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This is mostly a reminder to myself. We have a way to make Key PMCs at runtime from PIR or higher. Here is one example of that mechanism (it's ugly, but it does exist):
We either don't have, or I'm not aware of, a mechanism to create a Key which uses a register reference:
$P0[$I0; $S0] = $P1
This is going to be necessary for doing self-hosted compilation in places like PACT.
Creation and introspection of register-ref keys will be required. Currently trying to print arbitrary key constants loaded from a packfile will eventually cause the interpreter to die when it references a register that doesn't exist. This can't be worked around as any attempt to get the value redirects to the register in question.
Add in a note about introspection methods for Key PMC in #717.
I've added these methods to Key PMC and marked this experimental in api.yaml. I'm going to consider this issue closed. We can open a new ticket later if these methods need to be modified after testing and evaluation.