[PATCH 11/23] rust: introduce a c_str macro

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/hw/char/pl011/src/device.rs       |  5 ++-
 rust/hw/char/pl011/src/device_class.rs | 18 ++++-----
 rust/hw/char/pl011/src/lib.rs          |  6 ++-
 rust/qemu-api/meson.build              |  4 ++
 rust/qemu-api/src/c_str.rs             | 53 ++++++++++++++++++++++++++
 rust/qemu-api/src/lib.rs               |  1 +
 rust/qemu-api/src/vmstate.rs           |  2 +-
 rust/qemu-api/tests/tests.rs           |  8 ++--
 8 files changed, 78 insertions(+), 19 deletions(-)
 create mode 100644 rust/qemu-api/src/c_str.rs

diff --git a/rust/hw/char/pl011/src/device.rs b/rust/hw/char/pl011/src/device.rs
index 036757f7f3a..2d225d544de 100644
--- a/rust/hw/char/pl011/src/device.rs
+++ b/rust/hw/char/pl011/src/device.rs
@@ -10,6 +10,7 @@
 
 use qemu_api::{
     bindings::{self, *},
+    c_str,
     definitions::ObjectImpl,
 };
 
@@ -135,7 +136,7 @@ impl PL011State {
     /// location/instance. All its fields are expected to hold unitialized
     /// values with the sole exception of `parent_obj`.
     unsafe fn init(&mut self) {
-        const CLK_NAME: &CStr = c"clk";
+        const CLK_NAME: &CStr = c_str!("clk");
 
         let dev = addr_of_mut!(*self).cast::<DeviceState>();
         // SAFETY:
@@ -598,7 +599,7 @@ pub fn post_load(&mut self, _version_id: u32) -> Result<(), ()> {
         let dev: *mut DeviceState = qdev_new(PL011State::TYPE_INFO.name);
         let sysbus: *mut SysBusDevice = dev.cast::<SysBusDevice>();
 
-        qdev_prop_set_chr(dev, c"chardev".as_ptr(), chr);
+        qdev_prop_set_chr(dev, c_str!("chardev").as_ptr(), chr);
         sysbus_realize_and_unref(sysbus, addr_of!(error_fatal) as *mut *mut Error);
         sysbus_mmio_map(sysbus, 0, addr);
         sysbus_connect_irq(sysbus, 0, irq);
diff --git a/rust/hw/char/pl011/src/device_class.rs b/rust/hw/char/pl011/src/device_class.rs
index 27cabbd6651..8a2019e5345 100644
--- a/rust/hw/char/pl011/src/device_class.rs
+++ b/rust/hw/char/pl011/src/device_class.rs
@@ -2,14 +2,12 @@
 // Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use core::{
-    ffi::{c_int, c_void},
-    ptr::NonNull,
-};
+use core::ptr::NonNull;
+use std::os::raw::{c_int, c_void};
 
 use qemu_api::{
-    bindings::*, vmstate_clock, vmstate_fields, vmstate_int32, vmstate_subsections, vmstate_uint32,
-    vmstate_uint32_array, vmstate_unused, zeroable::Zeroable,
+    bindings::*, c_str, vmstate_clock, vmstate_fields, vmstate_int32, vmstate_subsections,
+    vmstate_uint32, vmstate_uint32_array, vmstate_unused, zeroable::Zeroable,
 };
 
 use crate::device::{PL011State, PL011_FIFO_DEPTH};
@@ -24,7 +22,7 @@ extern "C" fn pl011_clock_needed(opaque: *mut c_void) -> bool {
 
 /// Migration subsection for [`PL011State`] clock.
 pub static VMSTATE_PL011_CLOCK: VMStateDescription = VMStateDescription {
-    name: c"pl011/clock".as_ptr(),
+    name: c_str!("pl011/clock").as_ptr(),
     version_id: 1,
     minimum_version_id: 1,
     needed: Some(pl011_clock_needed),
@@ -43,7 +41,7 @@ extern "C" fn pl011_post_load(opaque: *mut c_void, version_id: c_int) -> c_int {
 }
 
 pub static VMSTATE_PL011: VMStateDescription = VMStateDescription {
-    name: c"pl011".as_ptr(),
+    name: c_str!("pl011").as_ptr(),
     version_id: 2,
     minimum_version_id: 2,
     post_load: Some(pl011_post_load),
@@ -74,14 +72,14 @@ extern "C" fn pl011_post_load(opaque: *mut c_void, version_id: c_int) -> c_int {
 qemu_api::declare_properties! {
     PL011_PROPERTIES,
     qemu_api::define_property!(
-        c"chardev",
+        c_str!("chardev"),
         PL011State,
         char_backend,
         unsafe { &qdev_prop_chr },
         CharBackend
     ),
     qemu_api::define_property!(
-        c"migrate-clk",
+        c_str!("migrate-clk"),
         PL011State,
         migrate_clock,
         unsafe { &qdev_prop_bool },
diff --git a/rust/hw/char/pl011/src/lib.rs b/rust/hw/char/pl011/src/lib.rs
index a539062aa30..a98d5cf9574 100644
--- a/rust/hw/char/pl011/src/lib.rs
+++ b/rust/hw/char/pl011/src/lib.rs
@@ -41,12 +41,14 @@
 extern crate bilge_impl;
 extern crate qemu_api;
 
+use qemu_api::c_str;
+
 pub mod device;
 pub mod device_class;
 pub mod memory_ops;
 
-pub const TYPE_PL011: &::std::ffi::CStr = c"pl011";
-pub const TYPE_PL011_LUMINARY: &::std::ffi::CStr = c"pl011_luminary";
+pub const TYPE_PL011: &::std::ffi::CStr = c_str!("pl011");
+pub const TYPE_PL011_LUMINARY: &::std::ffi::CStr = c_str!("pl011_luminary");
 
 /// Offset of each register from the base memory address of the device.
 ///
diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
index 3b849f7c413..c950b008d59 100644
--- a/rust/qemu-api/meson.build
+++ b/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,
diff --git a/rust/qemu-api/src/c_str.rs b/rust/qemu-api/src/c_str.rs
new file mode 100644
index 00000000000..cc6d8dc1211
--- /dev/null
+++ b/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);
+    }
+}
diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
index ed840ee2f72..e6bd953e10b 100644
--- a/rust/qemu-api/src/lib.rs
+++ b/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;
diff --git a/rust/qemu-api/src/vmstate.rs b/rust/qemu-api/src/vmstate.rs
index 5dd783a902a..0aaaef1c7c8 100644
--- a/rust/qemu-api/src/vmstate.rs
+++ b/rust/qemu-api/src/vmstate.rs
@@ -14,7 +14,7 @@
 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,
diff --git a/rust/qemu-api/tests/tests.rs b/rust/qemu-api/tests/tests.rs
index c7089f0cf21..381ac84657b 100644
--- a/rust/qemu-api/tests/tests.rs
+++ b/rust/qemu-api/tests/tests.rs
@@ -6,7 +6,7 @@
 
 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 @@
 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 @@ pub struct DummyClass {
     declare_properties! {
         DUMMY_PROPERTIES,
             define_property!(
-                c"migrate-clk",
+                c_str!("migrate-clk"),
                 DummyState,
                 migrate_clock,
                 unsafe { &qdev_prop_bool },
@@ -55,7 +55,7 @@ pub struct DummyClass {
     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;
-- 
2.47.0