AnyValueParser
We aren't enumerating arguments but values for an argument, so the name should reflect that. This will be important as part of #1807 when we have more specific attribute names.
`derive_ui.rs` did not have a marker for needing to be updated on MSRV changes, so it got missed.
unstable-v4
This is split out of #3661 as several changes I'm working on need it.