bors 9e6288efe0 Auto merge of #13516 - epage:msrv-add, r=ehuss
feat(add): Fallback to `rustc -v` when no MSRV is set

### What does this PR try to resolve?

#10653 made version-requirement selection respect MSRV as part of #9930.
This updates the implementation for the now-approved RFC specifies that we should respect `rustc -v` if there is no MSRV.

The messages also get a little bit of polish.

### How should we test and review this PR?

Tests are added in separate commits for easier viewing of behavior changes.

### Additional information
2024-03-03 22:07:46 +00:00
..
2024-02-21 20:20:41 -06:00