Nom parser for Rust items

Parse Rust structs and enums without a Syntex dependency, intended for use with Macros 1.1.

Designed for fast compile time.

• Compile time for syn (from scratch including all dependencies): 4 seconds
• Compile time for the syntex/quasi/aster stack: 60+ seconds

Usage with Macros 1.1

[dependencies]
syn = "0.8"
quote = "0.2"

[lib]
rustc-macro = true

#![feature(rustc_macro, rustc_macro_lib)]

extern crate rustc_macro;
use rustc_macro::TokenStream;

extern crate syn;

#[macro_use]
extern crate quote;

#[rustc_macro_derive(MyMacro)]
pub fn my_macro(input: TokenStream) -> TokenStream {
    let source = input.to_string();

    // Parse the string representation to an AST
    let ast = syn::parse_macro_input(&source).unwrap();

    // Build the output, possibly using quasi-quotation
    let expanded = quote! {
        // ...
    };

    // Parse back to a token stream and return it
    expanded.to_string().parse().unwrap()
}

Complete example

Suppose we have the following simple trait which returns the number of fields in a struct:

trait NumFields {
    fn num_fields() -> usize;
}

A complete Macros 1.1 implementation of #[derive(NumFields)] based on syn and quote looks like this:

#![feature(rustc_macro, rustc_macro_lib)]

extern crate rustc_macro;
use rustc_macro::TokenStream;

extern crate syn;

#[macro_use]
extern crate quote;

#[rustc_macro_derive(NumFields)]
pub fn num_fields(input: TokenStream) -> TokenStream {
    let source = input.to_string();

    // Parse the string representation to an AST
    let ast = syn::parse_macro_input(&source).unwrap();

    // Build the output
    let expanded = expand_num_fields(ast);

    // Parse back to a token stream and return it
    expanded.to_string().parse().unwrap()
}

fn expand_num_fields(ast: syn::MacroInput) -> quote::Tokens {
    let n = match ast.body {
        syn::Body::Struct(ref data) => data.fields().len(),
        syn::Body::Enum(_) => panic!("#[derive(NumFields)] can only be used with structs"),
    };

    // Used in the quasi-quotation below as `#name`
    let name = &ast.ident;

    // Helper is provided for handling complex generic types correctly and effortlessly
    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();

    quote! {
        // Preserve the input struct unmodified
        #ast

        // The generated impl
        impl #impl_generics ::mycrate::NumFields for #name #ty_generics #where_clause {
            fn num_fields() -> usize {
                #n
            }
        }
    }
}

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.