Remove T: Sized on ptr::is_null(), as_ref(), as_mut()

`NonZero::is_zero()` was already casting all pointers to thin `*mut u8`
to check for null.  It seems reasonable to apply that for `is_null()` in
general, and then unsized fat pointers can also be used with `as_ref()`
and `as_mut()` to get fat references.

src/libcore/nonzero.rs

@@ -28,8 +28,7 @@ macro_rules! impl_zeroable_for_pointer_types {
             unsafe impl<T: ?Sized> Zeroable for $Ptr {
                 #[inline]
                 fn is_zero(&self) -> bool {
-                    // Cast because `is_null` is only available on thin pointers
-                    (*self as *mut u8).is_null()
+                    (*self).is_null()
                 }
             }
         )+

src/libcore/ptr.rs

@@ -487,8 +487,10 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn is_null(self) -> bool where T: Sized {
-        self == null()
+    pub fn is_null(self) -> bool {
+        // Compare via a cast to a thin pointer, so fat pointers are only
+        // considering their "data" part for null-ness.
+        (self as *const u8) == null()
     }
 
     /// Returns `None` if the pointer is null, or else returns a reference to
@@ -519,7 +521,7 @@ 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 {
+    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
         if self.is_null() {
             None
         } else {
@@ -1118,8 +1120,10 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn is_null(self) -> bool where T: Sized {
-        self == null_mut()
+    pub fn is_null(self) -> bool {
+        // Compare via a cast to a thin pointer, so fat pointers are only
+        // considering their "data" part for null-ness.
+        (self as *mut u8) == null_mut()
     }
 
     /// Returns `None` if the pointer is null, or else returns a reference to
@@ -1150,7 +1154,7 @@ 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 {
+    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
         if self.is_null() {
             None
         } else {
@@ -1274,7 +1278,7 @@ 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 {
+    pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T> {
         if self.is_null() {
             None
         } else {

src/libcore/tests/ptr.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 use core::ptr::*;
+use core::slice;
 use core::cell::RefCell;
 
 #[test]
@@ -62,6 +63,28 @@ fn test_is_null() {
 
     let mq = unsafe { mp.offset(1) };
     assert!(!mq.is_null());
+
+    // Pointers to unsized types
+    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());
+
+    unsafe {
+        let ncs: *const [u8] = slice::from_raw_parts(null(), 0);
+        assert!(ncs.is_null());
+
+        let nms: *mut [u8] = slice::from_raw_parts_mut(null_mut(), 0);
+        assert!(nms.is_null());
+    }
 }
 
 #[test]
@@ -85,6 +108,26 @@ fn test_as_ref() {
             let p = &u as *const isize;
             assert_eq!(p.as_ref().unwrap(), &2);
         }
+
+        // Pointers to unsized types
+        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] = slice::from_raw_parts(null(), 0);
+        assert_eq!(ncs.as_ref(), None);
+
+        let nms: *mut [u8] = slice::from_raw_parts_mut(null_mut(), 0);
+        assert_eq!(nms.as_ref(), None);
     }
 }
 
@@ -103,6 +146,17 @@ fn test_as_mut() {
             let p = &mut u as *mut isize;
             assert!(p.as_mut().unwrap() == &mut 2);
         }
+
+        // Pointers to unsized types
+        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] = slice::from_raw_parts_mut(null_mut(), 0);
+        assert_eq!(nms.as_mut(), None);
     }
 }