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 +95,9 @@ pub trait ParseCallbacks: fmt::Debug + UnwindSafe {
     ) -> Option<ImplementsTrait> {
         None
     }
+
+    /// Provide a list of custom derive attributes.
+    fn add_derives(&self, _name: &str) -> Option<Vec<String>> {
+        None
+    }
 }

src/codegen/mod.rs

@@ -1997,6 +1997,16 @@ impl CodeGenerator for CompInfo {
         let mut derives: Vec<_> = derivable_traits.into();
         derives.extend(item.annotations().derives().iter().map(String::as_str));
 
+        // Check to see if there is a custom derives callback, and call that
+        // function to see if any custom derive attributes should be added.
+        let custom_derives;
+        if let Some(cb) = &ctx.options().parse_callbacks {
+            custom_derives = cb.add_derives(&canonical_name);
+            if let Some(custom_derive_vec) = &custom_derives {
+                derives.extend(custom_derive_vec.iter().map(|s| s.as_str()));
+            }
+        };
+
         if !derives.is_empty() {
             attributes.push(attributes::derives(&derives))
         }