Optimize EEx fn block detection

[preciz]

Assisted-by: Codex CLI:GPT-5.6 Sol

Scan reversed tokens once to identify whether a stab expression starts an anonymous function block.

Bench:

Mix.install([{:benchee, "~> 1.3"}])

defmodule FnBlockBench do
  def old(tokens) do
    fn_index = Enum.find_index(tokens, &match?({:fn, _}, &1)) || :infinity
    end_index = Enum.find_index(tokens, &match?({:end, _}, &1)) || :infinity
    end_index > fn_index
  end

  def new([{:fn, _} | _]), do: true
  def new([{:end, _} | _]), do: false
  def new([_ | tokens]), do: new(tokens)
  def new([]), do: false
end

token = {:identifier, {1, 1, nil}, :value}

inputs =
  Map.new([10, 100, 1_000], fn size ->
    {"#{size} preceding tokens", [token, {:fn, {1, 1, nil}} | List.duplicate(token, size)]}
  end)

Benchee.run(
  %{"old" => &FnBlockBench.old/1, "new" => &FnBlockBench.new/1},
  inputs: inputs,
  time: 2,
  warmup: 1
)

   Preceding tokens          New          Old    Difference
  ━━━━━━━━━━━━━━━━━━  ━━━━━━━━━━━  ━━━━━━━━━━━  ━━━━━━━━━━━━
                 10     28.47 ns    135.09 ns         4.75×
  ──────────────────  ───────────  ───────────  ────────────
                100     27.01 ns    719.61 ns        26.64×
  ──────────────────  ───────────  ───────────  ────────────
              1,000    0.0308 μs      6.62 μs       215.23×

[josevalim]

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