Cargo Features

[dependencies]
bitstream-io = { version = "2.5.0", default-features = false, features = ["std", "alloc"] }
default = std

The std feature is set by default whenever bitstream-io is added without default-features = false somewhere in the dependency tree.

std default
alloc = core2

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.

core2 alloc?