Implement defer

Closes #8086
Closes #12589

tpl/template.go

@@ -20,11 +20,13 @@ import (
 	"reflect"
 	"regexp"
 	"strings"
+	"sync"
 	"unicode"
 
 	bp "github.com/gohugoio/hugo/bufferpool"
 	"github.com/gohugoio/hugo/common/hcontext"
 	"github.com/gohugoio/hugo/identity"
+	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/output/layouts"
 
 	"github.com/gohugoio/hugo/output"
@@ -160,6 +162,11 @@ type TemplateFuncGetter interface {
 	GetFunc(name string) (reflect.Value, bool)
 }
 
+type RenderingContext struct {
+	Site       site
+	SiteOutIdx int
+}
+
 type contextKey string
 
 // Context manages values passed in the context to templates.
@@ -191,6 +198,15 @@ type page interface {
 	IsNode() bool
 }
 
+type site interface {
+	Language() *langs.Language
+}
+
+const (
+	HugoDeferredTemplatePrefix = "__hdeferred/"
+	HugoDeferredTemplateSuffix = "__d="
+)
+
 const hugoNewLinePlaceholder = "___hugonl_"
 
 var stripHTMLReplacerPre = strings.NewReplacer("\n", " ", "</p>", hugoNewLinePlaceholder, "<br>", hugoNewLinePlaceholder, "<br />", hugoNewLinePlaceholder)
@@ -228,3 +244,13 @@ func StripHTML(s string) string {
 
 	return s
 }
+
+type DeferredExecution struct {
+	Mu           sync.Mutex
+	Ctx          context.Context
+	TemplateName string
+	Data         any
+
+	Executed bool
+	Result   string
+}