rustc: Fix a leak in dependency= paths

With the addition of separate search paths to the compiler, it was intended that
applications such as Cargo could require a `--extern` flag per `extern crate`
directive in the source. The system can currently be subverted, however, due to
the `existing_match()` logic in the crate loader.

When loading crates we first attempt to match an `extern crate` directive
against all previously loaded crates to avoid reading metadata twice. This "hit
the cache if possible" step was erroneously leaking crates across the search
path boundaries, however. For example:

    extern crate b;
    extern crate a;

If `b` depends on `a`, then it will load crate `a` when the `extern crate b`
directive is being processed. When the compiler reaches `extern crate a` it will
use the previously loaded version no matter what. If the compiler was not
invoked with `-L crate=path/to/a`, it will still succeed.

This behavior is allowing `extern crate` declarations in Cargo without a
corresponding declaration in the manifest of a dependency, which is considered
a bug.

This commit fixes this problem by keeping track of the origin search path for a
crate. Crates loaded from the dependency search path are not candidates for
crates which are loaded from the crate search path.

As a result of this fix, this is a likely a breaking change for a number of
Cargo packages. If the compiler starts informing that a crate can no longer be
found, it likely means that the dependency was forgotten in your Cargo.toml.

[breaking-change]

src/test/run-make/compiler-lookup-paths-2/Makefile

@@ -0,0 +1,8 @@
+-include ../tools.mk
+
+all:
+	mkdir -p $(TMPDIR)/a $(TMPDIR)/b
+	$(RUSTC) a.rs && mv $(TMPDIR)/liba.rlib $(TMPDIR)/a
+	$(RUSTC) b.rs -L $(TMPDIR)/a && mv $(TMPDIR)/libb.rlib $(TMPDIR)/b
+	$(RUSTC) c.rs -L crate=$(TMPDIR)/b -L dependency=$(TMPDIR)/a \
+		&& exit 1 || exit 0

src/test/run-make/compiler-lookup-paths-2/a.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 = "lib"]

src/test/run-make/compiler-lookup-paths-2/b.rs

@@ -0,0 +1,12 @@
+// 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 = "lib"]
+extern crate a;

src/test/run-make/compiler-lookup-paths-2/c.rs

@@ -0,0 +1,13 @@
+// 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 = "lib"]
+extern crate b;
+extern crate a;

src/test/run-make/extern-flag-disambiguates/Makefile

@@ -17,8 +17,10 @@ all:
 	$(RUSTC) -C metadata=2 -C extra-filename=-2 a.rs
 	$(RUSTC) b.rs --extern a=$(TMPDIR)/liba-1.rlib
 	$(RUSTC) c.rs --extern a=$(TMPDIR)/liba-2.rlib
+	@echo before
 	$(RUSTC) --cfg before d.rs --extern a=$(TMPDIR)/liba-1.rlib
 	$(call RUN,d)
+	@echo after
 	$(RUSTC) --cfg after  d.rs --extern a=$(TMPDIR)/liba-1.rlib
 	$(call RUN,d)
 

src/test/run-make/extern-flag-disambiguates/a.rs

@@ -11,6 +11,6 @@
 #![crate_name = "a"]
 #![crate_type = "rlib"]
 
-static FOO: uint = 3;
+static FOO: usize = 3;
 
-pub fn token() -> &'static uint { &FOO }
+pub fn token() -> &'static usize { &FOO }

src/test/run-make/extern-flag-disambiguates/b.rs

@@ -13,7 +13,7 @@
 
 extern crate a;
 
-static FOO: uint = 3;
+static FOO: usize = 3;
 
-pub fn token() -> &'static uint { &FOO }
-pub fn a_token() -> &'static uint { a::token() }
+pub fn token() -> &'static usize { &FOO }
+pub fn a_token() -> &'static usize { a::token() }

src/test/run-make/extern-flag-disambiguates/c.rs

@@ -13,7 +13,7 @@
 
 extern crate a;
 
-static FOO: uint = 3;
+static FOO: usize = 3;
 
-pub fn token() -> &'static uint { &FOO }
-pub fn a_token() -> &'static uint { a::token() }
+pub fn token() -> &'static usize { &FOO }
+pub fn a_token() -> &'static usize { a::token() }

src/test/run-make/extern-flag-disambiguates/d.rs

@@ -13,7 +13,7 @@ extern crate b;
 extern crate c;
 #[cfg(after)] extern crate a;
 
-fn t(a: &'static uint) -> uint { a as *const _ as uint }
+fn t(a: &'static usize) -> usize { a as *const _ as usize }
 
 fn main() {
     assert!(t(a::token()) == t(b::a_token()));