rust: introduce a c_str macro

This allows CStr constants to be defined easily on Rust 1.63.0, while
checking that there are no embedded NULs.  c"" literals were only
stabilized in Rust 1.77.0.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/meson.build

@@ -3,6 +3,7 @@ _qemu_api_rs = static_library(
   structured_sources(
     [
       'src/lib.rs',
+      'src/c_str.rs',
       'src/definitions.rs',
       'src/device_class.rs',
       'src/vmstate.rs',
@@ -18,6 +19,9 @@ _qemu_api_rs = static_library(
   ],
 )
 
+rust.test('rust-qemu-api-tests', _qemu_api_rs,
+          suite: ['unit', 'rust'])
+
 qemu_api = declare_dependency(
   link_with: _qemu_api_rs,
   dependencies: qemu_api_macros,

rust/qemu-api/src/c_str.rs

@@ -0,0 +1,53 @@
+// Copyright 2024 Red Hat, Inc.
+// Author(s): Paolo Bonzini <pbonzini@redhat.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#[macro_export]
+/// Given a string constant _without_ embedded or trailing NULs, return
+/// a `CStr`.
+///
+/// Needed for compatibility with Rust <1.77.
+macro_rules! c_str {
+    ($str:expr) => {{
+        const STRING: &str = concat!($str, "\0");
+        const BYTES: &[u8] = STRING.as_bytes();
+
+        // "for" is not allowed in const context... oh well,
+        // everybody loves some lisp.  This could be turned into
+        // a procedural macro if this is a problem; alternatively
+        // Rust 1.72 makes CStr::from_bytes_with_nul a const function.
+        const fn f(b: &[u8], i: usize) {
+            if i == b.len() - 1 {
+            } else if b[i] == 0 {
+                panic!("c_str argument contains NUL")
+            } else {
+                f(b, i + 1)
+            }
+        }
+        f(BYTES, 0);
+
+        // SAFETY: absence of NULs apart from the final byte was checked above
+        unsafe { std::ffi::CStr::from_bytes_with_nul_unchecked(BYTES) }
+    }};
+}
+
+#[cfg(test)]
+mod tests {
+    use std::ffi::CStr;
+
+    use crate::c_str;
+
+    #[test]
+    fn test_cstr_macro() {
+        let good = c_str!("🦀");
+        let good_bytes = b"\xf0\x9f\xa6\x80\0";
+        assert_eq!(good.to_bytes_with_nul(), good_bytes);
+    }
+
+    #[test]
+    fn test_cstr_macro_const() {
+        const GOOD: &CStr = c_str!("🦀");
+        const GOOD_BYTES: &[u8] = b"\xf0\x9f\xa6\x80\0";
+        assert_eq!(GOOD.to_bytes_with_nul(), GOOD_BYTES);
+    }
+}

rust/qemu-api/src/lib.rs

@@ -29,6 +29,7 @@ unsafe impl Sync for bindings::VMStateDescription {}
 unsafe impl Sync for bindings::VMStateField {}
 unsafe impl Sync for bindings::VMStateInfo {}
 
+pub mod c_str;
 pub mod definitions;
 pub mod device_class;
 pub mod vmstate;

rust/qemu-api/src/vmstate.rs

@@ -15,7 +15,7 @@
 macro_rules! vmstate_unused_buffer {
     ($field_exists_fn:expr, $version_id:expr, $size:expr) => {{
         $crate::bindings::VMStateField {
-            name: c"unused".as_ptr(),
+            name: c_str!("unused").as_ptr(),
             err_hint: ::core::ptr::null(),
             offset: 0,
             size: $size,

rust/qemu-api/tests/tests.rs

@@ -6,7 +6,7 @@ use std::{ffi::CStr, os::raw::c_void};
 
 use qemu_api::{
     bindings::*,
-    declare_properties, define_property,
+    c_str, declare_properties, define_property,
     definitions::{Class, ObjectImpl},
     device_class_init,
     zeroable::Zeroable,
@@ -16,7 +16,7 @@ use qemu_api::{
 fn test_device_decl_macros() {
     // Test that macros can compile.
     pub static VMSTATE: VMStateDescription = VMStateDescription {
-        name: c"name".as_ptr(),
+        name: c_str!("name").as_ptr(),
         unmigratable: true,
         ..Zeroable::ZERO
     };
@@ -36,7 +36,7 @@ fn test_device_decl_macros() {
     declare_properties! {
         DUMMY_PROPERTIES,
             define_property!(
-                c"migrate-clk",
+                c_str!("migrate-clk"),
                 DummyState,
                 migrate_clock,
                 unsafe { &qdev_prop_bool },
@@ -55,7 +55,7 @@ fn test_device_decl_macros() {
     impl ObjectImpl for DummyState {
         type Class = DummyClass;
         const TYPE_INFO: qemu_api::bindings::TypeInfo = qemu_api::type_info! { Self };
-        const TYPE_NAME: &'static CStr = c"dummy";
+        const TYPE_NAME: &'static CStr = c_str!("dummy");
         const PARENT_TYPE_NAME: Option<&'static CStr> = Some(TYPE_DEVICE);
         const ABSTRACT: bool = false;
         const INSTANCE_INIT: Option<unsafe extern "C" fn(obj: *mut Object)> = None;