add custom derives callback

This callback allows us to specify arbitrary derive attributes for each
named struct. This is useful for adding things that can't easily be
implemented separately, such as `serde::Deserialize` or
`zerocopy::FromBytes`.

src/callbacks.rs

@@ -95,4 +95,12 @@ pub trait ParseCallbacks: fmt::Debug + UnwindSafe {
     ) -> Option<ImplementsTrait> {
         None
     }
+
+    /// Provide a list of custom derive attributes.
+    ///
+    /// If no additional attributes are wanted, this function should return an
+    /// empty `Vec`.
+    fn add_derives(&self, _name: &str) -> Vec<String> {
+        vec![]
+    }
 }

src/codegen/mod.rs

@@ -1997,6 +1997,15 @@ impl CodeGenerator for CompInfo {
         let mut derives: Vec<_> = derivable_traits.into();
         derives.extend(item.annotations().derives().iter().map(String::as_str));
 
+        // The custom derives callback may return a list of derive attributes;
+        // add them to the end of the list.
+        let custom_derives;
+        if let Some(cb) = &ctx.options().parse_callbacks {
+            custom_derives = cb.add_derives(&canonical_name);
+            // In most cases this will be a no-op, since custom_derives will be empty.
+            derives.extend(custom_derives.iter().map(|s| s.as_str()));
+        };
+
         if !derives.is_empty() {
             attributes.push(attributes::derives(&derives))
         }