Auto merge of #85176 - a1phyr:impl_clone_from, r=yaahc

Override `clone_from` for some types

Override `clone_from` method of the `Clone` trait for:
- `cell::RefCell`
- `cmp::Reverse`
- `io::Cursor`
- `mem::ManuallyDrop`

This can bring performance improvements.

Author: bors

library/core/src/cell.rs

@@ -1123,6 +1123,15 @@ impl<T: Clone> Clone for RefCell<T> {
     fn clone(&self) -> RefCell<T> {
         RefCell::new(self.borrow().clone())
     }
+
+    /// # Panics
+    ///
+    /// Panics if `other` is currently mutably borrowed.
+    #[inline]
+    #[track_caller]
+    fn clone_from(&mut self, other: &Self) {
+        self.get_mut().clone_from(&other.borrow())
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/cmp.rs

@@ -578,7 +578,7 @@ impl Ordering {
 /// v.sort_by_key(|&num| (num > 3, Reverse(num)));
 /// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
 /// ```
-#[derive(PartialEq, Eq, Debug, Copy, Clone, Default, Hash)]
+#[derive(PartialEq, Eq, Debug, Copy, Default, Hash)]
 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 #[repr(transparent)]
 pub struct Reverse<T>(#[stable(feature = "reverse_cmp_key", since = "1.19.0")] pub T);
@@ -616,6 +616,19 @@ impl<T: Ord> Ord for Reverse<T> {
     }
 }
 
+#[stable(feature = "reverse_cmp_key", since = "1.19.0")]
+impl<T: Clone> Clone for Reverse<T> {
+    #[inline]
+    fn clone(&self) -> Reverse<T> {
+        Reverse(self.0.clone())
+    }
+
+    #[inline]
+    fn clone_from(&mut self, other: &Self) {
+        self.0.clone_from(&other.0)
+    }
+}
+
 /// Trait for types that form a [total order](https://en.wikipedia.org/wiki/Total_order).
 ///
 /// An order is a total order if it is (for all `a`, `b` and `c`):

library/core/src/mem/manually_drop.rs

@@ -44,7 +44,7 @@ use crate::ptr;
 /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
 #[stable(feature = "manually_drop", since = "1.20.0")]
 #[lang = "manually_drop"]
-#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[repr(transparent)]
 pub struct ManuallyDrop<T: ?Sized> {
     value: T,
@@ -160,3 +160,16 @@ impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
         &mut self.value
     }
 }
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: Clone> Clone for ManuallyDrop<T> {
+    #[inline]
+    fn clone(&self) -> ManuallyDrop<T> {
+        ManuallyDrop { value: self.value.clone() }
+    }
+
+    #[inline]
+    fn clone_from(&mut self, other: &Self) {
+        self.value.clone_from(&other.value)
+    }
+}

library/std/src/io/cursor.rs

@@ -71,7 +71,7 @@ use core::convert::TryInto;
 /// }
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
-#[derive(Clone, Debug, Default, Eq, PartialEq)]
+#[derive(Debug, Default, Eq, PartialEq)]
 pub struct Cursor<T> {
     inner: T,
     pos: u64,
@@ -205,6 +205,23 @@ impl<T> Cursor<T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Clone for Cursor<T>
+where
+    T: Clone,
+{
+    #[inline]
+    fn clone(&self) -> Self {
+        Cursor { inner: self.inner.clone(), pos: self.pos }
+    }
+
+    #[inline]
+    fn clone_from(&mut self, other: &Self) {
+        self.inner.clone_from(&other.inner);
+        self.pos = other.pos;
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> io::Seek for Cursor<T>
 where