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Attribute on AST types for traits derived by AST codegen #70

Open overlookmotel opened 4 months ago

overlookmotel commented 4 months ago

Broken out from https://github.com/oxc-project/oxc/issues/4282#issuecomment-2229046078. @rzvxa said:

I had an idea for generated derives which I've never shared publicly. We might want to add an attribute such as #[expand_derive(...)] or #[derived(...)] to easily mark stuff like Hash, PartialEq, and now CloneIn derives.

It is a marker and wouldn't expand the derived macro base on the builtins(or proc_macro_derive), but it is a good way to telegraph that these are being derived for the said type although they are generated and not happening in the compiler.

overlookmotel commented 4 months ago

I like this idea. The way I could see it working is:

#[expand_derive(Foo)] is functionally equivalent to #[derive(Foo)], with only differences being:

  1. impl Foo for Ty is not inserted at the call site (the way #[derive] does) but in a separate file.
  2. impl is generated by codegen, rather than by macro expansion.

Perhaps this is what you meant anyway @rzvxa, but just to clarify:

I don't think #[expand_derive] should act purely as an informational comment. The codegen should use it. If #[expand_derive(Foo)] is present on a type, codegen will generate impl Foo, if it's not, it doesn't.

Naming: #[derived] is a bit ambiguous I think. I prefer #[expand_derive]. Or maybe #[static_derive], #[codegen_derive] or #[derive_by_codegen]?