Coherence Shenanigans

What this Rust coherence rule interview question tests

This is a medium-difficulty Rust question that probes understanding of the language's coherence rules—the constraints that prevent multiple trait implementations from conflicting. Firms building systems in Rust, especially in performance-critical domains, use questions like this to verify that candidates can reason about trait resolution without relying on trial-and-error compilation.

To answer correctly, you need to recognize which trait impl configuration violates Rust's orphan rule or creates an ambiguous overlap. The question rewards knowledge of when the compiler rejects an impl as incoherent, and why those rules exist: they ensure that any trait method call has exactly one valid implementation, regardless of which crates are linked together.

• Orphan rule and local trait requirements
• Overlapping implementations and blanket impls
• Generic type parameters and impl visibility