Deprecate using non-empty lists with the Collectable protocol (#7785)

Closes #7421.

6 files changed, 29 insertions, 11 deletions

diff --git a/lib/eex/test/eex_test.exs b/lib/eex/test/eex_test.exs
index 805f4adb3..8a9439af3 100644
--- a/lib/eex/test/eex_test.exs
+++ b/lib/eex/test/eex_test.exs
@@ -519,7 +519,7 @@ defmodule EExTest do
   end
 
   defp assert_eval(expected, actual, binding \\ [], opts \\ []) do
-    opts = Enum.into([file: __ENV__.file, engine: opts[:engine] || EEx.Engine], opts)
+    opts = Keyword.merge([file: __ENV__.file, engine: opts[:engine] || EEx.Engine], opts)
     result = EEx.eval_string(actual, binding, opts)
     assert result == expected
   end
diff --git a/lib/elixir/lib/collectable.ex b/lib/elixir/lib/collectable.ex
index 495eb848c..075254e2a 100644
--- a/lib/elixir/lib/collectable.ex
+++ b/lib/elixir/lib/collectable.ex
@@ -79,6 +79,16 @@ end
 
 defimpl Collectable, for: List do
   def into(original) do
+    if original != [] do
+      IO.warn(
+        "the Collectable protocol is deprecated for non-empty lists. The behaviour of " <>
+          "things like Enum.into/2 or \"for\" comprehensions with an :into option is incorrect " <>
+          "when collecting into non-empty lists. If you're collecting into a non-empty keyword " <>
+          "list, consider using Keyword.merge/2 instead. If you're collecting into a non-empty " <>
+          "list, consider concatenating the two lists with the ++ operator."
+      )
+    end
+
     fun = fn
       list, {:cont, x} -> [x | list]
       list, :done -> original ++ :lists.reverse(list)
diff --git a/lib/elixir/lib/enum.ex b/lib/elixir/lib/enum.ex
index 531896dbf..a1dbd921e 100644
--- a/lib/elixir/lib/enum.ex
+++ b/lib/elixir/lib/enum.ex
@@ -1164,10 +1164,14 @@ defmodule Enum do
   @doc """
   Inserts the given `enumerable` into a `collectable`.
 
+  Note that passing a non-empty list as the `collectable` is deprecated. If you're collecting
+  into a non-empty keyword list, consider using `Keyword.merge/2`. If you're collecting into a
+  non-empty list, consider something like `to_list(enumerable) ++ collectable`.
+
   ## Examples
 
-      iex> Enum.into([1, 2], [0])
-      [0, 1, 2]
+      iex> Enum.into([1, 2], [])
+      [1, 2]
 
       iex> Enum.into([a: 1, b: 2], %{})
       %{a: 1, b: 2}
@@ -1180,8 +1184,10 @@ defmodule Enum do
 
   """
   @spec into(Enumerable.t(), Collectable.t()) :: Collectable.t()
-  def into(enumerable, collectable) when is_list(collectable) do
-    collectable ++ to_list(enumerable)
+  def into(enumerable, collectable)
+
+  def into(enumerable, []) do
+    to_list(enumerable)
   end
 
   def into(%_{} = enumerable, collectable) do
diff --git a/lib/elixir/test/elixir/enum_test.exs b/lib/elixir/test/elixir/enum_test.exs
index 4df60680f..f9a625e21 100644
--- a/lib/elixir/test/elixir/enum_test.exs
+++ b/lib/elixir/test/elixir/enum_test.exs
@@ -1098,7 +1098,6 @@ defmodule EnumTest.Range do
   end
 
   test "into/2" do
-    assert Enum.into(3..5, [1, 2]) == [1, 2, 3, 4, 5]
     assert Enum.into(1..5, []) == [1, 2, 3, 4, 5]
   end
 
diff --git a/lib/ex_unit/lib/ex_unit/assertions.ex b/lib/ex_unit/lib/ex_unit/assertions.ex
index f8c46ada9..f1e5300da 100644
--- a/lib/ex_unit/lib/ex_unit/assertions.ex
+++ b/lib/ex_unit/lib/ex_unit/assertions.ex
@@ -584,10 +584,12 @@ defmodule ExUnit.Assertions do
   defp collect_vars_from_pattern({:when, _, [left, right]}) do
     pattern = collect_vars_from_pattern(left)
 
-    for {name, _, context} = var <- collect_vars_from_pattern(right),
-        Enum.any?(pattern, &match?({^name, _, ^context}, &1)),
-        into: pattern,
-        do: var
+    vars =
+      for {name, _, context} = var <- collect_vars_from_pattern(right),
+          Enum.any?(pattern, &match?({^name, _, ^context}, &1)),
+          do: var
+
+    pattern ++ vars
   end
 
   defp collect_vars_from_pattern(expr) do
diff --git a/lib/mix/lib/mix/compilers/elixir.ex b/lib/mix/lib/mix/compilers/elixir.ex
index 3ea777538..f18605368 100644
--- a/lib/mix/lib/mix/compilers/elixir.ex
+++ b/lib/mix/lib/mix/compilers/elixir.ex
@@ -82,9 +82,10 @@ defmodule Mix.Compilers.Elixir do
               {last_mtime, last_size} = Map.fetch!(sources_stats, source),
               times = Enum.map(external, &(sources_stats |> Map.fetch!(&1) |> elem(0))),
               size != last_size or Mix.Utils.stale?([last_mtime | times], [modified]),
-              into: new_paths,
               do: source
 
+        changed = new_paths ++ changed
+
         stale_local_deps = stale_local_deps(manifest, modified)
 
         {modules, structs, changed} =