staticcheck: misuse of slice make() and append()

[cgilmour]

A problem that seems to occur regularly is creating a slice with a length component, then only appending to it.
Example: https://play.golang.org/p/CehTV0VFKp

Generally, the intention would be making a zero-length slice with an initial capacity, then appending to it. The effect of this occurs only at runtime and the cause can be hard to spot when debugging and/or reading the related code -- it's a very subtle detail.

There may be cases where this was done intentionally to allocate some space for other functions to set the data, or to suit a particular protocol or data structure.
However, I expect that it's exceptionally rare compared to the common misuse.

A check that observes a slice created with make(slice-type, len) followed by only append()'s to that slice could be sufficient to detect the common mistake.

[mvdan]

What if you intentionally wanted a number of zero values at the beginning? I've needed to leave a leading 0 before, for example when populating a slice that will later be used for a binary search.

[cgilmour]

That consideration was noted in the third paragraph.

[mvdan]

I read that, but my point (which I didn't communicate very well) is that I think this would cause too many false positives.

However, I expect that it's exceptionally rare compared to the common misuse.

The example was to show a not-so-rare use :)

I'm also surprised that this issue would happen regularly. I can see it happening with people who are new to Go and aren't very familiar with how make works. But even then, it's fairly easy to spot that a slice has a chunk of zero elements at the beginning.

[ainar-g]

I am not new to Go, and I've committed this mistake several times over the last year. When you're tired you're just thinking "Hm, I should allocate space before appending." and you have a brain-fart and write

s2 := make([]int, len(s1))

instead of

s2 := make([]int, 0, len(s1))

before appending.

I think this should be a check.

[ainar-g]

FWIW, doctor, the future go vet replacement, reports this:

example.com/foo/bar.go:301:18: append after make(pq.Int64Array, n); did you mean make(pq.Int64Array, 0, n)?

[dominikh]

vet is starting to have more false positives than staticcheck ;)

[jcorbin]

Fwiw, I too am not new to go, and I still write make([]T, 42) way too often when I mean make([]T, 0, 42). In the case where i need to have some slots, it's usually more like make([]T, 1, N); I've never used make([]T, N), followed by only appends, and meant it.