Why does this macro invocation result in an unresolved name? - macros

Question

Here is a simplified version of a macro I am trying to implement for an RPC library I am working on:
#[macro_export]
macro_rules! msgpack_rpc {
(
$(
rpc $name:ident ( $( $arg:ident : $arg_ty:ty ),* ) -> $ret_ty:ty | $err_ty:ty;
)+
) => (
pub trait Service {
$(
fn $name ( &self, $( $arg : $arg_ty ),* ) -> Result<$ret_ty, $err_ty>;
)+
}
pub struct Server;
impl Server {
pub fn listen<S>(handle: &(), address: (), service: S)
-> ::std::io::Result<()>
where S: Service + Send + Sync + 'static {
let service = move |msg: &str| {
let result = match msg {
$(
stringify!($name) => {
service.$name($( $arg ),*)
.map(String::from)
.map_err(String::from)
}
),+,
_ => String::from("method not supported".into()),
};
};
Ok(())
}
}
)
}
msgpack_rpc! {
rpc echo(arg: i64) -> i64 | ();
}
The macro expansion fails to compile with this error:
error: unresolved name `arg` [--explain E0425]
--> <anon>:40:17
|>
40 |> rpc echo(arg: i64) -> i64 | ();
|> ^
<anon>:39:1: 41:2: note: in this expansion of msgpack_rpc! (defined in <anon>)
From reading similar questions, I know that macro_rules sometimes has problems expanding statements. However, I am confused as to why it is having trouble expanding items in this case.
Is there a workaround to fix the expansion?

Answer 1

You don't have any variable named arg in the context in which you call the function. This is the "unresolved arg" the compiler is complaining about.
stringify!($name) => {
$( let $arg = Default::default(); )*
service.$name($( $arg ),*)
.map(String::from)
.map_err(String::from)
}