Cargo Features

libseccomp has no features set by default.

libseccomp = { version = "0.3.0", features = ["const-syscall"] }
const-syscall = cfg-if

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

cfg-if const-syscall?