Augmented assignment creates a new collection under the hood

Reports augmented assignment (+=) expressions on a read-only Collection.

Augmented assignment (+=) expression on a read-only Collection temporarily allocates a new collection, which may hurt performance.

Change type to mutable quick-fix can be used to amend the code automatically.

Example:

  fun test() {
      var list = listOf(0)
      list += 42 // A new list is allocated here, equivalent to list = list + 42
  }

After the quick-fix is applied:

  fun test() {
      val list = mutableListOf(0)
      list += 42
  }

Locating this inspection

By ID

Can be used to locate inspection in e.g. Qodana configuration files, where you can quickly enable or disable it, or adjust its settings.

SuspiciousCollectionReassignment

Via Settings dialog

Path to the inspection settings via IntelliJ Platform IDE Settings dialog, when you need to adjust inspection settings directly from your IDE.

Settings or Preferences | Editor | Inspections | Kotlin | Probable bugs

Suppressing Inspection

You can suppress this inspection by placing the following comment marker before the code fragment where you no longer want messages from this inspection to appear:

//noinspection SuspiciousCollectionReassignment

More detailed instructions as well as other ways and options that you have can be found in the product documentation:

IntelliJ IDEA 2025.2

Inspection Details
By default bundled with:	IntelliJ IDEA 2025.2, Qodana for JVM 2025.2,

Last modified: 18 September 2025