split runs out of heartbeats with nested matches and as-patterns

[TwoFX]

Prerequisites

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Description

def foo (r : Nat) : Nat :=
  match r with
  | Nat.zero => 0
  | l@(Nat.succ _) =>
    match l with
    | 0 => 0
    | Nat.succ ll =>
      match ll with
      | Nat.succ _ => 0
      | _ => 0

/--
error: tactic 'simp' failed, nested error:
(deterministic) timeout at `whnf`, maximum number of heartbeats (200000) has been reached
Use `set_option maxHeartbeats <num>` to set the limit.
Additional diagnostic information may be available using the `set_option diagnostics true` command.
-/
#guard_msgs in
example {r : Nat} : foo r = 0 := by
  simp only [foo.eq_def]

Context

Found while working on the ordered tree.

Expected behavior: No error

Actual behavior: See above

Versions

4.15.0-nightly-2024-11-13 on live.lean-lang.org

Impact

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[nomeata]

The issue isn’t the unfolding, it’s what happens afterwards; and it seems to be related to creating the splitter =:

def foo (r : Nat) : Nat :=
  match r with
  | Nat.zero => 0
  | l@(Nat.succ _) =>
    match l with
    | 0 => 0
    | Nat.succ ll =>
      match ll with
      | Nat.succ _ => 0
      | _ => 0

set_option maxHeartbeats 20000

/--
error: (deterministic) timeout at `whnf`, maximum number of heartbeats (20000) has been reached
Use `set_option maxHeartbeats <num>` to set the limit.
Additional diagnostic information may be available using the `set_option diagnostics true` command.
-/
#guard_msgs in
example {r : Nat} : foo r = 0 := by
  unfold foo
  -- trace_state
  split
  · rfl
  · split
    · rfl
    · split