Serialization opt-ins

[LVMVRQUXL]

Description

In the build.gradle.kts file, we opt-in at the module level the experimental API of the kotlinx.serialization library.

languageSettings.optIn("kotlinx.serialization.ExperimentalSerializationApi")

But this is a bad idea because it simply removes all warning about these usages that could break at any point in time.
This way of disabling warnings linked to the use of experimental APIs also doesn't communicate the real stability of a declaration to consumers.

This is why we decide to remove this compiler argument for communicating the usage of an experimental API at the declaration level, by propagating the experimental annotation or marking the declaration with an Opt-in annotation.

Like suggested by the Kotlin team, we should choose between propagating an experimental annotation or marking a declaration with an Opt-in annotation:

• if the experimental type is part of our public API, we should propagate the annotation
• if the experimental declaration is an implementation detail, we should:
  • propagate the annotation if it's not in our control
  • opt-in the experimental declaration.

Checklist

• Wait for the issue Commits verification #82 to be resolved for working on safer commits.
• Remove the compiler option.
• Check if a declaration uses the experimental serialization API and do something (propagate/opt-in) with it if so.
• Update the Unreleased section in changelog.
• Document the decision-making about using experimental APIs in the discussions.