Convert derive_object to the same pattern of first making a
Result<proc_macro2::TokenStream, CompileError>, and then doing
.unwrap_or_else(Into::into) to support checking the validity of
the input.  Add is_c_repr to check that all QOM structs include
a #[repr(C)] attribute.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/qemu-api-macros/src/lib.rs | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/rust/qemu-api-macros/src/lib.rs b/rust/qemu-api-macros/src/lib.rs
index 74a8bc7503e..160b283d7fd 100644
--- a/rust/qemu-api-macros/src/lib.rs
+++ b/rust/qemu-api-macros/src/lib.rs
@@ -32,18 +32,23 @@ fn is_c_repr(input: &DeriveInput, msg: &str) -> Result<(), CompileError> {
     }
 }
 
-#[proc_macro_derive(Object)]
-pub fn derive_object(input: TokenStream) -> TokenStream {
-    let input = parse_macro_input!(input as DeriveInput);
-    let name = input.ident;
+fn derive_object_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, CompileError> {
+    is_c_repr(&input, "#[derive(Object)]")?;
 
-    let expanded = quote! {
+    let name = &input.ident;
+    Ok(quote! {
         ::qemu_api::module_init! {
             MODULE_INIT_QOM => unsafe {
                 ::qemu_api::bindings::type_register_static(&<#name as ::qemu_api::qom::ObjectImpl>::TYPE_INFO);
             }
         }
-    };
+    })
+}
+
+#[proc_macro_derive(Object)]
+pub fn derive_object(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let expanded = derive_object_or_error(input).unwrap_or_else(Into::into);
 
     TokenStream::from(expanded)
 }
-- 
2.47.1