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Rust’s external proofs do not support cyclic trait implementations

smallcultfollowing.com 7 min read Editor score 7/10

The post argues that cyclic trait implementations require modular proofs, where an implementation establishes its supertraits, rather than external proofs by callers. The distinction is useful context for engineers following Rust’s type-system design.

What

The post compares modular proofs, where an impl must establish its supertraits, with external proofs, where consuming code must establish them.

How

For `Magic: Copy`, modular checking rejects `impl Magic for String` because `String` is not `Copy`; valid impls let callers rely on the supertrait.

Why it matters

The argument concludes that cyclic Rust trait impls need modular proofs, since external proofs conflict with Rust's design.

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