RuntimeGenesiConfig: json macro added

[michalkucharczyk]

This PR adds build_struct_json_patch which helps to generate a JSON used for preset.

Here is doc and example:

polkadot-sdk/substrate/frame/support/src/generate_genesis_config.rs

Lines 168 to 266 in d868b85

	/// Creates a `RuntimeGenesisConfig` JSON patch, supporting recursive field initialization.
	///
	/// This macro creates a default `RuntimeGenesisConfig`, initializing specified fields (and nested
	/// fields) with the provided values. Any fields not explicitly given are initialized with their
	/// default values. The macro then serializes the fully initialized structure into a JSON blob,
	/// retaining only the fields that were explicitly provided, either partially or fully initialized.
	///
	/// The recursive nature of this macro allows nested structures within `RuntimeGenesisConfig`
	/// to be partially or fully initialized, and only the explicitly initialized fields are retained
	/// in the final JSON output.
	///
	/// This approach prevents errors from manually creating JSON objects, such as typos or
	/// inconsistencies with the `RuntimeGenesisConfig` structure, by relying on the actual
	/// struct definition. This ensures the generated JSON is valid and reflects any future changes
	/// to the structure.
	///
	/// This macro assumes that the `serde(rename_all="camelCase")` attribute is applied to
	/// `RuntimeGenesisConfig`, which is typical for frame-based runtimes.
	///
	/// # Example
	///
	/// ```rust
	/// use frame_support::generate_config;
	/// #[derive(Default, serde::Serialize, serde::Deserialize)]
	/// #[serde(rename_all = "camelCase")]
	/// struct RuntimeGenesisConfig {
	/// a_field: u32,
	/// b_field: B,
	/// c_field: u32,
	/// }
	///
	/// #[derive(Default, serde::Serialize, serde::Deserialize)]
	/// #[serde(rename_all = "camelCase")]
	/// struct B {
	/// i_field: u32,
	/// j_field: u32,
	/// }
	///
	/// impl B {
	/// fn new() -> Self {
	/// Self { i_field: 0, j_field: 2 }
	/// }
	/// }
	///
	/// assert_eq!(
	/// generate_config! ( RuntimeGenesisConfig {
	/// b_field: B {
	/// i_field: 2,
	/// }
	/// }),
	///
	/// serde_json::json!({
	/// "bField": {"iField": 2}
	/// })
	/// );
	///
	/// assert_eq!(
	/// generate_config! ( RuntimeGenesisConfig {
	/// a_field: 66,
	///
	/// }),
	/// serde_json::json!({
	/// "aField": 66,
	/// })
	/// );
	///
	/// assert_eq!(
	/// generate_config! ( RuntimeGenesisConfig {
	/// a_field: 66,
	/// b_field: B::new()
	///
	/// }),
	/// serde_json::json!({
	/// "aField": 66,
	/// "bField": {"iField": 0, "jField": 2}
	/// })
	/// );
	/// ```
	///
	/// In this example:
	/// ```ignore
	/// generate_config! ( RuntimeGenesisConfig {
	/// b_field: B {
	/// i_field: 2,
	/// }
	/// }),
	/// ```
	/// `b_field` is partially initialized, it will be expanded to:
	/// ```ignore
	/// RuntimeGenesisConfig {
	/// b_field {
	/// i_field: 2,
	/// ..Default::default()
	/// },
	/// ..Default::default()
	/// }
	/// ```
	/// while all other fields are initialized with default values. The macro serializes this, retaining
	/// only the provided fields.

And real-world usage:

polkadot-sdk/cumulus/parachains/runtimes/assets/asset-hub-rococo/src/genesis_config_presets.rs

Lines 37 to 59 in c92afa3

	build_struct_json_patch!(RuntimeGenesisConfig {
	balances: BalancesConfig {
	balances: endowed_accounts.iter().cloned().map(|k| (k, endowment)).collect(),
	},
	parachain_info: ParachainInfoConfig { parachain_id: id },
	collator_selection: CollatorSelectionConfig {
	invulnerables: invulnerables.iter().cloned().map(|(acc, _)| acc).collect(),
	candidacy_bond: ASSET_HUB_ROCOCO_ED * 16,
	},
	session: SessionConfig {
	keys: invulnerables
	.into_iter()
	.map(|(acc, aura)| {
	(
	acc.clone(), // account id
	acc, // validator id
	SessionKeys { aura }, // session keys
	)
	})
	.collect(),
	},
	polkadot_xcm: PolkadotXcmConfig { safe_xcm_version: Some(SAFE_XCM_VERSION) },
	})

Closes #5700

[michalkucharczyk]

I'll do one more renaming round.
Thinking about something like init_partial... which better reflects what macro actually does.

[skunert]

One thing I noticed while playing that is not supported is initilializing with full path, e.g. this fails:

use frame_support::build_struct_json_patch;

mod sub_module {
	#[derive(Default, serde::Serialize, serde::Deserialize)]
	#[serde(rename_all = "camelCase")]
	pub struct B {
		pub i_field: u32,
		pub j_field: u32,
	}
}
#[derive(Default, serde::Serialize, serde::Deserialize)]
#[serde(rename_all = "camelCase")]
struct RuntimeGenesisConfig {
	b_field: sub_module::B,
}

build_struct_json_patch!(RuntimeGenesisConfig { b_field: sub_module::B { i_field: 10 } });

The macro parses b_field as fully initialized and throws an error about missing j_field. Not sure how easy this is to fix.

[michalkucharczyk]

One thing I noticed while playing that is not supported is initilializing with full path, e.g. this fails:
...

S**t I forgot about tests for paths, should be fixed with d6c8854

[bkchr]

Machine! :D

Nice!

I left some comments around macro hygiene, for the macro itself you have enough tests.