Macro mark_compound

macro_rules! mark_compound {
    (struct $name:ident <$($T:ident),+> { $($field:ident),* $(,)? }) => { ... };
    (enum $name:ident <$($T:ident),+> { $($variant:ident $(($field:ident))?),* $(,)? }) => { ... };
}

🔬This is a nightly-only experimental API. (proc_macro_internals #27812)