This pertains to when a call is made to the function ExUnit.Assertions.assert/2 from within ExUnit.Assertions.translate_operator/6 (matched against the equality_check: false argument-set), and the first argument sent to it is unquote(call).

The call made to this function is from within a quote do: block, and the first argument sent is unquote(call), with the second being a keyword list.

In the ExUnit.Assertions.assert/2 function there is an unless expression that is to execute its code in the event that unquote(call), matched to the variable value, evaluates to false.

How can value ever be false?

When does unquote(call) have a chance to be evaluated?

Code for reference:-

assert/1

  defmacro assert(assertion) do
    if translated = translate_assertion(:assert, assertion, __CALLER__) do
      translated
    else
      {args, value} = extract_args(assertion, __CALLER__)

      quote generated: true do
        if value = unquote(value) do
          value
        else
          raise ExUnit.AssertionError,
            args: unquote(args),
            expr: unquote(escape_quoted(:assert, [], assertion)),
            message: "Expected truthy, got #{inspect(value)}"
        end
      end
    end
  end

translate_assertion

  defp translate_assertion(:assert, {operator, meta, [_, _]} = expr, caller)
       when operator in @operator do
    if match?([{_, Kernel}], Macro.Env.lookup_import(caller, {operator, 2})) do
      left = Macro.var(:left, __MODULE__)
      right = Macro.var(:right, __MODULE__)
      call = {operator, meta, [left, right]}
      equality_check? = operator in [:<, :>, :!==, :!=]
      message = "Assertion with #{operator} failed"
      translate_operator(:assert, expr, call, message, equality_check?, caller)
    end
  end

translate_operator

  defp translate_operator(kind, {op, meta, [left, right]} = expr, call, message, false, _caller) do
    expr = escape_quoted(kind, meta, expr)
    context = if op in [:===, :!==], do: :===, else: :==

    quote do
      left = unquote(left)
      right = unquote(right)

      ExUnit.Assertions.assert(unquote(call),
        left: left,
        right: right,
        expr: unquote(expr),
        message: unquote(message),
        context: unquote(context)
      )
    end
  end

assert/2

  def assert(value, opts) when is_list(opts) do
    unless value, do: raise(ExUnit.AssertionError, opts)
    true
  end