Cargo Features

[dependencies]
sssmc39 = { version = "0.0.3", default-features = false, features = ["ring_pbkdf2", "rust_crypto_pbkdf2"] }
default = ring_pbkdf2

Highly recommended to use the ring version of pbkdf2.
Do not the rust_crypto version where security is paramount

ring_pbkdf2 default = digest, ring
rust_crypto_pbkdf2 = pbkdf2

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.

ring ring_pbkdf2

Enables ring ^0.16

digest ring_pbkdf2

Enables digest ^0.8

pbkdf2 rust_crypto_pbkdf2?

Enables pbkdf2 ^0.3