[PATCH 02/12] rust: add a utility module for compile-time type checks

[Paolo Bonzini]

It is relatively common in the low-level qemu_api code to assert that
a field of a struct has a specific type; for example, it can be used
to ensure that the fields match what the qemu_api and C code expects
for safety.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/qemu-api/meson.build       |  1 +
 rust/qemu-api/src/assertions.rs | 90 +++++++++++++++++++++++++++++++++
 rust/qemu-api/src/lib.rs        |  1 +
 3 files changed, 92 insertions(+)
 create mode 100644 rust/qemu-api/src/assertions.rs

diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
index 9425ba7100c..60944a657de 100644
--- a/rust/qemu-api/meson.build
+++ b/rust/qemu-api/meson.build
@@ -15,6 +15,7 @@ _qemu_api_rs = static_library(
   structured_sources(
     [
       'src/lib.rs',
+      'src/assertions.rs',
       'src/bindings.rs',
       'src/bitops.rs',
       'src/callbacks.rs',
diff --git a/rust/qemu-api/src/assertions.rs b/rust/qemu-api/src/assertions.rs
new file mode 100644
index 00000000000..1a6cb3a6c7c
--- /dev/null
+++ b/rust/qemu-api/src/assertions.rs
@@ -0,0 +1,90 @@
+// Copyright 2024, Red Hat Inc.
+// Author(s): Paolo Bonzini <pbonzini@redhat.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! This module provides macros to check the equality of types and
+//! the type of `struct` fields.  This can be useful to ensure that
+//! types match the expectations of C code.
+
+// Based on https://stackoverflow.com/questions/64251852/x/70978292#70978292
+// (stackoverflow answers are released under MIT license).
+
+#[doc(hidden)]
+pub trait EqType {
+    type Itself;
+}
+
+impl<T> EqType for T {
+    type Itself = T;
+}
+
+/// Assert that two types are the same.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::assert_same_type;
+/// # use std::ops::Deref;
+/// assert_same_type!(u32, u32);
+/// assert_same_type!(<Box<u32> as Deref>::Target, u32);
+/// ```
+///
+/// Different types will cause a compile failure
+///
+/// ```compile_fail
+/// # use qemu_api::assert_same_type;
+/// assert_same_type!(&Box<u32>, &u32);
+/// ```
+#[macro_export]
+macro_rules! assert_same_type {
+    ($t1:ty, $t2:ty) => {
+        const _: () = {
+            #[allow(unused)]
+            fn assert_same_type(v: $t1) {
+                fn types_must_be_equal<T, U>(_: T)
+                where
+                    T: $crate::assertions::EqType<Itself = U>,
+                {
+                }
+                types_must_be_equal::<_, $t2>(v);
+            }
+        };
+    };
+}
+
+/// Assert that a field of a struct has the given type.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::assert_field_type;
+/// pub struct A {
+///     field1: u32,
+/// }
+///
+/// assert_field_type!(A, field1, u32);
+/// ```
+///
+/// Different types will cause a compile failure
+///
+/// ```compile_fail
+/// # use qemu_api::assert_field_type;
+/// # pub struct A { field1: u32 }
+/// assert_field_type!(A, field1, i32);
+/// ```
+#[macro_export]
+macro_rules! assert_field_type {
+    ($t:ty, $i:ident, $ti:ty) => {
+        const _: () = {
+            #[allow(unused)]
+            fn assert_field_type(v: $t) {
+                fn types_must_be_equal<T, U>(_: T)
+                where
+                    T: $crate::assertions::EqType<Itself = U>,
+                {
+                }
+                types_must_be_equal::<_, $ti>(v.$i);
+            }
+        };
+    };
+}
diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
index 4b43e02c0f9..83c6a987c05 100644
--- a/rust/qemu-api/src/lib.rs
+++ b/rust/qemu-api/src/lib.rs
@@ -12,6 +12,7 @@
 #[rustfmt::skip]
 pub mod prelude;
 
+pub mod assertions;
 pub mod bitops;
 pub mod c_str;
 pub mod callbacks;
-- 
2.47.1