Make runtime label extraction opt-in (#2815)

* Make runtime labeling opt-in

* Add test for EMOTION_RUNTIME_AUTO_LABEL

* Document EMOTION_RUNTIME_AUTO_LABEL

* Fix tests (maybe)

* Fix test again

* Fix eslint issue

* hoist the `runtimeAutoLabel` check

* move it back to allow for mocking

---------

Co-authored-by: Mateusz Burzyński <mateuszburzynski@gmail.com>

.changeset/weak-cooks-compare.md

@@ -0,0 +1,5 @@
+---
+'@emotion/react': minor
+---
+
+Automatic labeling at runtime is now an opt-in feature. Define `globalThis.EMOTION_RUNTIME_AUTO_LABEL = true` before Emotion gets initialized to enable it.

docs/labels.mdx

@@ -2,7 +2,7 @@
 title: 'Labels'
 ---
 
-Emotion adds a css property called `label`, the value of it is appended to the end of the class name, so it's more readable than a hash. `@emotion/babel-plugin` adds these labels automatically based on the variable name and other information, so they don't need to be manually specified.
+Emotion adds a CSS property called `label` which is appended to the generated class name to make it more readable. `@emotion/babel-plugin` adds these labels automatically based on the variable name and other information, so they don't need to be manually specified.
 
 ```jsx
 // @live
@@ -29,3 +29,16 @@ render(
   </div>
 )
 ```
+
+## Automatic Labeling at Runtime
+
+If you are not using `@emotion/babel-plugin`, you can still get automatic labels in development by setting the following global flag:
+
+```js
+globalThis.EMOTION_RUNTIME_AUTO_LABEL = true
+```
+
+This feature is opt-in because:
+
+- If you use server-side rendering and test your site in Safari, you may get spurious hydration warnings because the label computed on the server does not match the label computed in Safari.
+- This feature may degrade performance if the number of elements using the `css` prop is very large.

packages/react/__tests__/css.js

@@ -10,6 +10,10 @@ import createCache from '@emotion/cache'
 console.error = jest.fn()
 console.warn = jest.fn()
 
+beforeEach(() => {
+  delete globalThis.EMOTION_RUNTIME_AUTO_LABEL
+})
+
 afterEach(() => {
   jest.clearAllMocks()
   safeQuerySelector('body').innerHTML = ''
@@ -185,7 +189,27 @@ test('speedy option from a custom cache is inherited for <Global/> styles', () =
   expect(safeQuerySelector('body style').textContent).toEqual('')
 })
 
+it('does not autoLabel without babel or EMOTION_RUNTIME_AUTO_LABEL', () => {
+  let SomeComp = props => {
+    return (
+      <div
+        {...props}
+        css={{
+          color: 'hotpink'
+        }}
+      >
+        something
+      </div>
+    )
+  }
+  const tree = renderer.create(<SomeComp />)
+
+  expect(tree.toJSON().props.className).toMatch(/css-[^-]+/)
+})
+
 test('autoLabel without babel', () => {
+  globalThis.EMOTION_RUNTIME_AUTO_LABEL = true
+
   let SomeComp = props => {
     return (
       <div
@@ -204,6 +228,8 @@ test('autoLabel without babel', () => {
 })
 
 test('autoLabel without babel (sanitized)', () => {
+  globalThis.EMOTION_RUNTIME_AUTO_LABEL = true
+
   let SomeComp$ = props => {
     return (
       <div {...props} css={{ color: 'hotpink' }}>

packages/react/__tests__/rehydration.js

@@ -53,8 +53,8 @@ beforeEach(() => {
 test("cache created in render doesn't cause a hydration mismatch", () => {
   safeQuerySelector('body').innerHTML = [
     '<div id="root">',
-    '<style data-emotion="stl 1pdkrhd">.stl-1pdkrhd-App {color: hotpink;}</style>',
-    '<div class="stl-1pdkrhd-App">Hello world!</div>',
+    '<style data-emotion="stl 168r6j">.stl-1pdkrhd {color: hotpink;}</style>',
+    '<div class="stl-168r6j">Hello world!</div>',
     '</div>'
   ].join('')
 
@@ -141,7 +141,7 @@ test('initializing another Emotion instance should not move already moved styles
           data-s=""
         >
           
-          .stl-1pdkrhd-App{color:hotpink;}
+          .stl-168r6j{color:hotpink;}
         </style>
       </div>
     </head>
@@ -189,7 +189,7 @@ test('initializing another Emotion instance should not move already moved styles
           data-s=""
         >
           
-          .stl-1pdkrhd-App{color:hotpink;}
+          .stl-168r6j{color:hotpink;}
         </style>
       </div>
     </head>

packages/react/src/emotion-element.js

@@ -40,10 +40,16 @@ export const createEmotionProps = (
 
   newProps[typePropName] = type
 
-  // For performance, only call getLabelFromStackTrace in development and when
-  // the label hasn't already been computed
+  // Runtime labeling is an opt-in feature because:
+  // - It causes hydration warnings when using Safari and SSR
+  // - It can degrade performance if there are a huge number of elements
+  //
+  // Even if the flag is set, we still don't compute the label if it has already
+  // been determined by the Babel plugin.
   if (
     process.env.NODE_ENV !== 'production' &&
+    typeof globalThis !== 'undefined' &&
+    !!globalThis.EMOTION_RUNTIME_AUTO_LABEL &&
     !!props.css &&
     (typeof props.css !== 'object' ||
       typeof props.css.name !== 'string' ||