I'm confused by the `SuspiciousCollectionReassignm...

# stdlib

[mbonnin]

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?

[Paul Griffith]

The wording is a bit confusing, but the reason it’s “suspicious” is that the += operator is quietly allocating an entire new collection

[mbonnin]

Yea but sometimes you have no control over this

[mbonnin]

Typically Gradle var properties

[Paul Griffith]

🤷 it’s “suspicious”, not “wrong”

[mbonnin]

Yup

[Paul Griffith]

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

[Paul Griffith]

but there’s also nothing stopping you from suppressing and moving on 🙂

[mbonnin]

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

[mbonnin]

That clears it up, thanks!

[mbonnin]

https://github.com/JetBrains/intellij-community/pull/2063

[nkiesel]

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]"

[mbonnin]

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

[mbonnin]

I think the issue is with performance, not behaviour

[nkiesel]

the "MutableList" case behaves differently.

[mbonnin]

Ooo right, I see it now

[mbonnin]

The fact that l1 += 42 is not the same as l1 = l1 + 42 in the mutable case feels weird

[mcpiroman]

IMO this introspection could trigger just on += with `var MutableList`but no with `var List`nor val MutableList.

[Alexey Belkov [JB]]

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