Change dynamic cost arg to be var instead of literal

Author: saulshanabrook

src/set_cost.rs

@@ -140,7 +140,7 @@ struct EnableCostsDeclarations;
 impl UserDefinedCommand for EnableCostsDeclarations {
     fn update(&self, egraph: &mut EGraph, args: &[Expr]) -> Result<(), Error> {
         let span = args[0].span();
-        if let [GenericExpr::Lit(_, Literal::String(name))] = args {
+        if let [GenericExpr::Var(_, name)] = args {
             let function = egraph.get_function(name).ok_or_else(|| {
                 Error::ParseError(ParseError(
                     span.clone(),

tests/integration_test.rs

@@ -97,7 +97,7 @@ fn test_extract_enable_cost() {
             None,
             "
             (datatype E (Add E E) (Sub E E :cost 200) (Num i64))
-            (enable-dynamic-cost \"Num\")
+            (enable-dynamic-cost Num)
             (set-cost (Num 2) 1000)",
         )
         .unwrap();