cue/ast: add WalkVisitor to share with cue/ast/astutil

We have two copies of this walk code between the two packages.
Even though we have never exposed the APIs to end users before,
in practice the interface has been closely tied to their implementations
for some time now, so it seems fine to settle for it for now.

It's likely that once iterators in Go become widespread,
we will want to revisit what better APIs we want to expose here.
Adding WalkVisitor shouldn't preclude that, particularly since "Walk"
is not really a verb that one would use for an iterator API anyway.
That is being tracked in https://cuelang.org/issue/2953.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I5a6b7f2f12bb95a0850de86fbc2374cff480bdd6
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194006
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

cue/ast/astutil/resolve.go

@@ -75,14 +75,14 @@ type ErrFunc func(pos token.Pos, msg string, args ...interface{})
 // Resolve resolves all identifiers in a file. Unresolved identifiers are
 // recorded in Unresolved. It will not overwrite already resolved values.
 func Resolve(f *ast.File, errFn ErrFunc) {
-	walk(&scope{errFn: errFn, identFn: resolveIdent}, f)
+	ast.WalkVisitor(f, &scope{errFn: errFn, identFn: resolveIdent})
 }
 
 // Resolve resolves all identifiers in an expression.
 // It will not overwrite already resolved values.
 func ResolveExpr(e ast.Expr, errFn ErrFunc) {
 	f := &ast.File{}
-	walk(&scope{file: f, errFn: errFn, identFn: resolveIdent}, e)
+	ast.WalkVisitor(e, &scope{file: f, errFn: errFn, identFn: resolveIdent})
 }
 
 // A Scope maintains the set of named language entities declared
@@ -250,13 +250,13 @@ func (s *scope) lookup(name string) (p *scope, obj ast.Node, node entry) {
 }
 
 func (s *scope) After(n ast.Node) {}
-func (s *scope) Before(n ast.Node) (w visitor) {
+func (s *scope) Before(n ast.Node) (w ast.Visitor) {
 	switch x := n.(type) {
 	case *ast.File:
 		s := newScope(x, s, x, x.Decls)
 		// Support imports.
 		for _, d := range x.Decls {
-			walk(s, d)
+			ast.WalkVisitor(d, s)
 		}
 		return nil
 
@@ -265,7 +265,7 @@ func (s *scope) Before(n ast.Node) (w visitor) {
 
 	case *ast.Comprehension:
 		s = scopeClauses(s, x.Clauses)
-		walk(s, x.Value)
+		ast.WalkVisitor(x.Value, s)
 		return nil
 
 	case *ast.Field:
@@ -277,10 +277,10 @@ func (s *scope) Before(n ast.Node) (w visitor) {
 
 		switch label := n.(type) {
 		case *ast.ParenExpr:
-			walk(s, label)
+			ast.WalkVisitor(label, s)
 
 		case *ast.Interpolation:
-			walk(s, label)
+			ast.WalkVisitor(label, s)
 
 		case *ast.ListLit:
 			if len(label.Elts) != 1 {
@@ -317,7 +317,7 @@ func (s *scope) Before(n ast.Node) (w visitor) {
 					}
 				}
 			})
-			walk(s, expr)
+			ast.WalkVisitor(expr, s)
 		}
 
 		if n := x.Value; n != nil {
@@ -329,7 +329,7 @@ func (s *scope) Before(n ast.Node) (w visitor) {
 				n = alias.Expr
 			}
 			s.inField = true
-			walk(s, n)
+			ast.WalkVisitor(n, s)
 			s.inField = false
 		}
 
@@ -342,7 +342,7 @@ func (s *scope) Before(n ast.Node) (w visitor) {
 		delete(s.index, name) // The same name may still appear in another scope
 
 		if x.Expr != nil {
-			walk(s, x.Expr)
+			ast.WalkVisitor(x.Expr, s)
 		}
 		s.index[name] = saved
 		return nil
@@ -354,7 +354,7 @@ func (s *scope) Before(n ast.Node) (w visitor) {
 		delete(s.index, name) // The same name may still appear in another scope
 
 		if x.Expr != nil {
-			walk(s, x.Expr)
+			ast.WalkVisitor(x.Expr, s)
 		}
 		s.index[name] = saved
 		return nil
@@ -367,7 +367,7 @@ func (s *scope) Before(n ast.Node) (w visitor) {
 		// that resolve in a list.
 
 	case *ast.SelectorExpr:
-		walk(s, x.X)
+		ast.WalkVisitor(x.X, s)
 		return nil
 
 	case *ast.Ident:
@@ -410,20 +410,20 @@ func scopeClauses(s *scope, clauses []ast.Clause) *scope {
 	for _, c := range clauses {
 		switch x := c.(type) {
 		case *ast.ForClause:
-			walk(s, x.Source)
+			ast.WalkVisitor(x.Source, s)
 			s = newScope(s.file, s, x, nil)
 			if x.Key != nil {
 				s.insert(x.Key.Name, x.Key, x)
 			}
 			s.insert(x.Value.Name, x.Value, x)
 
 		case *ast.LetClause:
-			walk(s, x.Expr)
+			ast.WalkVisitor(x.Expr, s)
 			s = newScope(s.file, s, x, nil)
 			s.insert(x.Ident.Name, x.Ident, x)
 
 		default:
-			walk(s, c)
+			ast.WalkVisitor(c, s)
 		}
 	}
 	return s

cue/ast/astutil/sanitize.go

@@ -50,11 +50,11 @@ func Sanitize(f *ast.File) error {
 	}
 
 	// Gather all names.
-	walk(&scope{
+	ast.WalkVisitor(f, &scope{
 		errFn:   z.errf,
 		nameFn:  z.addName,
 		identFn: z.markUsed,
-	}, f)
+	})
 	if z.errs != nil {
 		return z.errs
 	}
@@ -67,7 +67,7 @@ func Sanitize(f *ast.File) error {
 		index:   make(map[string]entry),
 	}
 	z.fileScope = s
-	walk(s, f)
+	ast.WalkVisitor(f, s)
 	if z.errs != nil {
 		return z.errs
 	}