renderer: Helpful panic for unknown NodeKind

When rendering a Node, Renderer looks up the NodeRenderer for that
node's kind in a slice. That will panic with an "index out of range"
error if a node renderer for that node kind has not been registered.

    panic: runtime error: index out of range [1] with length 1

This changes Renderer to panic with a more helpful error message if the
node kind is unrecognized.

    panic: unrecognized node kind Document cannot be rendered:
    register a renderer for this node kind first

renderer/renderer.go

@@ -3,6 +3,7 @@ package renderer
 
 import (
 	"bufio"
+	"fmt"
 	"io"
 	"sync"
 
@@ -161,7 +162,14 @@ func (r *renderer) Render(w io.Writer, source []byte, n ast.Node) error {
 	err := ast.Walk(n, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
 		s := ast.WalkStatus(ast.WalkContinue)
 		var err error
-		f := r.nodeRendererFuncs[n.Kind()]
+
+		k := n.Kind()
+		if int(k) >= len(r.nodeRendererFuncs) {
+			panic(fmt.Sprintf("unrecognized node kind %v cannot be rendered: "+
+				"register a renderer for this node kind first", k))
+		}
+
+		f := r.nodeRendererFuncs[k]
 		if f != nil {
 			s, err = f(writer, source, n, entering)
 		}

renderer/renderer_test.go

@@ -0,0 +1,35 @@
+package renderer
+
+import (
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+)
+
+func TestRenderUnknownNode(t *testing.T) {
+	defer func() {
+		r := recover()
+		if r == nil {
+			t.Errorf("Render() should panic")
+			return
+		}
+
+		msg, ok := r.(string)
+		if !ok {
+			t.Errorf("Render() should panic with a string")
+		}
+
+		if !strings.Contains(msg, "unrecognized node kind Document") {
+			t.Errorf("Render() panicked with unexpected message: %v", msg)
+		}
+	}()
+
+	src := []byte("# Foo\n\nHello world")
+	node := parser.NewParser().Parse(text.NewReader(src))
+
+	r := NewRenderer()
+	r.Render(ioutil.Discard, src, node)
+}