A wrapper around the procedural macro API of the compiler's
This library serves two purposes:
Bring proc-macro-like functionality to other contexts like build.rs and main.rs. Types from
proc_macroare entirely specific to procedural macros and cannot ever exist in code outside of a procedural macro. Meanwhile
proc_macro2types may exist anywhere including non-macro code. By developing foundational libraries like syn and quote against
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_macrocan 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
 = "1.0"
The skeleton of a typical procedural macro typically looks like this:
extern crate proc_macro;
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,
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.