return &mut T from the arenas, not &T

The arenas write the value to memory and then return a non-aliasing
reference to it. The returned reference can be mutable and can be
coerced to an immutable one.

[breaking-change]

src/libarena/lib.rs

@@ -208,13 +208,13 @@ impl Arena {
     }
 
     #[inline]
-    fn alloc_copy<T>(&self, op: || -> T) -> &T {
+    fn alloc_copy<T>(&self, op: || -> T) -> &mut T {
         unsafe {
             let ptr = self.alloc_copy_inner(mem::size_of::<T>(),
                                             mem::min_align_of::<T>());
             let ptr = ptr as *mut T;
             ptr::write(&mut (*ptr), op());
-            return &*ptr;
+            return &mut *ptr;
         }
     }
 
@@ -262,7 +262,7 @@ impl Arena {
     }
 
     #[inline]
-    fn alloc_noncopy<T>(&self, op: || -> T) -> &T {
+    fn alloc_noncopy<T>(&self, op: || -> T) -> &mut T {
         unsafe {
             let tydesc = get_tydesc::<T>();
             let (ty_ptr, ptr) =
@@ -279,14 +279,14 @@ impl Arena {
             // the object is there.
             *ty_ptr = bitpack_tydesc_ptr(tydesc, true);
 
-            return &*ptr;
+            return &mut *ptr;
         }
     }
 
     /// Allocates a new item in the arena, using `op` to initialize the value,
     /// and returns a reference to it.
     #[inline]
-    pub fn alloc<T>(&self, op: || -> T) -> &T {
+    pub fn alloc<T>(&self, op: || -> T) -> &mut T {
         unsafe {
             if intrinsics::needs_drop::<T>() {
                 self.alloc_noncopy(op)
@@ -459,12 +459,12 @@ impl<T> TypedArena<T> {
 
     /// Allocates an object in the `TypedArena`, returning a reference to it.
     #[inline]
-    pub fn alloc(&self, object: T) -> &T {
+    pub fn alloc(&self, object: T) -> &mut T {
         if self.ptr == self.end {
             self.grow()
         }
 
-        let ptr: &T = unsafe {
+        let ptr: &mut T = unsafe {
             let ptr: &mut T = mem::transmute(self.ptr);
             ptr::write(ptr, object);
             self.ptr.set(self.ptr.get().offset(1));

src/librustc/middle/typeck/variance.rs

@@ -715,7 +715,7 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
             }
 
             _ => {
-                self.terms_cx.arena.alloc(|| TransformTerm(v1, v2))
+                &*self.terms_cx.arena.alloc(|| TransformTerm(v1, v2))
             }
         }
     }