Quick Answer

Marking experimental API usage explicitly

Understanding the Issue

Kotlin requires explicit opt-in for experimental features. You must annotate your code with @OptIn or @UseExperimental when using APIs marked as experimental.

The Problem

This code demonstrates the issue:

Kotlin Error 
// Problem: Using experimental API
@ExperimentalTime
fun measureTime() {
    val duration = TimeSource.Monotonic.measureTime { ... }
}

The Solution

Here's the corrected code:

Kotlin Fixed 
// Solution: Add opt-in annotation
@OptIn(ExperimentalTime::class)
fun measureTime() {
    val duration = TimeSource.Monotonic.measureTime { ... }
}

// Or enable for whole file:
@file:OptIn(ExperimentalTime::class)

Key Takeaways

Always explicitly opt-in when using experimental Kotlin APIs.