tpl/resources: Deprecate resources.PostProcess in favour of templates.Defer

Fixes #15086

tpl/resources/resources.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 
 	"github.com/gohugoio/hugo/common/hmaps"
+	"github.com/gohugoio/hugo/common/hugo"
 
 	"github.com/gohugoio/hugo/resources/postpub"
 
@@ -293,6 +294,9 @@ func (ns *Namespace) Minify(r resources.ResourceTransformer) (resource.Resource,
 }
 
 // PostProcess processes r after the build.
+//
+// Deprecated: Use templates.Defer instead.
 func (ns *Namespace) PostProcess(r resource.Resource) (postpub.PostPublishedResource, error) {
+	hugo.DeprecateWithLogger("resources.PostProcess", "Use templates.Defer instead. See https://gohugo.io/functions/templates/defer/", "v0.164.0", ns.deps.Log.Logger())
 	return ns.deps.ResourceSpec.PostProcess(r)
 }

tpl/resources/resources_integration_test.go

@@ -277,3 +277,22 @@ disableKinds = ['page','section','rss','sitemap','taxonomy','term']
 	)
 	b.AssertLogContains("! WARN  Dart Sass: hugo:vars")
 }
+
+// See issue 15086.
+func TestPostProcessDeprecated(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ["taxonomy", "term", "section", "RSS", "sitemap", "robotsTXT", "404"]
+-- assets/css/main.css --
+body { color: red; }
+-- layouts/home.html --
+{{ $r := resources.Get "css/main.css" | minify | resources.PostProcess }}
+{{ $r.RelPermalink }}
+`
+
+	b := hugolib.Test(t, files, hugolib.TestOptInfo())
+
+	b.AssertLogContains("resources.PostProcess was deprecated in Hugo v0.164.0")
+}