Error reusing variable with binary segment specifier

[yastanotheruser]

Elixir and Erlang/OTP versions

Erlang/OTP 27 [erts-15.2.7.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [jit:ns]

Elixir 1.20.0-dev (dd45d96) (compiled with Erlang/OTP 27)

Operating system

Alpine Linux Edge

Current behavior

Attempting to match an already bound binary variable and binding the same name, in a binary segment whose specifier references the previously bound value, results in an Erlang interpreter/compiler error.

iex(1)> a = "123"
iex(2)> <<a::binary-size(byte_size(a) - 1), b>> = "123"
** (ErlangError) Erlang error: {:illegal_pattern, {:bin, 2, [{:bin_element, 2, {:var, 2, :_a@1}, {:var, 2, :_a@1}, [:binary]}, {:bin_element, 2, {:var, 2, :_b@1}, :default, [:integer]}]}}
    (stdlib 6.2.2.2) erl_eval.erl:1414: :erl_eval.match/6
    iex:2: (file)

Expected behavior

Either display an error indicating such pattern isn't allowed, or allow referencing the old value.

[sabiwara]

@yastanotheruser thank you for the report! 💜

The right way to write this is to add the pin operator: <<a::binary-size(byte_size(^a) - 1), b>>.

I agree we should fix this to emit a proper warning and use the pre-match version of a rather than emitting incorrect erlang.
Just like we do on 1.19 when there is no variable being redefined:

iex> a="123"
iex> <<a2::binary-size(byte_size(a) - 1), b>> = "123"
warning: 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

[sabiwara]

I think have a fix, just need to write tests 🙂