Problems with creating an impl-block using a macro in Rust

Question

I want to create a macro generating a given formatter (Display, Debug,...) for a struct containing a single generic type.

macro_rules! create_formatter {
    ($type_name:ident<$gen_param:ident>, $trait:path) => {
        impl<$gen_param: $trait> $trait for $type_name<$gen_param> {
            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
                // isn't yet implemented 
            }
        }
    };
}

When I call the macro later in the code (create_formatter!(MyStruct<T>, std::fmt::Display);), the compiler gives the following feedback:

error: expected one of `,`, `=`, `>`, or `?`, found `std::fmt::Display`
--> test.rs:6:26
 |
6|         impl<$gen_param: $trait> $trait for $type_name<$gen_param> {
 |                          ^^^^^^^^

What am I doing wrong?

Answer 1

This does seem mysterious! It seems the problem is down to the way that the output of a macro is parsed: since it's been partly pre-parsed as a path during the macro processing, it no longer matches the parse rule for a trait bound. There was a bug raised a few weeks ago about this.

However, there is good news - it's been fixed! The example actually works in the beta or nightly compilers (playground), though note I had to rename $trait to $t as trait is a keyword.