Do not consider variables from pattern in bitstring spec

lib/elixir/src/elixir_bitstring.erl

@@ -30,7 +30,10 @@ expand(Meta, Args, S, E, RequireSize) ->
 expand(_BitstrMeta, _Fun, [], Acc, S, E, Alignment, _RequireSize) ->
   {lists:reverse(Acc), Alignment, S, E};
 expand(BitstrMeta, Fun, [{'::', Meta, [Left, Right]} | T], Acc, S, E, Alignment, RequireSize) ->
-  {ELeft, {SL, OriginalS}, EL} = expand_expr(Left, Fun, S, E),
+  % We don't want to consider variables added in the Left pattern inside the Right specs
+  {#elixir_ex{vars=BeforeVars}, _} = S,
+
+  {ELeft, {#elixir_ex{vars=AfterVars} = SL, OriginalS}, EL} = expand_expr(Left, Fun, S, E),
   validate_expr(ELeft, Meta, E),
 
   MatchOrRequireSize = RequireSize or is_match_size(T, EL),
@@ -40,10 +43,11 @@ expand(BitstrMeta, Fun, [{'::', Meta, [Left, Right]} | T], Acc, S, E, Alignment,
     {'^', _, [{_, _, _}]} -> {infer, ELeft};
     _ -> required
   end,
-  {ERight, EAlignment, SS, ES} = expand_specs(EType, Meta, Right, SL, OriginalS, EL, ExpectSize),
+
+  {ERight, EAlignment, SS, ES} = expand_specs(EType, Meta, Right, SL#elixir_ex{vars=BeforeVars}, OriginalS, EL, ExpectSize),
 
   EAcc = concat_or_prepend_bitstring(Meta, ELeft, ERight, Acc, ES, MatchOrRequireSize),
-  expand(BitstrMeta, Fun, T, EAcc, {SS, OriginalS}, ES, alignment(Alignment, EAlignment), RequireSize);
+  expand(BitstrMeta, Fun, T, EAcc, {SS#elixir_ex{vars=AfterVars}, OriginalS}, ES, alignment(Alignment, EAlignment), RequireSize);
 expand(BitstrMeta, Fun, [H | T], Acc, S, E, Alignment, RequireSize) ->
   Meta = extract_meta(H, BitstrMeta),
   {ELeft, {SS, OriginalS}, ES} = expand_expr(H, Fun, S, E),

lib/elixir/test/elixir/kernel/expansion_test.exs

@@ -2890,6 +2890,17 @@ defmodule Kernel.ExpansionTest do
       end)
     end
 
+    test "raises for variable used both in pattern and size" do
+      assert_compile_error(~r/undefined variable "foo"/, fn ->
+        code =
+          quote do
+            fn <<foo::size(foo)>> -> :ok end
+          end
+
+        expand(code, [])
+      end)
+    end
+
     test "raises for invalid unit" do
       message = ~r"unit in bitstring expects an integer as argument, got: :oops"
 

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -2237,6 +2237,20 @@ defmodule Kernel.WarningTest do
     purge(Sample)
   end
 
+  test "deprecate non-quoted variables in bitstring size modifiers" do
+    assert_warn_eval(
+      [
+        "the variable \"a\" is accessed inside size(...) of a bitstring but it was defined outside of the match",
+        "You must precede it with the pin operator"
+      ],
+      """
+      a = "foo"
+      <<a::binary-size(byte_size(a) - 1)>> <> _ = a
+      "fo" = a
+      """
+    )
+  end
+
   defp assert_compile_error(messages, string) do
     captured =
       capture_err(fn ->