Amended with the suggested syntax+improvements

Author: Ruster-a11y

src/liballoc/borrow.rs

@@ -480,17 +480,14 @@ impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
     }
 }
 
-impl<'a> AddAssign<char> for Cow<'a, str> {
+#[stable(feature = "cow_str_add_assign_char", since = "1.41.0")]
+impl AddAssign<char> for Cow<'_, str> {
     fn add_assign(&mut self, rhs: char) {
-        if self.is_empty() {
-            *self = rhs.to_string()
-        } else {
-            if let Cow::Borrowed(lhs) = *self {
-                let mut s = String::with_capacity(lhs.len() + rhs.len_utf8());
-                s.push_str(lhs);
-                *self = Cow::Owned(s);
-            }
-            self.to_mut().push(rhs);
+        if let Cow::Borrowed(lhs) = *self {
+            let mut s = String::with_capacity(lhs.len() + rhs.len_utf8());
+            s.push_str(lhs);
+            *self = Cow::Owned(s);
         }
+        self.to_mut().push(rhs);
     }
 }