Auto merge of #22023 - alexcrichton:oops-picked-the-wrong-plugin, r=nikomatsakis

The compiler would previously fall back to using `-L` and normal lookup paths if
a `--extern` path was specified but it did not match (wrong architecture, for
example). This commit removes this behavior and forces the hand of the crate
loader to *always* use the `--extern` path if specified, no matter whether it is
correct or not.

This fixes a bug today where the compiler's own libraries are favored in cross
compilation by accident. For example when a crate using the crates.io version of
`log` was cross compiled, Cargo would compile `log` for the target architecture.
When loading the macros, however, the compiler currently favors using the *host*
architecture (for plugins), and because the `--extern log=...` pointed at an
rlib for the target architecture, that lookup failed. The crate loader  then
fell back on `-L` paths to find the compiler-used `log` crate (the wrong one!)
and then a compile failure happened because the logging macros are slightly
different.

src/test/run-make/use-extern-for-plugins/Makefile

@@ -0,0 +1,13 @@
+-include ../tools.mk
+
+HOST := $(shell $(RUSTC) -vV | grep 'host:' | sed 's/host: //')
+ifeq ($(findstring i686,$(HOST)),i686)
+TARGET := $(subst i686,x86_64,$(HOST))
+else
+TARGET := $(subst x86_64,i686,$(HOST))
+endif
+
+all:
+	$(RUSTC) foo.rs -C extra-filename=-host
+	$(RUSTC) bar.rs -C extra-filename=-targ --target $(TARGET)
+	$(RUSTC) baz.rs --extern a=$(TMPDIR)/liba-targ.rlib --target $(TARGET)

src/test/run-make/use-extern-for-plugins/bar.rs

@@ -0,0 +1,19 @@
+// 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.
+
+#![feature(no_std)]
+#![no_std]
+#![crate_type = "lib"]
+#![crate_name = "a"]
+
+#[macro_export]
+macro_rules! bar {
+    () => ()
+}

src/test/run-make/use-extern-for-plugins/baz.rs

@@ -0,0 +1,18 @@
+// 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.
+
+#![feature(no_std)]
+#![no_std]
+#![crate_type = "lib"]
+
+#[macro_use]
+extern crate a;
+
+bar!();

src/test/run-make/use-extern-for-plugins/foo.rs

@@ -0,0 +1,19 @@
+// 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.
+
+#![feature(no_std)]
+#![no_std]
+#![crate_type = "lib"]
+#![crate_name = "a"]
+
+#[macro_export]
+macro_rules! foo {
+    () => ()
+}