Macro lazy_extern

macro_rules! lazy_extern {
    (
        libgroup $groupname:ident : $grouptype:ident;
        $(lib $libname:ident = $libpath:expr;)*
        $(
            $(#[$($meta:tt)*])*
            extern $abi:tt fn $fnname:ident($($argname:ident : $argty:ty),*) -> $retty:ty;
        )*
    ) => { ... };
    (
        @process_fn_meta
        $groupname:ident $grouptype:ident
        $abi:tt $fnname:ident($($argname:ident : $argty:ty),*; $retty:ty);
        $(; doc $doc:expr)*
    ) => { ... };
    (
        @process_fn_meta
        $groupname:ident $grouptype:ident
        $abi:tt $fnname:ident($($argname:ident : $argty:ty),*; $retty:ty);
        #[doc = $meta:expr]
        $(#[$($rest:tt)*])*
        $(; doc $doc:expr)*
    ) => { ... };
    (
        @process_fn_meta
        $groupname:ident $grouptype:ident
        $abi:tt $fnname:ident($($argname:ident : $argty:ty),*; $retty:ty);
        #[lib($libname:ident)]
        $(#[$($rest:tt)*])*
        $(; doc $doc:expr)*
    ) => { ... };
    (
        @process_fn_meta
        $groupname:ident $grouptype:ident
        $abi:tt $fnname:ident($($argname:ident : $argty:ty),*; $retty:ty);
        #[feature_test($feature:ident)]
        $(#[$($rest:tt)*])*
        $(; doc $doc:expr)*
    ) => { ... };
}

See the module documentation