cmd/compile/internal/syntax: support for alias declarations

Permits parsing of alias declarations with -newparser

	const/type/var/func T => p.T

but the compiler will reject it with an error. For now this
also accepts

	type T = p.T

so we can experiment with a type-alias only scenario.

- renamed _Arrow token to _Larrow (<-)
- introduced _Rarrow token (=>)
- introduced AliasDecl node
- extended scanner to accept _Rarrow
- extended parser and printer to handle alias declarations

Change-Id: I0170d10a87df8255db9186d466b6fd405228c38e
Reviewed-on: https://go-review.googlesource.com/29355
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/syntax/nodes.go

@@ -25,14 +25,21 @@ func (n *node) Line() uint32 {
 	return n.line
 }
 
+// TODO(gri) clean up init/initFrom once we have a good file pos story
 func (n *node) init(p *parser) {
 	n.pos = uint32(p.pos)
 	n.line = uint32(p.line)
 }
 
+func (n *node) initFrom(a *node) {
+	n.pos = a.pos
+	n.line = a.line
+}
+
 // ----------------------------------------------------------------------------
 // Files
 
+// package PkgName; DeclList[0], DeclList[1], ...
 type File struct {
 	PkgName  *Name
 	DeclList []Decl
@@ -49,6 +56,8 @@ type (
 		aDecl()
 	}
 
+	//              Path
+	// LocalPkgName Path
 	ImportDecl struct {
 		LocalPkgName *Name // including "."; nil means no rename present
 		Path         *BasicLit
@@ -56,6 +65,18 @@ type (
 		decl
 	}
 
+	// Name => Orig
+	AliasDecl struct {
+		Tok   token // Const, Type, Var, or Func
+		Name  *Name
+		Orig  Expr
+		Group *Group // nil means not part of a group
+		decl
+	}
+
+	// NameList
+	// NameList      = Values
+	// NameList Type = Values
 	ConstDecl struct {
 		NameList []*Name
 		Type     Expr   // nil means no type
@@ -64,13 +85,18 @@ type (
 		decl
 	}
 
+	// Name Type
 	TypeDecl struct {
 		Name  *Name
 		Type  Expr
+		Alias bool
 		Group *Group // nil means not part of a group
 		decl
 	}
 
+	// NameList Type
+	// NameList Type = Values
+	// NameList      = Values
 	VarDecl struct {
 		NameList []*Name
 		Type     Expr   // nil means no type
@@ -79,6 +105,10 @@ type (
 		decl
 	}
 
+	// func          Name Type { Body }
+	// func          Name Type
+	// func Receiver Name Type { Body }
+	// func Receiver Name Type
 	FuncDecl struct {
 		Attr    map[string]bool // go:attr map
 		Recv    *Field          // nil means regular function
@@ -418,6 +448,8 @@ func (simpleStmt) aSimpleStmt() {}
 // ----------------------------------------------------------------------------
 // Comments
 
+// TODO(gri) Consider renaming to CommentPos, CommentPlacement, etc.
+//           Kind = Above doesn't make much sense.
 type CommentKind uint
 
 const (