rule idea: flag if a rule is called "if"

[srenatus]

Regrettably, may policies using if that are missing a import future.keywords.if still parse successfully. However, they're rarely do what the author intended.

What they have in common is a rule named if. So, regal could flag that as a likely error, hinting that future.keywords.if should be imported.

Example:

package play

allow := true if {
    input.foo
}

With input = {"foo": true}, data.play evaluates to

{
  "if": true,
  "allow": true
}