Consider this real-world example from crc-rs:

[dependencies]
build_const = { version = "0.2", default-features = false }

[build-dependencies]
build_const = "0.2"

The build dependency, of course, needs (and has) the std feature. The runtime dependency does not. Moreover, on my platform, there is no std. However, cargo treats them as the same package, and as a result it is impossible to actually build crc-rs at all.