[GDScript 2] How to make an enumerated export variable with custom options

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:bust_in_silhouette: Asked By love_this_name

I want to make a tool script that can manipulate the layers on a TileMap. I want to have an export enumerator variable. This is easy enough with:
@export_enum(<options>) var my_enum: int

However, I can only seem to declare the enumerator’s options by hand in the script. I want to be able to pick between the TileMap’s layers, where there might be a different number for each TileMap. I can get the TileMap’s layers as either a number, array of strings or whatever easily enough, but how do I put it in the export enumerator?

Thanks.