Rollup merge of #40542 - abonander:issue_40535, r=jseyfried

Correctly get source for metatdata-only crate type

Closes #40535

However, I'm not sure how to approach writing a regression test since I'm still working on a reduced test case from the code that caused the ICE in the first place. It's not enough to have an unknown `extern crate` in a metadata crate, it depends on a few extra arguments but I'm not sure which yet.

Also replaced the `unwrap()` with a more informative `expect()`.

r? @jseyfried

src/librustc_metadata/creader.rs

@@ -236,7 +236,8 @@ impl<'a> CrateLoader<'a> {
             // path (this is a top-level dependency) as we don't want to
             // implicitly load anything inside the dependency lookup path.
             let prev_kind = source.dylib.as_ref().or(source.rlib.as_ref())
-                                  .unwrap().1;
+                                  .or(source.rmeta.as_ref())
+                                  .expect("No sources for crate").1;
             if ret.is_none() && (prev_kind == kind || prev_kind == PathKind::All) {
                 ret = Some(cnum);
             }

src/test/run-make/issue-40535/Makefile

@@ -0,0 +1,11 @@
+# The ICE occurred in the following situation:
+# * `foo` declares `extern crate bar, baz`, depends only on `bar` (forgetting `baz` in `Cargo.toml`)
+# * `bar` declares and depends on `extern crate baz`
+# * All crates built in metadata-only mode (`cargo check`)
+all:
+	# cc https://github.com/rust-lang/rust/issues/40623
+	$(RUSTC) baz.rs --emit=metadata --out-dir=$(TMPDIR)
+	$(RUSTC) bar.rs --emit=metadata --extern baz=$(TMPDIR)/libbaz.rmeta --out-dir=$(TMPDIR)
+	$(RUSTC) foo.rs --emit=metadata --extern bar=$(TMPDIR)/libbar.rmeta --out-dir=$(TMPDIR) 2>&1 | \
+	grep -vq "unexpectedly panicked"
+	# ^ Succeeds if it doesn't find the ICE message

src/test/run-make/issue-40535/bar.rs

@@ -0,0 +1,13 @@
+// Copyright 2017 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 baz;

src/test/run-make/issue-40535/baz.rs

@@ -0,0 +1,11 @@
+// Copyright 2017 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/issue-40535/foo.rs

@@ -0,0 +1,14 @@
+// Copyright 2017 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 bar;
+extern crate baz;