For rust, it is probably the try (?) operator. Fundamentally, it's just syntax sugar for a match statement with an early return in the Error or None cases, but it really improves the ergonomics of dealing with Result and Option types.
Interestingly, there was once a solid effort to add a try operator to Go. While the proposal was quite well received, upon closer inspection it was realized it would be essentially useless in the real world as, given how the rest of the language works, you almost never would want to simply early return with the value received. The data revealed that the vast majority of the code in the wild that the syntax sugar woul…
You have things a bit backwards. The try operator was not conceived until after sum types and traits were already stable parts of the language. The addition was simple, didn't really alter the language in a meaningful way and is almost purely syntactic sugar meant as a quality-of-life improvement for users. (ie. it hits pretty every single point in the article's definition of microfeatures)