You got a response - that the problem that you have is the same underlying problem as "Unwrap inner type when enum variant is known" (and two other options).
Your edit suggests that it did solve the problem, though it didn't answer the "I don't understand" aspect of the question.
If the solutions given in the linked duplicates didn't work, then specifically applying and showing how they didn't work would be the next step.
> Edit: Based on @Lukas solution, the following seems to compile, but I don't yet know whether it is doing what I want.
Stack Overflow itself is poorly suited for the "explain" type questions - especially if it is going to involve back and forth. The comments are intentionally poorly designed for that. The thing that SO comments are trying to avoid is the forum like "the real answer is buried deeply on page 10 of 15 of 30 comments per page on a phpforum". If the understanding is only uncovered after 50 back and forth comments, then the question and answer themselves are a poor fit for SO.
If you are after that type of interaction and a "here is the answer" isn't going to suffice then other formats may be better for your question. The corresponding difficulty is that Stack Overflow's framework can't be made to handle everything well. It handles Q&A well and doesn't try to handle conversations well.
Your question was marked as a dup because the solution that you are after is in those other Q&A sets.
If you need back and forth of explaining the syntax then /r/rust may be better. Alternatively, if you have something that works, get it reviewed on https://codereview.stackexchange.com/questions/tagged/rust or if you are after a more real time back and forth conversation, https://discord.gg/rust-lang
You will note that if you are trying to find an existing match to your problem, /r/rust and discord are poorly searchable and you might have to scroll through a bunch of comments to try to get the context around the problem... and that is what SO tries to solve.