alt Hacker News> That's am upper bound, as using a nightly compiler doesn't mean that a nightly compiler was needed.
To be fair it's not even a lower bound, as using a stable compiler doesn't imply the absence of nightly only feature (as in Cargo features, the ones you can enable on crates you depend on).
For the purposes of this discussion the question is not whether or not a crate exposes optional features that require a nightly compiler, but whether or not a crate makes use of the nightly compiler mandatory, which has become extremely rare in my experience. Perhaps it's more common in some embedded use cases, but if people want to make that assertion, I would ask that they either mention which libraries they're specifically talking about or which nightly features they're specifically referring to.