This PR implements `mvcgen invariants?` for providing initial invariant
skeletons for the user to flesh out. When the loop body has an early
return, it will helpfully suggest `Invariant.withEarlyReturn ...` as a
skeleton.
```lean
def mySum (l : List Nat) : Nat := Id.run do
let mut acc := 0
for x in l do
acc := acc + x
return acc
/--
info: Try this:
invariants
· ⇓⟨xs, acc⟩ => _
-/
#guard_msgs (info) in
theorem mySum_suggest_invariant (l : List Nat) : mySum l = l.sum := by
generalize h : mySum l = r
apply Id.of_wp_run_eq h
mvcgen invariants?
all_goals admit
def nodup (l : List Int) : Bool := Id.run do
let mut seen : HashSet Int := ∅
for x in l do
if x ∈ seen then
return false
seen := seen.insert x
return true
/--
info: Try this:
invariants
· Invariant.withEarlyReturn (onReturn := fun r acc => _) (onContinue := fun xs acc => _)
-/
#guard_msgs (info) in
theorem nodup_suggest_invariant (l : List Int) : nodup l ↔ l.Nodup := by
generalize h : nodup l = r
apply Id.of_wp_run_eq h
mvcgen invariants?
all_goals admit
```
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