panic on invalid environment variable

substrate/frame/metadata-hash-extension/src/lib.rs

@@ -71,7 +71,9 @@ enum MetadataHash {
 const RUNTIME_METADATA: Option<[u8; 32]> = if let Some(hex) = option_env!("RUNTIME_METADATA_HASH") {
 	match const_hex::const_decode_to_array(hex.as_bytes()) {
 		Ok(hex) => Some(hex),
-		Err(_) => None,
+		Err(_) => panic!("Invalid RUNTIME_METADATA_HASH environment variable: it must be a 32 \
+			bytes value in hexadecimal: e.g. 0x1234567890abcdef1234567890ABCDEF. Upper case or \
+			lower case, 0x prefix is optional."),
 	}
 } else {
 	None
@@ -160,13 +162,16 @@ impl<T: Config + Send + Sync> TransactionExtensionBase for CheckMetadataHash<T>
 
 		Ok(signed)
 	}
-	fn weight() -> Weight {
-		Weight::default()
-	}
 }
 impl<T: Config + Send + Sync> TransactionExtension<T::RuntimeCall> for CheckMetadataHash<T> {
 	type Val = ();
 	type Pre = ();
 
+	fn weight(&self, _: &T::RuntimeCall) -> Weight {
+		// The weight is the weight of implicit, it consists of a few match operation, it is
+		// negligible.
+		Weight::zero()
+	}
+
 	impl_tx_ext_default!(T::RuntimeCall; validate prepare);
 }