Add a test that demonstrates an incorrect return value when calling into rust with non-c-like-enums.

src/test/run-make-fulldeps/arguments-non-c-like-enum/Makefile

@@ -0,0 +1,7 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) --crate-type=staticlib nonclike.rs
+	$(CC) test.c $(call STATICLIB,nonclike) $(call OUT_EXE,test) \
+		$(EXTRACFLAGS) $(EXTRACXXFLAGS)
+	$(call RUN,test)

src/test/run-make-fulldeps/arguments-non-c-like-enum/nonclike.rs

@@ -0,0 +1,18 @@
+#![crate_type = "lib"]
+#![crate_name = "nonclike"]
+
+#[repr(C,u8)]
+pub enum T {
+    A(u64),
+    B,
+}
+
+#[no_mangle]
+pub extern "C" fn t_add(a: T, b: T) -> u64 {
+    match (a,b) {
+        (T::A(a), T::A(b)) => a + b,
+        (T::A(a), T::B) => a,
+        (T::B, T::A(b)) => b,
+        _ => 0,
+    }
+}

src/test/run-make-fulldeps/arguments-non-c-like-enum/test.c

@@ -0,0 +1,40 @@
+#include <stdint.h>
+#include <assert.h>
+
+#include <stdio.h>
+
+/* This is the code generated by cbindgen 0.12.1 for the `enum T` type
+ * in nonclike.rs . */
+enum T_Tag {
+  A,
+  B,
+};
+typedef uint8_t T_Tag;
+
+typedef struct {
+  uint64_t _0;
+} A_Body;
+
+typedef struct {
+  T_Tag tag;
+  union {
+    A_Body a;
+  };
+} T;
+
+/* This symbol is defined by the Rust staticlib built from
+ * nonclike.rs. */
+extern uint64_t t_add(T a, T b);
+
+int main(int argc, char *argv[]) {
+  (void)argc; (void)argv;
+
+  T x = { .tag = A, .a = { ._0 = 1 } };
+  T y = { .tag = A, .a = { ._0 = 10 } };
+
+  uint64_t r = t_add(x, y);
+
+  assert(11 == r);
+
+  return 0;
+}