stringer not used for map keys

[crawshaw]

When rendering type T, the cmp package uses the String method for slice values, but not for map keys.

package main

import (
	"fmt"

	"github.com/google/go-cmp/cmp"
)

type T [5]byte

func (t T) String() string { return string(t[:]) }

func main() {
	var t T
	copy(t[:], "world")
	m := map[T]string{t: "val"}

	s := []T{t}

	fmt.Println(cmp.Diff(nil, m))
	fmt.Println(cmp.Diff(nil, s))
}

Outputs:

  interface{}(
+ 	map[main.T]string{{0x77, 0x6f, 0x72, 0x6c, 0x64}: "val"},
  )

  interface{}(
+ 	[]main.T{s"world"},
  )

Try it here: https://play.golang.org/p/GS8f7ABvd61

If you agree this is a bug, I can take a crack at fixing it.

[dsnet]

Not calling String on keys was probably an oversight. Feel free to fix it.

It seems that enabling this is as trivial as removing this line:

go-cmp/cmp/report_reflect.go

Line 211 in 6f77996

opts.AvoidStringer = true

However, I can't remember why I would have set that explicitly to true.