converting between struct types with different struct tags breaks

[Wezery]

What version of Garble and Go are you using?

$ garble version
v0.10.2-0.20231009211313-45b591d8eb92
$ go version
go1.21.1 linux/amd64
go1.21.2 darwin/arm64

Faced struct cast issue with golang.org/x/crypto v0.14.0 ssh module

It uses two structs:

type pingMsg struct {
	Data string `sshtype:"192"`
}

type pongMsg struct {
	Data string `sshtype:"193"`
}

expected that they are "garbled" identically, but Data turns to garbled1 and garbled2.
Since in new ssh code used cast pongMsg(pingMsg) build fails with obvious error
:1: cannot convert kmohymSBLtRj (variable of type pingMsg) to type wWObn1U

During test founded this.
Example1. Builds and works fine

package main

import (
	"fmt"
)

type Test1 struct {
	Data string
}

type Test2 struct {
	Data string
}

func main() {
	t2 := Test2{Data:"2"}
	test := Test1(t2)
	fmt.Println(test.Data)
}

turns to

//line :1
package main

import (
	fmt "fw0ZR0h"
)

type UNSbt0XRHe struct {
	BPkrPRX string
}

type Ia2EJJoSk struct {
	BPkrPRX string
}

func main() {
	kE6sk8 := Ia2EJJoSk{BPkrPRX: "2"}
	j1aqHbggMN :=  /*line :1*/ UNSbt0XRHe(kE6sk8)
	 /*line :1*/ fmt.Ork_AO3wQag(j1aqHbggMN.BPkrPRX)
}

var _ = fmt.WspUe2S

Example2. Errors similar to ssh

package main

import (
	"fmt"
)

type Test1 struct {
	Data string `sshtype:"192"`
}

type Test2 struct {
	Data string `sshtype:"193"`
}

func main() {
	t2 := Test2{Data:"2"}
	test := Test1(t2)
	fmt.Println(test.Data)
}

turns to

//line :1
package main

import (
	fmt "fw0ZR0h"
)

type JeXhXvN struct {
	SN39xkW0LMFx string `sshtype:"192"`
}

type P4tGkaKgZ struct {
	WMEAOdA4bm string `sshtype:"193"`
}

func main() {
	neRq3vd0MLA := P4tGkaKgZ{WMEAOdA4bm: "2"}
	rGSFu5 :=  /*line :1*/ JeXhXvN(neRq3vd0MLA)
	 /*line :1*/ fmt.Ork_AO3wQag(rGSFu5.SN39xkW0LMFx)
}

var _ = fmt.WspUe2S

Text in back quotes mumbles logic of garble?

[mvdan]

Thanks for filing this bug - we've had a TODO about this edge case for some time, but you're the first one to run into the issue in practice :)

garble/hash.go

Lines 237 to 239 in 5e80f12

	// TODO: We should probably strip field tags here.
	// Do we need to do anything else to make a
	// struct type "canonical"?

[pielgrzym]

fix works :)