variable set in a loop but unused not failing compilation

[Tiago de Bem Natel de Moura]

Hello,

We had the bug below in production:
https://go.dev/play/p/-jewy7e7UcZ

Look at the `opt` variable inside `listGithubPullReviews`, it's set multiple times (inside the loop) but never used... it was supposed to be passed as the last argument of `ListReviews()`.

Why Go compiler is not giving an error for this case? AFAICS all of those `opt.Page = resp.NextPage` inside the loop cannot make any side effects, then it looks safe to assume the variable is only set and never used?

So, simplifying the problem, this is the non-failing case: https://go.dev/play/p/FLAIlVx_sSG

But if you change from a struct to a plain int, then the compiler gives: `./prog.go:6:2: a declared and not used`

Luckily, we were using a `ctx` with timeout otherwise it would be an infinite loop. The thing was tested but only for cases where all results came in a single page, then the loop aborted in the first iteration. I think this could be detected by the compiler, no?

[Axel Wagner]

Hi,

I'll note that this has nothing, really, to do with the loop, but with the fact that you are assigning a struct field.

For a simpler example, compare this: https://go.dev/play/p/MmR-AhOUQH3 with this: https://go.dev/play/p/Y1uoI8thYuV

If you only write to the entire variable, the compiler complains about that variable not being used. But if you set a field, the failure goes away.

This compilation error is based on this sentence from the spec:

> Implementation restriction: A compiler may make it illegal to declare a variable inside a function body if the variable is never used.

Note that it says "may" make it illegal. It's also not really defined what it means for a variable to be "never used".

TBH I don't really like these implementation-defined compilation failures. But it would be difficult to make them more strict. Perhaps this case could be a vet warning.

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[Tiago de Bem Natel de Moura]

Hi Axel,

Thanks for the explanation, it makes sense. Yes, I think it would be a good case for a `vet` warning.