enumeration: simplify standard types in variants.

Fixes #745

tests/rust/enum.rs

@@ -127,6 +127,12 @@ enum P {
     P1(u8, u8, u8),
 }
 
+#[repr(C)]
+enum Q {
+    Ok(Box<u32>),
+    Err(u32),
+}
+
 #[no_mangle]
 pub extern "C" fn root(
     opaque: *mut Opaque,
@@ -146,5 +152,6 @@ pub extern "C" fn root(
     n: N,
     o: O,
     p: P,
+    q: Q,
 ) {
 }

tests/rust/enum.toml

@@ -1,3 +1,18 @@
+header = """
+#if 0
+''' '
+#endif
+
+#ifdef __cplusplus
+template <typename T>
+using Box = T*;
+#endif
+
+#if 0
+' '''
+#endif
+"""
+
 trailer = """
 #if 0
 ''' '
@@ -15,5 +30,10 @@ static_assert(sizeof(CBINDGEN_STRUCT(P)) == 4, "unexpected size for P");
 #endif
 """
 
+[export]
+exclude = [
+  "Box",
+]
+
 [export.rename]
 "I" = "ExI"