Auto merge of #46094 - dtolnay:is_null, r=alexcrichton

Remove `T: Sized` on `ptr::is_null()`

Originally from #44932 -- this is purely a revert of the last commit of that PR, which was removing some changes from the previous commits in the PR. So a revert of a revert means this is code written by @cuviper!

@mikeyhew makes a compelling case in https://github.com/rust-lang/rfcs/issues/433#issuecomment-345495505 for why this is the right way to implement `is_null` for trait objects. And the behavior for slices makes sense to me as well.

```diff
  impl<T: ?Sized> *const T {
-     pub fn is_null(self) -> bool where T: Sized;
+     pub fn is_null(self) -> bool;
  }

  impl<T: ?Sized> *mut T {
-     pub fn is_null(self) -> bool where T: Sized;
+     pub fn is_null(self) -> bool;
  }

1 files changed, 33 insertions, 0 deletions

diff --git a/src/libcore/tests/ptr.rs b/src/libcore/tests/ptr.rs
index e93e9be0cd5..98436f0e1d1 100644
--- a/src/libcore/tests/ptr.rs
+++ b/src/libcore/tests/ptr.rs
@@ -62,6 +62,39 @@ fn test_is_null() {
 
     let mq = unsafe { mp.offset(1) };
     assert!(!mq.is_null());
+
+    // Pointers to unsized types -- slices
+    let s: &mut [u8] = &mut [1, 2, 3];
+    let cs: *const [u8] = s;
+    assert!(!cs.is_null());
+
+    let ms: *mut [u8] = s;
+    assert!(!ms.is_null());
+
+    let cz: *const [u8] = &[];
+    assert!(!cz.is_null());
+
+    let mz: *mut [u8] = &mut [];
+    assert!(!mz.is_null());
+
+    let ncs: *const [u8] = null::<[u8; 3]>();
+    assert!(ncs.is_null());
+
+    let nms: *mut [u8] = null_mut::<[u8; 3]>();
+    assert!(nms.is_null());
+
+    // Pointers to unsized types -- trait objects
+    let ci: *const ToString = &3;
+    assert!(!ci.is_null());
+
+    let mi: *mut ToString = &mut 3;
+    assert!(!mi.is_null());
+
+    let nci: *const ToString = null::<isize>();
+    assert!(nci.is_null());
+
+    let nmi: *mut ToString = null_mut::<isize>();
+    assert!(nmi.is_null());
 }
 
 #[test]