Enforce use of `if` and `contains` in rule declarations in OPA `1.0`

[johanfylling]

Currently it's, seemingly, arbitrary if a ref-head rule will produce a single-value or multi-value leaf:

package play

a[b] { b := 1 }

x.y[z] { z := 2 }

will produce

{
    "a": [
        1
    ],
    "x": {
        "y": {
            "2": true
        }
    }
}

All rules declared with if contribute to an object unless contains is also used, though:

package play
import future.keywords

a[b] if { b := 1 }

x.y[z] if { z := 2 }

-->

{
    "a": {
        "1": true
    },
    "x": {
        "y": {
            "2": true
        }
    }
}

and:

package play
import future.keywords

a contains b if { b := 1 }

x.y contains z if { z := 2 }

-->

{
    "a": [
        1
    ],
    "x": {
        "y": [
            2
        ]
    }
}

In a 0.x release of OPA, the future.strict import will be intrudoced, and will impose the same constraints on the module, which will hopefully aid in transitioning to OPA 1.0.

[johanfylling]

This will however not solve the issue with no-body rules. E.g:

package play

a.b

x.y.x

which will produce:

{
    "a": [
        "b"
    ],
    "x": {
        "y": {
            "x": true
        }
    }
}

Since the above rules don't have bodies, it's nonsensical to require the if keyword.

Is it possible to solve this without introducing a breaking change in OPA 1.0?