cstore: return an immutable borrow from visible_parent_map

Fixes rust-lang#41053.

Author: arielb1

src/librustc/middle/cstore.rs

@@ -172,7 +172,7 @@ pub trait CrateStore {
     fn stability(&self, def: DefId) -> Option<attr::Stability>;
     fn deprecation(&self, def: DefId) -> Option<attr::Deprecation>;
     fn visibility(&self, def: DefId) -> ty::Visibility;
-    fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>>;
+    fn visible_parent_map<'a>(&'a self) -> ::std::cell::Ref<'a, DefIdMap<DefId>>;
     fn item_generics_cloned(&self, def: DefId) -> ty::Generics;
     fn item_attrs(&self, def_id: DefId) -> Vec<ast::Attribute>;
     fn fn_arg_names(&self, did: DefId) -> Vec<ast::Name>;
@@ -302,7 +302,7 @@ impl CrateStore for DummyCrateStore {
     fn stability(&self, def: DefId) -> Option<attr::Stability> { bug!("stability") }
     fn deprecation(&self, def: DefId) -> Option<attr::Deprecation> { bug!("deprecation") }
     fn visibility(&self, def: DefId) -> ty::Visibility { bug!("visibility") }
-    fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>> {
+    fn visible_parent_map<'a>(&'a self) -> ::std::cell::Ref<'a, DefIdMap<DefId>> {
         bug!("visible_parent_map")
     }
     fn item_generics_cloned(&self, def: DefId) -> ty::Generics

src/librustc_metadata/cstore_impl.rs

@@ -507,12 +507,19 @@ impl CrateStore for cstore::CStore {
     /// Returns a map from a sufficiently visible external item (i.e. an external item that is
     /// visible from at least one local module) to a sufficiently visible parent (considering
     /// modules that re-export the external item to be parents).
-    fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>> {
-        let mut visible_parent_map = self.visible_parent_map.borrow_mut();
-        if !visible_parent_map.is_empty() { return visible_parent_map; }
+    fn visible_parent_map<'a>(&'a self) -> ::std::cell::Ref<'a, DefIdMap<DefId>> {
+        {
+            let visible_parent_map = self.visible_parent_map.borrow();
+            if !visible_parent_map.is_empty() {
+                return visible_parent_map;
+            }
+        }
 
         use std::collections::vec_deque::VecDeque;
         use std::collections::hash_map::Entry;
+
+        let mut visible_parent_map = self.visible_parent_map.borrow_mut();
+
         for cnum in (1 .. self.next_crate_num().as_usize()).map(CrateNum::new) {
             let cdata = self.get_crate_data(cnum);
 
@@ -556,6 +563,7 @@ impl CrateStore for cstore::CStore {
             }
         }
 
-        visible_parent_map
+        drop(visible_parent_map);
+        self.visible_parent_map.borrow()
     }
 }

src/test/run-pass/auxiliary/issue_41053.rs

@@ -0,0 +1,11 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub struct Test;

src/test/run-pass/issue-41053.rs

@@ -0,0 +1,30 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:issue_41053.rs
+
+pub trait Trait { fn foo(&self) {} }
+
+pub struct Foo;
+
+impl Iterator for Foo {
+    type Item = Box<Trait>;
+    fn next(&mut self) -> Option<Box<Trait>> {
+        extern crate issue_41053;
+        impl ::Trait for issue_41053::Test {
+            fn foo(&self) {}
+        }
+        Some(Box::new(issue_41053::Test))
+    }
+}
+
+fn main() {
+    Foo.next().unwrap().foo();
+}