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Niko Matsakis - baby steps0 Hot0 comments10 min read
Cylic trait implementations: motivation · baby steps
An exploration of cyclic trait implementations in Rust, explaining why the current trait system requires non-cyclic (inductive) proofs and what problems arise when cycles are permitted. The post introduces 'perfect derive' — generating where-clauses based on field types rather than type parameters — and shows why it requires cyclic reasoning. It also demonstrates why naively accepting all cycles is unsound, using a supertrait example where cyclic logic incorrectly proves String: Copy. The post sets up a series on distinguishing 'good' cyclic impls from 'bad' ones, with future posts to cover coinduction, modal logic, and specialization.
Source: https://smallcultfollowing.com/babysteps/blog/2026/08/10/cyclic-trait-solving. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.
