Rollup merge of rust-lang#143717 - Jules-Bertholet:pin-default, r=dtolnay

Add `Default` impls for `Pin`ned `Box`, `Rc`, `Arc`

Fixes rust-lang#143688.

`@rustbot` label T-libs-api needs-fcp

Also needs a crater run, as the `Box` impls could theoretically be breaking due to `#[fundamental]` (though a [cursory search](https://github.com/search?q=%2Fimpl%28%3C.*%3E%29%3F+Default+for+Pin%3C%2F+path%3A*.rs&type=code) suggests this is unlikely to cause issues).

Author: jhpratt

alloc/src/boxed.rs

@@ -1672,7 +1672,7 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Box<T> {
-    /// Creates a `Box<T>`, with the `Default` value for T.
+    /// Creates a `Box<T>`, with the `Default` value for `T`.
     #[inline]
     fn default() -> Self {
         let mut x: Box<mem::MaybeUninit<T>> = Box::new_uninit();
@@ -1695,6 +1695,7 @@ impl<T: Default> Default for Box<T> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Box<[T]> {
+    /// Creates an empty `[T]` inside a `Box`.
     #[inline]
     fn default() -> Self {
         let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
@@ -1716,6 +1717,19 @@ impl Default for Box<str> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Pin<Box<T>>
+where
+    T: ?Sized,
+    Box<T>: Default,
+{
+    #[inline]
+    fn default() -> Self {
+        Box::into_pin(Box::<T>::default())
+    }
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {

alloc/src/rc.rs

@@ -2357,7 +2357,7 @@ impl<T: Default> Default for Rc<T> {
     /// assert_eq!(*x, 0);
     /// ```
     #[inline]
-    fn default() -> Rc<T> {
+    fn default() -> Self {
         unsafe {
             Self::from_inner(
                 Box::leak(Box::write(
@@ -2373,7 +2373,7 @@ impl<T: Default> Default for Rc<T> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "more_rc_default_impls", since = "1.80.0")]
 impl Default for Rc<str> {
-    /// Creates an empty str inside an Rc
+    /// Creates an empty `str` inside an `Rc`.
     ///
     /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
@@ -2387,7 +2387,7 @@ impl Default for Rc<str> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "more_rc_default_impls", since = "1.80.0")]
 impl<T> Default for Rc<[T]> {
-    /// Creates an empty `[T]` inside an Rc
+    /// Creates an empty `[T]` inside an `Rc`.
     ///
     /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
@@ -2397,6 +2397,19 @@ impl<T> Default for Rc<[T]> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Pin<Rc<T>>
+where
+    T: ?Sized,
+    Rc<T>: Default,
+{
+    #[inline]
+    fn default() -> Self {
+        unsafe { Pin::new_unchecked(Rc::<T>::default()) }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 trait RcEqIdent<T: ?Sized + PartialEq, A: Allocator> {
     fn eq(&self, other: &Rc<T, A>) -> bool;

alloc/src/sync.rs

@@ -3654,6 +3654,19 @@ impl<T> Default for Arc<[T]> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Pin<Arc<T>>
+where
+    T: ?Sized,
+    Arc<T>: Default,
+{
+    #[inline]
+    fn default() -> Self {
+        unsafe { Pin::new_unchecked(Arc::<T>::default()) }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A> {
     fn hash<H: Hasher>(&self, state: &mut H) {