Ignore doesn't always work for map keys

[rogpeppe]

If there's an Ignore rule that matches a key in a map, I'd expect that key
to be ignored even if it doesn't exist in the map being compared to.

For example, the following code says that the two values aren't equal
even though I've specified that the "x" key should be ignored:

package main

import (
	"fmt"

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

func main() {
	a := map[string]int{
		"x": 1,
		"y": 2,
	}
	b := map[string]int{
		"y": 2,
	}
	fmt.Println(cmp.Diff(a, b, cmp.FilterPath(isKeyX, cmp.Ignore())))
}

func ignorePath(pat string) cmp.Option {
	return cmp.FilterPath(isKeyX, cmp.Ignore())
}

func isKeyX(p cmp.Path) bool {
	step, ok := p[len(p)-1].(cmp.MapIndex)
	return ok && step.Key().String() == "x"
}

It prints:

{map[string]int}["x"]:
	-: 1
	+: <non-existent>

If I add an "x" key (with any value) to b, then they compare equal as expected.

[dsnet]

The current behavior is documented on cmp.Equal:

Map keys are equal according to the == operator. To use custom comparisons for map keys, consider using cmpopts.SortMaps.

However, I believe the more general issue at hand is that there is no easy way for ignore to conceptually "remove" elements from a slice or map. #28 was trying to solve the application where the zero value of a map element should be removed.

I don't have a good answer for this :)

[dsnet]

See #28 (comment) which contains some ideas I explored and why I believe there were all problematic in some way.

[dsnet]

Fixed by #121