But for a feature like generators which is, I think, essentially user-facing syntactic sugar, I do think a count of usages is a pretty good measure of usefulness.
It's probably also worth noting that my data here is from published packages, which tends to skew towards reusable libraries and away from application code. So, if anything, I would expect this to be an over-count of their use if they primarily made libraries/frameworks useful on behalf of application developers.
One thing to remember is that Rust is a bit weird here; often times, features like this are "sugar" in a sense, but that sugar lets you write safe code, whereas the non-sugar version would force you to write unsafe. This means this kind of thing is a larger win in Rust than it would be in other languages.