[09/16] rust: introduce a c_str macro

Commit Message

Paolo Bonzini Oct. 15, 2024, 1:17 p.m. UTC

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 <[e-mail]>
---
 rust/hw/char/pl011/src/device.rs       |  3 +-
 rust/hw/char/pl011/src/device_class.rs | 10 +++--
 rust/hw/char/pl011/src/lib.rs          |  4 +-
 rust/qemu-api/meson.build              |  1 +
 rust/qemu-api/src/c_str.rs             | 52 ++++++++++++++++++++++++++
 rust/qemu-api/src/lib.rs               |  1 +
 rust/qemu-api/src/tests.rs             |  8 ++--
 7 files changed, 70 insertions(+), 9 deletions(-)
 create mode 100644 rust/qemu-api/src/c_str.rs

Patch

diff --git a/rust/hw/char/pl011/src/device.rs b/rust/hw/char/pl011/src/device.rs
index cd4c01c2336..55d933ee5e9 100644
--- a/rust/hw/char/pl011/src/device.rs
+++ b/rust/hw/char/pl011/src/device.rs
@@ -11,6 +11,7 @@ 
 
 use qemu_api::{
     bindings::{self, *},
+    c_str,
     definitions::ObjectImpl,
 };
 
@@ -99,7 +100,7 @@  impl qemu_api::definitions::Class for PL011Class {
 }
 
 #[used]
-pub static CLK_NAME: &CStr = c"clk";
+pub static CLK_NAME: &CStr = c_str!("clk");
 
 impl PL011State {
     /// Initializes a pre-allocated, unitialized instance of `PL011State`.
diff --git a/rust/hw/char/pl011/src/device_class.rs b/rust/hw/char/pl011/src/device_class.rs
index b7ab31af02d..a3d1b1e929a 100644
--- a/rust/hw/char/pl011/src/device_class.rs
+++ b/rust/hw/char/pl011/src/device_class.rs
@@ -4,7 +4,11 @@ 
 
 use core::ptr::NonNull;
 
-use qemu_api::{bindings::*, definitions::ObjectImpl};
+use qemu_api::{
+    bindings::*,
+    c_str,
+    definitions::ObjectImpl
+};
 
 use crate::device::PL011State;
 
@@ -18,14 +22,14 @@ 
 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 2b157868b0f..0a598e5629d 100644
--- a/rust/hw/char/pl011/src/lib.rs
+++ b/rust/hw/char/pl011/src/lib.rs
@@ -41,11 +41,13 @@ 
 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: &::std::ffi::CStr = c_str!("pl011");
 
 /// 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 436e2f1e836..b55931c6490 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/tests.rs',
diff --git a/rust/qemu-api/src/c_str.rs b/rust/qemu-api/src/c_str.rs
new file mode 100644
index 00000000000..0286dade306
--- /dev/null
+++ b/rust/qemu-api/src/c_str.rs
@@ -0,0 +1,52 @@ 
+// Copyright 2024 Red Hat, Inc.
+// Author(s): Paolo Bonzini <[e-mail]>
+// 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 == BYTES.len() - 1 {}
+            else if BYTES[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!("