Enforce that self doesn't escape from a class

Closes #2294

src/rustc/driver/driver.rs

@@ -186,6 +186,8 @@ fn compile_upto(sess: session, cfg: ast::crate_cfg,
          bind middle::check_loop::check_crate(ty_cx, crate));
     time(time_passes, "alt checking",
          bind middle::check_alt::check_crate(ty_cx, crate));
+    time(time_passes, "self checking",
+         bind middle::check_self::check_crate(ty_cx, crate));
     time(time_passes, "typestate checking",
          bind middle::tstate::ck::check_crate(ty_cx, crate));
     let (_root_map, mutbl_map) = time(

src/rustc/middle/check_self.rs

@@ -0,0 +1,60 @@
+/*
+  This module checks that within a class, "self" doesn't escape.
+  That is, it rejects any class in which "self" occurs other than
+  as the left-hand side of a field reference.
+ */
+import syntax::ast::*;
+import syntax::visit::*;
+import driver::session::session;
+import std::map::hashmap;
+import resolve::def_map;
+
+fn check_crate(cx: ty::ctxt, crate: @crate) {
+    visit_crate(*crate, cx, mk_vt(@{
+        visit_item: bind check_item(_, _, _)
+        with *default_visitor()
+    }));
+    cx.sess.abort_if_errors();
+}
+
+fn check_item(it: @item, &&cx: ty::ctxt, &&_v: vt<ty::ctxt>) {
+    alt it.node {
+      item_class(*) {
+          visit_item(it, cx, check_self_visitor());
+      }
+      _ {}
+    }
+}
+
+fn check_self_visitor() -> vt<ty::ctxt> {
+    mk_vt(@{
+        visit_expr: bind check_self_expr(_, _, _)
+                with *default_visitor()
+    })
+}
+
+fn check_self_expr(e: @expr, &&cx: ty::ctxt, &&v: vt<ty::ctxt>) {
+   alt e.node {
+     expr_field(@{node: expr_path(p),_},_,_) {
+       // self is ok here; don't descend
+     }
+     expr_path(_) {
+       alt cx.def_map.find(e.id) {
+          some(def_self(_)) {
+            cx.sess.span_err(e.span, "can't return self or store \
+              it in a data structure");
+          }
+          _ {}
+       }
+     }
+     _ { visit_expr(e, cx, v); }
+  }
+}
+
+// Local Variables:
+// mode: rust
+// fill-column: 78;
+// indent-tabs-mode: nil
+// c-basic-offset: 4
+// buffer-file-coding-system: utf-8-unix
+// End:

src/rustc/rustc.rc

@@ -55,6 +55,7 @@ mod middle {
     mod check_loop;
     mod check_alt;
     mod check_const;
+    mod check_self;
     mod lint;
     mod borrowck;
     mod alias;

src/test/compile-fail/issue-2294.rs

@@ -0,0 +1,6 @@
+class cat {
+  fn kitty() -> cat { self } //! ERROR: can't return self or store it in a data structure
+  new() { }
+}
+
+fn main() {}