use let to bind value from a macro expression

Author: tesuji

src/config/config_type.rs

@@ -324,9 +324,10 @@ macro_rules! create_config {
             #[allow(unreachable_pub)]
             /// Returns `true` if the config key was explicitly set and is the default value.
             pub fn is_default(&self, key: &str) -> bool {
+                let default = $def;
                 match key {
                     $(
-                        stringify!($i) => self.$i.1 && self.$i.2 == $def,
+                        stringify!($i) => self.$i.1 && self.$i.2 == default,
                     )+
                     _ => false,
                 }