Handle StorageEntry empty keys

[lexnv]

There are cases when the StorageEntry has an empty key.

For example, defining a custom pallet

#[pallet::config]
pub trait Config: frame_system::Config {
   type AssetNativeLocation: Parameter + Member + Default;
}

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

and implementing it

impl pallet_asset_registry::Config for Runtime {
    type AssetNativeLocation = ();

results in subxt generating an extra lifetime parameter for a parameter that does not exist.
For more details see #552.

This PR ensures that no lifetimes are added when dealing with empty keys.

Testing Done

Added the following patch to the substrate-node-template.

+++ b/pallets/template/src/lib.rs
@@ -24,6 +24,9 @@ pub mod pallet {
        pub trait Config: frame_system::Config {
                /// Because this pallet emits events, it depends on the runtime's definition of an event.
                type Event: From<Event<Self>> + IsType<<Self as frame_system::Config>::Event>;
+
+               /// AssetNativeLocation for () storage.
+               type AssetNativeLocation: Parameter + Member + Default + MaxEncodedLen;
        }

        #[pallet::pallet]
@@ -38,6 +41,12 @@ pub mod pallet {
        // https://docs.substrate.io/v3/runtime/storage#declaring-storage-items
        pub type Something<T> = StorageValue<_, u32>;

+       #[pallet::storage]
+       #[pallet::getter(fn location_assets)]
+       /// Local asset for native location.
+       pub type LocationAssets<T: Config> =
+               StorageMap<_, Blake2_128Concat, T::AssetNativeLocation, bool, OptionQuery>;
+
        // Pallets use events to inform users when important changes are made.
        // https://docs.substrate.io/v3/runtime/events-and-errors
        #[pallet::event]
diff --git a/runtime/src/lib.rs b/runtime/src/lib.rs
index 8a00054..4781e95 100644
--- a/runtime/src/lib.rs
+++ b/runtime/src/lib.rs
@@ -266,6 +266,7 @@ impl pallet_sudo::Config for Runtime {
 /// Configure the pallet-template in pallets/template.
 impl pallet_template::Config for Runtime {
        type Event = Event;
+       type AssetNativeLocation = ();

Before

Generated code does not compile

pub struct LocationAssets<'a>();

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

// Constructed as:
let entry = LocationAssets();

After

There is no extra lifetime parameter added.
Also, the constructor is modified to reflect the changes.

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

// Constructed as:
let entry = LocationAssets;

Closes #552.

[jsdw]

Looks good to me! The more I look at that code for generating storage entries, the more I think we need tohave a good pass over it to tidy it all up; it's all a bit of a mess nowadays as it keeps growing in complexity.

I'm working on a UI test for this too, but I think this code is OK to merge in isolation.

[jsdw]

Added a UI test in #567, so if that passes with this fix then this is good to go I think :)

[lexnv]

@jsdw Yep, the ui tests seem to pass with this PR.

The doc-tests seem to be failing currently, I ll have a look over those 😄