Compute the space occupied by bases, vptr, etc. based on offset of first field.
This amounts to giving up: rather than enumerating each source of data, we just check the offset of the first field (or end of the array). I did it this way because, best as I can tell, ASTRecordLayout does not export information about vtable pointers except whether one exists or not -- so, for example, it doesn't say how many there are. It's simpler and unifying for several cases to simply write down an opaque thunk. This also unifies the logic with the empty struct / sizeof one case, though really that might be a bad thing, as it prevents us from making the field public.
We do still need to e.g. check that any base classes exist at all, in order to decide whether there must be a private field.
PiperOrigin-RevId: 446904649
diff --git a/docs/struct_layout.md b/docs/struct_layout.md
index 457c66c..aecebd8 100644
--- a/docs/struct_layout.md
+++ b/docs/struct_layout.md
@@ -15,11 +15,22 @@
* Public subobjects must have the same offsets in C++ and Rust versions of the
structs.
-## Empty Structs
+## Non-field data
-In C++, an empty struct or class (e.g. `struct Empty{};`) has size `1`, while in
-Rust, it has size `0`. To make the layout match up, bindings for empty structs
-have a private `MaybeUninit<u8>` field.
+Rust bindings introduce a `__non_field_data: [MaybeUninit<u8>; N]` field to
+cover data within the object that is not part of individual fields. This
+includes:
+
+* Base classes.
+* VTable pointers.
+* Empty struct padding.
+
+### Empty Structs
+
+One notable special case of this is the empty struct padding. An empty struct or
+class (e.g. `struct Empty{};`) has size `1`, while in Rust, it has size `0`. To
+make the layout match up, bindings for empty structs will always enforce that
+the struct has size of at least 1, via `__non_field_data`.
(In C++, different array elements are guaranteed to have different addresses,
and also, arrays are guaranteed to be contiguous. Therefore, no object in C++
@@ -90,7 +101,7 @@
// and Rust wouldn't permit `z` to live inside of it.
// Nor do we align the array, for the same reason -- correct alignment must be
// achieved via the repr(align(2)) at the top.
- __base_class_subobjects : [MaybeUninit<u8>; 3];
+ __non_field_data : [MaybeUninit<u8>; 3];
pub z: i8,
}
```
