maybe it wasn't supposed to work before, but we have been doing this for awhile to take a mutableStateList and store it in a SavedStateHandle by calling .toList() on it. (since this mutablestatelist is in a viewmodel not a rememberSaveable). with 1.4 I know it exposes the internals instead of making a copy, but that results in this:

java.lang.IllegalArgumentException: Can't put value with type class androidx.compose.runtime.external.kotlinx.collections.immutable.implementations.immutableList.SmallPersistentVector into saved state
 	at androidx.lifecycle.SavedStateHandle.set(SavedStateHandle.kt:249)

(using lifecycle 2.6.0-rc01)

I can't use a mutableStateList with that though, I have to use simple state with a list member?