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Here is a lens diagram comparison between the EF 50mm f/1.8 STM and the upcoming Canon RF 50mm f/1.8. The new RF 50mm f/1.8 STM definitely looks like a brand new lens design and not simply making the EF mount version work with RF, which likely would never be worth the time and effort anyway.
I expect to see this lens announced next week, if not sooner.