Avoid latent (harmless) overflow in core::slice.

This overflow does not cause any problems; it just causes errors to be
signalled when compiling with `-C debug-assertions`.

Fix rust-lang#24997

Author: pnkfelix

src/libcore/slice.rs

@@ -632,7 +632,7 @@ fn size_from_ptr<T>(_: *const T) -> usize {
 
 
 // Use macro to be generic over const/mut
-macro_rules! slice_offset {
+macro_rules! slice_add_offset {
     ($ptr:expr, $by:expr) => {{
         let ptr = $ptr;
         if size_from_ptr(ptr) == 0 {
@@ -643,6 +643,17 @@ macro_rules! slice_offset {
     }};
 }
 
+macro_rules! slice_sub_offset {
+    ($ptr:expr, $by:expr) => {{
+        let ptr = $ptr;
+        if size_from_ptr(ptr) == 0 {
+            transmute(ptr as usize - $by)
+        } else {
+            ptr.offset(-$by)
+        }
+    }};
+}
+
 macro_rules! slice_ref {
     ($ptr:expr) => {{
         let ptr = $ptr;
@@ -672,7 +683,7 @@ macro_rules! iterator {
                         None
                     } else {
                         let old = self.ptr;
-                        self.ptr = slice_offset!(self.ptr, 1);
+                        self.ptr = slice_add_offset!(self.ptr, 1);
                         Some(slice_ref!(old))
                     }
                 }
@@ -714,7 +725,7 @@ macro_rules! iterator {
                     if self.end == self.ptr {
                         None
                     } else {
-                        self.end = slice_offset!(self.end, -1);
+                        self.end = slice_sub_offset!(self.end, 1);
                         Some(slice_ref!(self.end))
                     }
                 }
@@ -816,7 +827,7 @@ impl<'a, T> Iter<'a, T> {
     fn iter_nth(&mut self, n: usize) -> Option<&'a T> {
         match self.as_slice().get(n) {
             Some(elem_ref) => unsafe {
-                self.ptr = slice_offset!(elem_ref as *const _, 1);
+                self.ptr = slice_add_offset!(elem_ref as *const _, 1);
                 Some(slice_ref!(elem_ref))
             },
             None => {
@@ -959,7 +970,7 @@ impl<'a, T> IterMut<'a, T> {
     fn iter_nth(&mut self, n: usize) -> Option<&'a mut T> {
         match make_mut_slice!(T => &'a mut [T]: self.ptr, self.end).get_mut(n) {
             Some(elem_ref) => unsafe {
-                self.ptr = slice_offset!(elem_ref as *mut _, 1);
+                self.ptr = slice_add_offset!(elem_ref as *mut _, 1);
                 Some(slice_ref!(elem_ref))
             },
             None => {