RuntimeGenesiConfig: json macro added

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

@@ -24,6 +24,8 @@ use sp_core::{crypto::UncheckedInto, sr25519};
 use sp_genesis_builder::PresetId;
 use testnet_parachains_constants::rococo::{currency::UNITS as ROC, xcm_version::SAFE_XCM_VERSION};
 
+use frame_support::runtime_genesis_config_json;
+
 const ASSET_HUB_ROCOCO_ED: Balance = ExistentialDeposit::get();
 
 fn asset_hub_rococo_genesis(
@@ -32,7 +34,7 @@ fn asset_hub_rococo_genesis(
 	endowment: Balance,
 	id: ParaId,
 ) -> serde_json::Value {
-	let config = RuntimeGenesisConfig {
+	runtime_genesis_config_json!({
 		balances: BalancesConfig {
 			balances: endowed_accounts.iter().cloned().map(|k| (k, endowment)).collect(),
 		},
@@ -59,10 +61,7 @@ fn asset_hub_rococo_genesis(
 			safe_xcm_version: Some(SAFE_XCM_VERSION),
 			..Default::default()
 		},
-		..Default::default()
-	};
-
-	serde_json::to_value(config).expect("Could not build genesis config.")
+	})
 }
 
 /// Encapsulates names of predefined presets.

prdoc/pr_5813.prdoc

@@ -0,0 +1,16 @@
+title: "runtime_genesis_config_json macro added"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This PR adds a macro that allows to construct a RuntimeGenesisConfig preset 
+      containing only provided fields, while performing the validation of the
+      entire struct.
+
+      Related issue: https://github.com/paritytech/polkadot-sdk/issues/5700
+
+crates:
+  - name: frame-support
+    bump: minor
+  - name: asset-hub-rococo-runtime
+    bump: patch

substrate/frame/support/src/genesis_builder_helper.rs

@@ -55,3 +55,82 @@ where
 		preset_for_name,
 	)
 }
+
+/// Creates a `RuntimeGenesisConfig` JSON patch.
+///
+/// This macro creates a default `RuntimeGenesisConfig` initializing provided fields with given
+/// values, serialize it to JSON blob, and retain only the specified fields.
+///
+/// This macro helps to prevent errors that could occur from manually creating JSON objects, such
+/// as typos or discrepancies caused by future changes to the `RuntimeGenesisConfig` structure. By
+/// using the actual struct, it ensures that the JSON generated is valid and up-to-date.
+///
+/// This macro assumes that `serde(rename_all = "camelCase")` attribute is used for
+/// RuntimeGenesisConfig, what should be the case for frame-based runtimes.
+///
+/// # Example
+///
+/// ```rust
+/// use frame_support::runtime_genesis_config_json;
+/// #[derive(Default, serde::Serialize, serde::Deserialize)]
+/// #[serde(rename_all = "camelCase")]
+/// struct RuntimeGenesisConfig {
+///     a_field: u32,
+///     b_field: u32,
+///     c_field: u32,
+/// }
+/// assert_eq!(
+///     runtime_genesis_config_json! ({a_field:31, b_field:127}),
+///     serde_json::json!({"aField":31, "bField":127})
+/// );
+/// assert_eq!(
+///     runtime_genesis_config_json! ({a_field:31}),
+///     serde_json::json!({"aField":31})
+/// );
+/// ```
+#[macro_export]
+macro_rules! runtime_genesis_config_json {
+	({ $( $key:ident : $value:expr ),* $(,)? }) => {{
+        let config = RuntimeGenesisConfig {
+            $( $key : $value, )*
+            ..Default::default()
+        };
+
+		#[inline]
+        fn compare_keys(
+            mut snake_chars: core::str::Chars,
+            mut camel_chars: core::str::Chars,
+		) -> bool {
+			loop {
+				match (snake_chars.next(), camel_chars.next()) {
+					(None, None) => return true,
+					(None, Some(_)) | (Some(_), None) => return false,
+					(Some('_'), Some(c)) => {
+						if let Some(s) = snake_chars.next() {
+							if s.to_ascii_uppercase() != c {
+								return false;
+							};
+						};
+					}
+					(Some(s), Some(c)) => {
+						if c != s {
+							return false;
+						}
+					}
+				}
+			}
+		}
+
+		let mut json_value =
+			serde_json::to_value(config).expect("serialization to json should work. qed");
+		if let serde_json::Value::Object(ref mut map) = json_value {
+            let keys_to_keep : Vec<&'static str> = vec![ $( stringify!($key) ),* ];
+			map.retain(|json_key, _| {
+				keys_to_keep.iter().any(|&struct_key| {
+					compare_keys(struct_key.chars(), json_key.chars())
+				})
+			});
+		}
+        json_value
+    }};
+}