fn map_into<U>(self) -> MapInto<Self, U>
    where
        Self::Output: Into<U>,
        Self: Sized,
    {
        assert_future::<U, _>(MapInto::new(self))
    }
Map this future's output to a different type, returning a new future of
the resulting type.

This function is equivalent to calling `map(Into::into)` but allows naming
the return type.
futures-util/src/future/future/mod.rs