Rollup merge of rust-lang#140685 - viliml:patch-1, r=Mark-Simulacrum

Simplify `Vec::as_non_null` implementation and make it `const`

Tracking issue: rust-lang#130364.

Author: compiler-errors

library/alloc/src/raw_vec/mod.rs

@@ -287,7 +287,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     }
 
     #[inline]
-    pub(crate) fn non_null(&self) -> NonNull<T> {
+    pub(crate) const fn non_null(&self) -> NonNull<T> {
         self.inner.non_null()
     }
 

library/alloc/src/vec/mod.rs

@@ -1816,10 +1816,10 @@ impl<T, A: Allocator> Vec<T, A> {
     /// [`as_ptr`]: Vec::as_ptr
     /// [`as_non_null`]: Vec::as_non_null
     #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
+    #[rustc_const_unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
     #[inline]
-    pub fn as_non_null(&mut self) -> NonNull<T> {
-        // SAFETY: A `Vec` always has a non-null pointer.
-        unsafe { NonNull::new_unchecked(self.as_mut_ptr()) }
+    pub const fn as_non_null(&mut self) -> NonNull<T> {
+        self.buf.non_null()
     }
 
     /// Returns a reference to the underlying allocator.