Why 'externalized' proofs of cyclic trait impls does not work · baby steps

A Rust language design post examines two theoretical approaches for how the compiler verifies that supertrait obligations hold: 'modular' proofs, where an impl itself must prove its supertraits are satisfied, versus 'external' proofs, where the caller of the impl bears that responsibility. The author, previously favoring external proofs for their expressiveness, explains why they ultimately fail: they are incompatible with how unsafe impls work in Rust, since unsafe impls are trusted to prove their own safety conditions, creating an inconsistency if ordinary impls aren't trusted to prove supertrait conditions. The conclusion is that modular proofs, despite requiring careful handling of cyclic/recursive proof issues, are the only viable path forward, with coinductive modular proofs to be covered in a future post.

Source: https://smallcultfollowing.com/babysteps/blog/2026/10/10/modular-vs-external-proofs. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.