Convert string ref props to callback props

When enableRefAsProp is on, we should always use the props as the source
of truth for refs. Not a field on the fiber.

In the case of string refs, this presents a problem, because string refs
are not passed around internally as strings; they are converted to
callback refs. The ref used by the reconciler is not the same as the one
the user provided.

But since this is a deprecated feature anyway, what we can do is clone
the props object and replace it with the internal callback ref. Then we
can continue to use the props object as the source of truth.

This means the internal callback ref will leak into userspace. The
receiving component will receive a callback ref even though the parent
passed a string. Which is weird, but again, this is a deprecated
feature, and we're only leaving it around behind a flag so that Meta ca
keep using string refs temporarily while they finish migrating
their codebase.

packages/react-reconciler/src/ReactChildFiber.js

@@ -41,6 +41,7 @@ import {
   Fragment,
 } from './ReactWorkTags';
 import isArray from 'shared/isArray';
+import assign from 'shared/assign';
 import {checkPropStringCoercion} from 'shared/CheckStringCoercion';
 import {enableRefAsProp} from 'shared/ReactFeatureFlags';
 
@@ -276,13 +277,38 @@ function coerceRef(
       element,
       mixedRef,
     );
+
+    if (enableRefAsProp) {
+      // When enableRefAsProp is on, we should always use the props as the
+      // source of truth for refs. Not a field on the fiber.
+      //
+      // In the case of string refs, this presents a problem, because string
+      // refs are not passed around internally as strings; they are converted to
+      // callback refs. The ref used by the reconciler is not the same as the
+      // one the user provided.
+      //
+      // But since this is a deprecated feature anyway, what we can do is clone
+      // the props object and replace it with the internal callback ref. Then we
+      // can continue to use the props object as the source of truth.
+      //
+      // This means the internal callback ref will leak into userspace. The
+      // receiving component will receive a callback ref even though the parent
+      // passed a string. Which is weird, but again, this is a deprecated
+      // feature, and we're only leaving it around behind a flag so that Meta
+      // can keep using string refs temporarily while they finish migrating
+      // their codebase.
+      const userProvidedProps = workInProgress.pendingProps;
+      const propsWithInternalCallbackRef = assign({}, userProvidedProps);
+      propsWithInternalCallbackRef.ref = coercedRef;
+      workInProgress.pendingProps = propsWithInternalCallbackRef;
+    }
   } else {
     coercedRef = mixedRef;
   }
 
-    // TODO: If enableRefAsProp is on, we shouldn't use the `ref` field. We
-    // should always read the ref from the prop.
-    workInProgress.ref = coercedRef;
+  // TODO: If enableRefAsProp is on, we shouldn't use the `ref` field. We
+  // should always read the ref from the prop.
+  workInProgress.ref = coercedRef;
 }
 
 function throwOnInvalidObjectType(returnFiber: Fiber, newChild: Object) {

packages/react-reconciler/src/__tests__/ReactFiberRefs-test.js

@@ -84,4 +84,32 @@ describe('ReactFiberRefs', () => {
     expect(ref1.current).toBe(null);
     expect(ref2.current).not.toBe(null);
   });
+
+  // @gate enableRefAsProp
+  test('string ref props are converted to function refs', async () => {
+    let refProp;
+    function Child({ref}) {
+      refProp = ref;
+      return <div ref={ref} />;
+    }
+
+    let owner;
+    class Owner extends React.Component {
+      render() {
+        owner = this;
+        return <Child ref="child" />;
+      }
+    }
+
+    const root = ReactNoop.createRoot();
+    await act(() => root.render(<Owner />));
+
+    // When string refs aren't disabled, and enableRefAsProp is on, string refs
+    // the receiving component receives a callback ref, not the original string.
+    // This behavior should never be shipped to open source; it's only here to
+    // allow Meta to keep using string refs temporarily while they finish
+    // migrating their codebase.
+    expect(typeof refProp === 'function').toBe(true);
+    expect(owner.refs.child.type).toBe('div');
+  });
 });