Fail every time a Kotlin warning is found, except for "is deprecated" warnings. https://medium.com/dipien/fail-your-build-on-kotlin-warnings-96c4d9b3fd33

fail_on_kotlin_warning.gradle.kts

import org.gradle.api.Project
import org.gradle.api.internal.GradleInternal
import org.gradle.configurationcache.extensions.serviceOf
import org.gradle.internal.logging.events.operations.LogEventBuildOperationProgressDetails
import org.gradle.internal.operations.BuildOperationDescriptor
import org.gradle.internal.operations.BuildOperationListener
import org.gradle.internal.operations.BuildOperationListenerManager
import org.gradle.internal.operations.OperationFinishEvent
import org.gradle.internal.operations.OperationIdentifier
import org.gradle.internal.operations.OperationProgressEvent
import org.gradle.internal.operations.OperationStartEvent

val kotlinWarnings = mutableListOf<String>()
val nonKotlinWarnings = mutableListOf<String>()
val errors = mutableListOf<String>()
val buildOperationListener = object : BuildOperationListener {

    override fun started(buildOperation: BuildOperationDescriptor, startEvent: OperationStartEvent) { }

    override fun progress(operationIdentifier: OperationIdentifier, progressEvent: OperationProgressEvent) {
        val log = progressEvent.details
        if (log is LogEventBuildOperationProgressDetails) {
            if (log.logLevel == LogLevel.WARN) {
                if (log.message.contains("w:") && log.message.contains(".kt")) {
                    if (!log.message.contains("is deprecated") && !kotlinWarnings.contains(log.message)) {
                        kotlinWarnings.add(log.message)
                    }
                } else {
                    if (!nonKotlinWarnings.contains(log.message)) {
                        nonKotlinWarnings.add(log.message)
                    }
                }
            } else if (log.logLevel == LogLevel.ERROR) {
                if (!errors.contains(log.message)) {
                    errors.add(log.message)
                }
            }
        }
    }

    override fun finished(buildOperation: BuildOperationDescriptor, finishEvent: OperationFinishEvent) { }
}
val gradleInternal = project.gradle as GradleInternal
val buildOperationListenerManager = gradleInternal.serviceOf() as BuildOperationListenerManager?
buildOperationListenerManager?.addListener(buildOperationListener)
project.gradle.buildFinished { buildResult ->
    buildOperationListenerManager?.removeListener(buildOperationListener)

    if (buildResult.failure == null && nonKotlinWarnings.isNotEmpty()) {
        project.logger.warn("")
        project.logger.warn("================================= Project Warnings Summary ================================================================================")
        nonKotlinWarnings.forEach { project.logger.warn(it) }
        project.logger.warn("===========================================================================================================================================")
    }

    if (kotlinWarnings.isNotEmpty()) {
        project.logger.warn("")
        project.logger.warn("====================================== Kotlin Warnings ====================================================================================")
        kotlinWarnings.forEach { project.logger.warn(it) }
        project.logger.warn("===========================================================================================================================================")
        if (buildResult.failure == null) {
            throw RuntimeException("Kotlin warning found")
        }
    }

    if (buildResult.failure != null && errors.isNotEmpty()) {
        project.logger.warn("")
        project.logger.warn("================================= Project Errors Summary ==================================================================================")
        errors.forEach { project.logger.error(it) }
        project.logger.warn("===========================================================================================================================================")
    }
}