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I think the answer is very obvious here: Debug should not omit fields. serde should not skip fields. Which means that there needs to be a way to put flowinfo into textual representation. Since there are already incompatibilities in how ipv6 addresses are handled ("%enp4s0f0" is supported by ping but not by Rust), might as well make something up, like "1234::5678%9-Q0x42". Tools like "ping" accept "-Q 0x42" option already, so most network engineers would be able to guess what does "-Q0x42" tacked to the end of address means.

Since this is backward-compatible when QoS is not set, I'd just change Display to emit it. But I am guessing some people may complain, so special implementation for Debug would work too...