Fix std_detect_file_io

Author: Amanieu

crates/std_detect/src/lib.rs

@@ -24,6 +24,11 @@
 #[macro_use]
 extern crate std;
 
+// rust-lang/rust#83888: removing `extern crate` gives an error that `vec_spare>
+#[cfg_attr(feature = "std_detect_file_io", allow(unused_extern_crates))]
+#[cfg(feature = "std_detect_file_io")]
+extern crate alloc;
+
 #[doc(hidden)]
 #[unstable(feature = "stdsimd", issue = "27731")]
 pub mod detect;