test: delayed assignment performance issue (#12201)

This PR adds a benchmark that exposes a performance issue at
`instantiateMVars` when there are many nested delayed assignments.

tests/bench/delayed_assign.lean

@@ -0,0 +1,56 @@
+import Lean
+
+open Lean Meta
+
+def mkLE (i : Nat) : Expr :=
+  mkNatLE (mkNatLit 0) (mkBVar i)
+
+partial def solve (mvarId : MVarId) : MetaM Unit := do
+  let type ← instantiateMVars (← mvarId.getType)
+  if type.isForall then
+    let (_, mvarId) ← mvarId.intro1
+    solve mvarId
+  else if type.isAppOf ``And then
+    let [mvarId₁, mvarId₂] ← mvarId.applyConst ``And.intro | failure
+    solve mvarId₁
+    solve mvarId₂
+  else if type.isAppOf ``LE.le then
+    let [] ← mvarId.applyConst ``Nat.zero_le | failure
+  else
+    let [] ← mvarId.applyConst ``True.intro | failure
+
+partial def runBench (name : String) (n : Nat) (mk : Nat → MetaM MVarId) : MetaM Unit := do
+  let mvarId ← mk n
+  let startTime ← IO.monoNanosNow
+  solve mvarId
+  let endTime ← IO.monoNanosNow
+  let ms := (endTime - startTime).toFloat / 1000000.0
+  let startTime ← IO.monoNanosNow
+  discard <| instantiateMVars (mkMVar mvarId)
+  let endTime ← IO.monoNanosNow
+  let instMs := (endTime - startTime).toFloat / 1000000.0
+  IO.println s!"{name}_{n}: {ms} ms, instantiateMVars: {instMs} ms"
+
+def mkBench1 (n : Nat) : MetaM MVarId := do
+  let type := mkType n
+  return (← mkFreshExprSyntheticOpaqueMVar type).mvarId!
+where
+  mkResultType (i : Nat) : Expr :=
+    match i with
+    | 0 => mkConst ``True
+    | i+1 => mkAnd (mkLE i) (mkResultType i)
+
+  mkType (i : Nat) : Expr :=
+    match i with
+    | 0 => mkResultType n
+    | i+1 => .forallE `x Nat.mkType (mkAnd (mkType i) (mkLE (n - i - 1))) .default
+
+partial def bench1 (n : Nat) : MetaM Unit := do
+  runBench "bench1" n mkBench1
+
+run_meta do
+  IO.println "Example (n = 5):"
+  let ex ← (← mkBench1 5).getType
+  IO.println s!"{← ppExpr ex}"
+  for i in [10, 20, 40, 80, 100, 200, 300, 400, 500] do
+    bench1 i