Add a test for not resolving ambiguity for multiple --extern with same name

tests/run-make/duplicate-dependency-no-disambiguate/foo-v1.rs

@@ -0,0 +1 @@
+pub struct Foo;

tests/run-make/duplicate-dependency-no-disambiguate/foo-v2.rs

@@ -0,0 +1 @@
+pub struct Foo;

tests/run-make/duplicate-dependency-no-disambiguate/main.rs

@@ -0,0 +1,10 @@
+#![feature(custom_inner_attributes)]
+#![rustfmt::skip] // use_foo must be referenced before foo
+
+// Load foo-v2 through use-foo
+use use_foo as _;
+
+// Make sure we don't disambiguate this as foo-v2.
+use foo as _;
+
+fn main() {}

tests/run-make/duplicate-dependency-no-disambiguate/main.stderr

@@ -0,0 +1,12 @@
+error[E0464]: multiple candidates for `rlib` dependency `foo` found
+  --> main.rs:8:5
+   |
+LL | use foo as _;
+   |     ^^^
+   |
+   = note: candidate #1: /build-root/test/run-make/duplicate-dependency-no-disambiguate/rmake_out/libfoo-v1.rlib
+   = note: candidate #2: /build-root/test/run-make/duplicate-dependency-no-disambiguate/rmake_out/libfoo-v2.rlib
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0464`.

tests/run-make/duplicate-dependency-no-disambiguate/rmake.rs

@@ -0,0 +1,54 @@
+//@ needs-target-std
+
+use run_make_support::{Rustc, cwd, diff, regex, rust_lib_name, rustc};
+
+fn rustc_with_common_args() -> Rustc {
+    let mut rustc = rustc();
+    rustc.remap_path_prefix(cwd(), "$DIR");
+    rustc.edition("2018"); // Don't require `extern crate`
+    rustc
+}
+
+fn main() {
+    rustc_with_common_args()
+        .input("foo-v1.rs")
+        .crate_type("rlib")
+        .crate_name("foo")
+        .extra_filename("-v1")
+        .metadata("-v1")
+        .run();
+
+    rustc_with_common_args()
+        .input("foo-v2.rs")
+        .crate_type("rlib")
+        .crate_name("foo")
+        .extra_filename("-v2")
+        .metadata("-v2")
+        .run();
+
+    rustc_with_common_args()
+        .input("use-foo.rs")
+        .crate_type("rlib")
+        .extern_("foo", rust_lib_name("foo-v2"))
+        .run();
+
+    let stderr = rustc_with_common_args()
+        .input("main.rs")
+        .extern_("foo", rust_lib_name("foo-v1"))
+        .extern_("foo", rust_lib_name("foo-v2"))
+        .extern_("use_foo", rust_lib_name("use_foo"))
+        .library_search_path(cwd())
+        .ui_testing()
+        .run_fail()
+        .stderr_utf8();
+
+    diff()
+        .expected_file("main.stderr")
+        .normalize(
+            regex::escape(run_make_support::build_root().canonicalize().unwrap().to_str().unwrap()),
+            "/build-root",
+        )
+        .normalize(r"\\", "/")
+        .actual_text("(rustc)", &stderr)
+        .run();
+}

tests/run-make/duplicate-dependency-no-disambiguate/use-foo.rs

@@ -0,0 +1 @@
+use foo as _;