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diff --git a/proc-macro2/README.md b/proc-macro2/README.md new file mode 100644 index 0000000..19b0c3b --- /dev/null +++ b/proc-macro2/README.md @@ -0,0 +1,93 @@ +# 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> |