Crate safe_proc_macro2

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Expand description

This is a fork of proc_macro2 that with unsafe code removed. See the rejected PR.

A wrapper around the procedural macro API of the compiler’s proc_macro crate. This library serves two purposes:

  • Bring proc-macro-like functionality to other contexts like and Types from proc_macro are entirely specific to procedural macros and cannot ever exist in code outside of a procedural macro. Meanwhile proc_macro2 types may exist anywhere including non-macro code. By developing foundational libraries like syn and quote against proc_macro2 rather than proc_macro, the procedural macro ecosystem becomes easily applicable to many other use cases and we avoid reimplementing non-macro equivalents of those libraries.

  • Make procedural macros unit testable. As a consequence of being specific to procedural macros, nothing that uses proc_macro can be executed from a unit test. In order for helper libraries or components of a macro to be testable in isolation, they must be implemented using proc_macro2.


The skeleton of a typical procedural macro typically looks like this:

extern crate proc_macro;

pub fn my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let input = safe_proc_macro2::TokenStream::from(input);

    let output: safe_proc_macro2::TokenStream = {
        /* transform input */


If parsing with Syn, you’ll use parse_macro_input! instead to propagate parse errors correctly back to the compiler when parsing fails.

Unstable features

The default feature set of proc-macro2 tracks the most recent stable compiler API. Functionality in proc_macro that is not yet stable is not exposed by proc-macro2 by default.

To opt into the additional APIs available in the most recent nightly compiler, the procmacro2_semver_exempt config flag must be passed to rustc. We will polyfill those nightly-only APIs back to Rust 1.56.0. As these are unstable APIs that track the nightly compiler, minor versions of proc-macro2 may make breaking changes to them at any time.

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.

Semver exempt methods are marked as such in the proc-macro2 documentation.


Most types in this crate are !Sync because the underlying compiler types make use of thread-local memory, meaning they cannot be accessed from a different thread.


  • Items which do not have a correspondence to any API in the proc_macro crate, but are necessary to include in proc-macro2.
  • Public implementation details for the TokenStream type, such as iterators.


  • A delimited token stream.
  • A word of Rust code, which may be a keyword or legal variable name.
  • Error returned from TokenStream::from_str.
  • LineColumnspan-locations
    A line-column pair representing the start or end of a Span.
  • A literal string ("hello"), byte string (b"hello"), character ('a'), byte character (b'a'), an integer or floating point number with or without a suffix (1, 1u8, 2.3, 2.3f32).
  • A Punct is a single punctuation character like +, - or #.
  • SourceFileprocmacro2_semver_exempt
    The source file of a given Span.
  • A region of source code, along with macro expansion information.
  • An abstract stream of tokens, or more concretely a sequence of token trees.


  • Describes how a sequence of token trees is delimited.
  • Whether a Punct is followed immediately by another Punct or followed by another token or whitespace.
  • A single token or a delimited sequence of token trees (e.g. [1, (), ..]).