eslint-plugin: Add rule no-unused-vars-before-return

packages/eslint-plugin/configs/custom.js

@@ -1,5 +1,9 @@
 module.exports = {
+	plugins: [
+		'@wordpress',
+	],
 	rules: {
+		'@wordpress/no-unused-vars-before-return': 'error',
 		'no-restricted-syntax': [
 			'error',
 			{

packages/eslint-plugin/index.js

@@ -1,3 +1,4 @@
 module.exports = {
 	configs: require( './configs' ),
+	rules: require( './rules' ),
 };

packages/eslint-plugin/rules/index.js

@@ -0,0 +1 @@
+module.exports = require( 'requireindex' )( __dirname );

packages/eslint-plugin/rules/no-unused-vars-before-return.js

@@ -0,0 +1,55 @@
+module.exports = {
+	meta: {
+		type: 'problem',
+		schema: [],
+	},
+	create( context ) {
+		return {
+			ReturnStatement( node ) {
+				let functionScope = context.getScope();
+				while ( functionScope.type !== 'function' && functionScope.upper ) {
+					functionScope = functionScope.upper;
+				}
+
+				if ( ! functionScope ) {
+					return;
+				}
+
+				for ( const variable of functionScope.variables ) {
+					const isAssignmentCandidate = variable.defs.some( ( def ) => {
+						return (
+							def.node.type === 'VariableDeclarator' &&
+							// Allow declarations which are not initialized.
+							def.node.init &&
+							// Target function calls as "expensive".
+							def.node.init.type === 'CallExpression' &&
+							// Allow unused if part of an object destructuring.
+							def.node.id.type !== 'ObjectPattern' &&
+							// Only target assignments preceding `return`.
+							def.node.end < node.end
+						);
+					} );
+
+					if ( ! isAssignmentCandidate ) {
+						continue;
+					}
+
+					// The first entry in `references` is the declaration
+					// itself, which can be ignored.
+					const isUsedBeforeReturn = variable.references.slice( 1 ).some( ( reference ) => {
+						return reference.identifier.end < node.end;
+					} );
+
+					if ( isUsedBeforeReturn ) {
+						continue;
+					}
+
+					context.report(
+						node,
+						`Declared variable \`${ variable.name }\` is unused before a return path`
+					);
+				}
+			},
+		};
+	},
+};