Automatically impl From<&T> after defining From<T>

Question

Consider the following MRE:

struct A {}
struct B {}

impl From<A> for B {
    fn from(t: A) -> B { B {} }
}

Is it possible to automatically implement From<&A> for B?

Answer 1

No. You will have to write it yourself.

impl From<&A> for B {
    fn from(a: &A) -> B {
        unimplemented!("implement me")
    }
}

The only blanket implementation for From is impl<T> From<T> for T (the identity conversion), there is no #[derive] macro available (used for other "automatic" implementations), and no other macro that I'm aware of exists (though you could probably make one without much fuss if you really wanted it).