is there a single document describing rust's coherence rules? googling gives various blog posts and RFCs but I'm struggling to find a single definition of this
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The rebalancing coherence rfc lays out the reasoning pretty clearly. If we let you rely on ForeignType: !ForeignTrait, implementing any trait for any type would be a breaking change. So there's a balance chosen between permitting as much as possible while allowing lib evolution
In fact the error message you get in these cases should even way something like "foreign_crate may implement ForeignTrait for ForeignType in a future version"