go/ast: add BasicLit.ValueEnd field

// This program demonstrates the incorrect End position for a multiline raw
// string literal in Go 1.27.1. The change that adds BasicLit.ValueEnd is not
// yet available, so we show the problem that the proposal fixes.
package main

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
)

func main() {
	src := `package main
func main() {
	s := ` + "`Hello\nWorld`" + `
}`
	fset := token.NewFileSet()
	// Parse the source code.
	f, err := parser.ParseFile(fset, "example.go", src, 0)
	if err != nil {
		panic(err)
	}

	// Find the first BasicLit node (the raw string literal).
	var lit *ast.BasicLit
	ast.Inspect(f, func(n ast.Node) bool {
		if bl, ok := n.(*ast.BasicLit); ok && bl.Kind == token.STRING {
			lit = bl
			return false
		}
		return true
	})
	if lit == nil {
		panic("no string literal found")
	}

	// Print the literal value and its start position.
	fmt.Printf("Literal value: %q\n", lit.Value)
	fmt.Printf("Start position: %s\n", fset.Position(lit.Pos()))

	// End() uses ValuePos + len(Value), which is incorrect for multiline raw strings.
	fmt.Printf("End() position: %s\n", fset.Position(lit.End()))

	// Compute the correct end position by looking at the file content.
	// The file's content ends at the position of the closing backtick.
	// We find the index of the closing backtick in the source string.
	endIdx := len(src) - 1 // last character is the closing backtick
	endPos := fset.File(lit.Pos()).Pos(endIdx)
	fmt.Printf("Manual end position: %s\n", fset.Position(endPos))
}