It appears I am doing something off, as after mocking a module using mock, the module is completely unloaded from memory and subsequent code that depends on the module being there fails.

As a working proof of concept, the following script fails - the error is raised every time execution gets to run the last line, e.g. MyModule3.my_fun(1):

Mix.install [:mock]

defmodule MyModule1 do
  def my_fun(arg) do
    arg + 1
  end
end

defmodule MyModule2 do
  import Mock

  def my_fun(_arg) do
    with_mock(MyModule1, my_fun: fn _number -> 123 end) do
      MyModule1.my_fun(1)
    end
  end
end

MyModule1.my_fun(1) |> IO.inspect
MyModule2.my_fun(1) |> IO.inspect
MyModule1.my_fun(1) |> IO.inspect

The failure is:

$ elixir example.exs
2
123
** (UndefinedFunctionError) function MyModule1.my_fun/1 is undefined (module MyModule1 is not available)
    MyModule1.my_fun(1)
    example.exs:27: (file)
    (elixir 1.14.1) lib/code.ex:1245: Code.require_file/2
$

After digging a little, the mocking code appears to be reasonably straightforward, which leaves me quite puzzled as to what’s going on:

https://github.com/jjh42/mock/blob/f3580cb68692b41ee59e9784ba253473dab9420d/lib/mock.ex#L340-L368

Trivia:

elixir --version
Erlang/OTP 25 [erts-13.1.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Elixir 1.14.1 (compiled with Erlang/OTP 25)