go/printer: support for printing alias declarations

For #16339.

Change-Id: Ie2e3338b87e84f45cda0868213bbcd2dae9ab6e3
Reviewed-on: https://go-review.googlesource.com/30212
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/printer/nodes.go

@@ -1309,13 +1309,26 @@ func keepTypeColumn(specs []ast.Spec) []bool {
 		}
 	}
 
-	i0 := -1 // if i0 >= 0 we are in a run and i0 is the start of the run
-	var keepType bool
+	i0 := -1          // if i0 >= 0 we are in a run and i0 is the start of the run
+	var keepType bool // valid if we are in a run (i0 >= 0)
 	for i, s := range specs {
-		t := s.(*ast.ValueSpec)
-		if t.Values != nil {
+		var hasValues, hasType bool
+		switch t := s.(type) {
+		case *ast.AliasSpec:
+			// like a ValueSpec with values (alias origin), but no type
+			hasValues = true
+
+		case *ast.ValueSpec:
+			hasValues = len(t.Values) > 0
+			hasType = t.Type != nil
+
+		default:
+			panic("internal error: unexpected ast.Spec")
+		}
+
+		if hasValues {
 			if i0 < 0 {
-				// start of a run of ValueSpecs with non-nil Values
+				// start of a run with values
 				i0 = i
 				keepType = false
 			}
@@ -1326,7 +1339,7 @@ func keepTypeColumn(specs []ast.Spec) []bool {
 				i0 = -1
 			}
 		}
-		if t.Type != nil {
+		if hasType {
 			keepType = true
 		}
 	}
@@ -1338,6 +1351,25 @@ func keepTypeColumn(specs []ast.Spec) []bool {
 	return m
 }
 
+func (p *printer) aliasSpec(s *ast.AliasSpec, keepTypeCol bool) {
+	p.setComment(s.Doc)
+	p.expr(s.Name)
+	extraTabs := 3
+	if keepTypeCol {
+		p.print(vtab)
+		extraTabs--
+	}
+	p.print(vtab, token.ALIAS, blank)
+	p.expr(s.Orig)
+	extraTabs--
+	if s.Comment != nil {
+		for ; extraTabs > 0; extraTabs-- {
+			p.print(vtab)
+		}
+		p.setComment(s.Comment)
+	}
+}
+
 func (p *printer) valueSpec(s *ast.ValueSpec, keepType bool) {
 	p.setComment(s.Doc)
 	p.identList(s.Names, false) // always present
@@ -1421,6 +1453,17 @@ func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
 		p.setComment(s.Comment)
 		p.print(s.EndPos)
 
+	case *ast.AliasSpec:
+		p.setComment(s.Doc)
+		p.expr(s.Name)
+		if n == 1 {
+			p.print(blank)
+		} else {
+			p.print(vtab)
+		}
+		p.print(token.ALIAS, blank)
+		p.expr(s.Orig)
+
 	case *ast.ValueSpec:
 		if n != 1 {
 			p.internalError("expected n = 1; got", n)
@@ -1472,7 +1515,16 @@ func (p *printer) genDecl(d *ast.GenDecl) {
 						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
 					}
 					p.recordLine(&line)
-					p.valueSpec(s.(*ast.ValueSpec), keepType[i])
+					switch t := s.(type) {
+					case *ast.AliasSpec:
+						p.aliasSpec(t, keepType[i])
+
+					case *ast.ValueSpec:
+						p.valueSpec(t, keepType[i])
+
+					default:
+						p.internalError("unknown ast.Spec type: %T", t)
+					}
 				}
 			} else {
 				var line int

src/go/printer/testdata/declarations.golden

@@ -985,3 +985,49 @@ func _(struct {
 	x	int
 	y	int
 })	// no extra comma between } and )
+
+// alias declarations
+const c => C
+const c => p.C
+const (
+	a		= 123
+	b	int	= 456
+	c		=> foo
+	ddd		=> p.Foo
+)
+
+// TODO(gri) Currently = and => line up in the formatted output,
+//           but because = and => have different lengths, the
+//           text following doesn't line up. Consider putting that
+//           text into its own column.
+const (
+	a	int	= iota	// a comment
+	b		=> p.B	// b comment
+	c			// c comment
+	d	=> p.C		// d comment
+	e	=> p.E
+	f
+	g	float32	= 9.8
+)
+
+type c => C
+type c => p.C
+type (
+	s	struct{}
+	a	=> A
+	b	=> A
+	c	=> foo
+	ddd	=> p.Foo
+)
+
+var c => C
+var c => p.C
+var (
+	a		= 123
+	b	int	= 456
+	c		=> foo
+	ddd		=> p.Foo
+)
+
+func f => F
+func f_long => p.F

src/go/printer/testdata/declarations.input

@@ -999,3 +999,49 @@ func _(struct {
 	x int
 	y int
 }) // no extra comma between } and )
+
+// alias declarations
+const c => C
+const c => p.C
+const (
+	a = 123
+	b int = 456
+	c => foo
+	ddd => p.Foo
+)
+
+// TODO(gri) Currently = and => line up in the formatted output,
+//           but because = and => have different lengths, the
+//           text following doesn't line up. Consider putting that
+//           text into its own column.
+const (
+	a int = iota // a comment
+	b => p.B // b comment
+	c // c comment
+	d => p.C // d comment
+	e => p.E
+	f
+	g float32 = 9.8
+)
+
+type c => C
+type c => p.C
+type (
+	s struct{}
+	a => A
+	b = A
+	c = foo
+	ddd => p.Foo
+)
+
+var c => C
+var c => p.C
+var (
+	a = 123
+	b int = 456
+	c => foo
+	ddd => p.Foo
+)
+
+func f => F
+func f_long => p.F