Implement Default for raw pointers

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

@@ -984,3 +984,20 @@ fn test_ptr_metadata_in_const() {
     assert_eq!(SLICE_META, 3);
     assert_eq!(DYN_META.size_of(), 42);
 }
+
+#[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());
+}