cue/ast/astutil: use generics to remove some reflect

We don't need reflect to read and write nodePtr;
we can make it a pointer to a generic ast.Node instead.
While here, applyExprList didn't use its ptr argument.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ia28dfdcf6e371f9d1a2de9c0ce2302b83df4eb80
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1168830
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/ast/astutil/apply.go

@@ -199,7 +199,7 @@ type applyVisitor interface {
 
 // Helper functions for common node lists. They may be empty.
 
-func applyExprList(v applyVisitor, parent Cursor, ptr interface{}, list []ast.Expr) {
+func applyExprList(v applyVisitor, parent Cursor, list []ast.Expr) {
 	c := newCursor(parent, nil, nil)
 	for i, x := range list {
 		c.index = i
@@ -286,14 +286,12 @@ func applyDeclList(v applyVisitor, parent Cursor, list []ast.Decl) []ast.Decl {
 	return c.decls
 }
 
-func apply(v applyVisitor, parent Cursor, nodePtr interface{}) {
-	res := reflect.Indirect(reflect.ValueOf(nodePtr))
-	n := res.Interface()
-	node := n.(ast.Node)
+func apply[N ast.Node](v applyVisitor, parent Cursor, nodePtr *N) {
+	node := *nodePtr
 	c := newCursor(parent, node, nodePtr)
 	applyCursor(v, c)
-	if node != c.node {
-		res.Set(reflect.ValueOf(c.node))
+	if ast.Node(node) != c.node {
+		*nodePtr = c.node.(N)
 	}
 }
 
@@ -349,10 +347,10 @@ func applyCursor(v applyVisitor, c Cursor) {
 		// nothing to do
 
 	case *ast.Interpolation:
-		applyExprList(v, c, &n, n.Elts)
+		applyExprList(v, c, n.Elts)
 
 	case *ast.ListLit:
-		applyExprList(v, c, &n, n.Elts)
+		applyExprList(v, c, n.Elts)
 
 	case *ast.Ellipsis:
 		if n.Type != nil {
@@ -381,7 +379,7 @@ func applyCursor(v applyVisitor, c Cursor) {
 
 	case *ast.CallExpr:
 		apply(v, c, &n.Fun)
-		applyExprList(v, c, &n, n.Args)
+		applyExprList(v, c, n.Args)
 
 	case *ast.UnaryExpr:
 		apply(v, c, &n.X)