Make liveness print out a proper error message for moves out of a self field

This was a call to span_bug() before. I'm not sure about the other cases,
but the test case shows that the `vk_self` case can certainly arise with
a bad program, so it should be a span_err() thing and not a span_bug() thing.

Closes #2590

src/rustc/middle/liveness.rs

@@ -1669,7 +1669,14 @@ impl check_methods for @liveness {
                     #fmt["illegal move from field `%s`", *name]);
                 ret;
               }
-              vk_local(*) | vk_self | vk_implicit_ret {
+              vk_self {
+                self.tcx.sess.span_err(
+                    move_span,
+                    "illegal move from self (cannot move out of a field of \
+                       self)");
+                ret;
+              }
+              vk_local(*) | vk_implicit_ret {
                 self.tcx.sess.span_bug(
                     move_span,
                     #fmt["illegal reader (%?) for `%?`",

src/test/compile-fail/issue-2590.rs

@@ -0,0 +1,13 @@
+import dvec::dvec;
+
+type parser = {
+    tokens: dvec<int>,
+};
+
+impl parser for parser {
+    fn parse() -> [mut int] {
+        dvec::unwrap(self.tokens) //! ERROR illegal move from self
+    }
+}
+
+fn main() {}