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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yeah I'm starting to get the intersection of this problem and logic programming and why so much of the material on this topic is "we can't do negative reasoning"
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
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this chimes with a common approach in type theory which is that adding new fields to records should be safe
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Yeah, similar idea (though forwards compatibility is opt out instead of opt in here unlike with structs/enums). We're more restrictive than we need to be I think. Re-re-balancing coherence permitted a good bit more without changing the restrictions. I suspect more cases exist
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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"
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