Nom parser for Rust source code

Syn is a parsing library for parsing a stream of Rust tokens into a syntax tree of Rust source code.

Currently this library is geared toward the custom derive use case but contains some APIs that may be useful for Rust procedural macros more generally.

• Data structures — Syn provides a complete syntax tree that can represent any valid Rust source code. The syntax tree is rooted at syn::File which represents a full source file, but there are other entry points that may be useful to procedural macros including syn::Item, syn::Expr and syn::Type.

• Custom derives — Of particular interest to custom derives is syn::DeriveInput which is any of the three legal input items to a derive macro. An example below shows using this type in a library that can derive implementations of a trait of your own.

• Parser combinators — Parsing in Syn is built on a suite of public parser combinator macros that you can use for parsing any token-based syntax you dream up within a functionlike!(...) procedural macro. Every syntax tree node defined by Syn is individually parsable and may be used as a building block for custom syntaxes, or you may do it all yourself working from the most primitive tokens.

• Location information — Every token parsed by Syn is associated with a Span that tracks line and column information back to the source of that token. These spans allow a procedural macro to display detailed error messages pointing to all the right places in the user's code. There is an example of this below.

• Feature flags — Functionality is aggressively feature gated so your procedural macros enable only what they need, and do not pay in compile time for all the rest.

If you get stuck with anything involving procedural macros in Rust I am happy to provide help even if the issue is not related to Syn. Please file a ticket in this repo.

Version requirement: Syn supports any compiler version back to Rust's very first support for procedural macros in Rust 1.15.0. Some features especially around error reporting are only available in newer compilers or on the nightly channel.

Example of a custom derive

The canonical custom derive using Syn looks like this. We write an ordinary Rust function tagged with a proc_macro_derive attribute and the name of the trait we are deriving. Any time that derive appears in the user's code, the Rust compiler passes their data structure as tokens into our macro. We get to execute arbitrary Rust code to figure out what to do with those tokens, then hand some tokens back to the compiler to compile into the user's crate.

[dependencies]
syn = "0.12"
quote = "0.4"

[lib]
proc-macro = true

extern crate proc_macro;
extern crate syn;

#[macro_use]
extern crate quote;

use proc_macro::TokenStream;
use syn::DeriveInput;

#[proc_macro_derive(MyMacro)]
pub fn my_macro(input: TokenStream) -> TokenStream {
    // Parse the input tokens into a syntax tree
    let input: DeriveInput = syn::parse(input).unwrap();

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

    // Hand the output tokens back to the compiler
    expanded.into()
}

The heapsize example directory shows a complete working Macros 1.1 implementation of a custom derive. It works on any Rust compiler >=1.15.0. The example derives a HeapSize trait which computes an estimate of the amount of heap memory owned by a value.

pub trait HeapSize {
    /// Total number of bytes of heap memory owned by `self`.
    fn heap_size_of_children(&self) -> usize;
}

The custom derive allows users to write #[derive(HeapSize)] on data structures in their program.

#[derive(HeapSize)]
struct Demo<'a, T: ?Sized> {
    a: Box<T>,
    b: u8,
    c: &'a str,
    d: String,
}

Spans and error reporting

The heapsize2 example directory is an extension of the heapsize example that demonstrates some of the hygiene and error reporting properties of Macros 2.0. This example currently requires a nightly Rust compiler >=1.24.0-nightly but we are working to stabilize all of the APIs involved.

The token-based procedural macro API provides great control over where the compiler's error messages are displayed in user code. Consider the error the user sees if one of their field types does not implement HeapSize.

#[derive(HeapSize)]
struct Broken {
    ok: String,
    bad: std::thread::Thread,
}

In the Macros 1.1 string-based procedural macro world, the resulting error would point unhelpfully to the invocation of the derive macro and not to the actual problematic field.

error[E0599]: no method named `heap_size_of_children` found for type `std::thread::Thread` in the current scope
 --> src/main.rs:4:10
  |
4 | #[derive(HeapSize)]
  |          ^^^^^^^^

By tracking span information all the way through the expansion of a procedural macro as shown in the heapsize2 example, token-based macros in Syn are able to trigger errors that directly pinpoint the source of the problem.

error[E0277]: the trait bound `std::thread::Thread: HeapSize` is not satisfied
 --> src/main.rs:7:5
  |
7 |     bad: std::thread::Thread,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HeapSize` is not implemented for `std::thread::Thread`

Parsing a custom syntax using combinators

The lazy-static example directory shows the implementation of a functionlike!(...) procedural macro in which the input tokens are parsed using nom-style parser combinators.

The example reimplements the popular lazy_static crate from crates.io as a procedural macro.

lazy_static! {
    static ref USERNAME: Regex = Regex::new("^[a-z0-9_-]{3,16}$").unwrap();
}

The implementation shows how to trigger custom warnings and error messages on the macro input.

warning: come on, pick a more creative name
  --> src/main.rs:10:16
   |
10 |     static ref FOO: String = "lazy_static".to_owned();
   |                ^^^

Debugging

When developing a procedural macro it can be helpful to look at what the generated code looks like. Use cargo rustc -- -Zunstable-options --pretty=expanded or the cargo expand subcommand.

To show the expanded code for some crate that uses your procedural macro, run cargo expand from that crate. To show the expanded code for one of your own test cases, run cargo expand --test the_test_case where the last argument is the name of the test file without the .rs extension.

This write-up by Brandon W Maister discusses debugging in more detail: Debugging Rust's new Custom Derive system.

Optional features

Syn puts a lot of functionality behind optional features in order to optimize compile time for the most common use cases. The following features are available.

• derive (enabled by default) — Data structures for representing the possible input to a custom derive, including structs and enums and types.
• full — Data structures for representing the syntax tree of all valid Rust source code, including items and expressions.
• parsing (enabled by default) — Ability to parse input tokens into a syntax tree node of a chosen type.
• printing (enabled by default) — Ability to print a syntax tree node as tokens of Rust source code.
• visit — Trait for traversing a syntax tree.
• visit-mut — Trait for traversing and mutating in place a syntax tree.
• fold — Trait for transforming an owned syntax tree.
• clone-impls (enabled by default) — Clone impls for all syntax tree types.
• extra-traits — Debug, Eq, PartialEq, Hash impls for all syntax tree types.

Nightly features

By default Syn uses the proc-macro2 crate to emulate the nightly compiler's procedural macro API in a stable way that works all the way back to Rust 1.15.0. This shim makes it possible to write code without regard for whether the current compiler version supports the features we use.

On a nightly compiler, to eliminate the stable shim and use the compiler's proc-macro directly, add proc-macro2 to your Cargo.toml and set its "nightly" feature which bypasses the stable shim.

[dependencies]
syn = "0.12"
proc-macro2 = { version = "0.2", features = ["nightly"] }

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.