Add utility helper for creating utility rules that are automatically escaped and respect prefix/important options

__tests__/processPlugins.test.js

@@ -418,3 +418,55 @@ test("plugins can apply the user's chosen prefix", () => {
     }
   `)
 })
+
+test("utilities are escaped and automatically respect prefix and important options when created via `utility`", () => {
+  const [, utilities] = processPlugins({
+    plugins: [
+      function({ utility, addUtilities }) {
+        addUtilities([
+          utility('.rotate-1/4', {
+            transform: 'rotate(90deg)',
+          }),
+        ])
+      },
+    ],
+    options: {
+      prefix: 'tw-',
+      important: true,
+    },
+  })
+
+  expect(css(utilities)).toMatchCss(`
+    @variants {
+      .tw-rotate-1\\/4 {
+        transform: rotate(90deg) !important
+      }
+    }
+  `)
+})
+
+test("leading '.' is optional when creating utilities via `utility`", () => {
+  const [, utilities] = processPlugins({
+    plugins: [
+      function({ utility, addUtilities }) {
+        addUtilities([
+          utility('rotate-1/4', {
+            transform: 'rotate(90deg)',
+          }),
+        ])
+      },
+    ],
+    options: {
+      prefix: 'tw-',
+      important: true,
+    },
+  })
+
+  expect(css(utilities)).toMatchCss(`
+    @variants {
+      .tw-rotate-1\\/4 {
+        transform: rotate(90deg) !important
+      }
+    }
+  `)
+})

src/util/processPlugins.js

@@ -15,6 +15,20 @@ function defineRule(selector, properties) {
   return postcss.rule({ selector }).append(decls)
 }
 
+function defineUtility(selector, properties, options) {
+  if (selector.startsWith('.')) {
+    return defineUtility(selector.slice(1), properties, options)
+  }
+
+  const rule = defineRule(prefixSelector(options.prefix, `.${escapeClassName(selector)}`), properties)
+
+  if (options.important) {
+    rule.walkDecls(decl => (decl.important = true))
+  }
+
+  return rule
+}
+
 function defineAtRule(atRule, rules) {
   const [name, ...params] = atRule.split(' ')
 
@@ -34,6 +48,7 @@ export default function(config) {
     plugin({
       config: (path, defaultValue) => _.get(config, path, defaultValue),
       rule: defineRule,
+      utility: (selector, properties) => defineUtility(selector, properties, config.options),
       atRule: defineAtRule,
       e: escapeClassName,
       addUtilities: (utilities, variants = []) => {