An ABI that could express sum types would be a huge step forwards for the computing world.
.net and the JVM sort of achieve this, but at the cost of going managed for everything. That's mostly fine (and possibly entirely fine, to be honest; very few people who think they need non-GC actually do), but there are still a few diehards who insist on non-GC, and Rust is pretty much the first good language to have that option in a comprehensive way.
Rust though might be an example where such a thing wouldn't work (had it already existed), since the type system is legitimately different. But some things might work, and even a test suite you could develop against would be wonderful.