Support nested modules / namespacing

[sffc]

#[diplomat::bridge]
pub mod ffi {
    pub mod foo {
        pub struct Bar {
            pub x: u32,
        }
    }
}

The above code should generate a namespaced Bar object. Right now the whole module foo just gets ignored, and if I annotate it with another #[diplomat::bridge], it just puts Bar in the global (capi) namespace.

[Manishearth]

My hope was that namespacing could be done with an attribute, so you do something like #[diplomat::namespace(foo)] and it gets namespaced under foo, but you can still organize your code however you'd like.

It's also worth discussing if the namespacing should be done for everyone or if we should have a #[diplomat::cpp::namespace(foo)] attribute instead

it just puts Bar in the global (capi) namespace.

This isn't correct, the capi namespace is for the C headers. It puts it in the global namespace.

[Manishearth]

I think what we should do here is have diplomat::cpp::namespace(icu4x) and diplomat::c::namespace(ICU4X) (and nothing for JS), where the C++ implementation uses a C++ namespace (and can be applied to the bridge module or the individual type), and the C implementation just tacks on a prefix.

[Manishearth]

If we do this we should do this in the holistic backend attribute system designed in #235

[Manishearth]

n.b. currently we do not do any attribute merging in the AST except for CFGs (https://github.com/rust-diplomat/diplomat/blob/133a10534cad9f29e7b9ddfc1aea687c5e3b8274/core/src/ast/attrs.rs#L24)

For namespacing it would be nice to use a similar mechanism to "inherit" namespace and rename attributes from the module def.

[robertbastian]

I think there are two things here:

• Namespaces/packages/whatever in the foreign language
• A prefix for the C ABI symbols

I think we should deal with these separately. For Dart I don't need any fancy modularisation (because Dart doesn't support that), but I do want generate a Dart API without ICU4X.. prefixes. So in icu_capi I'd like to change

#[diplomat::bridge]
mod ffi {
    struct ICU4XLocale(...)
}

to

#[diplomat::bridge(prefix = "ICU4X")]
mod ffi {
    struct Locale(...)
}

to allow using nicer names in Dart.

[Manishearth]

I'd call it c_prefix but yeah

[Manishearth]

Splitting out C++ into https://github.com/rust-diplomat/diplomat/issues/427

[Manishearth]

Closing in favor of https://github.com/rust-diplomat/diplomat/issues/688