message: Cache based on contents hash
diff: @@ -163,10 +163,9 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
  // Older versions of Cargo used the single value of the hash of the HEAD commit as a `index_version`.
  // This is technically correct but a little too conservative. If a new commit is fetched all cached
  // files need to be regenerated even if a particular file was not changed.
  - // Cargo now reads the `index_version` in two parts the cache file is considered valid if `index_version`
  + // Cargo now checks the `index_version` in two parts the cache file is considered valid if `index_version`
  // ends with the hash of the HEAD commit OR if it starts with the hash of the file's contents.
  - // In the future cargo can write cached files with `index_version` = `git_file_hash + ":" + `git_commit_hash`,
  - // but for now it still uses `git_commit_hash` to be compatible with older Cargoes.
  + // Cargo writes the cached files with `index_version` = `git_file_hash` + ":" + `git_commit_hash`.
  fn load(
  &mut self,
  _root: &Path,
  @@ -178,7 +177,7 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
  }
  // Check if the cache is valid.
  let git_commit_hash = self.current_version();
  - if let (Some(c), Some(i)) = (git_commit_hash, index_version) {
  + if let (Some(i), Some(c)) = (index_version, git_commit_hash) {
  if i.ends_with(c.as_str()) {
  return Poll::Ready(Ok(LoadResponse::CacheValid));
  }
  @@ -212,9 +211,7 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
  Ok(LoadResponse::Data {
  raw_data: blob.content().to_vec(),
  - index_version: git_commit_hash.map(String::from),
  - // TODO: When the reading code has been stable for long enough (Say 8/2022)
  - // change to `git_file_hash + ":" + git_commit_hash`
  + index_version: git_commit_hash.map(|c| git_file_hash + c),
  })
  }
  