Cargo Features

[dependencies]
must_be_empty = { version = "0.1.1", default-features = false, features = ["only_on_release", "only_on_debug", "warn"] }
default = warn

The warn feature is set by default whenever must_be_empty is added without default-features = false somewhere in the dependency tree.

only_on_release
only_on_debug
warn default

must_be_empty has 4 features without comments.