Make <*const/mut T>::offset_from `const fn`

1 files changed, 17 insertions, 1 deletions

diff --git a/src/libcore/ptr/mod.rs b/src/libcore/ptr/mod.rs
index 13ccc9b252a..a7adaa6a509 100644
--- a/src/libcore/ptr/mod.rs
+++ b/src/libcore/ptr/mod.rs
@@ -1286,7 +1286,22 @@ impl<T: ?Sized> *const T {
     /// }
     /// ```
     #[unstable(feature = "ptr_offset_from", issue = "41079")]
+    #[cfg(not(bootstrap))]
+    #[rustc_const_unstable(feature = "const_ptr_offset_from")]
     #[inline]
+    pub const unsafe fn offset_from(self, origin: *const T) -> isize where T: Sized {
+        let pointee_size = mem::size_of::<T>();
+        let ok = 0 < pointee_size && pointee_size <= isize::max_value() as usize;
+        // assert that the pointee size is valid in a const eval compatible way
+        // FIXME: do this with a real assert at some point
+        [()][(!ok) as usize];
+        intrinsics::ptr_offset_from(self, origin)
+    }
+
+    #[unstable(feature = "ptr_offset_from", issue = "41079")]
+    #[inline]
+    #[cfg(bootstrap)]
+    /// bootstrap
     pub unsafe fn offset_from(self, origin: *const T) -> isize where T: Sized {
         let pointee_size = mem::size_of::<T>();
         assert!(0 < pointee_size && pointee_size <= isize::max_value() as usize);
@@ -2013,8 +2028,9 @@ impl<T: ?Sized> *mut T {
     /// }
     /// ```
     #[unstable(feature = "ptr_offset_from", issue = "41079")]
+    #[rustc_const_unstable(feature = "const_ptr_offset_from")]
     #[inline]
-    pub unsafe fn offset_from(self, origin: *const T) -> isize where T: Sized {
+    pub const unsafe fn offset_from(self, origin: *const T) -> isize where T: Sized {
         (self as *const T).offset_from(origin)
     }