librustc: Don't create &[T] slices with NULL as the ptr.

src/libcollections/vec.rs

@@ -27,6 +27,10 @@
 use slice::{MutableOrdVector, MutableVectorAllocating, CloneableVector};
 use slice::{Items, MutItems};
 
+
+#[doc(hidden)]
+pub static PTR_MARKER: u8 = 0;
+
 /// An owned, growable vector.
 ///
 /// # Examples
@@ -71,7 +75,13 @@ impl<T> Vec<T> {
     /// ```
     #[inline]
     pub fn new() -> Vec<T> {
-        Vec { len: 0, cap: 0, ptr: 0 as *mut 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.
+        Vec { len: 0, cap: 0, ptr: &PTR_MARKER as *const _ as *mut T }
     }
 
     /// Constructs a new, empty `Vec` with the specified capacity.
@@ -88,7 +98,7 @@ pub fn new() -> Vec<T> {
     #[inline]
     pub fn with_capacity(capacity: uint) -> Vec<T> {
         if mem::size_of::<T>() == 0 {
-            Vec { len: 0, cap: uint::MAX, ptr: 0 as *mut T }
+            Vec { len: 0, cap: uint::MAX, ptr: &PTR_MARKER as *const _ as *mut T }
         } else if capacity == 0 {
             Vec::new()
         } else {
@@ -1206,16 +1216,8 @@ pub fn init<'a>(&'a self) -> &'a [T] {
     /// would also make any pointers to it invalid.
     #[inline]
     pub fn as_ptr(&self) -> *const 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.
-        if mem::size_of::<T>() == 0 {
-            1 as *const T
-        } else {
         self.ptr as *const T
     }
-    }
 
     /// Returns a mutable unsafe pointer to the vector's buffer.
     ///
@@ -1226,13 +1228,8 @@ pub fn as_ptr(&self) -> *const T {
     /// would also make any pointers to it invalid.
     #[inline]
     pub fn as_mut_ptr(&mut self) -> *mut T {
-        // see above for the 0-size check
-        if mem::size_of::<T>() == 0 {
-            1 as *mut T
-        } else {
         self.ptr
     }
-    }
 
     /// Retains only the elements specified by the predicate.
     ///

src/librustc/middle/trans/tvec.rs

@@ -155,8 +155,9 @@ pub fn trans_slice_vstore<'a>(
     let llcount = C_uint(ccx, count);
     let llfixed;
     if count == 0 {
-        // Zero-length array: just use NULL as the data pointer
-        llfixed = C_null(vt.llunit_ty.ptr_to());
+        // Just create a zero-sized alloca to preserve
+        // the non-null invariant of the inner slice ptr
+        llfixed = base::arrayalloca(bcx, vt.llunit_ty, llcount);
     } else {
         // Make a fixed-length backing array and allocate it on the stack.
         llfixed = base::arrayalloca(bcx, vt.llunit_ty, llcount);