Elixir v1.20.0-rc.0 (and rc.1) released: type inference of all constructs

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I just finished upgrading a rather big project to 1.19.5 and resolving all warnings. After setting the elixir version to 1.20.0-rc.1 and running mix.compile the compiler revealed many new warnings

  • Found a lot of unused required modules. Mostly Logger and Ecto.Query (TIL you don’t need to require the Logger to make use of Logger.metadata)

  • Possible false positive or future deprecation? This warning was emitted, but pinning the variable here has no effect aside from removing the warning

    warning: the variable "chunk_size" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
    │
118 │     <<chunk::binary-size(chunk_size), rest::binary>> = binary

  • It found a couple of interesting, unused function clauses, similar to the cases below. In this particular case it was fine, but there are some cases where I cannot figure out the reason. Still a great addition to the compiler :+1:

    current = Enum.at(search_results, current_index)
highlight_term(current.sentence_text, current)}

         warning: this clause of defp highlight_term/2 is never used
     │
 194 │   defp highlight_term(text, nil), do: text

  • In this particular case it didn’t complain about the fact that we (accidentally) assumed that uri.query is a map, but we’re trying to pass a map to URI.decode_query. Still a great find :+1:

         warning: incompatible types given to URI.decode_query/1:

         URI.decode_query(query)

     given types:

         dynamic(map())

     but expected one of:

         binary()

     where "query" was given the type:

         # type: dynamic(map())
         # from: my_code.ex:187:33
         is_map(query)

     typing violation found at:
     │
 188 │            %{"latency" => latency_raw} <- URI.decode_query(query),

        uri = URI.parse(url)

    latency =
      with %{query: query} when is_map(query) <- uri,
           %{"latency" => latency_raw} <- URI.decode_query(query),
           {latency, ""} <- Integer.parse(latency_raw) do

I’ve also measured the performance of the compilation of this project a bit. ~190 dependencies and +800 files. Overall very happy with the results and MIX_OS_DEPS_COMPILE_PARTITION_COUNT is a blessing for sure
deps = mix deps.compile
project = mix compile

elixir_version stage partitions seconds
1.18.4-otp-28 deps unset 73.95
1.18.4-otp-28 project 11.57
1.19.3-otp-28 deps unset 68.80
1.19.3-otp-28 deps 2 41.78
1.19.3-otp-28 deps 4 31.60
1.19.3-otp-28 project 13.65
1.19.5-otp-28 deps unset 68.00
1.19.5-otp-28 deps 2 41.60
1.19.5-otp-28 deps 4 31.72
1.19.5-otp-28 project 12.88
1.20.0-rc.1-otp-28 deps unset 69.54
1.20.0-rc.1-otp-28 deps 2 41.56
1.20.0-rc.1-otp-28 deps 4 32.22
1.20.0-rc.1-otp-28 project 13.25
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