Function is_ipv6_ipv4_compatible 

pub fn is_ipv6_ipv4_compatible(ip: Ipv6Addr) -> bool

Returns true for the deprecated IPv4-compatible IPv6 form (::a.b.c.d, distinct from the IPv4-mapped ::ffff:a.b.c.d form already handled separately via to_ipv4_mapped()). Some stacks still route this form; treat any non-loopback, non-unspecified address here as embedding a target IPv4 address this guard has not yet validated.