Add test for foreign double unwind

src/test/run-make-fulldeps/foreign-double-unwind/Makefile

@@ -0,0 +1,10 @@
+-include ../tools.mk
+
+all: foo
+	$(call RUN,foo) | $(CGREP) -v unreachable
+
+foo: foo.rs $(call NATIVE_STATICLIB,foo)
+	$(RUSTC) $< -lfoo $(EXTRARSCXXFLAGS)
+
+$(TMPDIR)/libfoo.o: foo.cpp
+	$(call COMPILE_OBJ_CXX,$@,$<)

src/test/run-make-fulldeps/foreign-double-unwind/foo.cpp

@@ -0,0 +1,33 @@
+#include <cstdio>
+#include <exception>
+
+void println(const char* s) {
+    puts(s);
+    fflush(stdout);
+}
+
+struct outer_exception {};
+struct inner_exception {};
+
+extern "C" {
+    void throw_cxx_exception() {
+        if (std::uncaught_exception()) {
+            println("throwing inner C++ exception");
+            throw inner_exception();
+        } else {
+            println("throwing outer C++ exception");
+            throw outer_exception();
+        }
+    }
+
+    void cxx_catch_callback(void (*cb)()) {
+        try {
+            cb();
+            println("unreachable: callback returns");
+        } catch (outer_exception) {
+            println("unreachable: caught outer exception in catch (...)");
+        } catch (inner_exception) {
+            println("unreachable: caught inner exception in catch (...)");
+        }
+    }
+}

src/test/run-make-fulldeps/foreign-double-unwind/foo.rs

@@ -0,0 +1,26 @@
+// Tests that C++ double unwinding through Rust code will be properly guarded
+// against instead of exhibiting undefined behaviour.
+
+#![feature(c_unwind)]
+
+extern "C-unwind" {
+    fn throw_cxx_exception();
+    fn cxx_catch_callback(cb: extern "C-unwind" fn());
+}
+
+struct ThrowOnDrop;
+
+impl Drop for ThrowOnDrop {
+    fn drop(&mut self) {
+        unsafe { throw_cxx_exception() };
+    }
+}
+
+extern "C-unwind" fn test_double_unwind() {
+    let _a = ThrowOnDrop;
+    let _b = ThrowOnDrop;
+}
+
+fn main() {
+    unsafe { cxx_catch_callback(test_double_unwind) };
+}