Cargo Features

HPACK has no features set by default.

[dependencies]
hpack = { version = "0.3.0", features = ["interop_tests"] }
interop_tests = rustc-serialize

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.

rustc-serialize interop_tests?