validate map keys the same way as switch cases

[maratori]

Fix #35

In some cases switch by enum is too verbose and can be simplified by using map. WIth this PR exhaustive validates map keys as well.

type Enum int

const (
	A Enum = iota
	B
	C
)

func EnumToString(e Enum) string {
	return map[Enum]string{
		A: "A",
		B: "B",
		C: "C",
	}[e]
}

[maratori]

@nishanths I've decided to not refactor code in this PR.

1. I'm not sure how would you like to do it
2. PR is huge even without refactoring (however the most changes are in tests)

[nishanths]

Thank you so much. I will review it during the upcoming weekend.

[nishanths]

I looked at it quickly, and I couldn't find parts that would significantly benefit from refactoring right now, so I can think we can do it later if necessary. Did you have any parts in mind?

[maratori]

I don't have concrete suggestions for now. But here are some points of improvement:

• Following entities are declared in switch.go, but used in map.go as well:
  • type nodeVisitor
  • const resultXXX
  • func diagnosticEnumTypeName
  • func diagnosticMissingMembers
• Skipping generated files is duplicated in both switchStmtChecker and mapChecker
• mapChecker calls analyzeCaseClauseExpr

[nishanths]

Thank you, merged! And sorry for the delay.

I will follow up with a commit to add -maps flag for checking maps, so users have a way to opt-in/opt-out of checking maps.