Emit `private` and `protected` fields as opaque blobs of bytes.

Before this CL, fields with a type that is unsupported Crubit would be
emitted as a private field that is backed by an opaque blob of bytes
(represented by a Rust array `[u8; N]` of appropriate size).

After this CL, `private` and `protected` fields with a Crubit-supported type
are also emitted as an opaque blob of bytes.

This CL is desirable as a performance optimization - e.g. to avoid
generating bindings for template instantiations that are only ever used
in non-public fields.

PiperOrigin-RevId: 452627863

rs_bindings_from_cc/src_code_gen.rs

diff --git a/rs_bindings_from_cc/src_code_gen.rs b/rs_bindings_from_cc/src_code_gen.rs
index 2794f66..3e73c72 100644
--- a/rs_bindings_from_cc/src_code_gen.rs
+++ b/rs_bindings_from_cc/src_code_gen.rs
@@ -2934,12 +2934,14 @@
             rs_api,
             quote! {
                 #[derive(Clone, Copy)]
-                #[repr(C)]
+                #[repr(C, align(4))]
                 pub struct SomeStruct {
                     __non_field_data: [crate::rust_std::mem::MaybeUninit<u8>; 0],
                     pub public_int: i32,
-                    protected_int: i32,
-                    private_int: i32,
+                    #[doc = " Reason for representing this field as a blob of bytes:\n Types of non-public C++ fields can be elided away"]
+                    protected_int: [crate::rust_std::mem::MaybeUninit<u8>; 4],
+                    #[doc = " Reason for representing this field as a blob of bytes:\n Types of non-public C++ fields can be elided away"]
+                    private_int: [crate::rust_std::mem::MaybeUninit<u8>; 4],
                 }
             }
         );
@@ -4276,10 +4278,11 @@
             rs_api,
             quote! {
                 #[derive(Clone, Copy)]
-                #[repr(C)]
+                #[repr(C, align(8))]
                 pub union SomeUnionWithPrivateFields {
                     pub public_field: i32,
-                    private_field: i64,
+                    #[doc = " Reason for representing this field as a blob of bytes:\n Types of non-public C++ fields can be elided away"]
+                    private_field: [crate::rust_std::mem::MaybeUninit<u8>; 8],
                 }
             }
         );