Decouple setting the current fiber and phase

These are two distinct actions.
This helps centralize setting the current fiber in the scheduler.

I'm also removing a hack that's only useful in the corner case if you use unstable_renderSubtree() and top component has invalid child context type.
This makes the code more straightforward.

src/renderers/shared/fiber/ReactDebugCurrentFiber.js

@@ -55,9 +55,13 @@ function resetCurrentFiber() {
   ReactDebugCurrentFiber.phase = null;
 }
 
-function setCurrentFiber(fiber: Fiber | null, phase: LifeCyclePhase | null) {
+function setCurrentFiber(fiber: Fiber) {
   ReactDebugCurrentFrame.getCurrentStack = getCurrentFiberStackAddendum;
   ReactDebugCurrentFiber.current = fiber;
+  ReactDebugCurrentFiber.phase = null;
+}
+
+function setCurrentPhase(phase: LifeCyclePhase | null) {
   ReactDebugCurrentFiber.phase = phase;
 }
 
@@ -66,6 +70,7 @@ var ReactDebugCurrentFiber = {
   phase: (null: LifeCyclePhase | null),
   resetCurrentFiber,
   setCurrentFiber,
+  setCurrentPhase,
   getCurrentFiberOwnerName,
   getCurrentFiberStackAddendum,
 };

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -207,9 +207,9 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
     if (__DEV__) {
       ReactCurrentOwner.current = workInProgress;
-      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, 'render');
+      ReactDebugCurrentFiber.setCurrentPhase('render');
       nextChildren = fn(nextProps, context);
-      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
+      ReactDebugCurrentFiber.setCurrentPhase(null);
     } else {
       nextChildren = fn(nextProps, context);
     }
@@ -281,9 +281,9 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     ReactCurrentOwner.current = workInProgress;
     let nextChildren;
     if (__DEV__) {
-      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, 'render');
+      ReactDebugCurrentFiber.setCurrentPhase('render');
       nextChildren = instance.render();
-      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
+      ReactDebugCurrentFiber.setCurrentPhase(null);
     } else {
       nextChildren = instance.render();
     }
@@ -726,7 +726,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     }
 
     if (__DEV__) {
-      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
+      ReactDebugCurrentFiber.setCurrentFiber(workInProgress);
     }
 
     switch (workInProgress.tag) {

src/renderers/shared/fiber/ReactFiberCompleteWork.js

@@ -188,7 +188,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     renderPriority: PriorityLevel,
   ): Fiber | null {
     if (__DEV__) {
-      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
+      ReactDebugCurrentFiber.setCurrentFiber(workInProgress);
     }
 
     // Get the latest props.

src/renderers/shared/fiber/ReactFiberContext.js

@@ -89,13 +89,6 @@ exports.getMaskedContext = function(
 
   if (__DEV__) {
     const name = getComponentName(workInProgress) || 'Unknown';
-    if (workInProgress !== ReactDebugCurrentFiber.current) {
-      warning(
-        false,
-        'Expected the work in progress to match the currently processed fiber. ' +
-          'This error is likely caused by a bug in React. Please file an issue.',
-      );
-    }
     checkPropTypes(
       contextTypes,
       context,
@@ -158,11 +151,7 @@ exports.pushTopLevelContextObject = function(
   push(didPerformWorkStackCursor, didChange, fiber);
 };
 
-function processChildContext(
-  fiber: Fiber,
-  parentContext: Object,
-  isReconciling: boolean,
-): Object {
+function processChildContext(fiber: Fiber, parentContext: Object): Object {
   const instance = fiber.stateNode;
   const childContextTypes = fiber.type.childContextTypes;
 
@@ -189,19 +178,11 @@ function processChildContext(
 
   let childContext;
   if (__DEV__) {
-    // TODO: we only have to store the "previous" fiber and phase and restore them
-    // because this method can be called outside of reconciliation. We can remove this
-    // when we stop supporting unstable_renderSubtreeIntoContainer.
-    const previousCurrentFiber = ReactDebugCurrentFiber.current;
-    const previousCurrentPhase = ReactDebugCurrentFiber.phase;
-    ReactDebugCurrentFiber.setCurrentFiber(fiber, 'getChildContext');
+    ReactDebugCurrentFiber.setCurrentPhase('getChildContext');
     startPhaseTimer(fiber, 'getChildContext');
     childContext = instance.getChildContext();
     stopPhaseTimer();
-    ReactDebugCurrentFiber.setCurrentFiber(
-      previousCurrentFiber,
-      previousCurrentPhase,
-    );
+    ReactDebugCurrentFiber.setCurrentPhase(null);
   } else {
     childContext = instance.getChildContext();
   }
@@ -215,29 +196,18 @@ function processChildContext(
   }
   if (__DEV__) {
     const name = getComponentName(fiber) || 'Unknown';
-    // We can only provide accurate element stacks if we pass work-in-progress tree
-    // during the begin or complete phase. However currently this function is also
-    // called from unstable_renderSubtree legacy implementation. In this case it unsafe to
-    // assume anything about the given fiber. We won't pass it down if we aren't sure.
-    // TODO: remove this hack when we delete unstable_renderSubtree in Fiber.
-    const workInProgress = isReconciling ? fiber : null;
-    // TODO: we only have to store the "previous" fiber and phase and restore them
-    // because this method can be called outside of reconciliation. We can remove this
-    // when we stop supporting unstable_renderSubtreeIntoContainer.
-    const previousCurrentFiber = ReactDebugCurrentFiber.current;
-    const previousCurrentPhase = ReactDebugCurrentFiber.phase;
-    ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
     checkPropTypes(
       childContextTypes,
       childContext,
       'child context',
       name,
+      // In practice, there is one case in which we won't get a stack. It's when
+      // somebody calls unstable_renderSubtreeIntoContainer() and we process
+      // context from the parent component instance. The stack will be missing
+      // because it's outside of the reconciliation, and so the pointer has not
+      // been set. This is rare and doesn't matter. We'll also remove that API.
       ReactDebugCurrentFiber.getCurrentFiberStackAddendum,
     );
-    ReactDebugCurrentFiber.setCurrentFiber(
-      previousCurrentFiber,
-      previousCurrentPhase,
-    );
   }
 
   return {...parentContext, ...childContext};
@@ -285,11 +255,7 @@ exports.invalidateContextProvider = function(
     // Merge parent and own context.
     // Skip this if we're not updating due to sCU.
     // This avoids unnecessarily recomputing memoized values.
-    const mergedContext = processChildContext(
-      workInProgress,
-      previousContext,
-      true,
-    );
+    const mergedContext = processChildContext(workInProgress, previousContext);
     instance.__reactInternalMemoizedMergedChildContext = mergedContext;
 
     // Replace the old (or empty) context with the new one.

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -181,7 +181,7 @@ export type Reconciler<C, I, TI> = {
 getContextForSubtree._injectFiber(function(fiber: Fiber) {
   const parentContext = findCurrentUnmaskedContext(fiber);
   return isContextProvider(fiber)
-    ? processChildContext(fiber, parentContext, false)
+    ? processChildContext(fiber, parentContext)
     : parentContext;
 });
 

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -317,7 +317,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   function commitAllHostEffects() {
     while (nextEffect !== null) {
       if (__DEV__) {
-        ReactDebugCurrentFiber.setCurrentFiber(nextEffect, null);
+        ReactDebugCurrentFiber.setCurrentFiber(nextEffect);
         recordEffect();
       }