message: rename '--host' to '--index', add comments marked as TODO where host should be removed/repurposed
diff: @@ -5,7 +5,8 @@ use std::cmp;
  #[derive(Deserialize)]
  pub struct Options {
  - flag_host: Option<String>,
  + flag_index: Option<String>,
  + flag_host: Option<String>, // TODO: Depricated, remove
  flag_verbose: u32,
  flag_quiet: Option<bool>,
  flag_color: Option<String>,
  @@ -24,7 +25,8 @@ Usage:
  Options:
  -h, --help Print this message
  - --host HOST Host of a registry to search in
  + --index INDEX Registry index to search in
  + --host HOST DEPRICATED, renamed to '--index'
  -v, --verbose ... Use verbose output (-vv very verbose/build.rs output)
  -q, --quiet No output printed to stdout
  --color WHEN Coloring: auto, always, never
  @@ -40,12 +42,37 @@ pub fn execute(options: Options, config: &Config) -> CliResult {
  options.flag_frozen,
  options.flag_locked)?;
  let Options {
  - flag_host: host,
  + flag_index: index,
  + flag_host: host, // TODO: Depricated, remove
  flag_limit: limit,
  arg_query: query,
  ..
  } = options;
  - ops::search(&query.join("+"), config, host, cmp::min(100, limit.unwrap_or(10)) as u8)?;
  + // TODO: Depricated
  + // remove once it has been decided --host can be safely removed
  + // We may instead want to repurpose the host flag, as
  + // mentioned in this issue
  + // https://github.com/rust-lang/cargo/issues/4208
  +
  + let msg = "The flag '--host' is no longer valid.
  +
  +Previous versions of Cargo accepted this flag, but it is being
  +depricated. The flag is being renamed to 'index', as the flag
  +wants the location of the index in which to search. Please
  +use '--index' instead.
  +
  +This will soon become a hard error, so it's either recommended
  +to update to a fixed version or contact the upstream maintainer
  +about this warning.";
  +
  + let index = if host.clone().is_none() || host.clone().unwrap().is_empty() {
  + index
  + } else {
  + config.shell().warn(&msg)?;
  + host
  + };
  +
  + ops::search(&query.join("+"), config, index, cmp::min(100, limit.unwrap_or(10)) as u8)?;
  Ok(())
  }
  