4 releases

0.2.1 Jan 6, 2019
0.2.0 Jan 6, 2019
0.1.1 Jan 5, 2019
0.1.0 Jan 5, 2019

#323 in Procedural macros


362 lines


macro_rules-style syntax matching for procedural macros.

This crate is work-in-progress, incomplete, and probably buggy!


rules!(tokens => {
    ($finish:ident ($($found:ident)*) # [ $($inner:tt)* ] $($rest:tt)*) => {
        for f in found {
            do_something(finish, f, inner, rest[0]);
    (foo $($bar:expr)?) => {
        match bar {
            Some(e) => foo_with_expr(e),
            None => foo_no_expr(),

Using proc-macro-rules

Add proc-macro-rules = "0.2.1" to your Cargo.toml.

Import the rules macro with use proc_macro_rules::rules, then use with rules!(tokens => { branches }); where tokens is an expression which evaluates to a TokenStream (such as the argument in the definition of a procedural macro).

Each branch in branches should have the form ( pattern ) => { body } where pattern is a macro-rules-style pattern (using all the same syntax for meta-variables, AST nodes, repetition, etc.) and body is rust code executed when the pattern is matched. Within body, any meta-variables in the pattern are bound to variables of an appropriate type from either the proc_macro2 or syn crates. Where a meta-variable is inside a repetition or option clause, it will be wrapped in a Vec or Option, respectively.

For example, in the first branch in the above example ident has type syn::Ident and inner has type Vec<proc_macro2::TokenTree>.

Building and testing

Use cargo build to build, cargo test --all to test.


Contributions are very welcome! It would be great to know things which are missing or incorrect (in general we should have the same behaviour as macro_rules, so anything different is incorrect). Issues, code, docs, tests, and corrections are all welcome.


~41K SLoC