Auto infer size of matched variable in bitstrings

Author: sabiwara

lib/elixir/src/elixir_bitstring.erl

@@ -33,12 +33,22 @@ expand(_BitstrMeta, _Fun, [], Acc, S, E, Alignment, _RequireSize) ->
 expand(BitstrMeta, Fun, [{'::', Meta, [Left, Right]} | T], Acc, S, E, Alignment, RequireSize) ->
   {ELeft, {SL, OriginalS}, EL} = expand_expr(Meta, Left, Fun, S, E),
 
-  MatchOrRequireSize = RequireSize or is_match_size(T, EL),
-  EType = expr_type(ELeft),
-  {ERight, EAlignment, SS, ES} = expand_specs(EType, Meta, Right, SL, OriginalS, EL, MatchOrRequireSize),
-
-  EAcc = concat_or_prepend_bitstring(Meta, ELeft, ERight, Acc, ES, MatchOrRequireSize),
-  expand(BitstrMeta, Fun, T, EAcc, {SS, OriginalS}, ES, alignment(Alignment, EAlignment), RequireSize);
+  case {ELeft, Right} of
+    {{'^', _, [{var, _, _}]}, {Type, _, _}} when Type == binary; Type == bitstring ->
+      SizeFun = case Type of
+        binary -> byte_size;
+        bitstring -> bit_size
+      end,
+      ERight = {'-', Meta, [Right, {size, Meta, [{{'.', Meta, [erlang, SizeFun]}, Meta, [ELeft]}]}]},
+      EAcc = concat_or_prepend_bitstring(Meta, ELeft, ERight, Acc, EL, RequireSize),
+      expand(BitstrMeta, Fun, T, EAcc, {SL, OriginalS}, EL, unknown, RequireSize);
+    _ ->
+      MatchOrRequireSize = RequireSize or is_match_size(T, EL),
+      EType = expr_type(ELeft),
+      {ERight, EAlignment, SS, ES} = expand_specs(EType, Meta, Right, SL, OriginalS, EL, MatchOrRequireSize),
+      EAcc = concat_or_prepend_bitstring(Meta, ELeft, ERight, Acc, ES, MatchOrRequireSize),
+      expand(BitstrMeta, Fun, T, EAcc, {SS, OriginalS}, ES, alignment(Alignment, EAlignment), RequireSize)
+   end;
 expand(BitstrMeta, Fun, [H | T], Acc, S, E, Alignment, RequireSize) ->
   Meta = extract_meta(H, BitstrMeta),
   {ELeft, {SS, OriginalS}, ES} = expand_expr(Meta, H, Fun, S, E),

lib/elixir/test/elixir/kernel/binary_test.exs

@@ -269,6 +269,16 @@ defmodule Kernel.BinaryTest do
     assert <<1::size((^foo).bar)>> = <<1::5>>
   end
 
+  test "automatic size computation of matched bitsyntax variable" do
+    var = "foo"
+    <<^var::binary, rest::binary>> = "foobar"
+    assert rest == "bar"
+
+    var = <<0, 1>>
+    <<^var::bitstring, rest::bitstring>> = <<0, 1, 2, 3>>
+    assert rest == <<2, 3>>
+  end
+
   defmacro signed_16 do
     quote do
       big - signed - integer - unit(16)