slight Default cleanup for option

Author: lqd

compiler/rustc_session/src/config.rs

@@ -3420,9 +3420,10 @@ impl DumpMonoStatsFormat {
 
 /// `-Zpolonius` values, enabling the borrow checker polonius analysis, and which version: legacy,
 /// or future prototype.
-#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+#[derive(Clone, Copy, PartialEq, Hash, Debug, Default)]
 pub enum Polonius {
     /// The default value: disabled.
+    #[default]
     Off,
 
     /// Legacy version, using datalog and the `polonius-engine` crate. Historical value for `-Zpolonius`.
@@ -3432,12 +3433,6 @@ pub enum Polonius {
     Next,
 }
 
-impl Default for Polonius {
-    fn default() -> Self {
-        Polonius::Off
-    }
-}
-
 impl Polonius {
     /// Returns whether the legacy version of polonius is enabled
     pub fn is_legacy_enabled(&self) -> bool {