RFC: Memoize functional components by default

text/0069-memo-by-default.md

@@ -0,0 +1,69 @@
+-   Start Date: 2019-02-25
+-   RFC PR: (leave this empty)
+-   React Issue: (leave this empty)
+
+# Summary
+
+-   Make functional components implement `React.memo` by default
+-   Add `React.always` that flags the functional component to be treated as it is currently
+-   Add a warning to `React.memo` when it is used without defining `arePropsEqual`.
+
+# Basic example
+
+```js
+import { always } from 'react';
+
+function Button(props) {
+    // Component code
+}
+
+// for users who wish to opt out of memoization
+export default always(Button);
+```
+
+Only import `memo` if `arePropsEqual` is implemented.
+
+```js
+function arePropsEqual(prevProps, nextProps) {
+    return prevProps.color.id === nextProps.color.id;
+}
+
+// warning is given is arePropsEqual is undefined
+export default memo(Button, arePropsEqual);
+```
+
+# Motivation
+
+With hooks becoming more widely accepted, there will be many cases where the component tree could be made up of entirely functional components.
+
+In such cases, `React.memo` is needed to prevent children from rerendering.
+
+However, it can become easy to forget to memoize a functional component.
+
+In most cases, I believe reconciliation to be a more expensive operation than diffing props.
+
+# Detailed design
+
+`React.always` returns a component type that tells React "render the inner type and do not bail out".
+
+`React.always` accepts any valid component type as the first argument. This ensures that it can safely wrap an import without being concerned about its implementation details.
+
+# Drawbacks
+
+-   Breaking change and potentially slow.
+
+# Alternatives
+
+-   Call it something different like `unmemo`.
+
+# Adoption strategy
+
+This is a breaking change, therefore any functional componenets that rely on the render never bailing out will need to implement `React.always`. Components that implement `React.memo` will not have their implementation changed, however if `arePropsEqual` is not implemented, a warning will be given in developer mode.
+
+# How we teach this
+
+Many other UI libraries/frameworks advertise this sort of optimization as being selling point. I believe for most users this is expected to be done already.
+
+# Unresolved questions
+
+-   Will memoization by default be slower on average than reconciliation?