perf($compile): Lazily compile the transclude function

For transcluded directives, the transclude function can be lazily compiled
most of the time since the contents will not be needed until the
`transclude` function was actually invoked.  For example, the `transclude`
function that is passed to `ng-if` or `ng-switch-when` does not need to be
invoked until the condition that it's bound to has been matched.  For
complex trees or switch statements, this can represent significant
performance gains since compilation of branches is deferred, and that
compilation may never actually happen if it isn't needed.

There are two instances where compilation will not be lazy; when we scan
ahead in the array of directives to be processed and find at least two of
the following:

* A directive that is transcluded and does not allow multiple transclusion
* A directive that has templateUrl and replace: true

In both of those cases, we will need to continue eager compilation in
order to generate the multiple transclusion exception at the correct time.

Author: dcherman

src/ng/compile.js

@@ -1625,6 +1625,37 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
       };
     }
 
+    /**
+     * A function generator that is used to support both eager and lazy compilation
+     * linking function.
+     * @param eager
+     * @param $compileNodes
+     * @param transcludeFn
+     * @param maxPriority
+     * @param ignoreDirective
+     * @param previousCompileContext
+     * @returns {Function}
+     */
+    function compilationGenerator(eager, $compileNodes, transcludeFn, maxPriority, ignoreDirective, previousCompileContext) {
+        if (eager) {
+            return compile($compileNodes, transcludeFn, maxPriority, ignoreDirective, previousCompileContext);
+        }
+
+        var compiled;
+
+        return function() {
+            if (!compiled) {
+                compiled = compile($compileNodes, transcludeFn, maxPriority, ignoreDirective, previousCompileContext);
+
+                // Null out all of these references in order to make them eligible for garbage collection
+                // since this is a potentially long lived closure
+                $compileNodes = transcludeFn = previousCompileContext = null;
+            }
+
+            return compiled.apply(this, arguments);
+        };
+    }
+
     /**
      * Once the directives have been collected, their compile functions are executed. This method
      * is responsible for inlining directive templates as well as terminating the application
@@ -1669,6 +1700,8 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
           replaceDirective = originalReplaceDirective,
           childTranscludeFn = transcludeFn,
           linkFn,
+          didScanForMultipleTransclusion = false,
+          mightHaveMultipleTransclusionError = false,
           directiveValue;
 
       // executes all directives on the current element
@@ -1711,6 +1744,27 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
 
         directiveName = directive.name;
 
+        // If we encounter a condition that can result in transclusion on the directive,
+        // then scan ahead in the remaining directives for others that may cause a multiple
+        // transclusion error to be thrown during the compilation process.  If a matching directive
+        // is found, then we know that when we encounter a transcluded directive, we need to eagerly
+        // compile the `transclude` function rather than doing it lazily in order to throw
+        // exceptions at the correct time
+        if (!didScanForMultipleTransclusion && (directive.templateUrl && directive.replace)
+            || (directive.transclude && !directive.$$tlb)) {
+                var candidateDirective;
+
+                for (var scanningIndex = i + 1; candidateDirective = directives[scanningIndex++];) {
+                    if ((candidateDirective.transclude && !candidateDirective.$$tlb)
+                        || (candidateDirective.templateUrl && candidateDirective.replace)) {
+                        mightHaveMultipleTransclusionError = true;
+                        break;
+                    }
+                }
+
+                didScanForMultipleTransclusion = true;
+        }
+
         if (!directive.templateUrl && directive.controller) {
           directiveValue = directive.controller;
           controllerDirectives = controllerDirectives || createMap();
@@ -1740,7 +1794,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
             compileNode = $compileNode[0];
             replaceWith(jqCollection, sliceArgs($template), compileNode);
 
-            childTranscludeFn = compile($template, transcludeFn, terminalPriority,
+            childTranscludeFn = compilationGenerator(mightHaveMultipleTransclusionError, $template, transcludeFn, terminalPriority,
                                         replaceDirective && replaceDirective.name, {
                                           // Don't pass in:
                                           // - controllerDirectives - otherwise we'll create duplicates controllers
@@ -1754,7 +1808,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
           } else {
             $template = jqLite(jqLiteClone(compileNode)).contents();
             $compileNode.empty(); // clear contents
-            childTranscludeFn = compile($template, transcludeFn);
+            childTranscludeFn = compilationGenerator(mightHaveMultipleTransclusionError, $template, transcludeFn);
           }
         }