rustc: Fail better when the first argument to #fmt is not a string literal

src/comp/front/extfmt.rs

@@ -74,11 +74,12 @@ fn expr_to_str(@ast::expr expr) -> str {
                 case (ast::lit_str(?s)) {
                     ret s;
                 }
+                case (_) { /* fallthrough */ }
             }
         }
+        case (_) { /* fallthrough */ }
     }
-    // FIXME: Handle error correctly.
-    log_err "malformed #fmt call";
+    log_err "first argument to #fmt must be a string literal";
     fail;
 }
 

src/test/compile-fail/extfmt-non-literal.rs

@@ -0,0 +1,9 @@
+// xfail-stage0
+// error-pattern: literal
+
+fn main() {
+  // #fmt's first argument must be a literal.  Hopefully this
+  // restriction can be eased eventually to just require a
+  // compile-time constant.
+  auto x = #fmt("a" + "b");
+}