rust: add utility procedural macro crate

This commit adds a helper crate library, qemu-api-macros for derive (and
other procedural) macros to be used along qemu-api.

It needs to be a separate library because in Rust, procedural macros, or
macros that can generate arbitrary code, need to be special separate
compilation units.

Only one macro is introduced in this patch, #[derive(Object)]. It
generates a constructor to register a QOM TypeInfo on init and it must
be used on types that implement qemu_api::definitions::ObjectImpl trait.

Reviewed-by: Junjie Mao <junjie.mao@hotmail.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/r/dd645642406a6dc2060c6f3f17db2bc77ed67b59.1727961605.git.manos.pitsidianakis@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api-macros/src/lib.rs

@@ -0,0 +1,43 @@
+// Copyright 2024, Linaro Limited
+// Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+use proc_macro::TokenStream;
+use quote::{format_ident, quote};
+use syn::{parse_macro_input, DeriveInput};
+
+#[proc_macro_derive(Object)]
+pub fn derive_object(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    let name = input.ident;
+    let module_static = format_ident!("__{}_LOAD_MODULE", name);
+
+    let expanded = quote! {
+        #[allow(non_upper_case_globals)]
+        #[used]
+        #[cfg_attr(target_os = "linux", link_section = ".ctors")]
+        #[cfg_attr(target_os = "macos", link_section = "__DATA,__mod_init_func")]
+        #[cfg_attr(target_os = "windows", link_section = ".CRT$XCU")]
+        pub static #module_static: extern "C" fn() = {
+            extern "C" fn __register() {
+                unsafe {
+                    ::qemu_api::bindings::type_register_static(&<#name as ::qemu_api::definitions::ObjectImpl>::TYPE_INFO);
+                }
+            }
+
+            extern "C" fn __load() {
+                unsafe {
+                    ::qemu_api::bindings::register_module_init(
+                        Some(__register),
+                        ::qemu_api::bindings::module_init_type::MODULE_INIT_QOM
+                    );
+                }
+            }
+
+            __load
+        };
+    };
+
+    TokenStream::from(expanded)
+}