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101 lines
3.9 KiB
Markdown
101 lines
3.9 KiB
Markdown
# proc-macro2
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[](https://travis-ci.com/alexcrichton/proc-macro2)
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[](https://crates.io/crates/proc-macro2)
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[](https://docs.rs/proc-macro2)
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A wrapper around the procedural macro API of the compiler's `proc_macro` crate.
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This library serves three purposes:
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- **Bring proc-macro-like functionality to other contexts like build.rs and
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main.rs.** Types from `proc_macro` are entirely specific to procedural macros
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and cannot ever exist in code outside of a procedural macro. Meanwhile
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`proc_macro2` types may exist anywhere including non-macro code. By developing
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foundational libraries like [syn] and [quote] against `proc_macro2` rather
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than `proc_macro`, the procedural macro ecosystem becomes easily applicable to
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many other use cases and we avoid reimplementing non-macro equivalents of
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those libraries.
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- **Make procedural macros unit testable.** As a consequence of being specific
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to procedural macros, nothing that uses `proc_macro` can be executed from a
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unit test. In order for helper libraries or components of a macro to be
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testable in isolation, they must be implemented using `proc_macro2`.
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- **Provide the latest and greatest APIs across all compiler versions.**
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Procedural macros were first introduced to Rust in 1.15.0 with an extremely
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minimal interface. Since then, many improvements have landed to make macros
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more flexible and easier to write. This library tracks the procedural macro
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API of the most recent stable compiler but employs a polyfill to provide that
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API consistently across any compiler since 1.15.0.
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[syn]: https://github.com/dtolnay/syn
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[quote]: https://github.com/dtolnay/quote
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## Usage
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```toml
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[dependencies]
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proc-macro2 = "0.4"
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```
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The skeleton of a typical procedural macro typically looks like this:
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```rust
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extern crate proc_macro;
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#[proc_macro_derive(MyDerive)]
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pub fn my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
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let input = proc_macro2::TokenStream::from(input);
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let output: proc_macro2::TokenStream = {
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/* transform input */
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};
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proc_macro::TokenStream::from(output)
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}
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```
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If parsing with [Syn], you'll use [`parse_macro_input!`] instead to propagate
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parse errors correctly back to the compiler when parsing fails.
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[`parse_macro_input!`]: https://docs.rs/syn/0.15/syn/macro.parse_macro_input.html
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## Unstable features
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The default feature set of proc-macro2 tracks the most recent stable compiler
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API. Functionality in `proc_macro` that is not yet stable is not exposed by
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proc-macro2 by default.
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To opt into the additional APIs available in the most recent nightly compiler,
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the `procmacro2_semver_exempt` config flag must be passed to rustc. As usual, we
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will polyfill those nightly-only APIs all the way back to Rust 1.15.0. As these
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are unstable APIs that track the nightly compiler, minor versions of proc-macro2
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may make breaking changes to them at any time.
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```
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RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
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```
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Note that this must not only be done for your crate, but for any crate that
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depends on your crate. This infectious nature is intentional, as it serves as a
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reminder that you are outside of the normal semver guarantees.
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Semver exempt methods are marked as such in the proc-macro2 documentation.
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# License
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This project is licensed under either of
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* Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
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http://www.apache.org/licenses/LICENSE-2.0)
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* MIT license ([LICENSE-MIT](LICENSE-MIT) or
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http://opensource.org/licenses/MIT)
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at your option.
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### Contribution
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Unless you explicitly state otherwise, any contribution intentionally submitted
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for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
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be dual licensed as above, without any additional terms or conditions.
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