I was using rustc 1.19.0-nightly on master and received the following warning: ``` Compiling protover v0.0.1 (file:///Users/twilsonb/tor/tor-rust/src/rust/protover) error: use of unstable library feature 'retain_hash_collection' (see issue #36648) --> protover/protover.rs:259:10 | 259 | vers.retain(|x| !supported_versions.contains(x)); | ^^^^^^ | = help: add #![feature(retain_hash_collection)] to the crate attributes to enable ``` When I added the feature declaration, everything worked fine and the tests passed.