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# proc-macro2
[](https://travis-ci.com/alexcrichton/proc-macro2)
[](https://crates.io/crates/proc-macro2)
[](https://docs.rs/proc-macro2)
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/1.0/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 1.31.0. 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>
|
