This is error could have been caused by having `regex_macros` listed as a dependency. (Which has never worked on Rust stable.) (The failure mode is bad here. I

This is error could have been caused by having `regex_macros` listed as a dependency. (Which has never worked on Rust stable.) (The failure mode is bad here. Instead of reporting a compiler error on `regex_macros`, it's trying to compile `regex` with the `pattern` feature enabled, as specified in `regex_macros` Cargo.toml.) `regex` has worked on Rust stable since 1.0.