Detect conflicting module names in construct_runtime! (#7968)

substrate/frame/support/procedural/src/construct_runtime/mod.rs

@@ -62,6 +62,7 @@ impl Module {
 fn complete_modules(decl: impl Iterator<Item = ModuleDeclaration>) -> syn::Result<Vec<Module>> {
 	let mut indices = HashMap::new();
 	let mut last_index: Option<u8> = None;
+	let mut names = HashMap::new();
 
 	decl
 		.map(|module| {
@@ -88,6 +89,14 @@ fn complete_modules(decl: impl Iterator<Item = ModuleDeclaration>) -> syn::Resul
 				return Err(err);
 			}
 
+			if let Some(used_module) = names.insert(module.name.clone(), module.name.span()) {
+				let msg = "Two modules with the same name!";
+
+				let mut err = syn::Error::new(used_module, &msg);
+				err.combine(syn::Error::new(module.name.span(), &msg));
+				return Err(err);
+			}
+
 			Ok(Module {
 				name: module.name,
 				index: final_index,

substrate/frame/support/test/tests/construct_runtime_ui/conflicting_module_name.rs

@@ -0,0 +1,15 @@
+use frame_support::construct_runtime;
+
+construct_runtime! {
+	pub enum Runtime where
+		Block = Block,
+		NodeBlock = Block,
+		UncheckedExtrinsic = UncheckedExtrinsic
+	{
+		System: system::{Module},
+		Balance: balances::{Module},
+		Balance: balances::{Module},
+	}
+}
+
+fn main() {}

substrate/frame/support/test/tests/construct_runtime_ui/conflicting_module_name.stderr

@@ -0,0 +1,11 @@
+error: Two modules with the same name!
+  --> $DIR/conflicting_module_name.rs:10:3
+   |
+10 |         Balance: balances::{Module},
+   |         ^^^^^^^
+
+error: Two modules with the same name!
+  --> $DIR/conflicting_module_name.rs:11:3
+   |
+11 |         Balance: balances::{Module},
+   |         ^^^^^^^