I went with charlists and MapSets.

...
  |> Enum.map(&to_charlist/1)
  |> Enum.map(&MapSet.new/1)
  |> Enum.reduce(&MapSet.intersection/2)
...

I started with a MapSet but ended up with a :binary.match/2:

# split the rucksack into two equal compartments
{head, tail} = line |> String.split_at(line |> String.length() |> div(2))

head
|> String.codepoints()
|> Enum.reduce_while(:ok, fn code, _ ->
  case :binary.match(tail, code) do
    {_, 1} -> {:halt, code |> priority()}
    :nomatch -> {:cont, :ok}
  end
end)

Thanks! I made a thing in case anyone else wants to use it:

Mix.install([
  {:req_aoc, github: "mcrumm/req_aoc", ref: "28e3813"}
])

ReqAOC.fetch!({2022, 03, System.fetch_env!("LB_AOC_SESSION")})
# ==> "..."

I just pipe to the bitter end

defmodule Day03 do
  @priorities [?a..?z, ?A..?Z] |> Enum.concat() |> Enum.with_index(1) |> Map.new()

  def part1(input_path) do
    input_path
    |> File.stream!()
    |> Stream.map(&String.trim/1)
    |> Stream.map(&:binary.bin_to_list/1)
    |> Stream.map(&Enum.split(&1, div(length(&1), 2)))
    |> Stream.map(fn {f, r} ->
      Enum.uniq(f -- f -- r)
    end)
    |> Stream.flat_map(fn intersection ->
      Enum.map(intersection, &@priorities[&1])
    end)
    |> Enum.sum()
  end

  def part2(input_path) do
    input_path
    |> File.stream!()
    |> Stream.map(&String.trim/1)
    |> Stream.map(&:binary.bin_to_list/1)
    |> Stream.map(&MapSet.new/1)
    |> Stream.chunk_every(3)
    |> Stream.flat_map(fn group ->
      Enum.reduce(group, &MapSet.intersection/2)
    end)
    |> Stream.map(&@priorities[&1])
    |> Enum.sum()
  end
end

Nice touch to download the input from the interwebs