chore: add standalone function deprecation helper

lib/common/api/deprecate.js

@@ -35,6 +35,14 @@ const deprecate = {
     }
   },
 
+  function: (fn, newName) => {
+    const warn = warnOnce(fn.name, newName)
+    return function () {
+      warn()
+      fn.apply(this, arguments)
+    }
+  },
+
   event: (emitter, oldName, newName) => {
     const warn = newName.startsWith('-') /* internal event */
       ? warnOnce(`${oldName} event`)

spec/api-deprecations-spec.js

@@ -82,9 +82,35 @@ describe('deprecations', () => {
     expect(msg).to.include(prop)
   })
 
+  it('warns exactly once when a function is deprecated with no replacement', () => {
+    let msg
+    deprecations.setHandler(m => { msg = m })
+
+    function oldFn () { return 'hello' }
+    const deprecatedFn = deprecate.function(oldFn)
+    deprecatedFn()
+
+    expect(msg).to.be.a('string')
+    expect(msg).to.include('oldFn')
+  })
+
+  it('warns exactly once when a function is deprecated with a replacement', () => {
+    let msg
+    deprecations.setHandler(m => { msg = m })
+
+    function oldFn () { return 'hello' }
+    function newFn () { return 'goodbye' }
+    const deprecatedFn = deprecate.function(oldFn, newFn)
+    deprecatedFn()
+
+    expect(msg).to.be.a('string')
+    expect(msg).to.include('oldFn')
+    expect(msg).to.include('newFn')
+  })
+
   it('warns only once per item', () => {
     const messages = []
-    deprecations.setHandler(message => { messages.push(message) })
+    deprecations.setHandler(message => messages.push(message))
 
     const key = 'foo'
     const val = 'bar'
@@ -125,15 +151,15 @@ describe('deprecations', () => {
 
     const enableCallbackWarnings = () => {
       warnings = []
-      deprecations.setHandler(warning => { warnings.push(warning) })
+      deprecations.setHandler(warning => warnings.push(warning))
       process.enablePromiseAPIs = true
     }
 
     beforeEach(() => {
       deprecations.setHandler(null)
       process.throwDeprecation = true
 
-      promiseFunc = param => new Promise((resolve, reject) => { resolve(param) })
+      promiseFunc = param => new Promise((resolve, reject) => resolve(param))
     })
 
     it('acts as a pass-through for promise-based invocations', async () => {