Add render template hooks for headings

This commit also

* Renames previous types to be non-specific. (e.g. hookedRenderer rather
  than linkRenderer)

Resolves #6713

6 files changed, 202 insertions, 63 deletions

diff --git a/hugolib/page.go b/hugolib/page.go
index fddc25fa0..bd518c1e1 100644
--- a/hugolib/page.go
+++ b/hugolib/page.go
@@ -375,48 +375,54 @@ func (ps *pageState) initCommonProviders(pp pagePaths) error {
 	return nil
 }
 
-func (p *pageState) createRenderHooks(f output.Format) (*hooks.Render, error) {
-
+func (p *pageState) createRenderHooks(f output.Format) (*hooks.Renderers, error) {
 	layoutDescriptor := p.getLayoutDescriptor()
 	layoutDescriptor.RenderingHook = true
 	layoutDescriptor.LayoutOverride = false
 	layoutDescriptor.Layout = ""
 
+	var renderers hooks.Renderers
+
 	layoutDescriptor.Kind = "render-link"
-	linkTempl, linkTemplFound, err := p.s.Tmpl().LookupLayout(layoutDescriptor, f)
+	templ, templFound, err := p.s.Tmpl().LookupLayout(layoutDescriptor, f)
 	if err != nil {
 		return nil, err
 	}
+	if templFound {
+		renderers.LinkRenderer = hookRenderer{
+			templateHandler: p.s.Tmpl(),
+			Provider:        templ.(tpl.Info),
+			templ:           templ,
+		}
+	}
 
 	layoutDescriptor.Kind = "render-image"
-	imgTempl, imgTemplFound, err := p.s.Tmpl().LookupLayout(layoutDescriptor, f)
+	templ, templFound, err = p.s.Tmpl().LookupLayout(layoutDescriptor, f)
 	if err != nil {
 		return nil, err
 	}
-
-	var linkRenderer hooks.LinkRenderer
-	var imageRenderer hooks.LinkRenderer
-
-	if linkTemplFound {
-		linkRenderer = contentLinkRenderer{
+	if templFound {
+		renderers.ImageRenderer = hookRenderer{
 			templateHandler: p.s.Tmpl(),
-			Provider:        linkTempl.(tpl.Info),
-			templ:           linkTempl,
+			Provider:        templ.(tpl.Info),
+			templ:           templ,
 		}
 	}
 
-	if imgTemplFound {
-		imageRenderer = contentLinkRenderer{
+	layoutDescriptor.Kind = "render-heading"
+	templ, templFound, err = p.s.Tmpl().LookupLayout(layoutDescriptor, f)
+	if err != nil {
+		return nil, err
+	}
+	if templFound {
+		renderers.HeadingRenderer = hookRenderer{
 			templateHandler: p.s.Tmpl(),
-			Provider:        imgTempl.(tpl.Info),
-			templ:           imgTempl,
+			Provider:        templ.(tpl.Info),
+			templ:           templ,
 		}
 	}
 
-	return &hooks.Render{
-		LinkRenderer:  linkRenderer,
-		ImageRenderer: imageRenderer,
-	}, nil
+	return &renderers, nil
 }
 
 func (p *pageState) getLayoutDescriptor() output.LayoutDescriptor {
diff --git a/hugolib/page__per_output.go b/hugolib/page__per_output.go
index d7841f178..77a01801d 100644
--- a/hugolib/page__per_output.go
+++ b/hugolib/page__per_output.go
@@ -245,7 +245,7 @@ type pageContentOutput struct {
 	placeholdersEnabledInit sync.Once
 
 	// May be nil.
-	renderHooks *hooks.Render
+	renderHooks *hooks.Renderers
 	// Set if there are more than one output format variant
 	renderHooksHaveVariants bool // TODO(bep) reimplement this in another way, consolidate with shortcodes
 
diff --git a/hugolib/site.go b/hugolib/site.go
index 5688b5fac..34671443e 100644
--- a/hugolib/site.go
+++ b/hugolib/site.go
@@ -1650,14 +1650,20 @@ var infoOnMissingLayout = map[string]bool{
 	"404": true,
 }
 
-type contentLinkRenderer struct {
+// hookRenderer is the canonical implementation of all hooks.ITEMRenderer,
+// where ITEM is the thing being hooked.
+type hookRenderer struct {
 	templateHandler tpl.TemplateHandler
 	identity.Provider
 	templ tpl.Template
 }
 
-func (r contentLinkRenderer) Render(w io.Writer, ctx hooks.LinkContext) error {
-	return r.templateHandler.Execute(r.templ, w, ctx)
+func (hr hookRenderer) RenderLink(w io.Writer, ctx hooks.LinkContext) error {
+	return hr.templateHandler.Execute(hr.templ, w, ctx)
+}
+
+func (hr hookRenderer) RenderHeading(w io.Writer, ctx hooks.HeadingContext) error {
+	return hr.templateHandler.Execute(hr.templ, w, ctx)
 }
 
 func (s *Site) renderForTemplate(name, outputFormat string, d interface{}, w io.Writer, templ tpl.Template) (err error) {
diff --git a/markup/converter/converter.go b/markup/converter/converter.go
index 353775826..df4bad95c 100644
--- a/markup/converter/converter.go
+++ b/markup/converter/converter.go
@@ -126,7 +126,7 @@ type DocumentContext struct {
 type RenderContext struct {
 	Src         []byte
 	RenderTOC   bool
-	RenderHooks *hooks.Render
+	RenderHooks *hooks.Renderers
 }
 
 var (
diff --git a/markup/converter/hooks/hooks.go b/markup/converter/hooks/hooks.go
index 5dfb09e2d..ab26a6f1a 100644
--- a/markup/converter/hooks/hooks.go
+++ b/markup/converter/hooks/hooks.go
@@ -27,13 +27,41 @@ type LinkContext interface {
 	PlainText() string
 }
 
-type Render struct {
-	LinkRenderer  LinkRenderer
-	ImageRenderer LinkRenderer
+type LinkRenderer interface {
+	RenderLink(w io.Writer, ctx LinkContext) error
+	identity.Provider
+}
+
+// HeadingContext contains accessors to all attributes that a HeadingRenderer
+// can use to render a heading.
+type HeadingContext interface {
+	// Page is the page containing the heading.
+	Page() interface{}
+	// Level is the level of the header (i.e. 1 for top-level, 2 for sub-level, etc.).
+	Level() int
+	// Anchor is the HTML id assigned to the heading.
+	Anchor() string
+	// Text is the rendered (HTML) heading text, excluding the heading marker.
+	Text() string
+	// PlainText is the unrendered version of Text.
+	PlainText() string
+}
+
+// HeadingRenderer describes a uniquely identifiable rendering hook.
+type HeadingRenderer interface {
+	// Render writes the renderered content to w using the data in w.
+	RenderHeading(w io.Writer, ctx HeadingContext) error
+	identity.Provider
 }
 
-func (r *Render) Eq(other interface{}) bool {
-	ro, ok := other.(*Render)
+type Renderers struct {
+	LinkRenderer    LinkRenderer
+	ImageRenderer   LinkRenderer
+	HeadingRenderer HeadingRenderer
+}
+
+func (r *Renderers) Eq(other interface{}) bool {
+	ro, ok := other.(*Renderers)
 	if !ok {
 		return false
 	}
@@ -49,10 +77,9 @@ func (r *Render) Eq(other interface{}) bool {
 		return false
 	}
 
-	return true
-}
+	if r.HeadingRenderer.GetIdentity() != ro.HeadingRenderer.GetIdentity() {
+		return false
+	}
 
-type LinkRenderer interface {
-	Render(w io.Writer, ctx LinkContext) error
-	identity.Provider
+	return true
 }
diff --git a/markup/goldmark/render_link.go b/markup/goldmark/render_hooks.go
index c0269bedf..aaae68e7f 100644
--- a/markup/goldmark/render_link.go
+++ b/markup/goldmark/render_hooks.go
@@ -23,10 +23,10 @@ import (
 	"github.com/yuin/goldmark/util"
 )
 
-var _ renderer.SetOptioner = (*linkRenderer)(nil)
+var _ renderer.SetOptioner = (*hookedRenderer)(nil)
 
 func newLinkRenderer() renderer.NodeRenderer {
-	r := &linkRenderer{
+	r := &hookedRenderer{
 		Config: html.Config{
 			Writer: html.DefaultWriter,
 		},
@@ -70,23 +70,64 @@ func (ctx linkContext) Title() string {
 	return ctx.title
 }
 
-type linkRenderer struct {
+type headingContext struct {
+	page      interface{}
+	level     int
+	anchor    string
+	text      string
+	plainText string
+}
+
+func (ctx headingContext) Page() interface{} {
+	return ctx.page
+}
+
+func (ctx headingContext) Level() int {
+	return ctx.level
+}
+
+func (ctx headingContext) Anchor() string {
+	return ctx.anchor
+}
+
+func (ctx headingContext) Text() string {
+	return ctx.text
+}
+
+func (ctx headingContext) PlainText() string {
+	return ctx.plainText
+}
+
+type hookedRenderer struct {
 	html.Config
 }
 
-func (r *linkRenderer) SetOption(name renderer.OptionName, value interface{}) {
+func (r *hookedRenderer) SetOption(name renderer.OptionName, value interface{}) {
 	r.Config.SetOption(name, value)
 }
 
 // RegisterFuncs implements NodeRenderer.RegisterFuncs.
-func (r *linkRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+func (r *hookedRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
 	reg.Register(ast.KindLink, r.renderLink)
 	reg.Register(ast.KindImage, r.renderImage)
+	reg.Register(ast.KindHeading, r.renderHeading)
+}
+
+// https://github.com/yuin/goldmark/blob/b611cd333a492416b56aa8d94b04a67bf0096ab2/renderer/html/html.go#L404
+func (r *hookedRenderer) RenderAttributes(w util.BufWriter, node ast.Node) {
+
+	for _, attr := range node.Attributes() {
+		_, _ = w.WriteString(" ")
+		_, _ = w.Write(attr.Name)
+		_, _ = w.WriteString(`="`)
+		_, _ = w.Write(util.EscapeHTML(attr.Value.([]byte)))
+		_ = w.WriteByte('"')
+	}
 }
 
 // Fall back to the default Goldmark render funcs. Method below borrowed from:
 // https://github.com/yuin/goldmark/blob/b611cd333a492416b56aa8d94b04a67bf0096ab2/renderer/html/html.go#L404
-func (r *linkRenderer) renderDefaultImage(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+func (r *hookedRenderer) renderDefaultImage(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
 	if !entering {
 		return ast.WalkContinue, nil
 	}
@@ -111,9 +152,49 @@ func (r *linkRenderer) renderDefaultImage(w util.BufWriter, source []byte, node
 	return ast.WalkSkipChildren, nil
 }
 
+func (r *hookedRenderer) renderImage(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.Image)
+	var h *hooks.Renderers
+
+	ctx, ok := w.(*renderContext)
+	if ok {
+		h = ctx.RenderContext().RenderHooks
+		ok = h != nil && h.ImageRenderer != nil
+	}
+
+	if !ok {
+		return r.renderDefaultImage(w, source, node, entering)
+	}
+
+	if entering {
+		// Store the current pos so we can capture the rendered text.
+		ctx.pos = ctx.Buffer.Len()
+		return ast.WalkContinue, nil
+	}
+
+	text := ctx.Buffer.Bytes()[ctx.pos:]
+	ctx.Buffer.Truncate(ctx.pos)
+
+	err := h.ImageRenderer.RenderLink(
+		w,
+		linkContext{
+			page:        ctx.DocumentContext().Document,
+			destination: string(n.Destination),
+			title:       string(n.Title),
+			text:        string(text),
+			plainText:   string(n.Text(source)),
+		},
+	)
+
+	ctx.AddIdentity(h.ImageRenderer.GetIdentity())
+
+	return ast.WalkContinue, err
+
+}
+
 // Fall back to the default Goldmark render funcs. Method below borrowed from:
 // https://github.com/yuin/goldmark/blob/b611cd333a492416b56aa8d94b04a67bf0096ab2/renderer/html/html.go#L404
-func (r *linkRenderer) renderDefaultLink(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+func (r *hookedRenderer) renderDefaultLink(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
 	n := node.(*ast.Link)
 	if entering {
 		_, _ = w.WriteString("<a href=\"")
@@ -133,18 +214,18 @@ func (r *linkRenderer) renderDefaultLink(w util.BufWriter, source []byte, node a
 	return ast.WalkContinue, nil
 }
 
-func (r *linkRenderer) renderImage(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
-	n := node.(*ast.Image)
-	var h *hooks.Render
+func (r *hookedRenderer) renderLink(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.Link)
+	var h *hooks.Renderers
 
 	ctx, ok := w.(*renderContext)
 	if ok {
 		h = ctx.RenderContext().RenderHooks
-		ok = h != nil && h.ImageRenderer != nil
+		ok = h != nil && h.LinkRenderer != nil
 	}
 
 	if !ok {
-		return r.renderDefaultImage(w, source, node, entering)
+		return r.renderDefaultLink(w, source, node, entering)
 	}
 
 	if entering {
@@ -156,7 +237,7 @@ func (r *linkRenderer) renderImage(w util.BufWriter, source []byte, node ast.Nod
 	text := ctx.Buffer.Bytes()[ctx.pos:]
 	ctx.Buffer.Truncate(ctx.pos)
 
-	err := h.ImageRenderer.Render(
+	err := h.LinkRenderer.RenderLink(
 		w,
 		linkContext{
 			page:        ctx.DocumentContext().Document,
@@ -167,24 +248,40 @@ func (r *linkRenderer) renderImage(w util.BufWriter, source []byte, node ast.Nod
 		},
 	)
 
-	ctx.AddIdentity(h.ImageRenderer.GetIdentity())
+	ctx.AddIdentity(h.LinkRenderer.GetIdentity())
 
 	return ast.WalkContinue, err
+}
 
+func (r *hookedRenderer) renderDefaultHeading(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.Heading)
+	if entering {
+		_, _ = w.WriteString("<h")
+		_ = w.WriteByte("0123456"[n.Level])
+		if n.Attributes() != nil {
+			r.RenderAttributes(w, node)
+		}
+		_ = w.WriteByte('>')
+	} else {
+		_, _ = w.WriteString("</h")
+		_ = w.WriteByte("0123456"[n.Level])
+		_, _ = w.WriteString(">\n")
+	}
+	return ast.WalkContinue, nil
 }
 
-func (r *linkRenderer) renderLink(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
-	n := node.(*ast.Link)
-	var h *hooks.Render
+func (r *hookedRenderer) renderHeading(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.Heading)
+	var h *hooks.Renderers
 
 	ctx, ok := w.(*renderContext)
 	if ok {
 		h = ctx.RenderContext().RenderHooks
-		ok = h != nil && h.LinkRenderer != nil
+		ok = h != nil && h.HeadingRenderer != nil
 	}
 
 	if !ok {
-		return r.renderDefaultLink(w, source, node, entering)
+		return r.renderDefaultHeading(w, source, node, entering)
 	}
 
 	if entering {
@@ -195,22 +292,25 @@ func (r *linkRenderer) renderLink(w util.BufWriter, source []byte, node ast.Node
 
 	text := ctx.Buffer.Bytes()[ctx.pos:]
 	ctx.Buffer.Truncate(ctx.pos)
+	// All ast.Heading nodes are guaranteed to have an attribute called "id"
+	// that is an array of bytes that encode a valid string.
+	anchori, _ := n.AttributeString("id")
+	anchor := anchori.([]byte)
 
-	err := h.LinkRenderer.Render(
+	err := h.HeadingRenderer.RenderHeading(
 		w,
-		linkContext{
-			page:        ctx.DocumentContext().Document,
-			destination: string(n.Destination),
-			title:       string(n.Title),
-			text:        string(text),
-			plainText:   string(n.Text(source)),
+		headingContext{
+			page:      ctx.DocumentContext().Document,
+			level:     n.Level,
+			anchor:    string(anchor),
+			text:      string(text),
+			plainText: string(n.Text(source)),
 		},
 	)
 
-	ctx.AddIdentity(h.LinkRenderer.GetIdentity())
+	ctx.AddIdentity(h.HeadingRenderer.GetIdentity())
 
 	return ast.WalkContinue, err
-
 }
 
 type links struct {