Cargo Features

[dependencies]
ethereum-block = { version = "0.4.0", default-features = false, features = ["c-secp256k1", "rust-secp256k1"] }
default = rust-secp256k1

The rust-secp256k1 feature is set by default whenever ethereum-block is added without default-features = false somewhere in the dependency tree.

c-secp256k1 = secp256k1-plus
rust-secp256k1 default = libsecp256k1

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.

secp256k1-plus c-secp256k1?
libsecp256k1 rust-secp256k1

Enables libsecp256k1 ^0.1