How to use concat_idents macro

The concat_idents macro is removed and replaced by the macro_metavar_expr_concat feature using the ${concat(...)} syntax. Enable the new feature and use the metavariable expression inside your macro to join identifiers.

#![feature(macro_metavar_expr_concat)]

macro_rules! create_struct {
    ($name:ident) => {
        pub struct ${concat(First, $name)};
    };
}

create_struct!(Thing);

This expands to pub struct FirstThing;.

The old way to glue names together in macros is gone. You now use a special syntax that looks like a math formula to combine words directly inside your macro code. Think of it like snapping Lego blocks together to build a new name on the fly.