LayoutBuilder: Initialize BuilderItem::apply more directly

Also, add some timing information.

Change-Id: I5d38cac9e7ad0afb24504ff521f31d5202268462
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/libs/utils/builderutils.h

@@ -27,18 +27,16 @@ public:
     // Property setter
     template <typename Id, typename Arg>
     BuilderItem(IdAndArg<Id, Arg> && idarg)
-    {
+        : apply([&idarg](X *x) { doit(x, Id{}, idarg.arg); })
-        apply = [&idarg](X *x) { doit(x, Id{}, idarg.arg); };
+    {}
-    }
 
     // Nested child object
     template <typename Inner>
     BuilderItem(Inner && p)
-    {
+        : apply([&p](X *x) { doit(x, NestId{}, std::forward<Inner>(p)); })
-        apply = [&p](X *x) { doit(x, NestId{}, std::forward<Inner>(p)); };
+    {}
-    }
 
-    std::function<void(X *)> apply;
+    const std::function<void(X *)> apply;
 };
 
 #define QTC_DEFINE_BUILDER_SETTER(name, setter) \

tests/manual/layoutbuilder/v2/README.md

@@ -0,0 +1,37 @@
+
+This is an ad-hoc instruction count to judge the overhead imposed
+by Layoutbuilder for
+
+    Label ll {
+        text("World")
+    };
+
+
+(1) up to Label::Label
+(2) after new Label to up to doit()
+(3) tuple stuff in doit up to QLabel::setText parameter construction
+
+
+With
+
+    BuilderItem(IdAndArg<Id, Arg> && idarg)
+    {
+        apply = [&idarg](X *x) { doit(x, Id{}, idarg.arg); };
+    }
+
+
+    (1)   (2) + (3)    Sum
+-O0 547   259 + 802   1608
+-O2  24       23        47
+
+
+
+With
+
+    BuilderItem(IdAndArg<Id, Arg> && idarg)
+        : apply([&idarg](X *x) { doit(x, Id{}, idarg.arg); })
+    {}
+
+
+    (1)   (2) + (3)    Sum
+-O2  5        23        28

tests/manual/layoutbuilder/v2/lb.h

@@ -61,16 +61,14 @@ public:
     // Property setter
     template <typename Id, typename Arg>
     BuilderItem(IdAndArg<Id, Arg> && idarg)
-    {
+        : apply([&idarg](X *x) { doit(x, Id{}, idarg.arg); })
-        apply = [&idarg](X *x) { doit(x, Id{}, idarg.arg); };
+    {}
-    }
 
     // Nested child object
     template <typename Inner>
     BuilderItem(Inner && p)
-    {
+        : apply([&p](X *x) { doit(x, NestId{}, std::forward<Inner>(p)); })
-        apply = [&p](X *x) { doit(x, NestId{}, std::forward<Inner>(p)); };
+    {}
-    }
 
     std::function<void(X *)> apply;
 };