vec: avoid ptrtoint/inttoptr in the iterators

This results in throwing away alias analysis information, because LLVM
does *not* implement reasoning about these conversions yet.

We specialize zero-size types since a `getelementptr` offset will
return us the same pointer, making it broken as a simple counter.

Author: thestinger

src/libstd/vec.rs

@@ -849,10 +849,15 @@ impl<'self,T> ImmutableVector<'self, T> for &'self [T] {
     fn iter(self) -> VecIterator<'self, T> {
         unsafe {
             let p = vec::raw::to_ptr(self);
-            VecIterator{ptr: p,
-                        end: (p as uint + self.len() *
-                              sys::nonzero_size_of::<T>()) as *T,
-                        lifetime: cast::transmute(p)}
+            if sys::size_of::<T>() == 0 {
+                VecIterator{ptr: p,
+                            end: (p as uint + self.len()) as *T,
+                            lifetime: cast::transmute(p)}
+            } else {
+                VecIterator{ptr: p,
+                            end: p.offset(self.len() as int),
+                            lifetime: cast::transmute(p)}
+            }
         }
     }
 
@@ -1826,10 +1831,15 @@ impl<'self,T> MutableVector<'self, T> for &'self mut [T] {
     fn mut_iter(self) -> VecMutIterator<'self, T> {
         unsafe {
             let p = vec::raw::to_mut_ptr(self);
-            VecMutIterator{ptr: p,
-                           end: (p as uint + self.len() *
-                                 sys::nonzero_size_of::<T>()) as *mut T,
-                           lifetime: cast::transmute(p)}
+            if sys::size_of::<T>() == 0 {
+                VecMutIterator{ptr: p,
+                               end: (p as uint + self.len()) as *mut T,
+                               lifetime: cast::transmute(p)}
+            } else {
+                VecMutIterator{ptr: p,
+                               end: p.offset(self.len() as int),
+                               lifetime: cast::transmute(p)}
+            }
         }
     }