xref: /third_party/rust/crates/proc-macro2/README.md

# proc-macro2

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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 build.rs and
  main.rs.** 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`.

[syn]: https://github.com/dtolnay/syn
[quote]: https://github.com/dtolnay/quote

## Usage

```toml
[dependencies]
proc-macro2 = "1.0"
```

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

```rust
extern crate proc_macro;

#[proc_macro_derive(MyDerive)]
pub fn my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let input = proc_macro2::TokenStream::from(input);

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

    proc_macro::TokenStream::from(output)
}
```

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

[`parse_macro_input!`]: https://docs.rs/syn/2.0/syn/macro.parse_macro_input.html

## 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.

<br>

#### License

<sup>
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
</sup>

<br>

<sub>
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.
</sub>