Reject invalid custom and arbitrary variants

src/lib/generateRules.js

@@ -9,7 +9,7 @@ import * as sharedState from './sharedState'
 import { formatVariantSelector, finalizeSelector } from '../util/formatVariantSelector'
 import { asClass } from '../util/nameClass'
 import { normalize } from '../util/dataTypes'
-import { parseVariant } from './setupContextUtils'
+import { isValidVariantFormatString, parseVariant } from './setupContextUtils'
 import isValidArbitraryValue from '../util/isValidArbitraryValue'
 import { splitAtTopLevelOnly } from '../util/splitAtTopLevelOnly.js'
 
@@ -131,6 +131,10 @@ function applyVariant(variant, matches, context) {
   if (isArbitraryValue(variant) && !context.variantMap.has(variant)) {
     let selector = normalize(variant.slice(1, -1))
 
+    if (!isValidVariantFormatString(selector)) {
+      return []
+    }
+
     let fn = parseVariant(selector)
 
     let sort = Array.from(context.variantOrder.values()).pop() << 1n

src/lib/setupContextUtils.js

@@ -170,6 +170,10 @@ function withIdentifiers(styles) {
   })
 }
 
+export function isValidVariantFormatString(format) {
+  return format.startsWith('@') || format.includes('&')
+}
+
 export function parseVariant(variant) {
   variant = variant
     .replace(/\n+/g, '')
@@ -221,10 +225,24 @@ function buildPluginApi(tailwindConfig, context, { variantList, variantMap, offs
         if (typeof variantFunction !== 'string') {
           // Safelist public API functions
           return ({ modifySelectors, container, separator }) => {
-            return variantFunction({ modifySelectors, container, separator })
+            let result = variantFunction({ modifySelectors, container, separator })
+
+            if (typeof result === 'string' && !isValidVariantFormatString(result)) {
+              throw new Error(
+                `Your custom variant \`${variantName}\` has an invalid format string. Make sure it's an at-rule or contains a \`&\` placeholder.`
+              )
+            }
+
+            return result
           }
         }
 
+        if (!isValidVariantFormatString(variantFunction)) {
+          throw new Error(
+            `Your custom variant \`${variantName}\` has an invalid format string. Make sure it's an at-rule or contains a \`&\` placeholder.`
+          )
+        }
+
         return parseVariant(variantFunction)
       })
 

tests/arbitrary-variants.test.js

@@ -77,6 +77,34 @@ test('arbitrary variants with modifiers', () => {
   })
 })
 
+test('variants without & or an at-rule are ignored', () => {
+  let config = {
+    content: [
+      {
+        raw: html`
+          <div class="[div]:underline"></div>
+          <div class="[:hover]:underline"></div>
+          <div class="[wtf-bbq]:underline"></div>
+          <div class="[lol]:hover:underline"></div>
+        `,
+      },
+    ],
+    corePlugins: { preflight: false },
+  }
+
+  let input = css`
+    @tailwind base;
+    @tailwind components;
+    @tailwind utilities;
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      ${defaults}
+    `)
+  })
+})
+
 test('arbitrary variants are sorted after other variants', () => {
   let config = {
     content: [{ raw: html`<div class="[&>*]:underline underline lg:underline"></div>` }],

tests/variants.test.js

@@ -206,6 +206,44 @@ describe('custom advanced variants', () => {
       `)
     })
   })
+
+  test('variant format string must include at-rule or & (1)', async () => {
+    let config = {
+      content: [
+        {
+          raw: html` <div class="wtf-bbq:text-center"></div> `,
+        },
+      ],
+      plugins: [
+        function ({ addVariant }) {
+          addVariant('wtf-bbq', 'lol')
+        },
+      ],
+    }
+
+    await expect(run('@tailwind components;@tailwind utilities', config)).rejects.toThrowError(
+      "Your custom variant `wtf-bbq` has an invalid format string. Make sure it's an at-rule or contains a `&` placeholder."
+    )
+  })
+
+  test('variant format string must include at-rule or & (2)', async () => {
+    let config = {
+      content: [
+        {
+          raw: html` <div class="wtf-bbq:text-center"></div> `,
+        },
+      ],
+      plugins: [
+        function ({ addVariant }) {
+          addVariant('wtf-bbq', () => 'lol')
+        },
+      ],
+    }
+
+    await expect(run('@tailwind components;@tailwind utilities', config)).rejects.toThrowError(
+      "Your custom variant `wtf-bbq` has an invalid format string. Make sure it's an at-rule or contains a `&` placeholder."
+    )
+  })
 })
 
 test('stacked peer variants', async () => {