Acceptable for procedural macros to _write_ outside of source file?

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There are two usual approaches to this problem:

  • The simpler / faster approach is to have the #[gen_proto] generate hidden functions which are able to generate your desired stuff (usually behind a feature gate). Such functions are collected (either through life-before-main or through linker tricks) into a "compile-time" collection. You then have a special unit test that iterates through that collection and calls all those functions, with some given out path, effectively generating the desired file.

    This is the approach currently taken by ::safer_ffi, for instance:

    safer_ffi::headers - Rust

  • The more proper approach would be to write a CLI kind of application (tip: you could try to use GitHub - trailofbits/dylint: A tool for running Rust lints from dynamic libraries to hook onto lint capabilities to get the code exploration done for you) which traverses your code looking for the attribute / the marker, and have that tool be the one responsible for generating the files. This is what wasm_bindgen does, for instance.

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