proc-macro2 0.2.1

A stable implementation of the upcoming new `proc_macro` API. Comes with an option, off by default, to also reimplement itself in terms of the upstream unstable API.


Build Status Latest Version Rust Documentation

A small shim over the proc_macro crate in the compiler intended to multiplex the current stable interface (as of 2017-07-05) and the upcoming richer interface.

The upcoming support has features like:

  • Span information on tokens
  • No need to go in/out through strings
  • Structured input/output

The hope is that libraries ported to proc_macro2 will be trivial to port to the real proc_macro crate once the support on nightly is stabilize.


This crate by default compiles on the stable version of the compiler. It only uses the stable surface area of the proc_macro crate upstream in the compiler itself. Usage is done via:

proc-macro2 = "0.2"

followed by

extern crate proc_macro;
extern crate proc_macro2;

pub fn my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let input: proc_macro2::TokenStream = input.into();

    let output: proc_macro2::TokenStream = {
        /* transform input */


If you'd like you can enable the nightly feature in this crate. This will cause it to compile against the unstable and nightly-only features of the proc_macro crate. This in turn requires a nightly compiler. This should help preserve span information, however, coming in from the compiler itself.

You can enable this feature via:

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

Unstable Features

proc-macro2 supports exporting some methods from proc_macro which are currently highly unstable, and may not be stabilized in the first pass of proc_macro stabilizations. These features are not exported by default. Minor versions of proc-macro2 may make breaking changes to them at any time.

To enable these features, the procmacro2_semver_exempt config flag must be passed to rustc.

RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build

Note that this must not only be done for your crate, but for any crate that depends on your crate. This infectious nature is intentional, as it serves as a reminder that you are outside of the normal semver guarantees.


This project is licensed under either of

at your option.


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