rewrite unstable-flag-required to rmake

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -129,5 +129,4 @@ run-make/test-benches/Makefile
 run-make/thumb-none-cortex-m/Makefile
 run-make/thumb-none-qemu/Makefile
 run-make/translation/Makefile
-run-make/unstable-flag-required/Makefile
 run-make/x86_64-fortanix-unknown-sgx-lvi/Makefile

tests/run-make/unstable-flag-required/rmake.rs

@@ -0,0 +1,12 @@
+// The flag `--output-format` is unauthorized on beta and stable releases, which led
+// to confusion for maintainers doing testing on nightly. Tying it to an unstable flag
+// elucidates this, and this test checks that `--output-format` cannot be passed on its
+// own.
+// See https://github.com/rust-lang/rust/pull/82497
+
+use run_make_support::{diff, rustdoc};
+
+fn main() {
+    let out = rustdoc().output_format("json").input("x.html").run_fail().stderr_utf8();
+    diff().expected_file("output-format-json.stderr").actual_text("actual-json", out).run();
+}