Rollup merge of rust-lang#139535 - ChrisDenton:default-ptr, r=tgross35

Implement `Default` for raw pointers

ACP: rust-lang/libs-team#571

This is instantly stable so we will need an FCP here.

Closes rust-lang/rfcs#2464

Author: ChrisDenton

Diff for: library/core/src/ptr/const_ptr.rs

@@ -1739,3 +1739,11 @@ impl<T: ?Sized> PartialOrd for *const T {
         *self >= *other
     }
 }
+
+#[stable(feature = "raw_ptr_default", since = "CURRENT_RUSTC_VERSION")]
+impl<T: ?Sized + Thin> Default for *const T {
+    /// Returns the default value of [`null()`][crate::ptr::null].
+    fn default() -> Self {
+        crate::ptr::null()
+    }
+}

Diff for: library/core/src/ptr/mut_ptr.rs

@@ -2156,3 +2156,11 @@ impl<T: ?Sized> PartialOrd for *mut T {
         *self >= *other
     }
 }
+
+#[stable(feature = "raw_ptr_default", since = "CURRENT_RUSTC_VERSION")]
+impl<T: ?Sized + Thin> Default for *mut T {
+    /// Returns the default value of [`null_mut()`][crate::ptr::null_mut].
+    fn default() -> Self {
+        crate::ptr::null_mut()
+    }
+}

Diff for: library/coretests/tests/ptr.rs

@@ -1020,3 +1020,20 @@ fn test_ptr_swap_nonoverlapping_is_untyped() {
     ptr_swap_nonoverlapping_is_untyped_inner();
     const { ptr_swap_nonoverlapping_is_untyped_inner() };
 }
+
+#[test]
+fn test_ptr_default() {
+    #[derive(Default)]
+    struct PtrDefaultTest {
+        ptr: *const u64,
+    }
+    let default = PtrDefaultTest::default();
+    assert!(default.ptr.is_null());
+
+    #[derive(Default)]
+    struct PtrMutDefaultTest {
+        ptr: *mut u64,
+    }
+    let default = PtrMutDefaultTest::default();
+    assert!(default.ptr.is_null());
+}