Hint for language-region pair where language is expected

[laurmaedje]

Description

When writing #set text(lang: "en-gb"), we should have a hint that you should use the region field instead.

Use Case

Helps people fix the problem.

[Coekjan]

Hi, I wonder if this diagnostics shall be implemented in Lang::from_str. I see current diagnostics information "expected two or three letter language code (ISO 639-1/2/3)" comes from there.

[laurmaedje]

Unfortunately, the casting infrastructure doesn't support hints yet (FromValue always returns a StrResult). So, I think the only place we can do this at the moment is by augmenting the text.lang with a #[parse] annotation (you can search the codebase to see how it works).

[Coekjan]

I am thinking about when to hint something like "you should use region field". Here are some options I thought out:
Op1: The arg contains -
Op2: Op1 + the length of arg.split('-') is 2
Op3: Op2 + arg[0] can be parsed into Lang & arg[1] can be parsed into Region

Or does anyone have some other ideas?