I'm confused by the

SuspiciousCollectionReassignment

inspection. The description says:

fun test() {
      var list = listOf(0)
      list += 42 // new list is created, variable 'list' still contains only '0'
  }

But doing the same thing in playground yields

[0, 42]

. What am I missing?

The wording is a bit confusing, but the reason it’s “suspicious” is that the

+=

operator is quietly allocating an entire new collection

Yea but sometimes you have no control over this

🤷 it’s “suspicious”, not “wrong”

Yup

I think the preferred way to work around (rather than a suppress warnings) would be to make it explicit you’re setting to a new list, e.g.

list = list + 42

I've been suppressing this one for years, don't know why I decided to investigate more today 🙂

One reason I can see for "suspicious" is that

var l1 = listOf(0); var l2 = l1; l1 += 42

results in "l2=[0]", but a

var l1 = mutableListOf(0); var l2 = l1; l1 += 42

results in "l2=[0,42]"

It behaves exactly like int:

var l1 = 0; var l2 = l1; l1 += 42

results in "l2==0" so I'd argue this is behaving as expected

the "MutableList" case behaves differently.

Ooo right, I see it now

IMO this introspection could trigger just on

+=

with `var MutableList`but no with `var List`nor

val MutableList

.

See also https://youtrack.jetbrains.com/issue/KTIJ-15082/Useless-quickfix-Replace-with-ordinary-assignment-is-suggested-for-Augmented-assignment-creates-a-new-collection-under-the-hood