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I'm surprised by the ambiguity shown in the last example. Shouldn't the concrete return type for an impl Into<u32> be completely inaccessible outside of the function? Otherwise it would allow to introduce breaking changes without changing a function API.
But in this case, how would it be possible that impl_trait().into() is an u16?
I guess it's only accessible to the mem::size_of because it has an implicit Sized bound as well. So really the bound is impl Into + Sized and I don't think the code using the function is assuming anything else.
But I do agree it feels weird. I feel like the function should know its return type regardless of how it is used?