Optimize Partial Eval of default functions

[johanfylling]

During non-shallow PE, whether a default rule will be added to generated support module(s) is optimized based upon if its return value is compared/unified with a constant value. Because of how the compiler rewrites function calls, default functions don't have the same benefit, as the output argument will always be a non-constant variable without a bound value.

E.g.

package test

default f(x) := true

f(x) := y if {
   y := x.size < 100
}

p if {
   f(input.x) == false
}

is rewritten to

package test

default f(__local0__) := true

f(__local1__) := __local2__ if {
	__local5__ = __local1__.size
	__local3__ = __local5__ < 100
	__local2__ = __local3__
}

p if {
	__local6__ = input.x
	data.test.f(__local6__, __local4__)
	__local4__ = false
}

Possibly, we could update the compiler to reorder generated statements, so that unification happens before the call, so that PE can retrieve any bound constants.

p if {
	__local6__ = input.x
	__local4__ = false
	data.test.f(__local6__, __local4__)
}

This approach must however be investigated to explore whether it breaks any assumptions in other places of compile and eval.

Another alternative could be to detect when the generated output unification expr is superfluous, and can be replaced with a constant value:

p if {
	__local6__ = input.x
	data.test.f(__local6__, false)
}

[johanfylling]

For further context, see this discussion.