You're seeing just the macro is_connection_message, go back to Mint.HTTP module for more information.
Link to this macro

is_connection_message(conn, message)

View Source (macro) (since 1.1.0)

Macro to check that a given received message is intended for the given connection conn.

This guard is useful in receive loops or in callbacks that handle generic messages (such as a GenServer.handle_info/2 callback) so that you don't have to hand the message to and check for the :unknown_message return value.

This macro can be used in guards.

Note: this macro is only available if you compile Mint with Elixir 1.10.0 or greater (and OTP 21+, which is required by Elixir 1.10.0 and on).


require Mint.HTTP

{:ok, conn, request_ref} = Mint.HTTP.request(conn, "POST", "/", headers, "")

receive do
  message when Mint.HTTP.is_connection_message(conn, message) ->, message)

  other ->
    # This message is related to something else or to some other connection