Although it would be convenient, I don't think it is realistic to come up with an automatic tool for the majority case.
In most cases, the actual mapping between the custom type and the mojom type is not that mechanical.

Is it the most tedious case where a custom enum and a mojom enum have exactly the same values that is bothering you? :) That seems an easier problem to solve.

> the actual mapping between the custom type and the mojom type is not that mechanical

Uh I thought that:
- The struct being traitsed have to have at least the same fields of the mojom equivalent (and I think it's safe to assume the field names are the same).
- The fields are either pods (which have a direct mapping to mojo and hence their marshaling/unmarhaling is deterministic) or have to be recursively struct trait-ed

At least this has been the case for my code above. Out of curiosity what doesn't obey to this pattern?

#2: ... or Mojo array/map which have a 1:1 mapping with std::vector std::map

RE #2: Almost all traits do custom validation during deserialization. And it is not uncommon that mapping between custom / mojom struct fields are more complex than what is described in #2.

A simple example: https://cs.chromium.org/chromium/src/url/mojo/url_gurl_struct_traits.h
A complex example: https://cs.chromium.org/chromium/src/cc/ipc/quads_struct_traits.h

(And many others.)

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It doesn't look like the effort/gain ratio is high enough since individual traits vary in subtle ways.

I meant gain/effort :-)