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std::borrow::Cow is a very useful way to provide a value that can be copied on demand _for types that can be cloned_. In many cases it would make sense to:
- for cloneable types accept Cow<T>,
- for other types accept T.
We could make this work with Cow<B>, using a very weird construct (have B be essentially equivalent to Infeasible, but implement ToOwned with Owned = the_type_we_want). Is there a less cumbersome way?
