Add unsafe to extern blocks for Rust 2024 (#36)

## Description

This replaces the declaration of `extern "C"` blocks with `unsafe extern
"C"` as will be required by Rust 2024. Remove this when
rust-lang/rust-bindgen#2901 is merged and
[bindgen](https://crates.io/crates/bindgen) can generate these blocks
automatically as required.

Cargo.toml

@@ -51,6 +51,7 @@ include = [
 bindgen = { version = "0.69.4", features = ["experimental"] }
 cc = "1.0.83"
 cmake = "0.1.50"
+version_check = "0.9.5"
 
 [lints.rust]
 future_incompatible = { level = "warn", priority = -1 }

build.rs

@@ -1,5 +1,7 @@
 use std::{
     env,
+    fs::File,
+    io::{self, Write as _},
     path::{Path, PathBuf},
 };
 
@@ -16,6 +18,16 @@ const LIB_BASE: &str = "open62541";
 /// the `cc` build adds it as `rustc-link-lib` automatically.
 const LIB_EXT: &str = "open62541-ext";
 
+/// Pattern to search for compatibility with Edition 2024.
+///
+/// See also [`LEGACY_EXTERN_REPLACEMENT`].
+const LEGACY_EXTERN_PATTERN: &str = r#"extern "C" {"#;
+
+/// Replacement to use for compatibility with Edition 2024.
+///
+/// See also [`LEGACY_EXTERN_PATTERN`].
+const LEGACY_EXTERN_REPLACEMENT: &str = r#"unsafe extern "C" {"#;
+
 fn main() {
     let src = env::current_dir().expect("should get current directory");
 
@@ -122,9 +134,22 @@ fn main() {
         .expect("should generate `Bindings` instance");
 
     bindings
-        .write_to_file(out_bindings_rs)
+        .write_to_file(out_bindings_rs.clone())
         .expect("should write `bindings.rs`");
 
+    // Until <https://github.com/rust-lang/rust-bindgen/issues/2901> is resolved, we replace `extern
+    // "C"` with `unsafe extern "C"` manually here. Remove this when `bindgen` is able to do it.
+    if version_check::is_min_version("1.82.0") == Some(true) {
+        // We can only use `unsafe extern` starting with Rust 1.82.0. See
+        // <https://blog.rust-lang.org/2024/10/17/Rust-1.82.0.html#safe-items-with-unsafe-extern>.
+        replace_in_file(
+            &out_bindings_rs,
+            LEGACY_EXTERN_PATTERN,
+            LEGACY_EXTERN_REPLACEMENT,
+        )
+        .expect("should add unsafe to extern statements");
+    }
+
     // Build `extern.c` and our custom `wrapper.c` that both hold additional helpers that we want to
     // link in addition to the base `open62541` library.
     cc::Build::new()
@@ -184,3 +209,17 @@ impl bindgen::callbacks::ParseCallbacks for CustomCallbacks {
         original_item_name.strip_prefix("RS_").map(str::to_owned)
     }
 }
+
+/// Replaces all occurrences of pattern in file.
+///
+/// Note that this is not particularly efficient because it reads the entire file into memory before
+/// writing it back. Care should be taken when operating on large files.
+fn replace_in_file(path: &Path, pattern: &str, replacement: &str) -> io::Result<()> {
+    let buf = io::read_to_string(File::open(path)?)?;
+
+    let buf = buf.replace(pattern, replacement);
+
+    File::create(path)?.write_all(buf.as_bytes())?;
+
+    Ok(())
+}