rustc: Add a flag for specifying dependencies

This comit implements a new flag, --extern, which is used to specify where a
crate is located. The purpose of this flag is to bypass the normal crate
loading/matching of the compiler to point it directly at the right file.

This flag takes the form `--extern foo=bar` where `foo` is the name of a crate
and `bar` is the location at which to find the crate. Multiple `--extern`
directives are allowed with the same crate name to specify the rlib/dylib pair
for a crate. It is invalid to specify more than one rlib or more than one dylib,
and it's required that the crates are valid rust crates.

I have also added some extensive documentation to metadata::loader about how
crate loading should work.

RFC: 0035-remove-crate-id

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

@@ -0,0 +1,24 @@
+-include ../tools.mk
+
+# Attempt to build this dependency tree:
+#
+#	A.1   A.2
+#	 |\    |
+#	 | \   |
+#        B  \  C
+#         \ | /
+#          \|/
+#           D
+#
+# Note that A.1 and A.2 are crates with the same name.
+
+all:
+	$(RUSTC) -C metadata=1 -C extra-filename=-1 a.rs
+	$(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
+	$(RUSTC) --cfg before d.rs --extern a=$(TMPDIR)/liba-1.rlib
+	$(call RUN,d)
+	$(RUSTC) --cfg after  d.rs --extern a=$(TMPDIR)/liba-1.rlib
+	$(call RUN,d)
+

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

@@ -0,0 +1,6 @@
+#![crate_name = "a"]
+#![crate_type = "rlib"]
+
+static FOO: uint = 3;
+
+pub fn token() -> &'static uint { &FOO }

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

@@ -0,0 +1,9 @@
+#![crate_name = "b"]
+#![crate_type = "rlib"]
+
+extern crate a;
+
+static FOO: uint = 3;
+
+pub fn token() -> &'static uint { &FOO }
+pub fn a_token() -> &'static uint { a::token() }

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

@@ -0,0 +1,9 @@
+#![crate_name = "c"]
+#![crate_type = "rlib"]
+
+extern crate a;
+
+static FOO: uint = 3;
+
+pub fn token() -> &'static uint { &FOO }
+pub fn a_token() -> &'static uint { a::token() }

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

@@ -0,0 +1,11 @@
+#[cfg(before)] extern crate a;
+extern crate b;
+extern crate c;
+#[cfg(after)] extern crate a;
+
+fn t(a: &'static uint) -> uint { a as *const _ as uint }
+
+fn main() {
+    assert!(t(a::token()) == t(b::a_token()));
+    assert!(t(a::token()) != t(c::a_token()));
+}

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

@@ -0,0 +1,16 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) bar.rs --crate-type=rlib
+	$(RUSTC) bar.rs --crate-type=rlib -C extra-filename=-a
+	$(RUSTC) foo.rs --extern hello && exit 1 || exit 0
+	$(RUSTC) foo.rs --extern bar=no-exist && exit 1 || exit 0
+	$(RUSTC) foo.rs --extern bar=foo.rs && exit 1 || exit 0
+	$(RUSTC) foo.rs \
+		--extern bar=$(TMPDIR)/libbar.rlib \
+		--extern bar=$(TMPDIR)/libbar-a.rlib \
+		&& exit 1 || exit 0
+	$(RUSTC) foo.rs \
+		--extern bar=$(TMPDIR)/libbar.rlib \
+		--extern bar=$(TMPDIR)/libbar.rlib
+	$(RUSTC) foo.rs --extern bar=$(TMPDIR)/libbar.rlib

src/test/run-make/extern-flag-fun/foo.rs

@@ -0,0 +1,3 @@
+extern crate bar;
+
+fn main() {}

src/test/run-make/metadata-flag-frobs-symbols/Makefile

@@ -0,0 +1,10 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) foo.rs -C metadata=a -C extra-filename=-a
+	$(RUSTC) foo.rs -C metadata=b -C extra-filename=-b
+	$(RUSTC) bar.rs \
+		--extern foo1=$(TMPDIR)/libfoo-a.rlib \
+		--extern foo2=$(TMPDIR)/libfoo-b.rlib \
+		-Z print-link-args
+	$(call RUN,bar)

src/test/run-make/metadata-flag-frobs-symbols/bar.rs

@@ -0,0 +1,8 @@
+extern crate foo1;
+extern crate foo2;
+
+fn main() {
+    let a = foo1::foo();
+    let b = foo2::foo();
+    assert!(a as *const _ != b as *const _);
+}

src/test/run-make/metadata-flag-frobs-symbols/foo.rs

@@ -0,0 +1,6 @@
+#![crate_name = "foo"]
+#![crate_type = "rlib"]
+
+static FOO: uint = 3;
+
+pub fn foo() -> &'static uint { &FOO }