Original idea: dtolnay/trybuild#35
Suggested approach: dtolnay/trybuild#35 (comment)

Solution Summary

cargo expand should be able to expand the code of a test file somename.rs in the tests directory of a given crate via --test somename even though somename.rs exists but "somename" is not part of the test targets listed for the given crate by cargo-metadata.

Technical Details

The expansion should be done in a similar way as the trybuild crate handles building and testing of its tests:

• pull out the code under test (e.g. our somename.rs) into a temp. directory
• run rustc against that
  • just like trybuild would
  • using --pretty=expanded instead of building or testing anything