rewrite separate-link-fail to rmake.rs

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -202,7 +202,6 @@ run-make/rustdoc-io-error/Makefile
 run-make/sanitizer-cdylib-link/Makefile
 run-make/sanitizer-dylib-link/Makefile
 run-make/sanitizer-staticlib-link/Makefile
-run-make/separate-link-fail/Makefile
 run-make/sepcomp-cci-copies/Makefile
 run-make/sepcomp-inlining/Makefile
 run-make/sepcomp-separate/Makefile

tests/run-make/separate-link-fail/foo.rs

@@ -0,0 +1 @@
+fn main() {}

tests/run-make/separate-link-fail/rmake.rs

@@ -0,0 +1,15 @@
+// rustc usually wants Rust code as its input. The flag `link-only` is one
+// exception, where a .rlink file is instead requested. The compiler should
+// fail when the user is wrongly passing the original Rust code
+// instead of the generated .rlink file when this flag is on.
+// https://github.com/rust-lang/rust/issues/95297
+
+use run_make_support::rustc;
+
+fn main() {
+    rustc()
+        .arg("-Zlink-only")
+        .input("foo.rs")
+        .run_fail()
+        .assert_stderr_contains("The input does not look like a .rlink file");
+}