server/failtracer: don't assume only being fed two-elem calls

This was overly specific before, and quickly failed when fail events had
not been on of the right sort. Now, we're more lenient.

These would happen, as in https://github.com/orgs/open-policy-agent/discussions/722#discussioncomment-14812737,
when a fail event happened that's unrelated to unknowns lookup, but a
valid failure nonetheless. So this will not lead to a hint. Only fail
events of the previously-expected form could yield hints.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

Author: srenatus

v1/server/failtracer/failtracer.go

@@ -69,8 +69,7 @@ func (b *failTracer) Hints(unknowns []ast.Ref) []Hint {
 		var ref ast.Ref // when this is processed, only one input.X.Y ref is in the expression (SSA)
 		switch {
 		case expr.IsCall():
-			for i := range 2 {
-				op := expr.Operand(i)
+			for _, op := range expr.Operands() {
 				if r, ok := op.Value.(ast.Ref); ok && r.HasPrefix(ast.InputRootRef) {
 					ref = r
 				}

v1/server/failtracer/hints_test.go

@@ -38,6 +38,14 @@ func TestHints(t *testing.T) {
 				{Message: "input.fruit.price undefined, did you mean input.fruits.price?"},
 			},
 		},
+		{
+			note: "failing function call",
+			evts: []topdown.Event{
+				evtFromExpr(`data.policy.udf(__local1__)`),
+			},
+			unknowns: []string{"input.fruits"},
+			exp:      nil,
+		},
 		{
 			note: "all of input unknown, ignored",
 			evts: []topdown.Event{