Rollup merge of #146976 - npmccallum:clone, r=scottmcm

constify basic Clone impls

Author: Zalathar

library/core/src/clone.rs

@@ -575,7 +575,8 @@ mod impls {
 ($($t:ty)*) => {
  $(
  #[stable(feature = "rust1", since = "1.0.0")]
-implClonefor $t {
+ #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+implconstClonefor $t {
  #[inline(always)]
 fn clone(&self) -> Self{
 *self
@@ -593,23 +594,26 @@ mod impls {
 }
 
 #[unstable(feature = "never_type", issue = "35121")]
-implClonefor ! {
+#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+implconstClonefor ! {
 #[inline]
 fnclone(&self) -> Self{
 *self
 }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T:PointeeSized>Clonefor*constT{
+#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+impl<T:PointeeSized>constClonefor*constT{
 #[inline(always)]
 fnclone(&self) -> Self{
 *self
 }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T:PointeeSized>Clonefor*mutT{
+#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+impl<T:PointeeSized>constClonefor*mutT{
 #[inline(always)]
 fnclone(&self) -> Self{
 *self
@@ -618,7 +622,8 @@ mod impls {
 
 /// Shared references can be cloned, but mutable references *cannot*!
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T:PointeeSized>Clonefor&T{
+#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+impl<T:PointeeSized>constClonefor&T{
 #[inline(always)]
 #[rustc_diagnostic_item = "noop_method_clone"]
 fnclone(&self) -> Self{