add #[target_feature(enable = "sha3")] for the ARM backend

Author: newpavlov

keccak/src/lib.rs

@@ -56,8 +56,13 @@ impl Keccak {
     pub fn with_backend(&self, f: impl BackendClosure) {
         #[cfg(target_arch = "aarch64")]
         if self.armv8_sha3.get() {
-            // TODO: impl ARM backend
-            return f.call_once::<SoftBakend>();
+            #[target_feature(enable = "sha3")]
+            fn aarch64_sha3_inner(f: impl BackendClosure) {
+                // TODO: impl arm backend
+                f.call_once::<SoftBakend>();
+            }
+            // SAFETY: we checked target feature availability above
+            return unsafe { aarch64_sha3_inner(f) };
         }
         f.call_once::<SoftBakend>();
     }