False positive performance SpreadOperator in case of pass-through vararg

[christianhujer]

Expected Behavior

When using the spread operator to pass on a vararg array from a vararg function implementation to a vararg function call, the spread operator should not be reported.

Observed Behavior

The spread operator passing a vararg array from a vararg function implementation to a vararg function call is reported by Detekt as a performance issue because of the spread operator.

This is wrong because, in the case of passing vararg to vararg, the spread operator does not (anymore since Kotlin 1.1.60, 1.2-Beta2) create a new array and thus is not a performance issue. Refer to the following ticket on Kotlin: https://youtrack.jetbrains.com/issue/KT-20462

Steps to Reproduce

Call one vararg method from another using the spread operator.
A popular example is to call runApplication(args) from main(vararg args: String) in Spring applications.

The following code should be accepted as correct by detekt:

package com.example

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
import org.springframework.context.ApplicationContext

@SpringBootApplication
class ExampleApplication

lateinit var appContext: ApplicationContext
    private set

fun main(vararg args: String) {
    appContext = runApplication<ExampleApplication>(*args) // ⇐ Point of Interest
}

Context

The impact is low.
The situation of passing through vararg is relatively rare.
It is easy to deal with the situation by using a @SuppressWarnings("SpreadOperator") annotation on the corresponding method.

Your Environment

• Version of detekt used: 1.14.1
• Version of Gradle used (if applicable): 6.6.1
• Operating System and version: Kubuntu 20.04 LTS
• Link to your project (if it's a public repository): -

[christianhujer]

As an enhancement, the check could look at the actual byte code generated. If the spread operator leads to a new array instantiation, the spread operator is a performance issue and should be reported. If the spread operator passes through an existing array, the spread operator is not a performance issue and should not be reported.

[christianhujer]

Thank you, works like a charm now! ❤️