Switch to u64 types for value in cabi (#603)

minijinja-cabi/include/minijinja.h

@@ -92,7 +92,7 @@ typedef struct mj_value_iter mj_value_iter;
  Opaque value type.
  */
 typedef struct mj_value {
-  uintptr_t _opaque[3];
+  uint64_t _opaque[3];
 } mj_value;
 
 /*
@@ -414,7 +414,7 @@ MINIJINJA_API char *mj_value_to_str(struct mj_value value);
 MINIJINJA_API struct mj_value_iter *mj_value_try_iter(struct mj_value value);
 
 #ifdef __cplusplus
-}  // extern "C"
-#endif  // __cplusplus
+} // extern "C"
+#endif // __cplusplus
 
-#endif  /* _minijinja_h_included */
+#endif /* _minijinja_h_included */

minijinja-cabi/src/value.rs

@@ -39,7 +39,15 @@ where
 /// Opaque value type.
 #[repr(C)]
 pub struct mj_value {
-    _opaque: [usize; 3],
+    // Motivation on the size here: The size of `Value` is really not
+    // known and since cbindgen has no way to guarnatee us a matching
+    // size we have to be creative.  The dominating type size wise is
+    // most likely going to be SmallStr which is a u8+[u8; 22] plus the
+    // enum discriminant (u8).
+    //
+    // We are going with u64 here for alignment reasons which is likely
+    // to be a good default across platforms.
+    _opaque: [u64; 3],
 }
 
 impl mj_value {
@@ -51,7 +59,7 @@ impl mj_value {
 impl From<Value> for mj_value {
     fn from(value: Value) -> Self {
         mj_value {
-            _opaque: unsafe { transmute::<Value, [usize; 3]>(value) },
+            _opaque: unsafe { transmute::<Value, [u64; 3]>(value) },
         }
     }
 }