Automatically opt-in all experimental annotations for types publicly exposed by a benchmark class

[dkhalanskyjb]

I've used https://github.com/fzhinkin/benchmarks-template.

@State(Scope.Benchmark)
open class ExampleBenchmark {
    @Benchmark
    fun parse() =
        @OptIn(ExperimentalTime::class) Instant.parse("2023-10-01T16:12:42.546124Z")

    @Benchmark
    fun parseNoFrac() =
        @OptIn(ExperimentalTime::class) Instant.parse("2023-10-01T16:12:42Z")
}

then fails to compile with

> Task :compileJsBenchmarkKotlinJs FAILED
e: file:///home/jb/IdeaProjects/benchmark-Instant-parse/build/benchmarks/js/sources/kotlinx/benchmark/generated/org/example/ExampleBenchmark_Descriptor.kt:30:52 This declaration needs opt-in. Its usage must be marked with '@kotlin.time.ExperimentalTime' or '@OptIn(kotlin.time.ExperimentalTime::class)'
e: file:///home/jb/IdeaProjects/benchmark-Instant-parse/build/benchmarks/js/sources/kotlinx/benchmark/generated/org/example/ExampleBenchmark_Descriptor.kt:32:52 This declaration needs opt-in. Its usage must be marked with '@kotlin.time.ExperimentalTime' or '@OptIn(kotlin.time.ExperimentalTime::class)'

Moving OptIn somewhere outside also causes the compilation failure. The one way I've found to write this benchmark is to use block bodies for these functions:

    @Benchmark
    fun parse() {
        @OptIn(ExperimentalTime::class) Instant.parse("2023-10-01T16:12:42.546124Z")
    }

[fzhinkin]

The issue is not specific to K/JS and I was able to reproduce it with other targets as well.
The compilation fails because ExampleBenchmark::parse and ::parseNoFrac had an experimental return type (kotlin.time.Instant).

As a workaround that won't affect benchmark's semantics, the experimental annotation could be opted-in using a compiler flag (-opt-in=kotlin.time.ExperimentalTime).

It would be nice to add all necessary plugins to a generated plugin automatically, however, supporting it might be tricky:

• for klib-based targets (well, all non-JVM targets), we have to load all transitive dependencies in order to track down all experimental annotations; I'm not sure if it is worth support right now.
• for JVM target, we need to intercept JMH's code generator to emit all required opt-ins (further investigation is required here).

[dkhalanskyjb]

Putting the opt-in into the block also fixes the issue nicely, so it's not a big deal. I think the problem is only with the annotation processing.