Function const_value 

fn const_value(
    gcx: Gcx<'_>,
    expr: &Expr<'_>,
    lock: Option<(VariableId, Operand)>,
    seen: &mut BTreeSet<VariableId>,
) -> Option<Operand>

Constant-folds expr over literals, constant variables, casts, ! and ==/!=; lock supplies the value of the lock variable.