core: io: clean unused_variables lint under no_io_statics

library/core/src/io/error/os_functions.rs

@@ -6,7 +6,7 @@ use crate::fmt;
 /// The provided reference must point to data that is entirely constant; it must
 /// not be created during runtime.
 #[inline]
-pub(super) unsafe fn set_functions(f: &'static OsFunctions) {
+pub(super) unsafe fn set_functions(_f: &'static OsFunctions) {
     // FIXME: externally implementable items may allow for weak linkage, allowing
     // these methods to be overridden even when atomic pointers are not supported.
 }

tests/run-make/core-no-io-statics/rmake.rs

@@ -0,0 +1,14 @@
+// This test checks that the core library of Rust can be compiled when enabling
+// `--cfg no_io_statics`.
+
+use run_make_support::{rustc, source_root};
+
+fn main() {
+    rustc()
+        .edition("2024")
+        .arg("-Dwarnings")
+        .crate_type("rlib")
+        .input(source_root().join("library/core/src/lib.rs"))
+        .cfg("no_io_statics")
+        .run();
+}