cmd/cue: support stringable types as map keys

[addreas]

Ran into a case where metacontroller/metacontroller had a map where the key was a struct that implemented encoding.TextMarshaler and encoding.TextUnmarshaler. After some confusion because it was silently dropped (until i ran cue get go --verbose), i had a look and a fix seemed not too far off.

Not super excited about this patch, as it's a bit quick and dirty, but it did seem to do the trick.

[mvdan]

Apologies that we didn't reply to this PR quickly. Do you have an example or reproducer for this case? In particular, we would need a form of regression test to go with the change, to make sure that it works as expected and isn't broken in the future.

[addreas]

Not sure how to create a txtar for a cue get go command, and I've almost completely forgot about this whole thing as well. But the problem was that there existed a type that I wanted to cue get go on, but it was defined as type RelativeObjectMap map[GroupVersionKind]map[string]*unstructured.Unstructured and GroupVersionKind is a struct that implements encoding.TextMarshaler and encoding.TextUnmarshaler.

The minimal case should look something like this, I guess:

package test

type Convoluted map[Key]string

type Key struct {
	ActualKey string
}

// MarshalText is implementation of  encoding.TextMarshaler
func (k Key) MarshalText() ([]byte, error) {
	return []byte(k.ActualKey), nil
}

// UnmarshalText is implementation of encoding.TextUnmarshaler
func (k *Key) UnmarshalText(text []byte) error {
	*k = Key{
		ActualKey: string(text),
	}
	return nil
}

Happy to close this unless you think it's a feature worth the effort.