Optimize Option::clone to Copy when possible

Prior to this change, cloning an Option value would branch on whether the
Option was Some, even if it would be cheaper to just memcpy the bits.
Default the current implementation, and then specialize it for Copy types,
to avoid the branch.

Author: ridiculousfish

library/core/src/option.rs

@@ -1228,22 +1228,35 @@ fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone> Clone for Option<T> {
     #[inline]
-    fn clone(&self) -> Self {
+    default fn clone(&self) -> Self {
         match self {
             Some(x) => Some(x.clone()),
             None => None,
         }
     }
 
     #[inline]
-    fn clone_from(&mut self, source: &Self) {
+    default fn clone_from(&mut self, source: &Self) {
         match (self, source) {
             (Some(to), Some(from)) => to.clone_from(from),
             (to, from) => *to = from.clone(),
         }
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Copy> Clone for Option<T> {
+    #[inline]
+    fn clone(&self) -> Self {
+        *self
+    }
+
+    #[inline]
+    fn clone_from(&mut self, source: &Self) {
+        *self = *source
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Option<T> {
     /// Returns [`None`][Option::None].