macro_rules! from {
($owner:expr, $dependant:ident, $target:ty) => { ... };
}Convenience macro for constructing a Zc type via a Dependant's
From.
See Zc::new() for an example.
This macro creates an intermediate function to annotate the lifetime
required for the Construct trait as the compiler is not smart enough yet
to infer it for us. See issues 22340 and 70263.