message: Emit compile error when no runtime feature specified
diff: #![warn(clippy::unwrap_used, clippy::missing_errors_doc, clippy::missing_panics_doc)]
  #![forbid(unsafe_code)]
  +// Give a clear error when a required runtime error is not present. Would be better for this
  +// to be a fatal error preventing emission of further compile errors relating to lack of
  +// a runtime feature, but that does not seem currently possible:
  +// https://github.com/rust-lang/rust/issues/68838
  +
  +#[cfg(not(any(
  + feature = "runtime-tokio-hyper",
  + feature = "runtime-tokio-hyper-rustls",
  + feature = "runtime-blocking",
  + feature = "runtime-blocking-rustls",
  + feature = "runtime-async-std-surf",
  +)))]
  +compile_error!(
  + r"one of the following runtime features must be enabled:
  + [
  + 'runtime-tokio-hyper',
  + 'runtime-tokio-hyper-rustls',
  + 'runtime-blocking',
  + 'runtime-blocking-rustls',
  + 'runtime-async-std-surf'
  + ]"
  +);
  +
  mod client;
  mod error;
  mod ids;
  