x/tools/go/astutil/inspector: add Cursor.ParentEdge{Kind,Index} convenience accessors

// This program demonstrates how the new convenience accessors
// `ParentEdgeKind` and `ParentEdgeIndex` would be used once the
// change ships.  Since the real `golang.org/x/tools/go/ast/inspector`
// package is not available in the sandbox, we provide a minimal
// mock implementation that mimics the relevant API.

package main

import "fmt"

// --- Mock types to stand in for the real inspector package. ---

// edge.Kind is an enumeration of AST edge kinds.
type Kind int

const (
	CallExpr_Args Kind = iota
	SelectorExpr_Sel
)

// Cursor represents a position in an AST.  In the real package it
// contains many fields; here we only need the ParentEdge method.
type Cursor struct {
	kind  Kind
	index int
}

// ParentEdge returns the kind of edge from the cursor's parent to
// the cursor and the index of the cursor among its siblings.
func (c Cursor) ParentEdge() (Kind, int) {
	return c.kind, c.index
}

// ParentEdgeKind returns the first component of ParentEdge.
func (c Cursor) ParentEdgeKind() Kind {
	kind, _ := c.ParentEdge()
	return kind
}

// ParentEdgeIndex returns the second component of ParentEdge.
func (c Cursor) ParentEdgeIndex() int {
	_, idx := c.ParentEdge()
	return idx
}

// --- Example usage of the convenience accessors. ---

func main() {
	// Create a cursor that is the third argument to a function call.
	cur := Cursor{kind: CallExpr_Args, index: 2}

	// Using the new convenience methods.
	if cur.ParentEdgeKind() == CallExpr_Args {
		fmt.Println("Cursor is an argument to a call expression")
	}
	if cur.ParentEdgeIndex() == 2 {
		fmt.Println("Cursor is the third argument (index 2)")
	}
}