install stringer: go get -u -a golang.org/x/tools/cmd/stringer

permision_test.go

package core

import (
	"testing"
)

func TestPermissionFromName(t *testing.T) {
	for i := 0; i < len(_Permission_index)-1; i++ {
		left, right := _Permission_index[i], _Permission_index[i+1]
		name := _Permission_name[left:right]
		t.Run(name, func(t *testing.T) {
			p, ok := PermissionFromName(name)
			if !ok {
				t.Fatalf("permision name %q should be valid", name)
			}

			if expect, got := Permission(i), p; expect != got {
				t.Fatalf("name %s expect %v but got %v", name, expect, got)
			}
		})
	}
}

permission.go

package core

//go:generate stringer --type Permission

type Permission int

const (
	A Permission = iota
	B
	C
	D
	E
	F
)

func PermissionFromName(p string) (Permission, bool) {
	for i := 0; i < len(_Permission_index)-1; i++ {
		l, r := _Permission_index[i], _Permission_index[i+1]
		if p == _Permission_name[l:r] {
			return Permission(i), true
		}
	}

	return 0, false
}

permission_string.go

// Code generated by "stringer --type Permission"; DO NOT EDIT.

package core

import "strconv"

func _() {
	// An "invalid array index" compiler error signifies that the constant values have changed.
	// Re-run the stringer command to generate them again.
	var x [1]struct{}
	_ = x[A-0]
	_ = x[B-1]
	_ = x[C-2]
	_ = x[D-3]
	_ = x[E-4]
	_ = x[F-5]
}

const _Permission_name = "ABCDEF"

var _Permission_index = [...]uint8{0, 1, 2, 3, 4, 5, 6}

func (i Permission) String() string {
	if i < 0 || i >= Permission(len(_Permission_index)-1) {
		return "Permission(" + strconv.FormatInt(int64(i), 10) + ")"
	}
	return _Permission_name[_Permission_index[i]:_Permission_index[i+1]]
}