Rollup merge of rust-lang#73986 - RalfJung:raw-slice-as-ptr, r=sfackler

add (unchecked) indexing methods to raw (and NonNull) slices

This complements the existing (unstable) `len` method. Unfortunately, for non-null slices, we cannot call this method `as_ptr` as that overlaps with the existing method of the same name.

If this looks reasonable to accept, I propose to reuse the rust-lang#71146 tracking issue and rename the feature get to `slice_ptr_methods` or so.

Cc @SimonSapin
Fixes rust-lang#60639

src/libcore/lib.rs

@@ -149,6 +149,7 @@
 #![feature(associated_type_bounds)]
 #![feature(const_type_id)]
 #![feature(const_caller_location)]
+#![feature(slice_ptr_get)]
 #![feature(no_niche)] // rust-lang/rust#68303
 #![feature(unsafe_block_in_unsafe_fn)]
 #![deny(unsafe_op_in_unsafe_fn)]

src/libcore/ptr/const_ptr.rs

@@ -2,6 +2,7 @@ use super::*;
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics;
 use crate::mem;
+use crate::slice::SliceIndex;
 
 #[lang = "const_ptr"]
 impl<T: ?Sized> *const T {
@@ -826,6 +827,55 @@ impl<T> *const [T] {
         // Only `std` can make this guarantee.
         unsafe { Repr { rust: self }.raw }.len
     }
+
+    /// Returns a raw pointer to the slice's buffer.
+    ///
+    /// This is equivalent to casting `self` to `*const T`, but more type-safe.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(slice_ptr_get)]
+    /// use std::ptr;
+    ///
+    /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
+    /// assert_eq!(slice.as_ptr(), 0 as *const i8);
+    /// ```
+    #[inline]
+    #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
+    pub const fn as_ptr(self) -> *const T {
+        self as *const T
+    }
+
+    /// Returns a raw pointer to an element or subslice, without doing bounds
+    /// checking.
+    ///
+    /// Calling this method with an out-of-bounds index or when `self` is not dereferencable
+    /// is *[undefined behavior]* even if the resulting pointer is not used.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_ptr_get)]
+    ///
+    /// let x = &[1, 2, 4] as *const [i32];
+    ///
+    /// unsafe {
+    ///     assert_eq!(x.get_unchecked(1), x.as_ptr().add(1));
+    /// }
+    /// ```
+    #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[inline]
+    pub unsafe fn get_unchecked<I>(self, index: I) -> *const I::Output
+    where
+        I: SliceIndex<[T]>,
+    {
+        // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
+        unsafe { index.get_unchecked(self) }
+    }
 }
 
 // Equality for pointers

src/libcore/ptr/mut_ptr.rs

@@ -1,6 +1,7 @@
 use super::*;
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics;
+use crate::slice::SliceIndex;
 
 #[lang = "mut_ptr"]
 impl<T: ?Sized> *mut T {
@@ -1014,7 +1015,6 @@ impl<T> *mut [T] {
     ///
     /// ```rust
     /// #![feature(slice_ptr_len)]
-    ///
     /// use std::ptr;
     ///
     /// let slice: *mut [i8] = ptr::slice_from_raw_parts_mut(ptr::null_mut(), 3);
@@ -1028,6 +1028,55 @@ impl<T> *mut [T] {
         // Only `std` can make this guarantee.
         unsafe { Repr { rust_mut: self }.raw }.len
     }
+
+    /// Returns a raw pointer to the slice's buffer.
+    ///
+    /// This is equivalent to casting `self` to `*mut T`, but more type-safe.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(slice_ptr_get)]
+    /// use std::ptr;
+    ///
+    /// let slice: *mut [i8] = ptr::slice_from_raw_parts_mut(ptr::null_mut(), 3);
+    /// assert_eq!(slice.as_mut_ptr(), 0 as *mut i8);
+    /// ```
+    #[inline]
+    #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
+    pub const fn as_mut_ptr(self) -> *mut T {
+        self as *mut T
+    }
+
+    /// Returns a raw pointer to an element or subslice, without doing bounds
+    /// checking.
+    ///
+    /// Calling this method with an out-of-bounds index or when `self` is not dereferencable
+    /// is *[undefined behavior]* even if the resulting pointer is not used.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_ptr_get)]
+    ///
+    /// let x = &mut [1, 2, 4] as *mut [i32];
+    ///
+    /// unsafe {
+    ///     assert_eq!(x.get_unchecked_mut(1), x.as_mut_ptr().add(1));
+    /// }
+    /// ```
+    #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[inline]
+    pub unsafe fn get_unchecked_mut<I>(self, index: I) -> *mut I::Output
+    where
+        I: SliceIndex<[T]>,
+    {
+        // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
+        unsafe { index.get_unchecked_mut(self) }
+    }
 }
 
 // Equality for pointers

src/libcore/ptr/non_null.rs

@@ -6,6 +6,7 @@ use crate::marker::Unsize;
 use crate::mem;
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
 use crate::ptr::Unique;
+use crate::slice::SliceIndex;
 
 /// `*mut T` but non-zero and covariant.
 ///
@@ -192,7 +193,6 @@ impl<T> NonNull<[T]> {
     ///
     /// ```rust
     /// #![feature(slice_ptr_len, nonnull_slice_from_raw_parts)]
-    ///
     /// use std::ptr::NonNull;
     ///
     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
@@ -204,6 +204,57 @@ impl<T> NonNull<[T]> {
     pub const fn len(self) -> usize {
         self.as_ptr().len()
     }
+
+    /// Returns a non-null pointer to the slice's buffer.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
+    /// use std::ptr::NonNull;
+    ///
+    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
+    /// assert_eq!(slice.as_non_null_ptr(), NonNull::new(1 as *mut i8).unwrap());
+    /// ```
+    #[inline]
+    #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
+    pub const fn as_non_null_ptr(self) -> NonNull<T> {
+        // SAFETY: We know `self` is non-null.
+        unsafe { NonNull::new_unchecked(self.as_ptr().as_mut_ptr()) }
+    }
+
+    /// Returns a raw pointer to an element or subslice, without doing bounds
+    /// checking.
+    ///
+    /// Calling this method with an out-of-bounds index or when `self` is not dereferencable
+    /// is *[undefined behavior]* even if the resulting pointer is not used.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
+    /// use std::ptr::NonNull;
+    ///
+    /// let x = &mut [1, 2, 4];
+    /// let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());
+    ///
+    /// unsafe {
+    ///     assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
+    /// }
+    /// ```
+    #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[inline]
+    pub unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
+    where
+        I: SliceIndex<[T]>,
+    {
+        // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
+        // As a consequence, the resulting pointer cannot be NULL.
+        unsafe { NonNull::new_unchecked(self.as_ptr().get_unchecked_mut(index)) }
+    }
 }
 
 #[stable(feature = "nonnull", since = "1.25.0")]