Is there a way to have a public trait in a proc-macro crate?

Question

I have a proc-macro crate with a macro that, when expanded, needs to use custom trait implementations for Rust built-in types. I tried to define the trait in the same crate, but

Answer 1

The way this is usually dealt with is to not have users depend on your proc-macro crate at all.

Your problem can be solved with 3 crates:

• "internal" crate containing type and trait definitions that are used by the proc-macro
• proc-macro crate:
  • Depends on the internal crate, so it can use its types and traits
• "public" crate:
  • Depends on both the internal and proc-macro crates
  • Re-exports all types, traits and macros that you want your users to use

Whenever your macro mentions the shared types in its generated code, you need to use the fully-qualified names so that users don't also need to import them.

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Some popular examples of this pattern in the wild:

• thiserror depends on thiserror-impl which contains the actual macros
• pin-project depends on pin-project-internal which again contains the macros
• darling depends on darling-core and darling-macro, which itself also depends on darling-core