Rollup merge of #91823 - woppopo:const_ptr_as_ref, r=lcnr

Make `PTR::as_ref` and similar methods `const`.

Tracking issue: #91822
Feature gate: `#![feature(const_ptr_as_ref)]`

```rust
// core::ptr
impl<T: ?Sized> *const T {
    pub const unsafe fn as_ref<'a>(self) -> Option<&'a T>;
    pub const unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
    where
        T: Sized;
    pub const unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]>;
}

impl<T: ?Sized> *mut T {
    pub const unsafe fn as_ref<'a>(self) -> Option<&'a T>;
    pub const unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
    where
        T: Sized;
    pub const unsafe fn as_mut<'a>(self) -> Option<&'a mut T>;
    pub const unsafe fn as_uninit_mut<'a>(self) -> Option<&'a mut MaybeUninit<T>>
    where
        T: Sized;
    pub const unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]>;
    pub const unsafe fn as_uninit_slice_mut<'a>(self) -> Option<&'a mut [MaybeUninit<T>]>;
}

impl<T: Sized> NonNull<T> {
    pub const unsafe fn as_uninit_ref<'a>(&self) -> &'a MaybeUninit<T>;
    pub const unsafe fn as_uninit_mut<'a>(&mut self) -> &'a mut MaybeUninit<T>;
}

impl<T: ?Sized> NonNull<T> {
    pub const unsafe fn as_ref<'a>(&self) -> &'a T;
    pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T;
    pub const unsafe fn as_uninit_slice<'a>(&self) -> &'a [MaybeUninit<T>];
    pub const unsafe fn as_uninit_slice_mut<'a>(&self) -> &'a mut [MaybeUninit<T>];
}
```

Author: matthiaskrgr

library/core/src/ptr/const_ptr.rs

@@ -163,8 +163,9 @@ impl<T: ?Sized> *const T {
     /// }
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
     #[inline]
-    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
+    pub const unsafe fn as_ref<'a>(self) -> Option<&'a T> {
         // SAFETY: the caller must guarantee that `self` is valid
         // for a reference if it isn't null.
         if self.is_null() { None } else { unsafe { Some(&*self) } }
@@ -211,7 +212,8 @@ impl<T: ?Sized> *const T {
     /// ```
     #[inline]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
     where
         T: Sized,
     {
@@ -1068,7 +1070,8 @@ impl<T> *const [T] {
     /// [allocated object]: crate::ptr#allocated-object
     #[inline]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
         if self.is_null() {
             None
         } else {

library/core/src/ptr/mut_ptr.rs

@@ -166,8 +166,9 @@ impl<T: ?Sized> *mut T {
     /// }
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
     #[inline]
-    pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
+    pub const unsafe fn as_ref<'a>(self) -> Option<&'a T> {
         // SAFETY: the caller must guarantee that `self` is valid for a
         // reference if it isn't null.
         if self.is_null() { None } else { unsafe { Some(&*self) } }
@@ -217,7 +218,8 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[inline]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
     where
         T: Sized,
     {
@@ -411,8 +413,9 @@ impl<T: ?Sized> *mut T {
     /// println!("{:?}", s); // It'll print: "[4, 2, 3]".
     /// ```
     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
     #[inline]
-    pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T> {
+    pub const unsafe fn as_mut<'a>(self) -> Option<&'a mut T> {
         // SAFETY: the caller must guarantee that `self` is be valid for
         // a mutable reference if it isn't null.
         if self.is_null() { None } else { unsafe { Some(&mut *self) } }
@@ -446,7 +449,8 @@ impl<T: ?Sized> *mut T {
     /// [the module documentation]: crate::ptr#safety
     #[inline]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_mut<'a>(self) -> Option<&'a mut MaybeUninit<T>>
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_mut<'a>(self) -> Option<&'a mut MaybeUninit<T>>
     where
         T: Sized,
     {
@@ -1335,7 +1339,8 @@ impl<T> *mut [T] {
     /// [allocated object]: crate::ptr#allocated-object
     #[inline]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
         if self.is_null() {
             None
         } else {
@@ -1386,7 +1391,8 @@ impl<T> *mut [T] {
     /// [allocated object]: crate::ptr#allocated-object
     #[inline]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_slice_mut<'a>(self) -> Option<&'a mut [MaybeUninit<T>]> {
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_slice_mut<'a>(self) -> Option<&'a mut [MaybeUninit<T>]> {
         if self.is_null() {
             None
         } else {

library/core/src/ptr/non_null.rs

@@ -122,7 +122,8 @@ impl<T: Sized> NonNull<T> {
     #[inline]
     #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_ref<'a>(&self) -> &'a MaybeUninit<T> {
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_ref<'a>(&self) -> &'a MaybeUninit<T> {
         // SAFETY: the caller must guarantee that `self` meets all the
         // requirements for a reference.
         unsafe { &*self.cast().as_ptr() }
@@ -155,7 +156,8 @@ impl<T: Sized> NonNull<T> {
     #[inline]
     #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_mut<'a>(&mut self) -> &'a mut MaybeUninit<T> {
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_mut<'a>(&mut self) -> &'a mut MaybeUninit<T> {
         // SAFETY: the caller must guarantee that `self` meets all the
         // requirements for a reference.
         unsafe { &mut *self.cast().as_ptr() }
@@ -316,9 +318,10 @@ impl<T: ?Sized> NonNull<T> {
     ///
     /// [the module documentation]: crate::ptr#safety
     #[stable(feature = "nonnull", since = "1.25.0")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
     #[must_use]
     #[inline]
-    pub unsafe fn as_ref<'a>(&self) -> &'a T {
+    pub const unsafe fn as_ref<'a>(&self) -> &'a T {
         // SAFETY: the caller must guarantee that `self` meets all the
         // requirements for a reference.
         unsafe { &*self.as_ptr() }
@@ -366,9 +369,10 @@ impl<T: ?Sized> NonNull<T> {
     ///
     /// [the module documentation]: crate::ptr#safety
     #[stable(feature = "nonnull", since = "1.25.0")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
     #[must_use]
     #[inline]
-    pub unsafe fn as_mut<'a>(&mut self) -> &'a mut T {
+    pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T {
         // SAFETY: the caller must guarantee that `self` meets all the
         // requirements for a mutable reference.
         unsafe { &mut *self.as_ptr() }
@@ -534,7 +538,8 @@ impl<T> NonNull<[T]> {
     #[inline]
     #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_slice<'a>(&self) -> &'a [MaybeUninit<T>] {
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_slice<'a>(&self) -> &'a [MaybeUninit<T>] {
         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
         unsafe { slice::from_raw_parts(self.cast().as_ptr(), self.len()) }
     }
@@ -596,7 +601,8 @@ impl<T> NonNull<[T]> {
     #[inline]
     #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
-    pub unsafe fn as_uninit_slice_mut<'a>(&self) -> &'a mut [MaybeUninit<T>] {
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    pub const unsafe fn as_uninit_slice_mut<'a>(&self) -> &'a mut [MaybeUninit<T>] {
         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.
         unsafe { slice::from_raw_parts_mut(self.cast().as_ptr(), self.len()) }
     }