# binrw_derive

## Quick start for adding a new directive to binrw

In all cases, look to existing directives to follow established code and test
conventions.

1. Add a keyword for the new directive in `binrw::parser::keywords`.
2. Define the meta-type of the new directive in `binrw::parser::attrs`. If you
   need a new meta-type, add it to `meta_types` along with tests.
3. If the new directive needs a special final type (e.g. `CondEndian`), add
   that to a new `binrw::parser::types` module and export it from
   `binrw::parser::types`. New types must ultimately implement
   `binrw::parser::TrySet`, but can sometimes do so more simply (using trait
   generic impls) by implementing `From` or `TryFrom` instead.
4. Add the new directive as a field to the relevant structs in
   `binrw::parser::top_level_attrs` and `binrw::parser::field_level_attrs`.
5. If the new directive combines with other directives in ways that may be
   invalid, and the relationship cannot be expressed using an enum type
   (e.g. `ReadMode`), add validation in either `FromInput::push_field` (if the
   validation can occur immediately after the field is constructed) or
   `FromInput::validate` (if it can only be validated after the entire struct
   has been parsed).
6. Use the new fields to emit code in the appropriate places in
   `binrw::codegen::read_options` and `binrw::codegen::write_options`.
7. Add new integration tests in the `binrw` crate’s `tests` directory.
8. If the new directive generates new errors (e.g. from validation), add unit
   tests to validate those code paths in `binrw::parser::tests` (in `mod.rs`)
   and add identical trybuild tests to the `binrw` crate’s `tests/ui` directory.
   (A nightly compiler is required to run the trybuild tests; see the comment in
   `binrw::tests::ui` for more detail.)