hcl/printer: format single line objects to not have newline between

Fixes #161

See #161 and test cases for examples.

hcl/printer/nodes.go

@@ -171,7 +171,15 @@ func (p *printer) output(n interface{}) []byte {
 
 			buf.Write(p.output(t.Items[index]))
 			if index != len(t.Items)-1 {
-				buf.Write([]byte{newline, newline})
+				// Always write a newline to separate us from the next item
+				buf.WriteByte(newline)
+
+				// If the next item is an object that is exactly one line,
+				// then we don't output another newline.
+				next := t.Items[index+1]
+				if next.Pos().Line != t.Items[index].Pos().Line+1 || !p.isSingleLineObject(next) {
+					buf.WriteByte(newline)
+				}
 			}
 			index++
 		}
@@ -683,6 +691,36 @@ func (p *printer) heredocIndent(buf []byte) []byte {
 	return res
 }
 
+// isSingleLineObject tells whether the given object item is a single
+// line object such as "obj {}".
+//
+// A single line object:
+//
+//   * has no lead comments (hence multi-line)
+//   * has no assignment
+//   * has no values in the stanza (within {})
+//
+func (p *printer) isSingleLineObject(val *ast.ObjectItem) bool {
+	// If there is a lead comment, can't be one line
+	if val.LeadComment != nil {
+		return false
+	}
+
+	// If there is assignment, we always break by line
+	if val.Assign.IsValid() {
+		return false
+	}
+
+	// If it isn't an object type, then its not a single line object
+	ot, ok := val.Val.(*ast.ObjectType)
+	if !ok {
+		return false
+	}
+
+	// If the object has no items, it is single line!
+	return len(ot.List.Items) == 0
+}
+
 func lines(txt string) int {
 	endline := 1
 	for i := 0; i < len(txt); i++ {

hcl/printer/printer_test.go

@@ -44,6 +44,7 @@ var data = []entry{
 	{"empty_block.input", "empty_block.golden"},
 	{"list_of_objects.input", "list_of_objects.golden"},
 	{"multiline_string.input", "multiline_string.golden"},
+	{"object_singleline.input", "object_singleline.golden"},
 	{"object_with_heredoc.input", "object_with_heredoc.golden"},
 }
 

hcl/printer/testdata/empty_block.golden

@@ -1,5 +1,4 @@
 variable "foo" {}
-
 variable "foo" {}
 
 variable "foo" {

hcl/printer/testdata/object_singleline.golden

@@ -0,0 +1,20 @@
+variable "foo" {}
+variable "bar" {}
+variable "baz" {}
+
+variable "qux" {}
+
+variable "foo" {
+  foo = "bar"
+}
+
+variable "foo" {}
+
+# lead comment
+variable "bar" {}
+
+# Purposeful newline check below:
+
+variable "foo" {}
+
+variable "purposeful-newline" {}

hcl/printer/testdata/object_singleline.input

@@ -0,0 +1,16 @@
+variable "foo" {}
+variable "bar" {}
+variable "baz" {}
+
+variable "qux" {}
+variable "foo" { foo = "bar" }
+
+variable "foo" {}
+# lead comment
+variable "bar" {}
+
+# Purposeful newline check below:
+
+variable "foo" {}
+
+variable "purposeful-newline" {}