add warning when injections has been modified

Author: Kingwl

src/core/instance/inject.js

@@ -1,6 +1,7 @@
 /* @flow */
 
 import { hasSymbol } from 'core/util/env'
+import { warn } from '../util/index'
 import { defineReactive } from '../observer/index'
 
 export function initProvide (vm: Component) {
@@ -30,7 +31,18 @@ export function initInjections (vm: Component) {
       let source = vm
       while (source) {
         if (source._provided && provideKey in source._provided) {
-          defineReactive(vm, key, source._provided[provideKey])
+          if (process.env.NODE_ENV !== 'production') {
+            defineReactive(vm, key, source._provided[provideKey], () => {
+              warn(
+                `Avoid mutating a injections directly since the value will be ` +
+                `overwritten whenever the provided component re-renders. ` +
+                `injections being mutated: "${key}"`,
+                vm
+              )
+            })
+          } else {
+            defineReactive(vm, key, source._provided[provideKey])
+          }
           break
         }
         source = source.$parent

test/unit/features/options/inject.spec.js

@@ -196,4 +196,25 @@ describe('Options provide/inject', () => {
       })
     })
   })
+
+  it('should warn when injections has been modified', () => {
+    const key = 'foo'
+    const vm = new Vue({
+      provide: {
+        foo: 1
+      }
+    })
+
+    const child = new Vue({
+      parent: vm,
+      inject: ['foo']
+    })
+
+    expect(child.foo).toBe(1)
+    child.foo = 2
+    expect(
+      `Avoid mutating a injections directly since the value will be ` +
+      `overwritten whenever the provided component re-renders. ` +
+      `injections being mutated: "${key}"`).toHaveBeenWarned()
+  })
 })