After modifying Cargo.toml, Run fx update-rustc-third-party
You will need a modern OpenSSL and to export either PKG_CONFIG_PATH or CFLAGS and LDFLAGS in order for cargo to find it, before running fx update-rustc-third-party.
You can install using brew install openssl and observe brew info openssl for the above exports. It is recommended you only export these as-needed, not permanently, to avoid causing incompatibilities with other software.
This repository contains vendored copies of third party code used in Fuchsia.
Further documentation is available here:
/docs/development/languages/rust/third_party.md