Add flag to upgrade warnings to errors

[Timmmm]

Would you be ok with a --warnings-as-errors flag, like -Werror in C? I can implement it if so...

[Alasdair]

I'm not sure, I think that -Werror is often misused. The problem from my perspective is if people start doing things like using -Werror in CI, then I can't create new warnings without causing issues for users (even if that's arguably what they signed up for by using the flag).

[Timmmm]

Hmm what about if you have to specify a target version to enable errors for? I.e. --warnings-as-errors=0.18, which would mean any warnings added in version 0.18 or earlier become errors, but warnings added afterwards are just warnings. That way adding new warnings is never a breaking change.

It's a little bit like how Android target versions, or CMake policies or Rust editions work.

Would you have made some warnings errors by in the first place if backwards compatibility wasn't a concern? E.g. missing pure/impure, or missing register initialisers? If so maybe the best option is to not have this flag and instead require specifying an edition or target version. Then you can gradually transition those to errors for everyone.

[Alasdair]

That would be good, but it would require a lot of work to categorise all the warnings we have.

Would you have made some warnings errors by in the first place if backwards compatibility wasn't a concern?

Yes, often the idea is to make something a warning first then transition it to an error later.