Make Unique::as_ptr, NonNull::dangling and NonNull::cast const

Make `Unique::as_ptr` const without feature attribute as it's unstable
Make `NonNull::dangling` and `NonNull::cast` const with `feature = "const_ptr_nonnull"`

src/libcore/ptr.rs

@@ -2790,7 +2790,7 @@ impl<T: ?Sized> Unique<T> {
     }
 
     /// Acquires the underlying `*mut` pointer.
-    pub fn as_ptr(self) -> *mut T {
+    pub const fn as_ptr(self) -> *mut T {
         self.pointer as *mut T
     }
 
@@ -2903,7 +2903,8 @@ impl<T: Sized> NonNull<T> {
     /// some other means.
     #[stable(feature = "nonnull", since = "1.25.0")]
     #[inline]
-    pub fn dangling() -> Self {
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_ptr_nonnull"))]
+    pub const fn dangling() -> Self {
         unsafe {
             let ptr = mem::align_of::<T>() as *mut T;
             NonNull::new_unchecked(ptr)
@@ -2966,7 +2967,8 @@ impl<T: ?Sized> NonNull<T> {
     /// Cast to a pointer of another type
     #[stable(feature = "nonnull_cast", since = "1.27.0")]
     #[inline]
-    pub fn cast<U>(self) -> NonNull<U> {
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_ptr_nonnull"))]
+    pub const fn cast<U>(self) -> NonNull<U> {
         unsafe {
             NonNull::new_unchecked(self.as_ptr() as *mut U)
         }

src/test/run-pass/consts/const-ptr-nonnull.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(const_ptr_nonnull)]
+
+use std::ptr::NonNull;
+
+const DANGLING: NonNull<u32> = NonNull::dangling();
+const CASTED: NonNull<u32> = NonNull::cast(NonNull::<i32>::dangling());
+
+fn ident<T>(ident: T) -> T {
+    ident
+}
+
+pub fn main() {
+    assert_eq!(DANGLING, ident(NonNull::dangling()));
+    assert_eq!(CASTED, ident(NonNull::dangling()));
+}

src/test/run-pass/consts/const-ptr-unique.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(ptr_internals)]
+
+use std::ptr::Unique;
+
+const PTR: *mut u32 = Unique::empty().as_ptr();
+
+fn ident<T>(ident: T) -> T {
+    ident
+}
+
+pub fn main() {
+    assert_eq!(PTR, ident(Unique::<u32>::empty().as_ptr()));
+}

src/test/ui/consts/const-ptr-nonnull.rs

@@ -0,0 +1,11 @@
+use std::ptr::NonNull;
+
+fn main() {
+    let x: &'static NonNull<u32> = &(NonNull::dangling());
+    //~^ ERROR borrowed value does not live long enough
+
+    let mut i: i32 = 10;
+    let non_null = NonNull::new(&mut i).unwrap();
+    let x: &'static NonNull<u32> = &(non_null.cast());
+    //~^ ERROR borrowed value does not live long enough
+}

src/test/ui/consts/const-ptr-nonnull.stderr

@@ -0,0 +1,25 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/const-ptr-nonnull.rs:4:37
+   |
+LL |     let x: &'static NonNull<u32> = &(NonNull::dangling());
+   |                                     ^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/const-ptr-nonnull.rs:9:37
+   |
+LL |     let x: &'static NonNull<u32> = &(non_null.cast());
+   |                                     ^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+LL |     //~^ ERROR borrowed value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/consts/const-ptr-unique.rs

@@ -0,0 +1,10 @@
+#![feature(ptr_internals)]
+
+use std::ptr::Unique;
+
+fn main() {
+    let mut i: u32 = 10;
+    let unique = Unique::new(&mut i).unwrap();
+    let x: &'static *mut u32 = &(unique.as_ptr());
+    //~^ ERROR borrowed value does not live long enough
+}

src/test/ui/consts/const-ptr-unique.stderr

@@ -0,0 +1,14 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/const-ptr-unique.rs:8:33
+   |
+LL |     let x: &'static *mut u32 = &(unique.as_ptr());
+   |                                 ^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+LL |     //~^ ERROR borrowed value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.