[BUG] Wrong code generated for StorageEntry when the Key is ()

[shekohex]

Hello There, I guess I found a bug in the codegen for when the parameters of the StorageEntry is empty.

Error Message

error[E0392]: parameter `'a` is never used
     --> src/substrate/protocol_substrate_runtime.rs:29742:39
      |
29742 |             pub struct LocationAssets<'a>();
      |                                       ^^ unused parameter
      |
      = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`

Bug Overview

We have a pallet called AssetRegistry that looks like the following (simplified):

// ...snip...
#[pallet::config]
pub trait Config: frame_system::Config {
    type Event: From<Event<Self>> + IsType<<Self as frame_system::Config>::Event>;
    /// Asset type
    type AssetId: Parameter
        + Member
        + Default
        + Copy
        + BaseArithmetic
        + MaybeSerializeDeserialize
        + MaxEncodedLen;

    /// Asset location type
    type AssetNativeLocation: Parameter + Member + Default;

    /// Native Asset Id
    #[pallet::constant]
    type NativeAssetId: Get<Self::AssetId>;
}

// ...snip...

#[pallet::storage]
#[pallet::getter(fn locations)]
/// Native location of an asset.
pub type AssetLocations<T: Config> =
    StorageMap<_, Twox64Concat, T::AssetId, T::AssetNativeLocation, OptionQuery>;

#[pallet::storage]
#[pallet::getter(fn location_assets)]
/// Local asset for native location.
pub type LocationAssets<T: Config> =
    StorageMap<_, Blake2_128Concat, T::AssetNativeLocation, T::AssetId, OptionQuery>;

// ...snip...

and in our runtime, we are using it like this (simplified):

impl pallet_asset_registry::Config for Runtime {
    type AssetId = u32;
    type AssetNativeLocation = (); // **NOTICE THIS HERE**
    type Event = Event;
    type NativeAssetId = GetNativeCurrencyId;
}

The Bug

When we generate the code, from the metadata we noticed the following code:

• For AssetLocations:

pub struct AssetLocations<'a>(pub &'a ::core::primitive::u32);
impl ::subxt::StorageEntry for AssetLocations<'_> {
    const PALLET: &'static str = "AssetRegistry";
    const STORAGE: &'static str = "AssetLocations";
    type Value = ();
    fn key(&self) -> ::subxt::StorageEntryKey {
        ::subxt::StorageEntryKey::Map(vec![::subxt::StorageMapKey::new(
            &self.0,
            ::subxt::StorageHasher::Twox64Concat,
        )])
    }
}

Which looks correct (semantically and syntactically)

• For LocationAssets

pub struct LocationAssets<'a>();
impl ::subxt::StorageEntry for LocationAssets<'_> {
    const PALLET: &'static str = "AssetRegistry";
    const STORAGE: &'static str = "LocationAssets";
    type Value = ::core::primitive::u32;
    fn key(&self) -> ::subxt::StorageEntryKey {
        ::subxt::StorageEntryKey::Map(vec![::subxt::StorageMapKey::new(
            &(),
            ::subxt::StorageHasher::Blake2_128Concat,
        )])
    }
}

Which is also looking correct semantically but not syntactically

Ideas

So, this value is considered a tuple, but an empty one and I guess the code hits this path here:

subxt/codegen/src/api/storage.rs

Line 134 in 531e40d

pub struct #entry_struct_ident <'a>( #( pub &'a #field_types ),* );

but it does not take into account if the field_types is empty, we should check for that, and if so we should use core::marker::PhantomData for example.

Metadata

rustc version

rustc 1.60.0-nightly (498eeb72f 2022-01-31)
binary: rustc
commit-hash: 498eeb72f590e518e19746b346be53713689e207
commit-date: 2022-01-31
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

Substrate Runtime version/branch

version: 6.0.0
branch: polkadot-v0.9.22

subxt version

v0.21.0

[jsdw]

Thanks for reporting this and for the detailed writeup! Apologies; I've been on holiday and then abroad, but I'll have a look at this when I next get a chance!