Use typestate constraints for trans_be

trans_be now has a precondition that its expression argument
is a call expr. Obviously this code may be going away soon, but
I wanted to exercise typestate somehow and this was an easy one :-)

Author: catamorphism

src/comp/middle/trans.rs

@@ -4450,7 +4450,14 @@ fn trans_expr_out(cx: &@block_ctxt, e: &@ast::expr, output: out_method) ->
       ast::expr_cont. { ret trans_cont(e.span, cx); }
       ast::expr_ret(ex) { ret trans_ret(cx, ex); }
       ast::expr_put(ex) { ret trans_put(cx, ex); }
-      ast::expr_be(ex) { ret trans_be(cx, ex); }
+      ast::expr_be(ex) {
+        // Ideally, the expr_be tag would have a precondition
+        // that is_call_expr(ex) -- but we don't support that
+        // yet
+        // FIXME
+        check ast_util::is_call_expr(ex);
+        ret trans_be(cx, ex);
+      }
       ast::expr_anon_obj(anon_obj) {
         ret trans_anon_obj(cx, e.span, anon_obj, e.id);
       }
@@ -4776,10 +4783,10 @@ fn trans_ret(cx: &@block_ctxt, e: &option::t<@ast::expr>) -> result {
 
 fn build_return(bcx: &@block_ctxt) { bld::Br(bcx, bcx_fcx(bcx).llreturn); }
 
-fn trans_be(cx: &@block_ctxt, e: &@ast::expr) -> result {
-    // FIXME: This should be a typestate precondition
+// fn trans_be(cx: &@block_ctxt, e: &@ast::expr) -> result {
+fn trans_be(cx: &@block_ctxt, e: &@ast::expr)
+    : ast_util::is_call_expr(e) -> result {
 
-    assert (ast_util::is_call_expr(e));
     // FIXME: Turn this into a real tail call once
     // calling convention issues are settled
 

src/comp/syntax/ast_util.rs

@@ -167,8 +167,8 @@ fn is_exported(i: ident, m: _mod) -> bool {
     ret count == 0u && !nonlocal;
 }
 
-fn is_call_expr(e: @expr) -> bool {
-    alt e.node { expr_call(_, _) { ret true; } _ { ret false; } }
+pure fn is_call_expr(e: @expr) -> bool {
+    alt e.node { expr_call(_, _) { true } _ { false } }
 }
 
 fn is_constraint_arg(e: @expr) -> bool {