Make default Rust code compatible with 2024 edition (#7581)

examples/rust/default.rs

@@ -3,10 +3,10 @@
 // As of Rust 1.75, small functions are automatically
 // marked as `#[inline]` so they will not show up in
 // the output when compiling with optimisations. Use
-// `#[no_mangle]` or `#[inline(never)]` to work around
-// this issue.
+// `#[unsafe(no_mangle)]` or `#[inline(never)]` to
+// work around this issue.
 // See https://github.com/compiler-explorer/compiler-explorer/issues/5939
-#[no_mangle]
+#[unsafe(no_mangle)]
 pub fn square(num: i32) -> i32 {
     num * num
 }