rustc: Fix extern crate being order dependent

This commit fixes a bug where a crate could fail to compile depending on the
order of `extern crate` directives at the top of the crate. Specifically, if the
same crate is found at two locations, then if it's loaded first via `--extern`
it will not emit a duplicate warning, but if it's first loaded transitively
via a dep and *then* via `--extern` an error will be emitted.

The loader was tweaked to catch this scenario and coalesce the loading of these
two crates to prevent errors from being emitted.

src/librustc/metadata/creader.rs

@@ -368,7 +368,12 @@ impl<'a> CrateReader<'a> {
                      explicitly_linked: bool)
                          -> (ast::CrateNum, Rc<cstore::crate_metadata>,
                              cstore::CrateSource) {
-        match self.existing_match(name, hash, kind) {
+        enum LookupResult {
+            Previous(ast::CrateNum),
+            Loaded(loader::Library),
+        }
+        let result = match self.existing_match(name, hash, kind) {
+            Some(cnum) => LookupResult::Previous(cnum),
             None => {
                 let mut load_ctxt = loader::Context {
                     sess: self.sess,
@@ -386,16 +391,36 @@ impl<'a> CrateReader<'a> {
                     should_match_name: true,
                 };
                 let library = load_ctxt.load_library_crate();
-                self.register_crate(root, ident, name, span, library,
-                                    explicitly_linked)
+
+                // In the case that we're loading a crate, but not matching
+                // against a hash, we could load a crate which has the same hash
+                // as an already loaded crate. If this is the case prevent
+                // duplicates by just using the first crate.
+                let meta_hash = decoder::get_crate_hash(library.metadata
+                                                               .as_slice());
+                let mut result = LookupResult::Loaded(library);
+                self.sess.cstore.iter_crate_data(|cnum, data| {
+                    if data.name() == name && meta_hash == data.hash() {
+                        assert!(hash.is_none());
+                        result = LookupResult::Previous(cnum);
+                    }
+                });
+                result
             }
-            Some(cnum) => {
+        };
+
+        match result {
+            LookupResult::Previous(cnum) => {
                 let data = self.sess.cstore.get_crate_data(cnum);
                 if explicitly_linked && !data.explicitly_linked.get() {
                     data.explicitly_linked.set(explicitly_linked);
                 }
                 (cnum, data, self.sess.cstore.get_used_crate_source(cnum).unwrap())
             }
+            LookupResult::Loaded(library) => {
+                self.register_crate(root, ident, name, span, library,
+                                    explicitly_linked)
+            }
         }
     }
 

src/librustc/metadata/loader.rs

@@ -547,7 +547,12 @@ impl<'a> Context<'a> {
                     continue
                 }
             };
-            if ret.is_some() {
+            // If we've already found a candidate and we're not matching hashes,
+            // emit an error about duplicate candidates found. If we're matching
+            // based on a hash, however, then if we've gotten this far both
+            // candidates have the same hash, so they're not actually
+            // duplicates that we should warn about.
+            if ret.is_some() && self.hash.is_none() {
                 span_err!(self.sess, self.span, E0465,
                           "multiple {} candidates for `{}` found",
                           flavor, self.crate_name);

src/test/run-make/extern-multiple-copies/Makefile

@@ -0,0 +1,8 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) foo1.rs
+	$(RUSTC) foo2.rs
+	mkdir $(TMPDIR)/foo
+	cp $(TMPDIR)/libfoo1.rlib $(TMPDIR)/foo/libfoo1.rlib
+	$(RUSTC) bar.rs --extern foo1=$(TMPDIR)/libfoo1.rlib -L $(TMPDIR)/foo

src/test/run-make/extern-multiple-copies/bar.rs

@@ -0,0 +1,16 @@
+// Copyright 2015 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.
+
+extern crate foo2; // foo2 first to exhibit the bug
+extern crate foo1;
+
+fn main() {
+    /* ... */
+}

src/test/run-make/extern-multiple-copies/foo1.rs

@@ -0,0 +1,11 @@
+// Copyright 2015 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.
+
+#![crate_type = "rlib"]

src/test/run-make/extern-multiple-copies/foo2.rs

@@ -0,0 +1,11 @@
+// Copyright 2015 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.
+
+#![crate_type = "rlib"]