Make level field optional in the [lints] table

[Urgau]

What does this PR try to resolve?

This PR makes the level field in the individual lint under the [lints] optional if some other key (except priority since it would make sense to have it alone) is defined. The goal is to avoid a empty lint definition.

This is to allow special configs like check-cfg in the unexpected_cfgs to omit the level field.

Users would now be able to omit the level field, resulting in:

- unexpected_cfgs = { level = "warn", check-cfg = ['cfg(has_foo)'] }
+ unexpected_cfgs = { check-cfg = ['cfg(has_foo)'] }

How should we test and review this PR?

Test putting removing the level field with/without this PR.

Reviewing this PR should be straightforward, look at the tests that change to see the effect of this change.

Additional information

#13913 (comment)

r? @epage

[epage]

This is the big question for me: what should be allows

Today

lint_name = {}  # disallowed
lint_name = { config_name = "config_value" }  # disallowed
lint_name = { priority = 1 }  # disallowed
lint_name = { priority = 1, config_name = "config_value" }  # disallowed
lint_name = { level = "warn" }  # allowed
lint_name = { level = "warn", priority = 1 }  # allowed

With this PR

lint_name = {}  # disallowed
lint_name = { config_name = "config_value" }  # allowed
lint_name = { priority = 1 }  # disallowed
lint_name = { priority = 1, config_name = "config_value" }  # allowed
lint_name = { level = "warn" }  # allowed
lint_name = { level = "warn", priority = 1 }  # allowed

This allows priority without level. Should we allow that?

This requires level or config, should we bother with an empty table error?

I almost lean towards either keeping the requirement on level or dropping all requirements (though priority alone feels weird).

Thoughts?

[Urgau]

This allows priority without level. Should we allow that?

I don't think so, if priority is set then level must also be set, that's a oversight on my part. Will fix it.

This requires level or config, should we bother with an empty table error?

My stance is that it is useless and weird to have empty table.

[Urgau]

Fixed the priority field alone without the level field. It now bail out.

[epage]

We talked about this in today's Cargo team meeting and decided to defer this out.

• We can always merge this later but we cannot undo it. We should probably learn from the current behavior works before iterating on it further.
• There was some preference on the team that we should be explicit on the level when config is present.