go/ast: fix ast.Walk

- change Walk signature to use an ast.Node instead of interface{}
- add Pos functions to a couple of ast types to make them proper nodes
- explicit nil checks where a node can be nil; incorrect ASTs cause Walk to crash

For now ast.Walk is exercised extensively as part of godoc's indexer;
so we have some confidence in its correctness. But this needs a test,
eventually.

Fixes #1326.

R=rsc, r
CC=golang-dev
https://golang.org/cl/3481043

src/cmd/godoc/index.go

@@ -509,7 +509,7 @@ func (x *Indexer) visitSpec(spec ast.Spec, isVarDecl bool) {
 }
 
 
-func (x *Indexer) Visit(node interface{}) ast.Visitor {
+func (x *Indexer) Visit(node ast.Node) ast.Visitor {
 	// TODO(gri): methods in interface types are categorized as VarDecl
 	switch n := node.(type) {
 	case nil:

src/cmd/gofmt/simplify.go

@@ -12,7 +12,7 @@ import (
 
 type simplifier struct{}
 
-func (s *simplifier) Visit(node interface{}) ast.Visitor {
+func (s *simplifier) Visit(node ast.Node) ast.Visitor {
 	switch n := node.(type) {
 	case *ast.CompositeLit:
 		// array, slice, and map composite literals may be simplified
@@ -61,7 +61,7 @@ func (s *simplifier) Visit(node interface{}) ast.Visitor {
 }
 
 
-func simplify(node interface{}) {
+func simplify(node ast.Node) {
 	var s simplifier
 	ast.Walk(&s, node)
 }

src/cmd/govet/govet.go

@@ -139,7 +139,7 @@ func (f *File) checkFile(name string, file *ast.File) {
 }
 
 // Visit implements the ast.Visitor interface.
-func (f *File) Visit(node interface{}) ast.Visitor {
+func (f *File) Visit(node ast.Node) ast.Visitor {
 	// TODO: could return nil for nodes that cannot contain a CallExpr -
 	// will shortcut traversal.  Worthwhile?
 	switch n := node.(type) {

src/pkg/go/ast/ast.go

@@ -70,19 +70,20 @@ type Comment struct {
 }
 
 
-func (c *Comment) Pos() token.Pos {
-	return c.Slash
-}
+func (c *Comment) Pos() token.Pos { return c.Slash }
 
 
 // A CommentGroup represents a sequence of comments
 // with no other tokens and no empty lines between.
 //
 type CommentGroup struct {
-	List []*Comment
+	List []*Comment // len(List) > 0
 }
 
 
+func (g *CommentGroup) Pos() token.Pos { return g.List[0].Pos() }
+
+
 // ----------------------------------------------------------------------------
 // Expressions and types
 
@@ -115,6 +116,9 @@ type FieldList struct {
 }
 
 
+func (list *FieldList) Pos() token.Pos { return list.Opening }
+
+
 // NumFields returns the number of (named and anonymous fields) in a FieldList.
 func (f *FieldList) NumFields() int {
 	n := 0
@@ -294,7 +298,7 @@ type (
 	FuncType struct {
 		Func    token.Pos  // position of "func" keyword
 		Params  *FieldList // (incoming) parameters
-		Results *FieldList // (outgoing) results
+		Results *FieldList // (outgoing) results; or nil
 	}
 
 	// An InterfaceType node represents an interface type.
@@ -494,7 +498,7 @@ type (
 	BranchStmt struct {
 		TokPos token.Pos   // position of Tok
 		Tok    token.Token // keyword token (BREAK, CONTINUE, GOTO, FALLTHROUGH)
-		Label  *Ident
+		Label  *Ident      // label name; or nil
 	}
 
 	// A BlockStmt node represents a braced statement list.
@@ -507,10 +511,10 @@ type (
 	// An IfStmt node represents an if statement.
 	IfStmt struct {
 		If   token.Pos // position of "if" keyword
-		Init Stmt
-		Cond Expr
+		Init Stmt      // initalization statement; or nil
+		Cond Expr      // condition; or nil
 		Body *BlockStmt
-		Else Stmt
+		Else Stmt // else branch; or nil
 	}
 
 	// A CaseClause represents a case of an expression switch statement.
@@ -523,9 +527,9 @@ type (
 
 	// A SwitchStmt node represents an expression switch statement.
 	SwitchStmt struct {
-		Switch token.Pos // position of "switch" keyword
-		Init   Stmt
-		Tag    Expr
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initalization statement; or nil
+		Tag    Expr       // tag expression; or nil
 		Body   *BlockStmt // CaseClauses only
 	}
 
@@ -539,8 +543,8 @@ type (
 
 	// An TypeSwitchStmt node represents a type switch statement.
 	TypeSwitchStmt struct {
-		Switch token.Pos // position of "switch" keyword
-		Init   Stmt
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initalization statement; or nil
 		Assign Stmt       // x := y.(type)
 		Body   *BlockStmt // TypeCaseClauses only
 	}
@@ -563,9 +567,9 @@ type (
 	// A ForStmt represents a for statement.
 	ForStmt struct {
 		For  token.Pos // position of "for" keyword
-		Init Stmt
-		Cond Expr
-		Post Stmt
+		Init Stmt      // initalization statement; or nil
+		Cond Expr      // condition; or nil
+		Post Stmt      // post iteration statement; or nil
 		Body *BlockStmt
 	}
 
@@ -780,3 +784,13 @@ type Package struct {
 	Scope *Scope           // package scope; or nil
 	Files map[string]*File // Go source files by filename
 }
+
+
+func (p *Package) Pos() (pos token.Pos) {
+	// get the position of the package clause of the first file, if any
+	for _, f := range p.Files {
+		pos = f.Pos()
+		break
+	}
+	return
+}

src/pkg/go/ast/walk.go

@@ -10,51 +10,57 @@ import "fmt"
 // If the result visitor w is not nil, Walk visits each of the children
 // of node with the visitor w, followed by a call of w.Visit(nil).
 type Visitor interface {
-	Visit(node interface{}) (w Visitor)
+	Visit(node Node) (w Visitor)
 }
 
 
-func walkIdent(v Visitor, x *Ident) {
-	if x != nil {
+// Helper functions for common node lists. They may be empty.
+
+func walkIdentList(v Visitor, list []*Ident) {
+	for _, x := range list {
 		Walk(v, x)
 	}
 }
 
 
-func walkCommentGroup(v Visitor, g *CommentGroup) {
-	if g != nil {
-		Walk(v, g)
+func walkExprList(v Visitor, list []Expr) {
+	for _, x := range list {
+		Walk(v, x)
 	}
 }
 
 
-func walkBlockStmt(v Visitor, b *BlockStmt) {
-	if b != nil {
-		Walk(v, b)
+func walkStmtList(v Visitor, list []Stmt) {
+	for _, x := range list {
+		Walk(v, x)
 	}
 }
 
 
-// Walk traverses an AST in depth-first order: If node != nil, it
-// invokes v.Visit(node). If the visitor w returned by v.Visit(node) is
-// not nil, Walk visits each of the children of node with the visitor w,
-// followed by a call of w.Visit(nil).
-//
-// Walk may be called with any of the named ast node types. It also
-// accepts arguments of type []*Field, []*Ident, []Expr, []Stmt and []Decl;
-// the respective children are the slice elements.
-//
-func Walk(v Visitor, node interface{}) {
-	if node == nil {
-		return
+func walkDeclList(v Visitor, list []Decl) {
+	for _, x := range list {
+		Walk(v, x)
 	}
+}
+
+
+// TODO(gri): Investigate if providing a closure to Walk leads to
+//            simpler use (and may help eliminate Inspect in turn).
+
+// Walk traverses an AST in depth-first order: It starts by calling
+// v.Visit(node); node must not be nil. If the visitor w returned by
+// v.Visit(node) is not nil, Walk is invoked recursively with visitor
+// w for each of the non-nil children of node, followed by a call of
+// w.Visit(nil).
+//
+func Walk(v Visitor, node Node) {
 	if v = v.Visit(node); v == nil {
 		return
 	}
 
 	// walk children
 	// (the order of the cases matches the order
-	// of the corresponding declaration in ast.go)
+	// of the corresponding node types in ast.go)
 	switch n := node.(type) {
 	// Comments and fields
 	case *Comment:
@@ -66,11 +72,17 @@ func Walk(v Visitor, node interface{}) {
 		}
 
 	case *Field:
-		walkCommentGroup(v, n.Doc)
-		Walk(v, n.Names)
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
 		Walk(v, n.Type)
-		Walk(v, n.Tag)
-		walkCommentGroup(v, n.Comment)
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
 
 	case *FieldList:
 		for _, f := range n.List {
@@ -82,21 +94,21 @@ func Walk(v Visitor, node interface{}) {
 		// nothing to do
 
 	case *FuncLit:
-		if n != nil {
-			Walk(v, n.Type)
-		}
-		walkBlockStmt(v, n.Body)
+		Walk(v, n.Type)
+		Walk(v, n.Body)
 
 	case *CompositeLit:
-		Walk(v, n.Type)
-		Walk(v, n.Elts)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Elts)
 
 	case *ParenExpr:
 		Walk(v, n.X)
 
 	case *SelectorExpr:
 		Walk(v, n.X)
-		walkIdent(v, n.Sel)
+		Walk(v, n.Sel)
 
 	case *IndexExpr:
 		Walk(v, n.X)
@@ -104,16 +116,22 @@ func Walk(v Visitor, node interface{}) {
 
 	case *SliceExpr:
 		Walk(v, n.X)
-		Walk(v, n.Index)
-		Walk(v, n.End)
+		if n.Index != nil {
+			Walk(v, n.Index)
+		}
+		if n.End != nil {
+			Walk(v, n.End)
+		}
 
 	case *TypeAssertExpr:
 		Walk(v, n.X)
-		Walk(v, n.Type)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
 
 	case *CallExpr:
 		Walk(v, n.Fun)
-		Walk(v, n.Args)
+		walkExprList(v, n.Args)
 
 	case *StarExpr:
 		Walk(v, n.X)
@@ -131,7 +149,9 @@ func Walk(v Visitor, node interface{}) {
 
 	// Types
 	case *ArrayType:
-		Walk(v, n.Len)
+		if n.Len != nil {
+			Walk(v, n.Len)
+		}
 		Walk(v, n.Elt)
 
 	case *StructType:
@@ -164,7 +184,7 @@ func Walk(v Visitor, node interface{}) {
 		// nothing to do
 
 	case *LabeledStmt:
-		walkIdent(v, n.Label)
+		Walk(v, n.Label)
 		Walk(v, n.Stmt)
 
 	case *ExprStmt:
@@ -174,148 +194,177 @@ func Walk(v Visitor, node interface{}) {
 		Walk(v, n.X)
 
 	case *AssignStmt:
-		Walk(v, n.Lhs)
-		Walk(v, n.Rhs)
+		walkExprList(v, n.Lhs)
+		walkExprList(v, n.Rhs)
 
 	case *GoStmt:
-		if n.Call != nil {
-			Walk(v, n.Call)
-		}
+		Walk(v, n.Call)
 
 	case *DeferStmt:
-		if n.Call != nil {
-			Walk(v, n.Call)
-		}
+		Walk(v, n.Call)
 
 	case *ReturnStmt:
-		Walk(v, n.Results)
+		walkExprList(v, n.Results)
 
 	case *BranchStmt:
-		walkIdent(v, n.Label)
+		if n.Label != nil {
+			Walk(v, n.Label)
+		}
 
 	case *BlockStmt:
-		Walk(v, n.List)
+		walkStmtList(v, n.List)
 
 	case *IfStmt:
-		Walk(v, n.Init)
-		Walk(v, n.Cond)
-		walkBlockStmt(v, n.Body)
-		Walk(v, n.Else)
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Cond != nil {
+			Walk(v, n.Cond)
+		}
+		Walk(v, n.Body)
+		if n.Else != nil {
+			Walk(v, n.Else)
+		}
 
 	case *CaseClause:
-		Walk(v, n.Values)
-		Walk(v, n.Body)
+		walkExprList(v, n.Values)
+		walkStmtList(v, n.Body)
 
 	case *SwitchStmt:
-		Walk(v, n.Init)
-		Walk(v, n.Tag)
-		walkBlockStmt(v, n.Body)
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		Walk(v, n.Body)
 
 	case *TypeCaseClause:
-		Walk(v, n.Types)
-		Walk(v, n.Body)
+		for _, x := range n.Types {
+			Walk(v, x)
+		}
+		walkStmtList(v, n.Body)
 
 	case *TypeSwitchStmt:
-		Walk(v, n.Init)
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
 		Walk(v, n.Assign)
-		walkBlockStmt(v, n.Body)
-
-	case *CommClause:
-		Walk(v, n.Lhs)
-		Walk(v, n.Rhs)
 		Walk(v, n.Body)
 
+	case *CommClause:
+		if n.Lhs != nil {
+			Walk(v, n.Lhs)
+		}
+		if n.Rhs != nil {
+			Walk(v, n.Rhs)
+		}
+		walkStmtList(v, n.Body)
+
 	case *SelectStmt:
-		walkBlockStmt(v, n.Body)
+		Walk(v, n.Body)
 
 	case *ForStmt:
-		Walk(v, n.Init)
-		Walk(v, n.Cond)
-		Walk(v, n.Post)
-		walkBlockStmt(v, n.Body)
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Cond != nil {
+			Walk(v, n.Cond)
+		}
+		if n.Post != nil {
+			Walk(v, n.Post)
+		}
+		Walk(v, n.Body)
 
 	case *RangeStmt:
 		Walk(v, n.Key)
-		Walk(v, n.Value)
+		if n.Value != nil {
+			Walk(v, n.Value)
+		}
 		Walk(v, n.X)
-		walkBlockStmt(v, n.Body)
+		Walk(v, n.Body)
 
 	// Declarations
 	case *ImportSpec:
-		walkCommentGroup(v, n.Doc)
-		walkIdent(v, n.Name)
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		if n.Name != nil {
+			Walk(v, n.Name)
+		}
 		Walk(v, n.Path)
-		walkCommentGroup(v, n.Comment)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
 
 	case *ValueSpec:
-		walkCommentGroup(v, n.Doc)
-		Walk(v, n.Names)
-		Walk(v, n.Type)
-		Walk(v, n.Values)
-		walkCommentGroup(v, n.Comment)
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Values)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
 
 	case *TypeSpec:
-		walkCommentGroup(v, n.Doc)
-		walkIdent(v, n.Name)
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
 		Walk(v, n.Type)
-		walkCommentGroup(v, n.Comment)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
 
 	case *BadDecl:
 		// nothing to do
 
 	case *GenDecl:
-		walkCommentGroup(v, n.Doc)
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
 		for _, s := range n.Specs {
 			Walk(v, s)
 		}
 
 	case *FuncDecl:
-		walkCommentGroup(v, n.Doc)
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
 		if n.Recv != nil {
 			Walk(v, n.Recv)
 		}
-		walkIdent(v, n.Name)
-		if n.Type != nil {
-			Walk(v, n.Type)
+		Walk(v, n.Name)
+		Walk(v, n.Type)
+		if n.Body != nil {
+			Walk(v, n.Body)
 		}
-		walkBlockStmt(v, n.Body)
 
 	// Files and packages
 	case *File:
-		walkCommentGroup(v, n.Doc)
-		walkIdent(v, n.Name)
-		Walk(v, n.Decls)
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
+		walkDeclList(v, n.Decls)
 		for _, g := range n.Comments {
 			Walk(v, g)
 		}
+		// don't walk n.Comments - they have been
+		// visited already through the individual
+		// nodes
 
 	case *Package:
 		for _, f := range n.Files {
 			Walk(v, f)
 		}
 
-	case []*Ident:
-		for _, x := range n {
-			Walk(v, x)
-		}
-
-	case []Expr:
-		for _, x := range n {
-			Walk(v, x)
-		}
-
-	case []Stmt:
-		for _, x := range n {
-			Walk(v, x)
-		}
-
-	case []Decl:
-		for _, x := range n {
-			Walk(v, x)
-		}
-
 	default:
-		fmt.Printf("ast.Walk: unexpected type %T", n)
+		fmt.Printf("ast.Walk: unexpected node type %T", n)
 		panic("ast.Walk")
 	}
 
@@ -323,20 +372,20 @@ func Walk(v Visitor, node interface{}) {
 }
 
 
-type inspector func(node interface{}) bool
+type inspector func(Node) bool
 
-func (f inspector) Visit(node interface{}) Visitor {
-	if node != nil && f(node) {
+func (f inspector) Visit(node Node) Visitor {
+	if f(node) {
 		return f
 	}
 	return nil
 }
 
 
-// Inspect traverses an AST in depth-first order: If node != nil, it
-// invokes f(node). If f returns true, inspect invokes f for all the
-// non-nil children of node, recursively.
+// Inspect traverses an AST in depth-first order: It starts by calling
+// f(node); node must not be nil. If f returns true, Inspect invokes f
+// for all the non-nil children of node, recursively.
 //
-func Inspect(ast interface{}, f func(node interface{}) bool) {
-	Walk(inspector(f), ast)
+func Inspect(node Node, f func(Node) bool) {
+	Walk(inspector(f), node)
 }

src/pkg/go/printer/nodes.go

@@ -994,7 +994,7 @@ func stripParens(x ast.Expr) ast.Expr {
 		// parentheses must not be stripped if there are any
 		// unparenthesized composite literals starting with
 		// a type name
-		ast.Inspect(px.X, func(node interface{}) bool {
+		ast.Inspect(px.X, func(node ast.Node) bool {
 			switch x := node.(type) {
 			case *ast.ParenExpr:
 				// parentheses protect enclosed composite literals