[net-types] Change IP Display to match rust #112606

This CL changes the Display impl for net_types::ip::Ipv6Addr to match
std::net::Ipv6Addr and re-enables the `test_ipv6_display` test which was
disabled in Ie459a53094b958b752e75e5cf86baa6f55bdade6.

The change follows rust-lang/rust#112606, which
stops formatting IPv4-compatible addresses as "::a.b.c.d". IPv4-mapped
addresses continue to be treated specially ("::ffff:a.b.c.d").

Test: fx test net_types_lib_test -- ip::tests::test_ipv6_display
Bug: 126927
Change-Id: I854f2bf8ba42b3e1eedceb2a5a70e8ca1da17a4b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/873716
Reviewed-by: Joshua Liebow-Feeser <joshlf@google.com>
Commit-Queue: Mitchell Kember <mkember@google.com>

src/connectivity/lib/net-types/src/ip.rs

@@ -1933,16 +1933,13 @@ impl Display for Ipv6Addr {
         // by creating a scratch buffer, calling `fmt_inner` on that scratch
         // buffer, and then satisfying those requirements.
         fn fmt_inner<W: fmt::Write>(addr: &Ipv6Addr, w: &mut W) -> Result<(), fmt::Error> {
-            // We special-case the unspecified and localhost addresses because
-            // both addresses are valid IPv4-compatible addresses, and so if we
-            // don't special case them, they'll get printed as IPv4-compatible
-            // addresses ("::0.0.0.0" and "::0.0.0.1") respectively.
+            // We special-case the unspecified address, localhost address, and
+            // IPv4-mapped addresses, but not IPv4-compatible addresses. We
+            // follow Rust's behavior here: https://github.com/rust-lang/rust/pull/112606
             if !addr.is_specified() {
                 write!(w, "::")
             } else if addr.is_loopback() {
                 write!(w, "::1")
-            } else if let Some(v4) = addr.to_ipv4_compatible() {
-                write!(w, "::{}", v4)
             } else if let Some(v4) = addr.to_ipv4_mapped() {
                 write!(w, "::ffff:{}", v4)
             } else {
@@ -3658,9 +3655,6 @@ mod tests {
     }
 
     #[test]
-    // TODO(fxbug.dev/129316): Temporarily disabled for toolchain roll.
-    // Change to match https://github.com/rust-lang/rust/pull/112606 and re-enable.
-    #[ignore]
     fn test_ipv6_display() {
         // Test that `addr` is formatted the same by our `Display` impl as by
         // the standard library's `Display` impl. Optionally test that it