Function try_init_global_dns_resolver 

pub fn try_init_global_dns_resolver(
    resolver: impl DnsResolver,
) -> Result<(), BoxDnsResolver>

Tries to initialise the global DnsResolver.

This returns the input DnsResolver boxed but useless back, in case the global DnsResolver was already set.

You can use init_global_dns_resolver should you want to panic on failure instead.