]

If you need full reflection support, then you'll likely need to include metadata for system types. Generally, I avoid that as that's a dependency I don't want to be responsible to track down. 😊

[kennykerr]

Looks good - thanks!