Auto merge of #44932 - cuviper:unsized-ptr-is_null, r=alexcrichton

Remove `T: Sized` on pointer `as_ref()` and `as_mut()`

`NonZero::is_zero()` was already casting all pointers to thin `*mut u8` to check for null.  The same test on unsized fat pointers can also be used with `as_ref()` and `as_mut()` to get fat references.

(This PR formerly changed `is_null()` too, but checking just the data pointer is not obviously correct for trait objects, especially if `*const self` sorts of methods are ever allowed.)

src/libcore/ptr.rs

@@ -517,8 +517,10 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
     #[inline]
-    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> where T: Sized {
-        if self.is_null() {
+    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
+        // Check for null via a cast to a thin pointer, so fat pointers are only
+        // considering their "data" part for null-ness.
+        if (self as *const u8).is_null() {
             None
         } else {
             Some(&*self)
@@ -1148,8 +1150,10 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
     #[inline]
-    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> where T: Sized {
-        if self.is_null() {
+    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
+        // Check for null via a cast to a thin pointer, so fat pointers are only
+        // considering their "data" part for null-ness.
+        if (self as *const u8).is_null() {
             None
         } else {
             Some(&*self)
@@ -1272,8 +1276,10 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
     #[inline]
-    pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T> where T: Sized {
-        if self.is_null() {
+    pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T> {
+        // Check for null via a cast to a thin pointer, so fat pointers are only
+        // considering their "data" part for null-ness.
+        if (self as *mut u8).is_null() {
             None
         } else {
             Some(&mut *self)

src/libcore/tests/ptr.rs

@@ -85,6 +85,39 @@ fn test_as_ref() {
             let p = &u as *const isize;
             assert_eq!(p.as_ref().unwrap(), &2);
         }
+
+        // Pointers to unsized types -- slices
+        let s: &mut [u8] = &mut [1, 2, 3];
+        let cs: *const [u8] = s;
+        assert_eq!(cs.as_ref(), Some(&*s));
+
+        let ms: *mut [u8] = s;
+        assert_eq!(ms.as_ref(), Some(&*s));
+
+        let cz: *const [u8] = &[];
+        assert_eq!(cz.as_ref(), Some(&[][..]));
+
+        let mz: *mut [u8] = &mut [];
+        assert_eq!(mz.as_ref(), Some(&[][..]));
+
+        let ncs: *const [u8] = null::<[u8; 3]>();
+        assert_eq!(ncs.as_ref(), None);
+
+        let nms: *mut [u8] = null_mut::<[u8; 3]>();
+        assert_eq!(nms.as_ref(), None);
+
+        // Pointers to unsized types -- trait objects
+        let ci: *const ToString = &3;
+        assert!(ci.as_ref().is_some());
+
+        let mi: *mut ToString = &mut 3;
+        assert!(mi.as_ref().is_some());
+
+        let nci: *const ToString = null::<isize>();
+        assert!(nci.as_ref().is_none());
+
+        let nmi: *mut ToString = null_mut::<isize>();
+        assert!(nmi.as_ref().is_none());
     }
 }
 
@@ -103,6 +136,24 @@ fn test_as_mut() {
             let p = &mut u as *mut isize;
             assert!(p.as_mut().unwrap() == &mut 2);
         }
+
+        // Pointers to unsized types -- slices
+        let s: &mut [u8] = &mut [1, 2, 3];
+        let ms: *mut [u8] = s;
+        assert_eq!(ms.as_mut(), Some(s));
+
+        let mz: *mut [u8] = &mut [];
+        assert_eq!(mz.as_mut(), Some(&mut [][..]));
+
+        let nms: *mut [u8] = null_mut::<[u8; 3]>();
+        assert_eq!(nms.as_mut(), None);
+
+        // Pointers to unsized types -- trait objects
+        let mi: *mut ToString = &mut 3;
+        assert!(mi.as_mut().is_some());
+
+        let nmi: *mut ToString = null_mut::<isize>();
+        assert!(nmi.as_mut().is_none());
     }
 }