go/printer: indent lone comments in composite lits

If a composite literal contains any comments on their own lines without
any elements, the printer would unindent the comments.

The comments in this edge case are written when the closing '}' is
written. Indent and outdent first so that the indentation is
interspersed before the comment is written.

Also note that the go/printer golden tests don't show the exact same
behaviour that gofmt does. Added a TODO to figure this out in a separate
CL.

While at it, ensure that the tree conforms to gofmt. The changes are
unrelated to this indentation fix, however.

Fixes #22355.

Change-Id: I5ac25ac6de95a236f1e123479127cc4dd71e93fe
Reviewed-on: https://go-review.googlesource.com/74232
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/printer/nodes.go

@@ -865,7 +865,9 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 		if len(x.Elts) > 0 {
 			mode |= noExtraBlank
 		}
-		p.print(mode, x.Rbrace, token.RBRACE, mode)
+		// need the initial indent to print lone comments with
+		// the proper level of indentation
+		p.print(indent, unindent, mode, x.Rbrace, token.RBRACE, mode)
 		p.level--
 
 	case *ast.Ellipsis:

src/go/printer/testdata/comments.golden

@@ -831,4 +831,32 @@ func _() {
 	*/
 }
 
+var _ = []T{ /* lone comment */ }
+
+var _ = []T{
+	/* lone comment */
+}
+
+var _ = []T{
+	// lone comments
+	// in composite lit
+}
+
+var _ = [][]T{
+	{
+		// lone comments
+		// in composite lit
+	},
+}
+
+// TODO: gofmt doesn't add these tabs; make it so that these golden
+// tests run the printer in a way that it's exactly like gofmt.
+
+var _ = []T{	// lone comment
+}
+
+var _ = []T{	// lone comments
+	// in composite lit
+}
+
 /* This comment is the last entry in this file. It must be printed and should be followed by a newline */

src/go/printer/testdata/comments.input

@@ -832,4 +832,32 @@ func _() {
 */
 }
 
+var _ = []T{/* lone comment */}
+
+var _ = []T{
+/* lone comment */
+}
+
+var _ = []T{
+// lone comments
+// in composite lit
+}
+
+var _ = [][]T{
+	{
+		// lone comments
+		// in composite lit
+	},
+}
+
+// TODO: gofmt doesn't add these tabs; make it so that these golden
+// tests run the printer in a way that it's exactly like gofmt.
+
+var _ = []T{// lone comment
+}
+
+var _ = []T{// lone comments
+// in composite lit
+}
+
 /* This comment is the last entry in this file. It must be printed and should be followed by a newline */

src/math/big/int.go

@@ -533,7 +533,6 @@ func (z *Int) GCD(x, y, a, b *Int) *Int {
 // See Jebelean, "Improving the multiprecision Euclidean algorithm",
 // Design and Implementation of Symbolic Computation Systems, pp 45-58.
 func (z *Int) lehmerGCD(a, b *Int) *Int {
-
 	// ensure a >= b
 	if a.abs.cmp(b.abs) < 0 {
 		a, b = b, a
@@ -551,7 +550,6 @@ func (z *Int) lehmerGCD(a, b *Int) *Int {
 
 	// loop invariant A >= B
 	for len(B.abs) > 1 {
-
 		// initialize the digits
 		var a1, a2, u0, u1, u2, v0, v1, v2 Word