When I tried to use androidx.compose.ui.platform.LocalSoftwareKeyboardController in my RSS Feed Reader app, it turned out it is an @ExperimentalComposeUiApi which has this @RequiresOptIn() annotation.

@RequiresOptIn("This API is experimental and is likely to change in the future.")
annotation class ExperimentalComposeUiApi

That means, in order to use this LocalSoftwareKeyboardController, I need to add @OptIn() annotation in the class or else you will see a warning/error like below which is defined by the RequiresOptIn() annotation above.

This API is experimental and is likely to change in the future.

So, I added this @OptIn(ExperimentalComposeUiApi::class).

@OptIn(ExperimentalComposeUiApi::class)
@Composable
fun ArticlesTopBar(
    navHostController: NavHostController,
    viewModel: MainViewModel,
    onArticlesSearch: () -> Unit,
) {
    val keyboardController = 
    val keyboardController = LocalSoftwareKeyboardController.current
    /*...*/
}

After that, I got this warning.

Warning:(21, 6) This class can only be used with the compiler argument '-opt-in=kotlin.RequiresOptIn'

To fix this warning, I added this compiler argument in the build.gradle (module level) based on the official documentation here.

android {

    /*...*/

    tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
        kotlinOptions {
            freeCompilerArgs += "-Xopt-in=kotlin.RequiresOptIn"
        }
    }
}

[Updated: July 01, 2022] - It looks like this -Xopt-in=kotlin.RequiresOptIn compiler argument is not needed anymore in Kotlin 1.7.0. See here.

While developing same Simple REST API app, I also encountered the same issue because the APIs from com.jakewharton.retrofit2.converter.kotlinx.serialization packages are marked with @ExperimentalSerializationApi annotation, which requires me to specify the@OptIn() annotation.

@RequiresOptIn(level = RequiresOptIn.Level.WARNING)
public annotation class ExperimentalSerializationApi

So I did the same as above to get rid of the warnings.