hcl/printer: file with only comments formats properly

hcl/parser/parser_test.go

@@ -282,6 +282,29 @@ func TestObjectKey(t *testing.T) {
 	}
 }
 
+func TestCommentGroup(t *testing.T) {
+	var cases = []struct {
+		src    string
+		groups int
+	}{
+		{"# Hello\n# World", 1},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.src, func(t *testing.T) {
+			p := newParser([]byte(tc.src))
+			file, err := p.Parse()
+			if err != nil {
+				t.Fatalf("parse error: %s", err)
+			}
+
+			if len(file.Comments) != tc.groups {
+				t.Fatalf("bad: %#v", file.Comments)
+			}
+		})
+	}
+}
+
 // Official HCL tests
 func TestParse(t *testing.T) {
 	cases := []struct {

hcl/printer/nodes.go

@@ -123,7 +123,10 @@ func (p *printer) output(n interface{}) []byte {
 
 					if comment.Pos().After(p.prev) && comment.Pos().Before(nextItem) {
 						// if we hit the end add newlines so we can print the comment
-						if index == len(t.Items) {
+						// we don't do this if prev is invalid which means the
+						// beginning of the file since the first comment should
+						// be at the first line.
+						if p.prev.IsValid() && index == len(t.Items) {
 							buf.Write([]byte{newline, newline})
 						}
 

hcl/printer/printer_test.go

@@ -33,6 +33,7 @@ var data = []entry{
 	{"list.input", "list.golden"},
 	{"comment.input", "comment.golden"},
 	{"comment_aligned.input", "comment_aligned.golden"},
+	{"comment_newline.input", "comment_newline.golden"},
 	{"comment_standalone.input", "comment_standalone.golden"},
 	{"empty_block.input", "empty_block.golden"},
 	{"list_of_objects.input", "list_of_objects.golden"},

hcl/printer/testdata/comment_newline.golden

@@ -0,0 +1,3 @@
+# Hello
+# World
+

hcl/printer/testdata/comment_newline.input

@@ -0,0 +1,2 @@
+# Hello
+# World