Derive value_parser for Option<PasswordHashString>; empty string converts to None

[overhacked]

Hello, I've got a app that generates an ephemeral password if --auth-token=TOKEN is not supplied on the command line, but if the user explicitly supplies an empty auth token --auth-token="", then the Option<PasswordHashString> should be set to None. I'm not currently able to accomplish this with value_parser, because it is expected to return the inner type.

I found a reply from a few years ago by @epage that alludes there is a way to get a value_parser to return an Option, but I couldn't find any more details. Do you have any suggestions?

[epage]

You would need to set your type to Option<Option<PasswordHashString>> with a parse function that returns Option<PasswordHashString>.

[overhacked]

So that's what I tried, and I get the error: Mismatch between definition and access of `maybe_string`. Could not downcast to alloc::string::String, need to downcast to core::option::Option<alloc::string::String>

Link to playground minimal reproducer: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a59e07ee059dfedaa67953dcd0a51aed

Am I thinking about something in the wrong way?

[epage]

I forgot we support Option<Option<T>> (source), requiring Option<Option<Option<T>> (playground) or use the hack at that previous link (playground)

To force any inferred type (like Vec) to be treated as T, you can refer to the type by another means, like using std::vec::Vec instead of Vec. For improving this, see #4626.

None of this is great. We are exploring alternatives in #4626 but they all have different, annoying trade offs so far.