Fix #265: New Rule: export used types

[maco]

No description provided.

[elbrujohalcon]

Great job! The only thing missing, from my perspective, is to make the rule run on beam files and test that.

[paulo-ferraz-oliveira]

Should we mention, somewhere, that exported types can also be opaque, as a general guidance?

[erszcz]

@paulo-ferraz-oliveira

Should we mention, somewhere, that exported types can also be opaque, as a general guidance?

I'm not sure of the intention of the question as a bystander, but FWIW opaque types have to be exported to be useful at all. In the local module they're just types, i.e. no difference between -type and -opaque (i.e. you can pattern match and construct an opaque in the module that defines it without constraints), they only have special treatment when referred to as external types, but for that to be possible, they have to be exported.

[paulo-ferraz-oliveira]

but FWIW opaque types have to be exported to be useful at all

I know 😄, there's even a compiler warning if I'm not mistaken.

What I'm hinting at is that this rule will make consumers just go "Oh, I need to add an export_type here" and do it without thinking about the consequences of potentially exposing a non-opaque. If it was they're intention before, sure, the type should already be opaque and exposed; if it wasn't, now that it's being exposed there are consequences to it.

[elbrujohalcon]

Once we have a new release, we should run it on OTP itself.
This rule in particular encapsulates something that people were asking for a while back.

[paulo-ferraz-oliveira]

Here's a pointer from the OTP team and exporting types: "Our policy is to be restrictive with exporting types. Only input and return types should be necessary to export." which correlates with the rule.