Add stackframe information to deprecation message.

Having the userland method line/file should help in fixing deprecation
warnings. I've not used Debugger here as it would add a dependency to
cakephp/core that will not be easy to satisfy for standalone packages.

src/Core/functions.php

@@ -254,10 +254,28 @@ function env($key, $default = null)
      * Helper method for outputting deprecation warnings
      *
      * @param string $message The message to output as a deprecation warning.
+     * @param int $stackFrame The stack frame to include in the error. Defaults to 2
+     *   as that should point to application/plugin code.
      * @return void
      */
-    function deprecationWarning($message)
+    function deprecationWarning($message, $stackFrame = 2)
     {
+        if (!Configure::read('debug')) {
+            return;
+        }
+        $trace = debug_backtrace();
+        if (isset($trace[$stackFrame])) {
+            $frame = $trace[$stackFrame];
+            $frame += ['file' => '[internal]', 'line' => '??'];
+
+            $message = sprintf(
+                '%s - %s, line: %s',
+                $message,
+                $frame['file'],
+                $frame['line']
+            );
+        }
+
         trigger_error($message, E_USER_DEPRECATED);
     }
 }

tests/TestCase/Core/FunctionsTest.php

@@ -14,6 +14,7 @@
  */
 namespace Cake\Test\TestCase\Core;
 
+use Cake\Core\Configure;
 use Cake\TestSuite\TestCase;
 
 /**
@@ -44,12 +45,40 @@ public function testEnv()
     }
 
     /**
+     * Test error messages coming out when debug is on
+     *
      * @expectedException PHPUnit\Framework\Error\Deprecated
-     * @expectedExceptionMessage This is going away
+     * @expectedExceptionMessage This is going away - [internal], line: ??
      */
-    public function testDeprecationWarning()
+    public function testDeprecationWarningDebugEnabled()
     {
+        Configure::write('debug', true);
+        error_reporting(E_ALL);
+        deprecationWarning('This is going away', 1);
+    }
+
+    /**
+     * Test error messages coming out when debug is on
+     *
+     * @expectedException PHPUnit\Framework\Error\Deprecated
+     * @expectedExceptionMessageRegExp /This is going away - (.*?)\/TestCase.php, line\: \d+/
+     */
+    public function testDeprecationWarningDebugEnabledDefaultFrame()
+    {
+        Configure::write('debug', true);
         error_reporting(E_ALL);
         deprecationWarning('This is going away');
     }
+
+    /**
+     * Test no error when debug is off.
+     *
+     * @return void
+     */
+    public function testDeprecationWarningDebugDisabled()
+    {
+        Configure::write('debug', false);
+        error_reporting(E_ALL);
+        $this->assertNull(deprecationWarning('This is going away'));
+    }
 }