Add a pass that checks that blocks are only used in safe ways

Closes #1188

src/comp/middle/block_use.rs

@@ -0,0 +1,41 @@
+import syntax::visit;
+import syntax::ast::*;
+
+type ctx = {tcx: ty::ctxt, mutable allow_block: bool};
+
+fn check_crate(tcx: ty::ctxt, crate: @crate) {
+    let cx = {tcx: tcx, mutable allow_block: false};
+    let v = visit::mk_vt(@{visit_expr: visit_expr
+                           with *visit::default_visitor()});
+    visit::visit_crate(*crate, cx, v);
+}
+
+fn visit_expr(ex: @expr, cx: ctx, v: visit::vt<ctx>) {
+    if !cx.allow_block {
+        alt ty::struct(cx.tcx, ty::expr_ty(cx.tcx, ex)) {
+          ty::ty_fn(proto_block., _, _, _, _) {
+            cx.tcx.sess.span_err(ex.span, "expressions with block type \
+                can only appear in callee or (by-ref) argument position");
+          }
+          _ {}
+        }
+    }
+    let outer = cx.allow_block;
+    alt ex.node {
+      expr_call(f, args, _) {
+        cx.allow_block = true;
+        v.visit_expr(f, cx, v);
+        let i = 0u;
+        for arg_t in ty::ty_fn_args(cx.tcx, ty::expr_ty(cx.tcx, f)) {
+            cx.allow_block = arg_t.mode == by_ref;
+            v.visit_expr(args[i], cx, v);
+            i += 1u;
+        }
+      }
+      _ {
+        cx.allow_block = false;
+        visit::visit_expr(ex, cx, v);
+      }
+    }
+    cx.allow_block = outer;
+}