diff --git a/src/tools/tidy/src/allowed_run_make_makefiles.txt b/src/tools/tidy/src/allowed_run_make_makefiles.txt
index 36f7f68ef7b35..229dc87574a13 100644
--- a/src/tools/tidy/src/allowed_run_make_makefiles.txt
+++ b/src/tools/tidy/src/allowed_run_make_makefiles.txt
@@ -13,7 +13,6 @@ run-make/dep-info/Makefile
 run-make/dump-ice-to-disk/Makefile
 run-make/emit-to-stdout/Makefile
 run-make/export-executable-symbols/Makefile
-run-make/extern-flag-disambiguates/Makefile
 run-make/extern-fn-reachable/Makefile
 run-make/fmt-write-bloat/Makefile
 run-make/foreign-double-unwind/Makefile
diff --git a/tests/run-make/README.md b/tests/run-make/README.md
index a6c1b4b7db76a..4035990347389 100644
--- a/tests/run-make/README.md
+++ b/tests/run-make/README.md
@@ -41,3 +41,8 @@ The setup for the `rmake.rs` version is a 3-stage process:
 
 [`run_make_support`]: ../../src/tools/run-make-support
 [extern_prelude]: https://doc.rust-lang.org/reference/names/preludes.html#extern-prelude
+
+### Formatting
+
+Note that files under `tests/` are not formatted by `./x fmt`,
+use `rustfmt tests/path/to/file.rs` to format a specific file if desired.
diff --git a/tests/run-make/extern-flag-disambiguates/Makefile b/tests/run-make/extern-flag-disambiguates/Makefile
deleted file mode 100644
index e54a537ecd062..0000000000000
--- a/tests/run-make/extern-flag-disambiguates/Makefile
+++ /dev/null
@@ -1,26 +0,0 @@
-# ignore-cross-compile
-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
-	@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)
diff --git a/tests/run-make/extern-flag-disambiguates/rmake.rs b/tests/run-make/extern-flag-disambiguates/rmake.rs
new file mode 100644
index 0000000000000..2d7d7f69f66fa
--- /dev/null
+++ b/tests/run-make/extern-flag-disambiguates/rmake.rs
@@ -0,0 +1,30 @@
+//@ ignore-cross-compile
+
+use run_make_support::{cwd, run, rustc};
+
+// 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.
+
+// original Makefile at https://github.com/rust-lang/rust/issues/14469
+
+fn main() {
+    rustc().metadata("1").extra_filename("-1").input("a.rs").run();
+    rustc().metadata("2").extra_filename("-2").input("a.rs").run();
+    rustc().input("b.rs").extern_("a", "liba-1.rlib").run();
+    rustc().input("c.rs").extern_("a", "liba-2.rlib").run();
+    println!("before");
+    rustc().cfg("before").input("d.rs").extern_("a", "liba-1.rlib").run();
+    run("d");
+    println!("after");
+    rustc().cfg("after").input("d.rs").extern_("a", "liba-1.rlib").run();
+    run("d");
+}