[chore] improve illegal declaration error message

package-lock.json

@@ -5,7 +5,7 @@
   "requires": true,
   "packages": {
     "": {
-      "version": "3.41.0",
+      "version": "3.42.5",
       "license": "MIT",
       "devDependencies": {
         "@ampproject/remapping": "^0.3.0",

src/compiler/compile/Component.ts

@@ -587,7 +587,7 @@ export default class Component {
 
 		scope.declarations.forEach((node, name) => {
 			if (name[0] === '$') {
-				return this.error(node as any, compiler_errors.illegal_declaration);
+				return this.error(node as any, compiler_errors.illegal_declaration(name));
 			}
 
 			const writable = node.type === 'VariableDeclaration' && (node.kind === 'var' || node.kind === 'let');
@@ -660,7 +660,7 @@ export default class Component {
 
 		instance_scope.declarations.forEach((node, name) => {
 			if (name[0] === '$') {
-				return this.error(node as any, compiler_errors.illegal_declaration);
+				return this.error(node as any, compiler_errors.illegal_declaration(name));
 			}
 
 			const writable = node.type === 'VariableDeclaration' && (node.kind === 'var' || node.kind === 'let');

src/compiler/compile/compiler_errors.ts

@@ -174,10 +174,10 @@ export default {
 		code: 'default-export',
 		message: 'A component cannot have a default export'
 	},
-	illegal_declaration: {
+	illegal_declaration: (name: string) => ({
 		code: 'illegal-declaration',
-		message: 'The $ prefix is reserved, and cannot be used for variable and import names'
-	},
+		message: `The $ prefix is reserved, and cannot be used for variable and import names, but found ${name}`
+	}),
 	illegal_subscription: {
 		code: 'illegal-subscription',
 		message: 'Cannot reference store value inside <script context="module">'

test/runtime/samples/store-prevent-user-declarations/_config.js

@@ -5,5 +5,5 @@ export default {
 		count: writable(0)
 	},
 
-	error: 'The $ prefix is reserved, and cannot be used for variable and import names'
+	error: 'The $ prefix is reserved, and cannot be used for variable and import names, but found $count'
 };