Nested Macro not expanding correctly

Question

I am trying to nest macro calls to ease converting between rust and c ffi types.

The inner macro generates calls to conversion functions from type names like this:

macro_rules! type_conversion {
    ($arg_name:ident, bool, BOOL) => (crate::type_wrappers::type_conversion::convert_rust_bool($arg_name));
    ($arg_name:ident, BOOL, bool) => (crate::type_wrappers::type_conversion::convert_c_bool($arg_name));
}

The outer macro now uses this to generate further conversions. It basically works like this:

macro_rules! outer {
    ($arg_name:ident, $type1:ty, $type2:ty) => (type_conversion!($arg_name, $type1, $type2));
}

Now, when i try to use the outer macro like this:

fn outer() {
    outer!(hello_world, bool, BOOL);
}

I get this Error:

error: no rules expected the token `bool`
   --> src\type_wrappers\type_conversion.rs:252:77
    |
202 | macro_rules! type_conversion {
    | ---------------------------- when calling this macro
...
252 |     ($arg_name:ident, $type1:ty, $type2:ty) => (type_conversion!($arg_name, $type1, $type2));
    |                                                                             ^^^^^^ no rules expected this token in macro call
...
256 |     outer!(hello_world, bool, BOOL);
    |     ------------------------------- in this macro invocation

I cannot make sense out of this error, especially since CLions Macro expansion tool expands this into:

type_conversion!(hello_world , bool , BOOL )

Which is what i would expect and also, what the Error suggests.

Where is the Error in those Macros?

@Kitsu Ah,test! was meant to be outer!. I edited the question. — Erik Schulze

Kitsu Kitsu · Accepted Answer · 2020-06-29T02:13:05

I guess the ty macro type cannot be casted to identifier (which bool & BOOL parameter are) and thus matched. So with ident type it works (playground):

macro_rules! outer {
    ($arg_name:ident, $type1:ident, $type2:ident) => {
        type_conversion!($arg_name, $type1, $type2)
    };
}

Nested Macro not expanding correctly

1 Answers