Add --printUnusedTemplates

Fixes #9502

5 files changed, 154 insertions, 8 deletions

diff --git a/tpl/template.go b/tpl/template.go
index 0375b4a17..c5a6a44c0 100644
--- a/tpl/template.go
+++ b/tpl/template.go
@@ -44,6 +44,11 @@ type TemplateFinder interface {
 	TemplateLookupVariant
 }
 
+// UnusedTemplatesProvider lists unused templates if the build is configured to track those.
+type UnusedTemplatesProvider interface {
+	UnusedTemplates() []FileInfo
+}
+
 // TemplateHandler finds and executes templates.
 type TemplateHandler interface {
 	TemplateFinder
diff --git a/tpl/template_info.go b/tpl/template_info.go
index d9b438138..c21c0ae7d 100644
--- a/tpl/template_info.go
+++ b/tpl/template_info.go
@@ -27,6 +27,11 @@ type Info interface {
 	identity.Provider
 }
 
+type FileInfo interface {
+	Name() string
+	Filename() string
+}
+
 type InfoManager interface {
 	ParseInfo() ParseInfo
 
@@ -65,10 +70,6 @@ func (info ParseInfo) IsZero() bool {
 	return info.Config.Version == 0
 }
 
-// Info holds some info extracted from a parsed template.
-type Info1 struct {
-}
-
 type ParseConfig struct {
 	Version int
 }
diff --git a/tpl/tplimpl/integration_test.go b/tpl/tplimpl/integration_test.go
new file mode 100644
index 000000000..f71e28bc1
--- /dev/null
+++ b/tpl/tplimpl/integration_test.go
@@ -0,0 +1,61 @@
+package tplimpl_test
+
+import (
+	"path/filepath"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/hugolib"
+	"github.com/gohugoio/hugo/tpl"
+)
+
+func TestPrintUnusedTemplates(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+baseURL = 'http://example.com/'
+printUnusedTemplates=true
+-- content/p1.md --
+---
+title: "P1"
+---
+{{< usedshortcode >}}
+-- layouts/baseof.html --
+{{ block "main" . }}{{ end }}
+-- layouts/baseof.json --
+{{ block "main" . }}{{ end }}
+-- layouts/index.html --
+{{ define "main" }}FOO{{ end }}
+-- layouts/_default/single.json --
+-- layouts/_default/single.html --
+{{ define "main" }}MAIN{{ end }}
+-- layouts/post/single.html --
+{{ define "main" }}MAIN{{ end }}
+-- layouts/partials/usedpartial.html --
+-- layouts/partials/unusedpartial.html --
+-- layouts/shortcodes/usedshortcode.html --
+{{ partial "usedpartial.html" }}
+-- layouts/shortcodes/unusedshortcode.html --
+
+	`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   true,
+		},
+	)
+	b.Build()
+
+	unused := b.H.Tmpl().(tpl.UnusedTemplatesProvider).UnusedTemplates()
+
+	var names []string
+	for _, tmpl := range unused {
+		names = append(names, tmpl.Name())
+	}
+
+	b.Assert(names, qt.DeepEquals, []string{"_default/single.json", "baseof.json", "partials/unusedpartial.html", "post/single.html", "shortcodes/unusedshortcode.html"})
+	b.Assert(unused[0].Filename(), qt.Equals, filepath.Join(b.Cfg.WorkingDir, "layouts/_default/single.json"))
+}
diff --git a/tpl/tplimpl/template.go b/tpl/tplimpl/template.go
index 66ef7fc21..9d2911ee9 100644
--- a/tpl/tplimpl/template.go
+++ b/tpl/tplimpl/template.go
@@ -67,10 +67,11 @@ var embeddedTemplatesAliases = map[string][]string{
 }
 
 var (
-	_ tpl.TemplateManager    = (*templateExec)(nil)
-	_ tpl.TemplateHandler    = (*templateExec)(nil)
-	_ tpl.TemplateFuncGetter = (*templateExec)(nil)
-	_ tpl.TemplateFinder     = (*templateExec)(nil)
+	_ tpl.TemplateManager         = (*templateExec)(nil)
+	_ tpl.TemplateHandler         = (*templateExec)(nil)
+	_ tpl.TemplateFuncGetter      = (*templateExec)(nil)
+	_ tpl.TemplateFinder          = (*templateExec)(nil)
+	_ tpl.UnusedTemplatesProvider = (*templateExec)(nil)
 
 	_ tpl.Template = (*templateState)(nil)
 	_ tpl.Info     = (*templateState)(nil)
@@ -130,6 +131,11 @@ func newTemplateExec(d *deps.Deps) (*templateExec, error) {
 		funcMap[k] = v.Interface()
 	}
 
+	var templateUsageTracker map[string]templateInfo
+	if d.Cfg.GetBool("printUnusedTemplates") {
+		templateUsageTracker = make(map[string]templateInfo)
+	}
+
 	h := &templateHandler{
 		nameBaseTemplateName: make(map[string]string),
 		transformNotFound:    make(map[string]*templateState),
@@ -146,6 +152,8 @@ func newTemplateExec(d *deps.Deps) (*templateExec, error) {
 		layoutHandler:       output.NewLayoutHandler(),
 		layoutsFs:           d.BaseFs.Layouts.Fs,
 		layoutTemplateCache: make(map[layoutCacheKey]tpl.Template),
+
+		templateUsageTracker: templateUsageTracker,
 	}
 
 	if err := h.loadEmbedded(); err != nil {
@@ -225,13 +233,72 @@ func (t *templateExec) Execute(templ tpl.Template, wr io.Writer, data interface{
 		defer t.Metrics.MeasureSince(templ.Name(), time.Now())
 	}
 
+	if t.templateUsageTracker != nil {
+		if ts, ok := templ.(*templateState); ok {
+			t.templateUsageTrackerMu.Lock()
+			if _, found := t.templateUsageTracker[ts.Name()]; !found {
+				t.templateUsageTracker[ts.Name()] = ts.info
+			}
+
+			if !ts.baseInfo.IsZero() {
+				if _, found := t.templateUsageTracker[ts.baseInfo.name]; !found {
+					t.templateUsageTracker[ts.baseInfo.name] = ts.baseInfo
+				}
+			}
+			t.templateUsageTrackerMu.Unlock()
+		}
+	}
+
 	execErr := t.executor.Execute(templ, wr, data)
 	if execErr != nil {
 		execErr = t.addFileContext(templ, execErr)
 	}
+
 	return execErr
 }
 
+// TODO1
+func (t *templateExec) UnusedTemplates() []tpl.FileInfo {
+	if t.templateUsageTracker == nil {
+		return nil
+	}
+	var unused []tpl.FileInfo
+
+	for _, ti := range t.needsBaseof {
+		if _, found := t.templateUsageTracker[ti.name]; !found {
+			unused = append(unused, ti)
+		}
+	}
+
+	for _, ti := range t.baseof {
+		if _, found := t.templateUsageTracker[ti.name]; !found {
+			unused = append(unused, ti)
+		}
+	}
+
+	for _, ts := range t.main.templates {
+		ti := ts.info
+		if strings.HasPrefix(ti.name, "_internal/") {
+			continue
+		}
+		if strings.HasPrefix(ti.name, "partials/inline/pagination") {
+			// TODO(bep) we need to fix this. These are internal partials, but
+			// they may also be defined in the project, which currently could
+			// lead to some false negatives.
+			continue
+		}
+		if _, found := t.templateUsageTracker[ti.name]; !found {
+			unused = append(unused, ti)
+		}
+	}
+
+	sort.Slice(unused, func(i, j int) bool {
+		return unused[i].Name() < unused[j].Name()
+	})
+
+	return unused
+}
+
 func (t *templateExec) GetFunc(name string) (reflect.Value, bool) {
 	v, found := t.funcs[name]
 	return v, found
@@ -285,6 +352,10 @@ type templateHandler struct {
 	// Note that for shortcodes that same information is embedded in the
 	// shortcodeTemplates type.
 	templateInfo map[string]tpl.Info
+
+	// May be nil.
+	templateUsageTracker   map[string]templateInfo
+	templateUsageTrackerMu sync.Mutex
 }
 
 // AddTemplate parses and adds a template to the collection.
diff --git a/tpl/tplimpl/template_errors.go b/tpl/tplimpl/template_errors.go
index df80726f5..06d895536 100644
--- a/tpl/tplimpl/template_errors.go
+++ b/tpl/tplimpl/template_errors.go
@@ -34,6 +34,14 @@ type templateInfo struct {
 	realFilename string
 }
 
+func (t templateInfo) Name() string {
+	return t.name
+}
+
+func (t templateInfo) Filename() string {
+	return t.realFilename
+}
+
 func (t templateInfo) IsZero() bool {
 	return t.name == ""
 }