Collect HTML elements during the build to use in PurgeCSS etc.

The main use case for this is to use with resources.PostProcess and resources.PostCSS with purgecss.

You would normally set it up to extract keywords from your templates, doing it from the full /public takes forever for bigger sites.

Doing the template thing misses dynamically created class names etc., and it's hard/impossible to set up in when using themes.

You can enable this in your site config:

```toml
[build]
  writeStats = true
```

It will then write a `hugo_stats.json` file to the project root as part of the build.

If you're only using this for the production build, you should consider putting it below `config/production`.

You can then set it up with PostCSS like this:

```js
const purgecss = require('@fullhuman/postcss-purgecss')({
    content: [ './hugo_stats.json' ],
    defaultExtractor: (content) => {
        let els = JSON.parse(content).htmlElements;
        return els.tags.concat(els.classes, els.ids);
    }
});

module.exports = {
    plugins: [
        require('tailwindcss'),
        require('autoprefixer'),
        ...(process.env.HUGO_ENVIRONMENT === 'production' ? [ purgecss ] : [])
    ]
};
```

Fixes #6999

1 files changed, 59 insertions, 3 deletions

diff --git a/hugolib/hugo_sites_build.go b/hugolib/hugo_sites_build.go
index 6a65605fc..fac20e883 100644
--- a/hugolib/hugo_sites_build.go
+++ b/hugolib/hugo_sites_build.go
@@ -16,11 +16,17 @@ package hugolib
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"fmt"
 	"os"
+	"path/filepath"
 	"runtime/trace"
 	"strings"
 
+	"github.com/gohugoio/hugo/publisher"
+
+	"github.com/gohugoio/hugo/hugofs"
+
 	"github.com/gohugoio/hugo/common/para"
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/resources/postpub"
@@ -146,10 +152,10 @@ func (h *HugoSites) Build(config BuildCfg, events ...fsnotify.Event) error {
 		if err != nil {
 			h.SendError(err)
 		}
-	}
 
-	if err := h.postProcess(); err != nil {
-		h.SendError(err)
+		if err = h.postProcess(); err != nil {
+			h.SendError(err)
+		}
 	}
 
 	if h.Metrics != nil {
@@ -337,6 +343,12 @@ func (h *HugoSites) render(config *BuildCfg) error {
 }
 
 func (h *HugoSites) postProcess() error {
+	// Make sure to write any build stats to disk first so it's available
+	// to the post processors.
+	if err := h.writeBuildStats(); err != nil {
+		return err
+	}
+
 	var toPostProcess []resource.OriginProvider
 	for _, s := range h.Sites {
 		for _, v := range s.ResourceSpec.PostProcessResources {
@@ -422,3 +434,47 @@ func (h *HugoSites) postProcess() error {
 	return g.Wait()
 
 }
+
+type publishStats struct {
+	CSSClasses string `json:"cssClasses"`
+}
+
+func (h *HugoSites) writeBuildStats() error {
+	if !h.ResourceSpec.BuildConfig.WriteStats {
+		return nil
+	}
+
+	htmlElements := &publisher.HTMLElements{}
+	for _, s := range h.Sites {
+		stats := s.publisher.PublishStats()
+		htmlElements.Merge(stats.HTMLElements)
+	}
+
+	htmlElements.Sort()
+
+	stats := publisher.PublishStats{
+		HTMLElements: *htmlElements,
+	}
+
+	js, err := json.MarshalIndent(stats, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	filename := filepath.Join(h.WorkingDir, "hugo_stats.json")
+
+	// Make sure it's always written to the OS fs.
+	if err := afero.WriteFile(hugofs.Os, filename, js, 0666); err != nil {
+		return err
+	}
+
+	// Write to the destination, too, if a mem fs is in play.
+	if h.Fs.Source != hugofs.Os {
+		if err := afero.WriteFile(h.Fs.Destination, filename, js, 0666); err != nil {
+			return err
+		}
+	}
+
+	return nil
+
+}