libcore: Fix Items iterator for zero sized types.

Author: luqmana

src/libcollections/vec.rs

@@ -75,12 +75,10 @@ impl<T> Vec<T> {
     /// ```
     #[inline]
     pub fn new() -> Vec<T> {
-        // If we have a 0-sized vector, then the base pointer should not be NULL
-        // because an iterator over the slice will attempt to yield the base
-        // pointer as the first element in the vector, but this will end up
-        // being Some(NULL) which is optimized to None. So instead we set ptr
-        // to some arbitrary non-null value which is fine since we never call
-        // deallocate on the ptr if cap is 0.
+        // We want ptr to never be NULL so instead we set it to some arbitrary
+        // non-null value which is fine since we never call deallocate on the ptr
+        // if cap is 0. The reason for this is because the pointer of a slice
+        // being NULL would break the null pointer optimization for enums.
         Vec { len: 0, cap: 0, ptr: &PTR_MARKER as *const _ as *mut T }
     }
 

src/libcore/slice.rs

@@ -884,17 +884,20 @@ macro_rules! iterator {
                     if self.ptr == self.end {
                         None
                     } else {
-                        let old = self.ptr;
-                        self.ptr = if mem::size_of::<T>() == 0 {
+                        if mem::size_of::<T>() == 0 {
                             // purposefully don't use 'ptr.offset' because for
                             // vectors with 0-size elements this would return the
                             // same pointer.
-                            transmute(self.ptr as uint + 1)
+                            self.ptr = transmute(self.ptr as uint + 1);
+
+                            // Use a non-null pointer value
+                            Some(transmute(1u))
                         } else {
-                            self.ptr.offset(1)
-                        };
+                            let old = self.ptr;
+                            self.ptr = self.ptr.offset(1);
 
-                        Some(transmute(old))
+                            Some(transmute(old))
+                        }
                     }
                 }
             }
@@ -916,13 +919,17 @@ macro_rules! iterator {
                     if self.end == self.ptr {
                         None
                     } else {
-                        self.end = if mem::size_of::<T>() == 0 {
+                        if mem::size_of::<T>() == 0 {
                             // See above for why 'ptr.offset' isn't used
-                            transmute(self.end as uint - 1)
+                            self.end = transmute(self.end as uint - 1);
+
+                            // Use a non-null pointer value
+                            Some(transmute(1u))
                         } else {
-                            self.end.offset(-1)
-                        };
-                        Some(transmute(self.end))
+                            self.end = self.end.offset(-1);
+
+                            Some(transmute(self.end))
+                        }
                     }
                 }
             }
@@ -956,7 +963,12 @@ impl<'a, T> RandomAccessIterator<&'a T> for Items<'a, T> {
     fn idx(&mut self, index: uint) -> Option<&'a T> {
         unsafe {
             if index < self.indexable() {
-                transmute(self.ptr.offset(index as int))
+                if mem::size_of::<T>() == 0 {
+                    // Use a non-null pointer value
+                    Some(transmute(1u))
+                } else {
+                    Some(transmute(self.ptr.offset(index as int)))
+                }
             } else {
                 None
             }