I'm actually still a little bit unsure about MapCurve specifically. My reasoning was that it should behave like it owns a fn() -> S since it actually owns a curve with that output type; the fn(S) -> T phantom type looks reasonable but is also just going to be redundant in all real cases, because F: Fn(S) -> T whenever the struct is a curve (so, for instance, it could literally be a function pointer fn(S) -> T).

I'm actually still a little bit unsure about MapCurve specifically. My reasoning was that it should behave like it owns a fn() -> S since it actually owns a curve with that output type; the fn(S) -> T phantom type looks reasonable but is also just going to be redundant in all real cases, because F: Fn(S) -> T whenever the struct is a curve (so, for instance, it could literally be a function pointer fn(S) -> T).

When would MapCurve not be a curve? Is there a reason F isn't defined with the Fn(S) -> T bound?

When would MapCurve not be a curve? Is there a reason F isn't defined with the Fn(S) -> T bound?

It's often regarded as good practice not to include trait bounds on struct definitions unless they are required for the struct itself to make sense (e.g. by using associated types in fields). (If anything, this helps avoid needless duplication of trait bounds, but there are other reasons as well.)