Fixes error message when _ is used without dev mode (#13886)

* Initial changes

* Adds UI test for error when _ is used without dev_mode

* Minor

* ".git/.scripts/commands/fmt/fmt.sh"

* Adds test to verify hasher

* Fixes error message when _ is used without dev mode

* Updates test

* Addresses review comment

---------

Co-authored-by: command-bot <>

substrate/frame/support/procedural/src/pallet/parse/storage.rs

@@ -514,10 +514,16 @@ fn process_unnamed_generics(
 	})?;
 
 	let use_default_hasher = |arg_pos| {
-		if let Some(arg) = retrieve_arg(arg_pos).ok() {
-			dev_mode && syn::parse2::<syn::Token![_]>(arg.to_token_stream()).is_ok()
+		let arg = retrieve_arg(arg_pos)?;
+		if syn::parse2::<syn::Token![_]>(arg.to_token_stream()).is_ok() {
+			if dev_mode {
+				Ok(true)
+			} else {
+				let msg = "`_` can only be used in dev_mode. Please specify an appropriate hasher.";
+				Err(syn::Error::new(arg.span(), msg))
+			}
 		} else {
-			false
+			Ok(false)
 		}
 	};
 
@@ -528,13 +534,13 @@ fn process_unnamed_generics(
 			None,
 			Metadata::Map { key: retrieve_arg(2)?, value: retrieve_arg(3)? },
 			retrieve_arg(4).ok(),
-			use_default_hasher(1),
+			use_default_hasher(1)?,
 		),
 		StorageKind::CountedMap => (
 			None,
 			Metadata::CountedMap { key: retrieve_arg(2)?, value: retrieve_arg(3)? },
 			retrieve_arg(4).ok(),
-			use_default_hasher(1),
+			use_default_hasher(1)?,
 		),
 		StorageKind::DoubleMap => (
 			None,
@@ -544,7 +550,7 @@ fn process_unnamed_generics(
 				value: retrieve_arg(5)?,
 			},
 			retrieve_arg(6).ok(),
-			use_default_hasher(1) && use_default_hasher(3),
+			use_default_hasher(1)? && use_default_hasher(3)?,
 		),
 		StorageKind::NMap => {
 			let keygen = retrieve_arg(1)?;

substrate/frame/support/test/tests/pallet_ui/dev_mode_without_arg_default_hasher.stderr

@@ -1,19 +1,11 @@
-error[E0121]: the placeholder `_` is not allowed within types on item signatures for type aliases
+error: `_` can only be used in dev_mode. Please specify an appropriate hasher.
   --> tests/pallet_ui/dev_mode_without_arg_default_hasher.rs:21:47
    |
 21 |     type MyStorageMap<T: Config> = StorageMap<_, _, u32, u64>;
-   |                                                  ^ not allowed in type signatures
+   |                                                  ^
 
-error[E0121]: the placeholder `_` is not allowed within types on item signatures for type aliases
-  --> tests/pallet_ui/dev_mode_without_arg_default_hasher.rs:24:59
-   |
-24 |     type MyStorageDoubleMap<T: Config> = StorageDoubleMap<_, _, u32, _, u64, u64>;
-   |                                                              ^       ^ not allowed in type signatures
-   |                                                              |
-   |                                                              not allowed in type signatures
-
-error[E0121]: the placeholder `_` is not allowed within types on item signatures for type aliases
-  --> tests/pallet_ui/dev_mode_without_arg_default_hasher.rs:27:61
-   |
-27 |     type MyCountedStorageMap<T: Config> = CountedStorageMap<_, _, u32, u64>;
-   |                                                                ^ not allowed in type signatures
+error[E0432]: unresolved import `pallet`
+ --> tests/pallet_ui/dev_mode_without_arg_default_hasher.rs:3:9
+  |
+3 | pub use pallet::*;
+  |         ^^^^^^ help: a similar path exists: `test_pallet::pallet`