feat: shorthand for retrieving custom constants from Const, Value

Closes #654

src/ops/constant.rs

@@ -76,6 +76,11 @@ impl Const {
             .unzip();
         Self::new(Value::tuple(values), Type::new_tuple(types)).unwrap()
     }
+
+    /// For a Const holding a CustomConst, extract the CustomConst by downcasting.
+    pub fn get_custom_value<T: CustomConst>(&self) -> Option<&T> {
+        self.value().get_custom_value()
+    }
 }
 
 impl OpName for Const {
@@ -123,7 +128,7 @@ mod test {
             prelude::{ConstUsize, USIZE_T},
             ExtensionId, ExtensionSet,
         },
-        std_extensions::arithmetic::float_types::FLOAT64_TYPE,
+        std_extensions::arithmetic::float_types::{ConstF64, FLOAT64_TYPE},
         type_row,
         types::test::test_registry,
         types::type_param::TypeArg,
@@ -197,11 +202,21 @@ mod test {
             tuple_ty.check_type(&tuple_val2),
             Err(ConstTypeError::ValueCheckFail(ty, tv2)) => ty == tuple_ty && tv2 == tuple_val2
         );
-        let tuple_val3 = Value::tuple([int_value, serialized_float(3.3), serialized_float(2.0)]);
+        let tuple_val3 = Value::tuple([
+            int_value.clone(),
+            serialized_float(3.3),
+            serialized_float(2.0),
+        ]);
         assert_eq!(
             tuple_ty.check_type(&tuple_val3),
             Err(ConstTypeError::TupleWrongLength)
         );
+
+        let op = Const::new(int_value, USIZE_T).unwrap();
+
+        assert_eq!(op.get_custom_value(), Some(&ConstUsize::new(257)));
+        let try_float: Option<&ConstF64> = op.get_custom_value();
+        assert!(try_float.is_none());
     }
 
     #[test]

src/values.rs

@@ -127,6 +127,18 @@ impl Value {
             val: PrimValue::Extension { c: (Box::new(c),) },
         }
     }
+
+    /// For a Const holding a CustomConst, extract the CustomConst by downcasting.
+    pub fn get_custom_value<T: CustomConst>(&self) -> Option<&T> {
+        if let Value::Prim {
+            val: PrimValue::Extension { c: (custom,) },
+        } = self
+        {
+            custom.downcast_ref()
+        } else {
+            None
+        }
+    }
 }
 
 impl<T: CustomConst> From<T> for Value {