It's used only as a dev dependency in 3 crates.

Depender (with downloads and own rev deps) spl-merkle-tree-reference version
9.0K 6 spl-concurrent-merkle-tree dev ^0.1.0
7.6K 5 mpl-bubblegum dev ^0.1.0
200 2 tensor-whitelist dev ^0.1.0