Cargo Features

[dependencies]
ckb-merkle-mountain-range = { version = "0.6.0", default-features = false, features = ["std"] }
default = std

The std feature is set by default whenever ckb-merkle-mountain-range is added without default-features = false somewhere in the dependency tree.

std default

ckb-merkle-mountain-range has 2 features without comments.