feat: add a Selectable.one benchmark (#14251)

This PR adds a benchmark for an instantly succeeding `Selectable.one`.

Author: hargoniX

tests/compile_bench/select.lean

@@ -0,0 +1,39 @@
+import Std.Async.Select
+
+/-!
+Benchmarks `Selectable.one` on a pair of custom `Selectable`s that resolve
+instantly via their non-blocking `tryFn`, exercising the fast path of the
+selection protocol in a configurable iteration loop.
+-/
+
+open Std Async
+
+/-- A `Selector` that instantly resolves with `value` through its non-blocking `tryFn`. -/
+def instantSelector (value : α) : Selector α where
+  tryFn := pure (some value)
+  registerFn _ := pure ()
+  unregisterFn := pure ()
+
+/-- A `Selectable` built from `instantSelector` that yields `value` unchanged. -/
+def instantSelectable (value : Nat) : Selectable Nat :=
+  .case (instantSelector value) pure
+
+def bench (iterations : Nat) : Async Nat := do
+  let mut acc := 0
+  for i in *...iterations do
+    acc := acc + (← Selectable.one #[instantSelectable i, instantSelectable (i + 1)])
+  return acc
+
+def run (name : String) (iterations : Nat) : IO Unit := do
+  let t1 ← IO.monoMsNow
+  let acc ← (bench iterations).block
+  let t2 ← IO.monoMsNow
+  let time : Float := (t2 - t1).toFloat / 1000.0
+  IO.println s!"measurement: {name} {time} s"
+  -- Consume `acc` so the loop cannot be optimized away; the bound is never hit.
+  if acc > iterations * iterations + iterations then
+    throw <| .userError "unreachable"
+
+def main (args : List String) : IO Unit := do
+  let iterations := args[0]!.toNat!
+  run "select_one_two" iterations

tests/compile_bench/select.lean.init.sh

@@ -0,0 +1,5 @@
+TEST_ARGS=( 1000 )
+
+if [[ -n $TEST_BENCH ]]; then
+  TEST_ARGS=( 400000 )
+fi

tests/compile_bench/select.lean.out.expected

@@ -0,0 +1 @@
+measurement: select_one_two ...