Unary predicate used to retain elements.
The predicate function is called once for each distinct value, and
true if this is the
first call to the predicate on the second application of the operation.
Evaluate the predicate for the given element
impl<V> !RefUnwindSafe for Predicate<V>
impl<V> !UnwindSafe for Predicate<V>
type Error = Infallible
The type returned in the event of a conversion error.