Rustfmt: Unstable feature warnings are printed multiple times

[rossmacarthur]

When I run cargo fmt on stable the warnings about unstable features seem to be printed multiple times, one for each file that is being linted.

Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_doc_attributes = true`, unstable features are only available in nightly channel.
Warning: can't set `format_strings = true`, unstable features are only available in nightly channel.
Warning: can't set `merge_imports = true`, unstable features are only available in nightly channel.
Warning: can't set `reorder_impl_items = true`, unstable features are only available in nightly channel.
Warning: can't set `condense_wildcard_suffixes = true`, unstable features are only available in nightly channel.
Warning: can't set `unstable_features = true`, unstable features are only available in nightly channel.
Warning: can't set `wrap_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_comments = true`, unstable features are only available in nightly channel.
Warning: can't set `normalize_doc_attributes = true`, unstable features are only available in nightly channel.
Warning: can't set `format_strings = true`, unstable features are only available in nightly channel.
Warning: can't set `merge_imports = true`, unstable features are only available in nightly channel.
Warning: can't set `reorder_impl_items = true`, unstable features are only available in nightly channel.
...

[calebcartwright]

Do you have multiple rustfmt config files by any chance?

[topecongiro]

We should handle these kinds of errors and warnings more deliberately. In this particular case, instead of eprintln!-ing each warning, we should store them to a map and emit them afterward.