Deprecate struct update syntax

The struct update syntax was added early in Elixir to help validate
at compile-time that the update keys were valid. However, for a couple
releases already, Elixir's static analysis can perform such validation
more reliably and find more error if you pattern match on the struct
when the variable is defined instead.

This deprecation simplifies the language and pushes developers to
better alternatives.

Closes #13974.

Author: josevalim

lib/elixir/lib/exception.ex

@@ -1387,6 +1387,8 @@ defmodule BadFunctionError do
 end
 
 defmodule BadStructError do
+  @moduledoc deprecated:
+               "This exception is deprecated alongside the struct update syntax that raises it"
   defexception [:struct, :term]
 
   @impl true

lib/elixir/lib/kernel/special_forms.ex

@@ -98,8 +98,8 @@ defmodule Kernel.SpecialForms do
 
       %User{}
 
-  Underneath a struct is just a map with a `:__struct__` key
-  pointing to the `User` module:
+  Underneath a struct is a map with a `:__struct__` key pointing
+  to the `User` module, where the keys are validated at compile-time:
 
       %User{} == %{__struct__: User, name: "john", age: 27}
 
@@ -118,16 +118,7 @@ defmodule Kernel.SpecialForms do
 
       %User{full_name: "john doe"}
 
-  An update operation specific for structs is also available:
-
-      %User{user | age: 28}
-
-  Once again, the syntax above will guarantee the given keys
-  are valid at compilation time and it will guarantee at runtime
-  the given argument is a struct, failing with `BadStructError`
-  otherwise. The map update syntax can also be used for updating
-  structs, and it is useful when you want to update any struct,
-  regardless of their name, as long as they have matching fields:
+  The map update syntax can also be used for updating structs:
 
       %{user | age: 28}
 

lib/elixir/lib/map_set.ex

@@ -440,8 +440,8 @@ defmodule MapSet do
   defimpl Inspect do
     import Inspect.Algebra
 
-    def inspect(map_set, opts) do
-      opts = %Inspect.Opts{opts | charlists: :as_lists}
+    def inspect(map_set, %Inspect.Opts{} = opts) do
+      opts = %{opts | charlists: :as_lists}
       concat(["MapSet.new(", Inspect.List.inspect(MapSet.to_list(map_set), opts), ")"])
     end
   end

lib/elixir/lib/module/types/expr.ex

@@ -225,48 +225,23 @@ defmodule Module.Types.Expr do
   end
 
   # %Struct{map | ...}
+  # This syntax is deprecated, so we simply traverse.
   def of_expr(
-        {:%, struct_meta, [module, {:%{}, _, [{:|, update_meta, [map, args]}]}]} = struct,
-        expected,
+        {:%, _, [_, {:%{}, _, [{:|, _, [map, args]}]}]} = struct,
+        _expected,
         expr,
         stack,
         context
       ) do
-    if stack.mode == :traversal do
-      {_, context} = of_expr(map, term(), struct, stack, context)
+    {_, context} = of_expr(map, term(), struct, stack, context)
 
-      context =
-        Enum.reduce(args, context, fn {key, value}, context when is_atom(key) ->
-          {_, context} = of_expr(value, term(), expr, stack, context)
-          context
-        end)
+    context =
+      Enum.reduce(args, context, fn {key, value}, context when is_atom(key) ->
+        {_, context} = of_expr(value, term(), expr, stack, context)
+        context
+      end)
 
-      {dynamic(), context}
-    else
-      {info, context} = Of.struct_info(module, struct_meta, stack, context)
-      struct_type = Of.struct_type(module, info)
-      {map_type, context} = of_expr(map, struct_type, struct, stack, context)
-
-      if compatible?(map_type, struct_type) do
-        map_type = map_put!(map_type, :__struct__, atom([module]))
-
-        Enum.reduce(args, {map_type, context}, fn
-          {key, value}, {map_type, context} when is_atom(key) ->
-            # TODO: Once we support typed structs, we need to type check them here.
-            expected_value_type =
-              case map_fetch(expected, key) do
-                {_, expected_value_type} -> expected_value_type
-                _ -> term()
-              end
-
-            {value_type, context} = of_expr(value, expected_value_type, expr, stack, context)
-            {map_put!(map_type, key, value_type), context}
-        end)
-      else
-        warning = {:badstruct, struct, struct_type, map_type, context}
-        {error_type(), error(__MODULE__, warning, update_meta, stack, context)}
-      end
-    end
+    {dynamic(), context}
   end
 
   # %{...}
@@ -792,13 +767,6 @@ defmodule Module.Types.Expr do
   defp flatten_when({:when, _meta, [left, right]}), do: [left | flatten_when(right)]
   defp flatten_when(other), do: [other]
 
-  defp map_put!(map_type, key, value_type) do
-    case map_put(map_type, key, value_type) do
-      {:ok, descr} -> descr
-      error -> raise "unexpected #{inspect(error)}"
-    end
-  end
-
   defp repack_match(left_expr, {:=, meta, [new_left, new_right]}),
     do: repack_match({:=, meta, [left_expr, new_left]}, new_right)
 
@@ -807,31 +775,6 @@ defmodule Module.Types.Expr do
 
   ## Warning formatting
 
-  def format_diagnostic({:badstruct, expr, expected_type, actual_type, context}) do
-    traces = collect_traces(expr, context)
-
-    %{
-      details: %{typing_traces: traces},
-      message:
-        IO.iodata_to_binary([
-          """
-          incompatible types in struct update:
-
-              #{expr_to_string(expr) |> indent(4)}
-
-          expected type:
-
-              #{to_quoted_string(expected_type) |> indent(4)}
-
-          but got type:
-
-              #{to_quoted_string(actual_type) |> indent(4)}
-          """,
-          format_traces(traces)
-        ])
-    }
-  end
-
   def format_diagnostic({:badmap, type, expr, context}) do
     traces = collect_traces(expr, context)
 

lib/elixir/src/elixir_erl_pass.erl

@@ -477,6 +477,7 @@ translate_struct(Ann, Name, {'%{}', _, [{'|', _, [Update, Assocs]}]}, S) ->
   Map = {map, Ann, [{map_field_exact, Ann, {atom, Ann, '__struct__'}, {atom, Ann, Name}}]},
 
   Match = {match, Ann, Var, Map},
+  %% Once this is removed, we should remove badstruct handling from elixir_erl_try
   Error = {tuple, Ann, [{atom, Ann, badstruct}, {atom, Ann, Name}, Var]},
 
   {TUpdate, TU} = translate(Update, Ann, VS),

lib/elixir/src/elixir_map.erl

@@ -15,21 +15,29 @@ expand_map(Meta, Args, S, E) ->
   {{'%{}', Meta, EArgs}, SE, EE}.
 
 expand_struct(Meta, Left, {'%{}', MapMeta, MapArgs}, S, #{context := Context} = E) ->
-  CleanMapArgs = clean_struct_key_from_map_args(Meta, MapArgs, E),
+  CleanMapArgs = delete_struct_key(Meta, MapArgs, E),
   {[ELeft, ERight], SE, EE} = elixir_expand:expand_args([Left, {'%{}', MapMeta, CleanMapArgs}], S, E),
 
   case validate_struct(ELeft, Context) of
     true when is_atom(ELeft) ->
-      case extract_struct_assocs(Meta, ERight, E) of
-        {expand, MapMeta, Assocs} when Context /= match -> %% Expand
+      case ERight of
+        {'%{}', MapMeta, [{'|', _, [_, Assocs]}]} ->
+          %% The update syntax for structs is deprecated,
+          %% so we return only the update syntax downstream.
+          %% TODO: Remove me on Elixir v2.0
+          file_warn(MapMeta, ?key(E, file), ?MODULE, {deprecated_update, ELeft, ERight}),
+          _ = load_struct_info(Meta, ELeft, Assocs, EE),
+          {{'%', Meta, [ELeft, ERight]}, SE, EE};
+
+        {'%{}', MapMeta, Assocs} when Context /= match ->
           AssocKeys = [K || {K, _} <- Assocs],
           Struct = load_struct(Meta, ELeft, Assocs, EE),
           Keys = ['__struct__'] ++ AssocKeys,
           WithoutKeys = lists:sort(maps:to_list(maps:without(Keys, Struct))),
           StructAssocs = elixir_quote:escape(WithoutKeys, none, false),
           {{'%', Meta, [ELeft, {'%{}', MapMeta, StructAssocs ++ Assocs}]}, SE, EE};
 
-        {_, _, Assocs} -> %% Update or match
+        {'%{}', MapMeta, Assocs} ->
           _ = load_struct_info(Meta, ELeft, Assocs, EE),
           {{'%', Meta, [ELeft, ERight]}, SE, EE}
       end;
@@ -46,12 +54,12 @@ expand_struct(Meta, Left, {'%{}', MapMeta, MapArgs}, S, #{context := Context} =
 expand_struct(Meta, _Left, Right, _S, E) ->
   file_error(Meta, E, ?MODULE, {non_map_after_struct, Right}).
 
-clean_struct_key_from_map_args(Meta, [{'|', PipeMeta, [Left, MapAssocs]}], E) ->
-  [{'|', PipeMeta, [Left, clean_struct_key_from_map_assocs(Meta, MapAssocs, E)]}];
-clean_struct_key_from_map_args(Meta, MapAssocs, E) ->
-  clean_struct_key_from_map_assocs(Meta, MapAssocs, E).
+delete_struct_key(Meta, [{'|', PipeMeta, [Left, MapAssocs]}], E) ->
+  [{'|', PipeMeta, [Left, delete_struct_key_assoc(Meta, MapAssocs, E)]}];
+delete_struct_key(Meta, MapAssocs, E) ->
+  delete_struct_key_assoc(Meta, MapAssocs, E).
 
-clean_struct_key_from_map_assocs(Meta, Assocs, E) ->
+delete_struct_key_assoc(Meta, Assocs, E) ->
   case lists:keytake('__struct__', 1, Assocs) of
     {value, _, CleanAssocs} ->
       file_warn(Meta, ?key(E, file), ?MODULE, ignored_struct_key_in_struct),
@@ -110,16 +118,6 @@ validate_kv(Meta, KV, Original, #{context := Context} = E) ->
       file_error(Meta, E, ?MODULE, {not_kv_pair, lists:nth(Index, Original)})
   end, {1, #{}}, KV).
 
-extract_struct_assocs(_, {'%{}', Meta, [{'|', _, [_, Assocs]}]}, _) ->
-  {update, Meta, delete_struct_key(Assocs)};
-extract_struct_assocs(_, {'%{}', Meta, Assocs}, _) ->
-  {expand, Meta, delete_struct_key(Assocs)};
-extract_struct_assocs(Meta, Other, E) ->
-  file_error(Meta, E, ?MODULE, {non_map_after_struct, Other}).
-
-delete_struct_key(Assocs) ->
-  lists:keydelete('__struct__', 1, Assocs).
-
 validate_struct({'^', _, [{Var, _, Ctx}]}, match) when is_atom(Var), is_atom(Ctx) -> true;
 validate_struct({Var, _Meta, Ctx}, match) when is_atom(Var), is_atom(Ctx) -> true;
 validate_struct(Atom, _) when is_atom(Atom) -> true;
@@ -301,4 +299,9 @@ format_error({invalid_key_for_struct, Key}) ->
   io_lib:format("invalid key ~ts for struct, struct keys must be atoms, got: ",
                 ['Elixir.Macro':to_string(Key)]);
 format_error(ignored_struct_key_in_struct) ->
-  "key :__struct__ is ignored when using structs".
+  "key :__struct__ is ignored when using structs";
+format_error({deprecated_update, Struct, MapUpdate}) ->
+  io_lib:format("the struct update syntax is deprecated:\n\n~ts\n\n"
+                "Instead, prefer to pattern matching on structs when the variable is first defined and "
+                "use the regular map update syntax instead:\n\n~ts\n",
+                ['Elixir.Macro':to_string({'%', [], [Struct, MapUpdate]}), 'Elixir.Macro':to_string(MapUpdate)]).

lib/elixir/test/elixir/calendar/datetime_test.exs

@@ -483,8 +483,7 @@ defmodule DateTimeTest do
     assert DateTime.to_unix(gregorian_0) == -62_167_219_200
     assert DateTime.to_unix(Map.from_struct(gregorian_0)) == -62_167_219_200
 
-    min_datetime = %DateTime{gregorian_0 | year: -9999}
-
+    min_datetime = %{gregorian_0 | year: -9999}
     assert DateTime.to_unix(min_datetime) == -377_705_116_800
   end
 

lib/elixir/test/elixir/map_test.exs

@@ -283,12 +283,8 @@ defmodule MapTest do
 
   test "structs" do
     assert %ExternalUser{} == %{__struct__: ExternalUser, name: "john", age: 27}
-
     assert %ExternalUser{name: "meg"} == %{__struct__: ExternalUser, name: "meg", age: 27}
 
-    user = %ExternalUser{}
-    assert %ExternalUser{user | name: "meg"} == %{__struct__: ExternalUser, name: "meg", age: 27}
-
     %ExternalUser{name: name} = %ExternalUser{}
     assert name == "john"
   end

lib/elixir/test/elixir/module/types/expr_test.exs

@@ -875,87 +875,6 @@ defmodule Module.Types.ExprTest do
              """
     end
 
-    test "updating structs" do
-      assert typecheck!([x], %Point{x | x: :zero}) ==
-               dynamic(
-                 closed_map(__struct__: atom([Point]), x: atom([:zero]), y: term(), z: term())
-               )
-
-      assert typecheck!([x], %Point{%Point{x | x: :zero} | y: :one}) ==
-               dynamic(
-                 closed_map(
-                   __struct__: atom([Point]),
-                   x: atom([:zero]),
-                   y: atom([:one]),
-                   z: term()
-                 )
-               )
-
-      assert typeerror!(
-               (
-                 x = %{x: 0}
-                 %Point{x | x: :zero}
-               )
-             ) ==
-               ~l"""
-               incompatible types in struct update:
-
-                   %Point{x | x: :zero}
-
-               expected type:
-
-                   dynamic(%Point{x: term(), y: term(), z: term()})
-
-               but got type:
-
-                   %{x: integer()}
-
-               where "x" was given the type:
-
-                   # type: %{x: integer()}
-                   # from: types_test.ex:LINE-4
-                   x = %{x: 0}
-               """
-    end
-
-    test "inference on struct update" do
-      assert typecheck!(
-               [x],
-               (
-                 %Point{x | x: :zero}
-                 x
-               )
-             ) ==
-               dynamic(closed_map(__struct__: atom([Point]), x: term(), y: term(), z: term()))
-
-      assert typeerror!(
-               [x],
-               (
-                 x.w
-                 %Point{x | x: :zero}
-               )
-             ) ==
-               ~l"""
-               incompatible types in struct update:
-
-                   %Point{x | x: :zero}
-
-               expected type:
-
-                   dynamic(%Point{x: term(), y: term(), z: term()})
-
-               but got type:
-
-                   dynamic(%{..., w: term()})
-
-               where "x" was given the type:
-
-                   # type: dynamic(%{..., w: term()})
-                   # from: types_test.ex:LINE-4
-                   x.w
-               """
-    end
-
     test "nested map" do
       assert typecheck!([x = %{}], x.foo.bar) == dynamic()
     end

lib/ex_unit/lib/ex_unit/diff.ex

@@ -218,19 +218,19 @@ defmodule ExUnit.Diff do
   # Guards
 
   defp diff_guard({:when, _, [expression, clause]}, right, env) do
-    {diff_expression, post_env} = diff_quoted(expression, right, nil, env)
+    {diff, post_env} = diff_quoted(expression, right, nil, env)
 
     {guard_clause, guard_equivalent?} =
-      if diff_expression.equivalent? do
+      if diff.equivalent? do
         bindings = Map.merge(post_env.pins, post_env.current_vars)
         diff_guard_clause(clause, bindings)
       else
         {clause, false}
       end
 
-    diff = %__MODULE__{
-      diff_expression
-      | left: {:when, [], [diff_expression.left, guard_clause]},
+    diff = %{
+      diff
+      | left: {:when, [], [diff.left, guard_clause]},
         equivalent?: guard_equivalent?
     }
 
@@ -825,10 +825,10 @@ defmodule ExUnit.Diff do
   end
 
   defp diff_string_concat(left, nil, indexes, _left_length, right, env) do
-    {parsed_diff, parsed_post_env} = diff_string(left, right, ?", env)
-    left_diff = rebuild_concat_string(parsed_diff.left, nil, indexes)
+    {diff, parsed_post_env} = diff_string(left, right, ?", env)
+    left_diff = rebuild_concat_string(diff.left, nil, indexes)
 
-    diff = %__MODULE__{parsed_diff | left: left_diff}
+    diff = %{diff | left: left_diff}
     {diff, parsed_post_env}
   end