Fix work with less files after #360

index.js

@@ -106,7 +106,7 @@ const ruleFunction = (expectation, options, context) => {
 
     let prettierSource;
 
-    const source = root.toString();
+    const source = root.toString(result.opts.syntax);
     try {
       prettierSource = await prettier.format(source, prettierOptions);
     } catch (err) {

package.json

@@ -46,6 +46,7 @@
     "eslint-plugin-prettier": "^5.0.0",
     "postcss": "^8.4.25",
     "postcss-html": "^1.5.0",
+    "postcss-less": "^6.0.0",
     "postcss-markdown": "^1.2.0",
     "postcss-scss": "^4.0.6",
     "postcss-styled-syntax": "^0.5.0",

test/fixtures/check.invalid.less

@@ -0,0 +1,13 @@
+.foo {
+  background-image: url("x");
+}
+
+@map: {
+  alpha: 10;
+  beta: 20;
+  gamma: 30
+};
+
+.node {
+.someMixinName(value); // inline
+}

test/fixtures/stylelint.config.js

@@ -15,6 +15,10 @@ export default {
       files: ['**/*.scss'],
       customSyntax: 'postcss-scss',
     },
+    {
+      files: ['**/*.less'],
+      customSyntax: 'postcss-less',
+    },
     {
       files: ['**/*.{js,jsx,tsx}'],
       customSyntax: 'postcss-styled-syntax',

test/stylelint-prettier-e2e.test.js

@@ -44,6 +44,20 @@ describe('E2E Tests', () => {
     assert.strictEqual(result.status, 2);
   });
 
+  test('LESS files', () => {
+    const result = runStylelint('*.less');
+
+    const expectedResult = `
+::error file=check.invalid.less,line=2,col=25,endLine=2,endColumn=28,title=Stylelint problem::Replace ""x"" with "'x'" (prettier/prettier)
+::error file=check.invalid.less,line=8,col=12,endLine=8,endColumn=13,title=Stylelint problem::Insert ";" (prettier/prettier)
+::error file=check.invalid.less,line=12,col=1,endLine=12,endColumn=2,title=Stylelint problem::Insert "··" (prettier/prettier)
+`.trim();
+
+    assert.strictEqual(result.output, '');
+    assert.strictEqual(result.error, expectedResult);
+    assert.strictEqual(result.status, 2);
+  });
+
   /**
    * Don't act upon html-like files, as prettier already handles them as whole
    * files