rewrite extern-flag-fun to rmake

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -43,7 +43,6 @@ run-make/error-writing-dependencies/Makefile
 run-make/export-executable-symbols/Makefile
 run-make/extern-diff-internal-name/Makefile
 run-make/extern-flag-disambiguates/Makefile
-run-make/extern-flag-fun/Makefile
 run-make/extern-flag-pathless/Makefile
 run-make/extern-flag-rename-transitive/Makefile
 run-make/extern-fn-explicit-align/Makefile

tests/run-make/extern-flag-fun/rmake.rs

@@ -0,0 +1,36 @@
+// The --extern flag can override the default crate search of
+// the compiler and directly fetch a given path. There are a few rules
+// to follow: for example, there can't be more than one rlib, the crates must
+// be valid ("no-exist" in this test), and private crates can't be loaded
+// as non-private. This test checks that these rules are enforced.
+// See https://github.com/rust-lang/rust/pull/15319
+
+//@ ignore-cross-compile
+
+use run_make_support::{rust_lib_name, rustc};
+
+fn main() {
+    rustc().input("bar.rs").crate_type("rlib").run();
+    rustc().input("bar.rs").crate_type("rlib").extra_filename("-a").run();
+    rustc().input("bar-alt.rs").crate_type("rlib").run();
+    rustc().input("foo.rs").extern_("bar", "no-exist").run_fail();
+    rustc().input("foo.rs").extern_("bar", "foo.rs").run_fail();
+    rustc()
+        .input("foo.rs")
+        .extern_("bar", rust_lib_name("bar"))
+        .extern_("bar", rust_lib_name("bar-alt"))
+        .run_fail();
+    rustc()
+        .input("foo.rs")
+        .extern_("bar", rust_lib_name("bar"))
+        .extern_("bar", rust_lib_name("bar-a"))
+        .run();
+    rustc().input("foo.rs").extern_("bar", rust_lib_name("bar")).run();
+    // Try to be sneaky and load a private crate from with a non-private name.
+    rustc().input("rustc.rs").arg("-Zforce-unstable-if-unmarked").crate_type("rlib").run();
+    rustc()
+        .input("gated_unstable.rs")
+        .extern_("alloc", rust_lib_name("rustc"))
+        .run_fail()
+        .assert_stderr_contains("rustc_private");
+}