Implement size_hint on the GroupBy and GroupByMut Iterators

Author: Kerollmops

library/core/src/slice/iter.rs

@@ -3009,6 +3009,15 @@ where P: FnMut(&T, &T) -> bool,
             Some(head)
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.slice.is_empty() {
+            (0, Some(0))
+        } else {
+            (1, Some(self.slice.len()))
+        }
+    }
 }
 
 #[unstable(feature = "slice_group_by", issue = "0")]
@@ -3080,6 +3089,15 @@ where P: FnMut(&T, &T) -> bool,
             Some(head)
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.slice.is_empty() {
+            (0, Some(0))
+        } else {
+            (1, Some(self.slice.len()))
+        }
+    }
 }
 
 #[unstable(feature = "slice_group_by", issue = "0")]