Miss Dada  🏳️‍⚧️

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

but would I be wrong in saying that "coherence" is the set of rules that means rustc can determine a single `fn` to dispatch to, for every method/function call site, without ambiguity?

and it's what necessitates things like the orphan rules, `fundamental`, the "conflicting implementation" error for blanket impls for different traits...

further to this, what's the best explanation of `fundamental` in existence? I'm trying to explain why you can't have an impl for `&mut T` if you also have one for `F where F: FnMut(T)`, but you can have one for `&mut i64` because that type will never impl FnMut

Rust prevents you from negative reasoning on foreign traits/types for forwards compatibility. A few types/traits are allowed to break this rule for ergonomic reasons, the trade-off being that implementing a trait for a fundamental type is a breaking change.

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"