Macro define_reify_functions

macro_rules! define_reify_functions {
    ($(
        fn $name:ident $(<$($param:ident),*>)?
            for $(extern $abi:tt)? fn($($arg:ident: $arg_ty:ty),*) -> $ret_ty:ty;
    )+) => { ... };
}

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