Rollup merge of #71500 - josephlr:offset, r=oli-obk,RalfJung

Make pointer offset methods/intrinsics const

Implements #71499 using [the implementations from miri](https://github.com/rust-lang/miri/blob/52f5d202bdcfe8986f0615845f8d1647ab8a2c6a/src/shims/intrinsics.rs#L96-L112).

I added some tests what's allowed and what's UB. Let me know if any other cases should be added.

CC: @RalfJung @oli-obk

4 files changed, 27 insertions, 12 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 9006e4cfaf7..2d97fecf8a7 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -1314,6 +1314,7 @@ extern "rust-intrinsic" {
     /// The stabilized version of this intrinsic is
     /// [`std::pointer::offset`](../../std/primitive.pointer.html#method.offset).
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     pub fn offset<T>(dst: *const T, offset: isize) -> *const T;
 
     /// Calculates the offset from a pointer, potentially wrapping.
@@ -1331,6 +1332,7 @@ extern "rust-intrinsic" {
     /// The stabilized version of this intrinsic is
     /// [`std::pointer::wrapping_offset`](../../std/primitive.pointer.html#method.wrapping_offset).
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     pub fn arith_offset<T>(dst: *const T, offset: isize) -> *const T;
 
     /// Equivalent to the appropriate `llvm.memcpy.p0i8.0i8.*` intrinsic, with
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index ca13433caec..7d21f9a9a66 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -85,6 +85,7 @@
 #![feature(const_panic)]
 #![feature(const_fn_union)]
 #![feature(const_generics)]
+#![feature(const_ptr_offset)]
 #![feature(const_ptr_offset_from)]
 #![feature(const_result)]
 #![feature(const_slice_from_raw_parts)]
diff --git a/src/libcore/ptr/const_ptr.rs b/src/libcore/ptr/const_ptr.rs
index 85ba5fc0638..835183d171a 100644
--- a/src/libcore/ptr/const_ptr.rs
+++ b/src/libcore/ptr/const_ptr.rs
@@ -151,8 +151,9 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub unsafe fn offset(self, count: isize) -> *const T
+    pub const unsafe fn offset(self, count: isize) -> *const T
     where
         T: Sized,
     {
@@ -210,8 +211,9 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub fn wrapping_offset(self, count: isize) -> *const T
+    pub const fn wrapping_offset(self, count: isize) -> *const T
     where
         T: Sized,
     {
@@ -393,8 +395,9 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub unsafe fn add(self, count: usize) -> Self
+    pub const unsafe fn add(self, count: usize) -> Self
     where
         T: Sized,
     {
@@ -455,8 +458,9 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub unsafe fn sub(self, count: usize) -> Self
+    pub const unsafe fn sub(self, count: usize) -> Self
     where
         T: Sized,
     {
@@ -511,8 +515,9 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub fn wrapping_add(self, count: usize) -> Self
+    pub const fn wrapping_add(self, count: usize) -> Self
     where
         T: Sized,
     {
@@ -567,8 +572,9 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub fn wrapping_sub(self, count: usize) -> Self
+    pub const fn wrapping_sub(self, count: usize) -> Self
     where
         T: Sized,
     {
diff --git a/src/libcore/ptr/mut_ptr.rs b/src/libcore/ptr/mut_ptr.rs
index 0781d7e6cac..40b5e4e2234 100644
--- a/src/libcore/ptr/mut_ptr.rs
+++ b/src/libcore/ptr/mut_ptr.rs
@@ -145,8 +145,9 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub unsafe fn offset(self, count: isize) -> *mut T
+    pub const unsafe fn offset(self, count: isize) -> *mut T
     where
         T: Sized,
     {
@@ -203,8 +204,9 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub fn wrapping_offset(self, count: isize) -> *mut T
+    pub const fn wrapping_offset(self, count: isize) -> *mut T
     where
         T: Sized,
     {
@@ -439,8 +441,9 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub unsafe fn add(self, count: usize) -> Self
+    pub const unsafe fn add(self, count: usize) -> Self
     where
         T: Sized,
     {
@@ -501,8 +504,9 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub unsafe fn sub(self, count: usize) -> Self
+    pub const unsafe fn sub(self, count: usize) -> Self
     where
         T: Sized,
     {
@@ -557,8 +561,9 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub fn wrapping_add(self, count: usize) -> Self
+    pub const fn wrapping_add(self, count: usize) -> Self
     where
         T: Sized,
     {
@@ -613,8 +618,9 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
+    #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
     #[inline]
-    pub fn wrapping_sub(self, count: usize) -> Self
+    pub const fn wrapping_sub(self, count: usize) -> Self
     where
         T: Sized,
     {