Rust has been wrestling with coherence more-or-less since we added methods; our current rule, the “orphan rule”, is safe but overly strict. Roughly speaking, the rule says that one can only implement foreign traits (that is, traits defined by one of your dependencies) for local types (that is, types that you define). The goal of this rule was to help foster the crates.io ecosystem — we wanted to ensure that you could grab any two crates and use them together, without worrying that they ...
