rust/qemu-api-macros: add unit tests

Add unit tests to check Derive macro output for expected error messages,
or for expected correct codegen output.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250704-rust_add_derive_macro_unit_tests-v1-2-ebd47fa7f78f@linaro.org
[Remove usage of MacroError. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 143 insertions, 0 deletions

diff --git a/rust/qemu-api-macros/meson.build b/rust/qemu-api-macros/meson.build
index 8610ce1c84..2152bcb99b 100644
--- a/rust/qemu-api-macros/meson.build
+++ b/rust/qemu-api-macros/meson.build
@@ -17,3 +17,6 @@ _qemu_api_macros_rs = rust.proc_macro(
 qemu_api_macros = declare_dependency(
   link_with: _qemu_api_macros_rs,
 )
+
+rust.test('rust-qemu-api-macros-tests', _qemu_api_macros_rs,
+          suite: ['unit', 'rust'])
diff --git a/rust/qemu-api-macros/src/lib.rs b/rust/qemu-api-macros/src/lib.rs
index 5bbf8c6127..b525d89c09 100644
--- a/rust/qemu-api-macros/src/lib.rs
+++ b/rust/qemu-api-macros/src/lib.rs
@@ -11,6 +11,9 @@ use syn::{
 mod bits;
 use bits::BitsConstInternal;
 
+#[cfg(test)]
+mod tests;
+
 fn get_fields<'a>(
     input: &'a DeriveInput,
     msg: &str,
diff --git a/rust/qemu-api-macros/src/tests.rs b/rust/qemu-api-macros/src/tests.rs
new file mode 100644
index 0000000000..d6dcd62fcf
--- /dev/null
+++ b/rust/qemu-api-macros/src/tests.rs
@@ -0,0 +1,137 @@
+// Copyright 2025, Linaro Limited
+// Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+use quote::quote;
+
+use super::*;
+
+macro_rules! derive_compile_fail {
+    ($derive_fn:ident, $input:expr, $error_msg:expr) => {{
+        let input: proc_macro2::TokenStream = $input;
+        let error_msg: &str = $error_msg;
+        let derive_fn: fn(input: syn::DeriveInput) -> Result<proc_macro2::TokenStream, syn::Error> =
+            $derive_fn;
+
+        let input: syn::DeriveInput = syn::parse2(input).unwrap();
+        let result = derive_fn(input);
+        let err = result.unwrap_err().into_compile_error();
+        assert_eq!(
+            err.to_string(),
+            quote! { ::core::compile_error! { #error_msg } }.to_string()
+        );
+    }};
+}
+
+macro_rules! derive_compile {
+    ($derive_fn:ident, $input:expr, $($expected:tt)*) => {{
+        let input: proc_macro2::TokenStream = $input;
+        let expected: proc_macro2::TokenStream = $($expected)*;
+        let derive_fn: fn(input: syn::DeriveInput) -> Result<proc_macro2::TokenStream, syn::Error> =
+            $derive_fn;
+
+        let input: syn::DeriveInput = syn::parse2(input).unwrap();
+        let result = derive_fn(input).unwrap();
+        assert_eq!(result.to_string(), expected.to_string());
+    }};
+}
+
+#[test]
+fn test_derive_object() {
+    derive_compile_fail!(
+        derive_object_or_error,
+        quote! {
+            #[derive(Object)]
+            struct Foo {
+                _unused: [u8; 0],
+            }
+        },
+        "#[repr(C)] required for #[derive(Object)]"
+    );
+    derive_compile!(
+        derive_object_or_error,
+        quote! {
+            #[derive(Object)]
+            #[repr(C)]
+            struct Foo {
+                _unused: [u8; 0],
+            }
+        },
+        quote! {
+            ::qemu_api::assert_field_type!(
+                Foo,
+                _unused,
+                ::qemu_api::qom::ParentField<<Foo as ::qemu_api::qom::ObjectImpl>::ParentType>
+            );
+            ::qemu_api::module_init! {
+                MODULE_INIT_QOM => unsafe {
+                    ::qemu_api::bindings::type_register_static(&<Foo as ::qemu_api::qom::ObjectImpl>::TYPE_INFO);
+                }
+            }
+        }
+    );
+}
+
+#[test]
+fn test_derive_tryinto() {
+    derive_compile_fail!(
+        derive_tryinto_or_error,
+        quote! {
+            #[derive(TryInto)]
+            struct Foo {
+                _unused: [u8; 0],
+            }
+        },
+        "#[repr(u8/u16/u32/u64) required for #[derive(TryInto)]"
+    );
+    derive_compile!(
+        derive_tryinto_or_error,
+        quote! {
+            #[derive(TryInto)]
+            #[repr(u8)]
+            enum Foo {
+                First = 0,
+                Second,
+            }
+        },
+        quote! {
+            impl Foo {
+                #[allow(dead_code)]
+                pub const fn into_bits(self) -> u8 {
+                    self as u8
+                }
+
+                #[allow(dead_code)]
+                pub const fn from_bits(value: u8) -> Self {
+                    match ({
+                        const First: u8 = Foo::First as u8;
+                        const Second: u8 = Foo::Second as u8;
+                        match value {
+                            First => core::result::Result::Ok(Foo::First),
+                            Second => core::result::Result::Ok(Foo::Second),
+                            _ => core::result::Result::Err(value),
+                        }
+                    }) {
+                        Ok(x) => x,
+                        Err(_) => panic!("invalid value for Foo"),
+                    }
+                }
+            }
+
+            impl core::convert::TryFrom<u8> for Foo {
+                type Error = u8;
+
+                #[allow(ambiguous_associated_items)]
+                fn try_from(value: u8) -> Result<Self, u8> {
+                    const First: u8 = Foo::First as u8;
+                    const Second: u8 = Foo::Second as u8;
+                    match value {
+                        First => core::result::Result::Ok(Foo::First),
+                        Second => core::result::Result::Ok(Foo::Second),
+                        _ => core::result::Result::Err(value),
+                    }
+                }
+            }
+        }
+    );
+}