Make `rcases` use the `induction`/`cases` framework
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Description
Currently rcases uses Lean.MVarId.cases, but the cases tactic uses its own system of applying casesOn.
The rcases tactic should use the same system as cases.
Note that #3747 introduced a hack into Lean.MVarId.cases via the useNatCasesAuxOn argument for the "beautiful Nat induction" project. This should be removed, since this has a general solution via @[induction_eliminator] and @[cases_eliminator] in induction/cases. (The Lean.MVarId.cases is a low-level function that should not use user-defined eliminators.)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by comparing Lean.MVarId.cases with the cases tactic's casesOn application path, then inspect how induction/cases eliminators are selected. Trace the useNatCasesAuxOn argument introduced by #3747. Done means rcases uses the same cases framework, while the low-level MVarId.cases hack is removed without breaking the intended Nat induction behavior.
Written by the indexing model from the issue text.
Assessment
- Domain
- compilers
- Issue type
- Refactor
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100