rustc: Make return value checking warnings a little prettier. Also introduce a new "note" diagnostic level.

src/comp/driver/session.rs

@@ -102,6 +102,11 @@ state obj session(ast::crate_num cnum,
         emit_diagnostic(sp, msg, "warning", 11u8, cm);
     }
 
+    fn span_note(span sp, str msg) {
+        // FIXME: Use constants, but rustboot doesn't know how to export them.
+        emit_diagnostic(sp, msg, "note", 10u8, cm);
+    }
+
     fn bug(str msg) {
         log_err #fmt("error: internal compiler error %s", msg);
         fail;

src/comp/middle/tstate/ck.rs

@@ -141,9 +141,11 @@ fn check_states_against_conditions(&fn_ctxt fcx, &_fn f, &ann a) -> () {
         /* fcx.ccx.tcx.sess.span_err(f.body.span, "Function " +
            fcx.name + " may not return. Its declared return type is "
            + util.common.ty_to_str(*f.decl.output)); */
-        log_err("WARNING: Function " +
-           fcx.name + " may not return. Its declared return type is "
-                + ty_to_str(*f.decl.output));
+        fcx.ccx.tcx.sess.span_warn(f.body.span, "not all control paths " +
+            "return a value");
+        fcx.ccx.tcx.sess.span_note(f.decl.output.span,
+            "see declared return type of '" + ty_to_str(*f.decl.output) +
+            "'");
     }
 
 }