rewrite and rename issue-20626 to rmake

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -60,7 +60,6 @@ run-make/issue-109934-lto-debuginfo/Makefile
 run-make/issue-14698/Makefile
 run-make/issue-15460/Makefile
 run-make/issue-18943/Makefile
-run-make/issue-20626/Makefile
 run-make/issue-22131/Makefile
 run-make/issue-25581/Makefile
 run-make/issue-26006/Makefile

tests/run-make/issue-20626/Makefile

@@ -1,9 +0,0 @@
-# ignore-cross-compile
-include ../tools.mk
-
-# Test output to be four
-# The original error only occurred when printing, not when comparing using assert!
-
-all:
-	$(RUSTC) foo.rs -O
-	[ `$(call RUN,foo)` = "4" ]

tests/run-make/raw-fn-pointer-opt-undefined-behavior/rmake.rs

@@ -0,0 +1,16 @@
+// Despite the absence of any unsafe Rust code, foo.rs in this test would,
+// because of the raw function pointer,
+// cause undefined behavior and fail to print the expected result, "4" -
+// only when activating optimizations (opt-level 2). This test checks
+// that this bug does not make a resurgence.
+// Note that the bug cannot be observed in an assert_eq!, only in the stdout.
+// See https://github.com/rust-lang/rust/issues/20626
+
+//@ ignore-cross-compile
+
+use run_make_support::{run, rustc};
+
+fn main() {
+    rustc().input("foo.rs").opt().run();
+    run("foo").assert_stdout_equals("4");
+}