Function is_expression_filter 

pub fn is_expression_filter(filter: &Value) -> bool

Whether filter is already a modern expression filter (as opposed to a legacy filter needing conversion). A direct port of isExpressionFilter.

The rule errs toward reporting legacy for the ambiguous shapes an old style might carry (e.g. ["in", "color", "red"]) so they are converted; authors can force expression interpretation with a ["literal", …] operand.