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>

3 files changed, 55 insertions, 1 deletions

diff --git a/rust/qemu-api/src/c_str.rs b/rust/qemu-api/src/c_str.rs
new file mode 100644
index 0000000000..4cd96da0b4
--- /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 ed840ee2f7..e6bd953e10 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 4e06e40505..9c252ce18e 100644
--- a/rust/qemu-api/src/vmstate.rs
+++ b/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,