Use a plain map for Ecto.Multi names to avoid Dialyzer opaqueness violations

[roelandvanbatenburg]

Summary

Ecto.Multi.new() still triggers call_without_opaque on Elixir 1.20.4 / Erlang/OTP 29.1, so #4707 isn't fully fixed by the Elixir 1.20 MapSet change. Elixir has since documented @dialyzer :no_opaque as the workaround instead of fixing it further (elixir-lang/elixir#15673, #15674), which would mean every Ecto.Multi caller has to add it.

The cause: defstruct names: MapSet.new() is inlined as a %MapSet{map: %{}} literal. Dialyzer infers the opaque :sets.set/1 for MapSet internals from the :sets calls in MapSet.put/2 / MapSet.member?/2, which flows into the success typing of Ecto.Multi.update/3 and friends.

This replaces the internal MapSet with a plain map (similar to #4708):

-  defstruct operations: [], names: MapSet.new()
+  defstruct operations: [], names: %{}

-  @typep names :: MapSet.t()
+  @typep names :: %{optional(name) => true}

 add_operation
-  MapSet.member?(names, name) / MapSet.put(names, name)
+  Map.has_key?(names, name)   / Map.put(names, name, true)

 merge_structs / merge_results
-  MapSet.intersection |> MapSet.to_list, MapSet.union
+  common_names/2,                        Map.merge

Evidence

Minimal repro without Ecto, on Elixir 1.20.4 / OTP 29.1:

defmodule Holder do
  defstruct names: MapSet.new()
  @type t :: %__MODULE__{names: MapSet.t()}
  @spec new() :: t
  def new, do: %__MODULE__{}
  @spec add(t, term) :: t
  def add(%__MODULE__{names: names} = h, name), do: %{h | names: MapSet.put(names, name)}
end

Holder.add(Holder.new(), :x)                          # call_without_opaque
Holder.add(%Holder{names: MapSet.new(list)}, :x)      # ok (runtime value, not inlined)

Downstream app calling Ecto.Multi.new() |> Ecto.Multi.update(:user, changeset):

• Before: lib/my_app/accounts.ex:236:23:call_without_opaque ... Call does not have expected term of type %Ecto.Multi{names: %MapSet{map: :sets.set(_)}, ...} (with opaque subterms)
  After: Total errors: 0 — done (passed successfully)

mix test on Elixir 1.20.4 / OTP 29.1: 1609 passed (97 doctests, 1512 tests). The multi.names assertions in test/ecto/multi_test.exs are updated to the map shape.

Merge Danger

Door: two-way

Blast Radius: small

names is an internal field (@typep). Only code that reads multi.names directly and expects a MapSet would notice; nothing in ecto_sql does.