If you make any error in your Cargo.toml, the whole file is underlined in red:

The error is of the form:

failed to parse manifest at `toml-rs/Cargo.toml`
Feature `preserve_order` includes `indexmaap` which is neither a dependency nor another feature

So clearly, there could be a span for the error, pointing to indexmaap. If we underlined only that it would be much nicer. But this is only an example, my request is about the general feature.

Yes, this will need changes in Cargo. But I still rather file the issue here because this is the tool that suffers most from it.