cmd/compile: ++ counts as using a variable, shouldn't

[heschi]

What version of Go are you using (go version)?

1.9.2, as seen on the playground.

Does this issue reproduce with the latest release?

Yes.

What did you do?

https://play.golang.org/p/2RuYMrSLjt3:

package main

func main() {
	x := 0
	x++
}

What did you expect to see?

A compiler error saying that x is unused. For the purposes of removing useless code, ++ isn't any more of a reason to have a variable than the initial assignment. += behaves the same way.

What did you see instead?

Nothing, apparently because x++ counts as using the variable. go vet also seems content.

It's hard to believe this has never been reported before, but the closest bug Alan and I could find is https://golang.org/issue/10989, which is rather different.

I don't know if it's worth catching this as a compile error, but it ought to at least be reported by vet?

[as]

Working as designed, because x++ is equivalent to x=x+1 and that reduces to x=x, which is coherent with the current definition of usage.

https://play.golang.org/p/jCqJ8XwMJhs

[mvdan]

One error that can be given is that the last assigned value is unused. For example, when one does x := 0; x = x + 1, @dominikh's staticcheck reports this value of x is never used. That is pointed at the second statement, not at the first, as it concerns the last assigned value and not the declaration.

It is a different error, but perhaps one that would be easier to integrate into vet and other static analysis tools.

[heschi]

That's #6072, which I personally feel is a weaker case than this. I suppose if the best way to implement this check is an SSA analysis then both really are the same.

[adonovan]

FWIW, the github.com/gordonklaus/ineffassign analyzer reports this mistake, and it doesn't use SSA. (It constructs its own control-flow graph similar to ctrlflow.)

$ cat a.go

package main

func main() {
	x := 0
	x++
}

$ go run github.com/gordonklaus/ineffassign@latest ./a.go 
a.go:6:2: ineffectual assignment to x