Currently, Cargo warns when I ask for a feature of another crate that does not exist. However, it seems that it treats all dependencies also as features. For optional dependencies, this makes sense, and indeed the documentation says "A feature of a package is either an optional dependency, or a set of other features."

So, I can depend on regex = { version = "0.2.2", features = ["memchr"]} and there is no warning or error that this (a) doesn't make any sense, and (b) memchr is not technically a feature according to the definition of the docs (and indeed, it should not be).