Convert mutable statics error to have error code and add explanation.

Also changes 'owned pointers' => 'boxes' in the error message.

src/librustc/diagnostics.rs

@@ -880,6 +880,19 @@ From [RFC 246]:
  > required that all references be borrowed.
 
 [RFC 246]: https://github.com/rust-lang/rfcs/pull/246
+"##,
+
+E0397: r##"
+It is not allowed for a mutable static to allocate or have destructors. For
+example:
+
+```
+// error: mutable statics are not allowed to have boxes
+static mut FOO: Option<Box<usize>> = None;
+
+// error: mutable statics are not allowed to have destructors
+static mut BAR: Option<Vec<i32>> = None;
+```
 "##
 
 }

src/librustc/middle/check_const.rs

@@ -249,13 +249,13 @@ impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
         let suffix = if tcontents.has_dtor() {
             "destructors"
         } else if tcontents.owns_owned() {
-            "owned pointers"
+            "boxes"
         } else {
             return
         };
 
-        self.tcx.sess.span_err(e.span, &format!("mutable statics are not allowed \
-                                                 to have {}", suffix));
+        span_err!(self.tcx.sess, e.span, E0397,
+                 "mutable statics are not allowed to have {}", suffix);
     }
 
     fn check_static_type(&self, e: &ast::Expr) {

src/test/compile-fail/static-mut-not-constant.rs

@@ -12,6 +12,6 @@
 
 static mut a: Box<isize> = box 3;
 //~^ ERROR allocations are not allowed in statics
-//~^^ ERROR mutable statics are not allowed to have owned pointers
+//~^^ ERROR mutable statics are not allowed to have boxes
 
 fn main() {}