将元素添加到 Go AST 后乱序

下面的测试try 使用AST向 struct 中添加字段.字段添加正确，但注释添加顺序混乱.我想这个职位可能需要手动指定，但到目前为止，我还没有找到答案.

这里有个不及格的测试:http://play.golang.org/p/RID4N30FZK

以下是代码:

package generator

import (
    "bytes"
    "fmt"
    "go/ast"
    "go/parser"
    "go/printer"
    "go/token"
    "testing"
)

func TestAst(t *testing.T) {

    source := `package a

// B comment
type B struct {
    // C comment
    C string
}`

    fset := token.NewFileSet()
    file, err := parser.ParseFile(fset, "", []byte(source), parser.ParseComments)
    if err != nil {
        t.Error(err)
    }

    v := &visitor{
        file: file,
    }
    ast.Walk(v, file)

    var output []byte
    buf := bytes.NewBuffer(output)
    if err := printer.Fprint(buf, fset, file); err != nil {
        t.Error(err)
    }

    expected := `package a

// B comment
type B struct {
    // C comment
    C string
    // D comment
    D int
    // E comment
    E float64
}
`

    if buf.String() != expected {
        t.Error(fmt.Sprintf("Test failed. Expected:\n%s\nGot:\n%s", expected, buf.String()))
    }

    /*
    actual output = `package a

// B comment
type B struct {
    // C comment
    // D comment
    // E comment
    C   string
    D   int
    E   float64
}
`
    */

}

type visitor struct {
    file *ast.File
}

func (v *visitor) Visit(node ast.Node) (w ast.Visitor) {

    if node == nil {
        return v
    }

    switch n := node.(type) {
    case *ast.GenDecl:
        if n.Tok != token.TYPE {
            break
        }
        ts := n.Specs[0].(*ast.TypeSpec)
        if ts.Name.Name == "B" {
            fields := ts.Type.(*ast.StructType).Fields
            addStructField(fields, v.file, "int", "D", "D comment")
            addStructField(fields, v.file, "float64", "E", "E comment")
        }
    }

    return v
}

func addStructField(fields *ast.FieldList, file *ast.File, typ string, name string, comment string) {
    c := &ast.Comment{Text: fmt.Sprint("// ", comment)}
    cg := &ast.CommentGroup{List: []*ast.Comment{c}}
    f := &ast.Field{
        Doc:   cg,
        Names: []*ast.Ident{ast.NewIdent(name)},
        Type:  ast.NewIdent(typ),
    }
    fields.List = append(fields.List, f)
    file.Comments = append(file.Comments, cg)
}