Rollup merge of #91804 - woppopo:const_clone, r=oli-obk

Make some `Clone` impls `const`

Tracking issue: #91805
`Clone::clone_from` and some impls (Option, Result) bounded on `~const Drop`.

```rust
// core::clone
impl const Clone for INTEGER
impl const Clone for FLOAT
impl const Clone for bool
impl const Clone for char
impl const Clone for !
impl<T: ?Sized> const Clone for *const T
impl<T: ?Sized> const Clone for *mut T
impl<T: ?Sized> const Clone for &T

// core::option
impl<T> const Clone for Option<T>
where
    T: ~const Clone + ~const Drop

// core::result
impl<T, E> const Clone for Result<T, E>
where
    T: ~const Clone + ~const Drop,
    E: ~const Clone + ~const Drop,

// core::convert
impl const Clone for Infallible

// core::ptr
impl<T: ?Sized> const Clone for NonNull<T>
impl<T: ?Sized> const Clone for Unique<T>
```

library/core/src/clone.rs

@@ -127,7 +127,11 @@ pub trait Clone: Sized {
     /// allocations.
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn clone_from(&mut self, source: &Self) {
+    #[default_method_body_is_const]
+    fn clone_from(&mut self, source: &Self)
+    where
+        Self: ~const Drop,
+    {
         *self = source.clone()
     }
 }
@@ -178,7 +182,8 @@ mod impls {
         ($($t:ty)*) => {
             $(
                 #[stable(feature = "rust1", since = "1.0.0")]
-                impl Clone for $t {
+                #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+                impl const Clone for $t {
                     #[inline]
                     fn clone(&self) -> Self {
                         *self
@@ -196,7 +201,8 @@ mod impls {
     }
 
     #[unstable(feature = "never_type", issue = "35121")]
-    impl Clone for ! {
+    #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+    impl const Clone for ! {
         #[inline]
         fn clone(&self) -> Self {
             *self
@@ -204,7 +210,8 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T: ?Sized> Clone for *const T {
+    #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+    impl<T: ?Sized> const Clone for *const T {
         #[inline]
         fn clone(&self) -> Self {
             *self
@@ -212,7 +219,8 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T: ?Sized> Clone for *mut T {
+    #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+    impl<T: ?Sized> const Clone for *mut T {
         #[inline]
         fn clone(&self) -> Self {
             *self
@@ -221,7 +229,8 @@ mod impls {
 
     /// Shared references can be cloned, but mutable references *cannot*!
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T: ?Sized> Clone for &T {
+    #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+    impl<T: ?Sized> const Clone for &T {
         #[inline]
         #[rustc_diagnostic_item = "noop_method_clone"]
         fn clone(&self) -> Self {

library/core/src/convert/mod.rs

@@ -690,7 +690,8 @@ impl AsMut<str> for str {
 pub enum Infallible {}
 
 #[stable(feature = "convert_infallible", since = "1.34.0")]
-impl Clone for Infallible {
+#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+impl const Clone for Infallible {
     fn clone(&self) -> Infallible {
         match *self {}
     }

library/core/src/lib.rs

@@ -104,6 +104,7 @@
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_char_convert)]
+#![feature(const_clone)]
 #![feature(const_discriminant)]
 #![feature(const_eval_select)]
 #![feature(const_float_bits_conv)]

library/core/src/option.rs

@@ -1857,7 +1857,11 @@ const fn expect_failed(msg: &str) -> ! {
 /////////////////////////////////////////////////////////////////////////////
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Clone> Clone for Option<T> {
+#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+impl<T> const Clone for Option<T>
+where
+    T: ~const Clone + ~const Drop,
+{
     #[inline]
     fn clone(&self) -> Self {
         match self {

library/core/src/ptr/non_null.rs

@@ -642,7 +642,8 @@ impl<T> NonNull<[T]> {
 }
 
 #[stable(feature = "nonnull", since = "1.25.0")]
-impl<T: ?Sized> Clone for NonNull<T> {
+#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+impl<T: ?Sized> const Clone for NonNull<T> {
     #[inline]
     fn clone(&self) -> Self {
         *self

library/core/src/ptr/unique.rs

@@ -146,7 +146,8 @@ impl<T: ?Sized> Unique<T> {
 }
 
 #[unstable(feature = "ptr_internals", issue = "none")]
-impl<T: ?Sized> Clone for Unique<T> {
+#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+impl<T: ?Sized> const Clone for Unique<T> {
     #[inline]
     fn clone(&self) -> Self {
         *self

library/core/src/result.rs

@@ -1801,7 +1801,12 @@ fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
 /////////////////////////////////////////////////////////////////////////////
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Clone, E: Clone> Clone for Result<T, E> {
+#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+impl<T, E> const Clone for Result<T, E>
+where
+    T: ~const Clone + ~const Drop,
+    E: ~const Clone + ~const Drop,
+{
     #[inline]
     fn clone(&self) -> Self {
         match self {