expanding a def-id is not a read

Across crates only, converting a def-id into its def-key or def-path was
considered a read. This caused spurious reads when computing the symbol
name for some item.

src/librustc_metadata/csearch.rs

@@ -425,13 +425,21 @@ impl<'tcx> CrateStore<'tcx> for cstore::CStore {
     /// parent `DefId` as well as some idea of what kind of data the
     /// `DefId` refers to.
     fn def_key(&self, def: DefId) -> hir_map::DefKey {
-        self.dep_graph.read(DepNode::MetaData(def));
+        // Note: loading the def-key (or def-path) for a def-id is not
+        // a *read* of its metadata. This is because the def-id is
+        // really just an interned shorthand for a def-path, which is the
+        // canonical name for an item.
+        //
+        // self.dep_graph.read(DepNode::MetaData(def));
         let cdata = self.get_crate_data(def.krate);
         decoder::def_key(&cdata, def.index)
     }
 
     fn relative_def_path(&self, def: DefId) -> hir_map::DefPath {
-        self.dep_graph.read(DepNode::MetaData(def));
+        // See `Note` above in `def_key()` for why this read is
+        // commented out:
+        //
+        // self.dep_graph.read(DepNode::MetaData(def));
         let cdata = self.get_crate_data(def.krate);
         decoder::def_path(&cdata, def.index)
     }

src/test/incremental/remove-private-item-cross-crate/auxiliary/a.rs

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+#![allow(warnings)]
+#![crate_name = "a"]
+#![crate_type = "rlib"]
+
+pub fn foo(b: u8) -> u32 { b as u32 }
+
+#[cfg(rpass1)]
+fn bar() { }

src/test/incremental/remove-private-item-cross-crate/main.rs

@@ -0,0 +1,28 @@
+// Copyright 2016 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.
+
+// Test that we are able to reuse `main` even though a private
+// item was removed from the root module of crate`a`.
+
+// revisions:rpass1 rpass2
+// aux-build:a.rs
+
+#![feature(rustc_attrs)]
+#![crate_type = "bin"]
+#![rustc_partition_reused(module="main", cfg="rpass2")]
+
+extern crate a;
+
+pub fn main() {
+    let vec: Vec<u8> = vec![0, 1, 2, 3];
+    for &b in &vec {
+        println!("{}", a::foo(b));
+    }
+}