mock: hide implementation of Anything

[dolmen]

mock.Anything is currently implemented as:

const (
          // Anything is used in Diff and Assert when the argument being tested
          // shouldn't be taken into consideration. 
          Anything = "mock.Anything"
)

This has multiple problems:

• the implementation details are public. This is a design mistake.
• as the sentinel value is just an untyped string there is a non-zero risk of user code conflict

I propose to hide the implementation of Anything by making it a special value of a private type anything:

type anything struct{}

// Anything is used in Diff and Assert when the argument being tested
// shouldn't be taken into consideration. 
var Anything anything

[brackendawson]

This has bugged me every time I've seen it. I'm scared of this because is meets the definition of a breaking change, annoyingly.

Attention need to be paid to assertion failures where this string const is used, seeing it here annoys me too, and we can change this whenever we want, it's not breaking.

package main

import (
	"testing"

	"github.com/stretchr/testify/mock"
)

type myMock struct {
	mock.Mock
}

func (m *myMock) Do(stuff, things string) {
	m.Called()
}

func TestIfy(t *testing.T) {
	m := myMock{}
	m.Test(t)
	m.On("Do", mock.Anything, "cubes").Return()
	m.Do("anything", "spheres")
}

play

=== RUN   TestIfy
    mock.go:334: 
        
        mock: Unexpected Method Call
        -----------------------------
        
        Do()
        		
        
        The closest call I have is: 
        
        Do(string,string)
        		0: "mock.Anything"
        		1: "cubes"
        
        Provided 2 arguments, mocked for 0 arguments
        Diff: 0: PASS:  (Missing) == (string=mock.Anything)
        	1: FAIL:  (Missing) != (string=cubes)
--- FAIL: TestIfy (0.00s)
FAIL