fix: predefinition preprocessing: float .mdata out of non-unary applications #3204
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both Structural and WF recursion have a preprocess module (since #2818 at least). Back then I found that betareducing in WF, like it happens for structural, bytes because it would drop `let_fun` annotations, which are of course worth preserving. Now that #2973 is landed let's see if we can do it now. I hope that this fixes an issue where code like ``` theorem size_rev {α} (arr : Array α) (i : Nat) : (rev arr i).size = arr.size := by unfold rev split · rw [size_rev] rw [Array.size_swap] · rfl termination_by arr.size - i decreasing_by simp_wf; omega ``` fails but works with `size_rev _ _`. It fails due to some lambda-redexes that maybe go away. (Maybe we can also/additinally/alternatively track down where that redex is created and use `.beta` instead of `.app`.)
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Recursive predefinitions contains “rec app” markers as mdata in the predefinitions,
but sometimes these get in the way of termination checking, when you have
Therefore, the
preprocesspass floats them out of applications (originallyonly for structural recursion, since #2818 also for well-founded recursion).
But the code was incomplete: Because
Meta.transformcallspostonf x yonlyonce (and not also on
f x) one has to float out of nested applications as well.A consequence of this can be that in a recursive proof,
rw [foo]does not workalthough
rw [foo _ _]does.Also adding the testcase where @david-christiansen and I stumbled over this
(Maybe the two preprocess modules can be combined, now that #2973 is landed, will try that
in a follow-up).