Rollup merge of #122492 - GrigorenkoPV:ptr_as_ref_unchecked, r=workingjubilee

Implement ptr_as_ref_unchecked

Implementation of #122034.

Prefixed the feature name with `ptr_` for clarity.

Linked const-unstability to #91822, so the post there should probably be updated to mentions the 3 new methods when/if this PR is merged.

Author: matthiaskrgr

library/core/src/ptr/const_ptr.rs

@@ -358,6 +358,54 @@ impl<T: ?Sized> *const T {
         if self.is_null() { None } else { unsafe { Some(&*self) } }
     }
 
+    /// Returns a shared reference to the value behind the pointer.
+    /// If the pointer may be null or the value may be uninitialized, [`as_uninit_ref`] must be used instead.
+    /// If the pointer may be null, but the value is known to have been initialized, [`as_ref`] must be used instead.
+    ///
+    /// [`as_ref`]: #method.as_ref
+    /// [`as_uninit_ref`]: #method.as_uninit_ref
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferenceable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, while this reference exists, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    /// (The part about being initialized is not yet fully decided, but until
+    /// it is, the only safe approach is to ensure that they are indeed initialized.)
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ptr_as_ref_unchecked)]
+    /// let ptr: *const u8 = &10u8 as *const u8;
+    ///
+    /// unsafe {
+    ///     println!("We got back the value: {}!", ptr.as_ref_unchecked());
+    /// }
+    /// ```
+    // FIXME: mention it in the docs for `as_ref` and `as_uninit_ref` once stabilized.
+    #[unstable(feature = "ptr_as_ref_unchecked", issue = "122034")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    #[inline]
+    #[must_use]
+    pub const unsafe fn as_ref_unchecked<'a>(self) -> &'a T {
+        // SAFETY: the caller must guarantee that `self` is valid for a reference
+        unsafe { &*self }
+    }
+
     /// Returns `None` if the pointer is null, or else returns a shared reference to
     /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
     /// that the value has to be initialized.

library/core/src/ptr/mut_ptr.rs

@@ -367,6 +367,57 @@ impl<T: ?Sized> *mut T {
         if self.is_null() { None } else { unsafe { Some(&*self) } }
     }
 
+    /// Returns a shared reference to the value behind the pointer.
+    /// If the pointer may be null or the value may be uninitialized, [`as_uninit_ref`] must be used instead.
+    /// If the pointer may be null, but the value is known to have been initialized, [`as_ref`] must be used instead.
+    ///
+    /// For the mutable counterpart see [`as_mut_unchecked`].
+    ///
+    /// [`as_ref`]: #method.as_ref
+    /// [`as_uninit_ref`]: #method.as_uninit_ref
+    /// [`as_mut_unchecked`]: #method.as_mut_unchecked
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferenceable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, while this reference exists, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    /// (The part about being initialized is not yet fully decided, but until
+    /// it is, the only safe approach is to ensure that they are indeed initialized.)
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ptr_as_ref_unchecked)]
+    /// let ptr: *mut u8 = &mut 10u8 as *mut u8;
+    ///
+    /// unsafe {
+    ///     println!("We got back the value: {}!", ptr.as_ref_unchecked());
+    /// }
+    /// ```
+    // FIXME: mention it in the docs for `as_ref` and `as_uninit_ref` once stabilized.
+    #[unstable(feature = "ptr_as_ref_unchecked", issue = "122034")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    #[inline]
+    #[must_use]
+    pub const unsafe fn as_ref_unchecked<'a>(self) -> &'a T {
+        // SAFETY: the caller must guarantee that `self` is valid for a reference
+        unsafe { &*self }
+    }
+
     /// Returns `None` if the pointer is null, or else returns a shared reference to
     /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
     /// that the value has to be initialized.
@@ -688,6 +739,58 @@ impl<T: ?Sized> *mut T {
         if self.is_null() { None } else { unsafe { Some(&mut *self) } }
     }
 
+    /// Returns a unique reference to the value behind the pointer.
+    /// If the pointer may be null or the value may be uninitialized, [`as_uninit_mut`] must be used instead.
+    /// If the pointer may be null, but the value is known to have been initialized, [`as_mut`] must be used instead.
+    ///
+    /// For the shared counterpart see [`as_ref_unchecked`].
+    ///
+    /// [`as_mut`]: #method.as_mut
+    /// [`as_uninit_mut`]: #method.as_uninit_mut
+    /// [`as_ref_unchecked`]: #method.as_mut_unchecked
+    ///
+    /// # Safety
+    ///
+    /// When calling this method, you have to ensure that all of the following is true:
+    ///
+    /// * The pointer must be properly aligned.
+    ///
+    /// * It must be "dereferenceable" in the sense defined in [the module documentation].
+    ///
+    /// * The pointer must point to an initialized instance of `T`.
+    ///
+    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
+    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
+    ///   In particular, while this reference exists, the memory the pointer points to must
+    ///   not get mutated (except inside `UnsafeCell`).
+    ///
+    /// This applies even if the result of this method is unused!
+    /// (The part about being initialized is not yet fully decided, but until
+    /// it is, the only safe approach is to ensure that they are indeed initialized.)
+    ///
+    /// [the module documentation]: crate::ptr#safety
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ptr_as_ref_unchecked)]
+    /// let mut s = [1, 2, 3];
+    /// let ptr: *mut u32 = s.as_mut_ptr();
+    /// let first_value = unsafe { ptr.as_mut_unchecked() };
+    /// *first_value = 4;
+    /// # assert_eq!(s, [4, 2, 3]);
+    /// println!("{s:?}"); // It'll print: "[4, 2, 3]".
+    /// ```
+    // FIXME: mention it in the docs for `as_mut` and `as_uninit_mut` once stabilized.
+    #[unstable(feature = "ptr_as_ref_unchecked", issue = "122034")]
+    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    #[inline]
+    #[must_use]
+    pub const unsafe fn as_mut_unchecked<'a>(self) -> &'a mut T {
+        // SAFETY: the caller must guarantee that `self` is valid for a reference
+        unsafe { &mut *self }
+    }
+
     /// Returns `None` if the pointer is null, or else returns a unique reference to
     /// the value wrapped in `Some`. In contrast to [`as_mut`], this does not require
     /// that the value has to be initialized.