Document that is_human_readable == false is a breaking change

serde/src/de/mod.rs

@@ -1016,6 +1016,9 @@ pub trait Deserializer<'de>: Sized {
     ///
     /// Some formats are not intended to be human readable. For these formats
     /// a type being serialized may opt to serialize into a more compact form.
+    ///
+    /// NOTE: Implementing this method and returning `false` is considered a breaking
+    /// change as it may alter how any given type tries to deserialize itself.
     fn is_human_readable(&self) -> bool { true }
 }
 

serde/src/ser/impls.rs

@@ -506,9 +506,16 @@ impl Serialize for net::IpAddr {
     where
         S: Serializer,
     {
-        match *self {
-            net::IpAddr::V4(ref a) => a.serialize(serializer),
-            net::IpAddr::V6(ref a) => a.serialize(serializer),
+        if serializer.is_human_readable() {
+            match *self {
+                net::IpAddr::V4(ref a) => a.serialize(serializer),
+                net::IpAddr::V6(ref a) => a.serialize(serializer),
+            }
+        } else {
+            match *self {
+                net::IpAddr::V4(ref a) => (0u8, a).serialize(serializer),
+                net::IpAddr::V6(ref a) => (1u8, a).serialize(serializer),
+            }
         }
     }
 }

serde/src/ser/mod.rs

@@ -1368,6 +1368,9 @@ pub trait Serializer: Sized {
     ///
     /// Some formats are not intended to be human readable. For these formats
     /// a type being serialized may opt to serialize into a more compact form.
+    ///
+    /// NOTE: Implementing this method and returning `false` is considered a breaking
+    /// change as it may alter how any given type tries to serialize itself.
     fn is_human_readable(&self) -> bool { true }
 }