kodraus / syn

Nom parser for Rust source code

Nom parser for Rust source code

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 = "0.10"
quote = "0.3"

[lib]
proc-macro = true

#![feature(proc_macro, proc_macro_lib)]

extern crate proc_macro;
use proc_macro::TokenStream;

extern crate syn;

#[macro_use]
extern crate quote;

#[proc_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(proc_macro, proc_macro_lib)]

extern crate proc_macro;
use proc_macro::TokenStream;

extern crate syn;

#[macro_use]
extern crate quote;

#[proc_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);

    // Return the original input struct unmodified, and the
    // generated impl along with it
    quote!(#ast #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! {
        // The generated impl
        impl #impl_generics ::mycrate::NumFields for #name #ty_generics #where_clause {
            fn num_fields() -> usize {
                #n
            }
        }
    }
}

Optional features

Syn puts a lot of functionality behind optional features in order to optimize compile time for the most common use cases. These are the available features and their effect on compile time. Dependencies are included in the compile times.

Features	Compile time	Functionality
(none)	1 sec	The data structures representing the AST of Rust structs, enums, and types.
parsing	4 sec	Parsing Rust source code containing structs and enums into an AST.
printing	2 sec	Printing an AST of structs and enums as Rust source code.
parsing, printing	4 sec	This is the default. Parsing and printing of Rust structs and enums. This is typically what you want for implementing Macros 1.1 custom derives.
full	2 sec	The data structures representing the full AST of all possible Rust code.
full, parsing	7 sec	Parsing any valid Rust source code to an AST.
full, printing	4 sec	Turning an AST into Rust source code.
full, parsing, printing	8 sec	Parsing and printing any Rust syntax.
full, parsing, printing, expand	9 sec	Expansion of custom derives in a file of Rust code. This is typically what you want for expanding custom derives on stable Rust using a build script.
full, parsing, printing, expand, pretty	60 sec	Expansion of custom derives with pretty-printed output. This is what you want when iterating on or debugging a custom derive, but the pretty printing should be disabled once you get everything working.

Custom derives on stable Rust

Syn supports a way of expanding custom derives from a build script, similar to what Serde is able to do with serde_codegen. The advantage of using Syn for this purpose rather than Syntex is much faster compile time.

Continuing with the NumFields example from above, it can be extended to support stable Rust like this. One or more custom derives are added to a Registry, which is then able to expand those derives in a source file at a particular path and write the expanded output to a different path. A custom derive is represented by the CustomDerive trait which takes a MacroInput (either a struct or an enum) and expands it into zero or more new items and maybe a modified or unmodified instance of the original input.

pub fn expand_file<S, D>(src: S, dst: D) -> Result<(), String>
    where S: AsRef<Path>,
          D: AsRef<Path>
{
    let mut registry = syn::Registry::new();
    registry.add_derive("NumFields", |input| {
        let tokens = expand_num_fields(&input);
        let items = syn::parse_items(&tokens.to_string()).unwrap();
        Ok(syn::Expanded {
            new_items: items,
            original: Some(input),
        })
    });
    registry.expand_file(src, dst)
}

The codegen can be invoked from a build script as follows.

extern crate your_codegen;

use std::env;
use std::path::Path;

fn main() {
    let out_dir = env::var_os("OUT_DIR").unwrap();

    let src = Path::new("src/codegen_types.in.rs");
    let dst = Path::new(&out_dir).join("codegen_types.rs");

    your_codegen::expand_file(&src, &dst).unwrap();
}

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.