I am not aware of anything Credo does to remember results of prior runs, but it does allow you to ignore specific lines or files, if you are willing able to go through the code and apply those exceptions.

You can use ReviewDog to filter out failures limiting them to only the errors in current diff.

As a kind of a workaround, you could use git to select only files that changed relative to the base branch for credo checks. It’s still coarse grained but allows for incrementally improving credo compliance of the code base (however that assumes branch-based workflow so YMMV).

NOTE: I didn’t run this code (it’s a heavily edited and simplified version of an existing solution). It should give an idea though.

defmodule Mix.Tasks.Credo.Ci do
  use Mix.Task

  @impl true
  def run(_) do
    if System.cmd("git", ["rev-parse", "--abbrev-ref", "HEAD"]) == {"master", 0} do
      Mix.Task.run("credo", [])
    else
      {head_sha, 0} = System.cmd("git", ["rev-parse", "HEAD"])

      {closest_ancestor_sha, 0} =
        System.cmd("git", [
          "merge-base",
          String.trim(head_sha),
          "origin/master",
        ])

      {diff, 0} = System.cmd("git", ["diff", "--name-only", String.trim(closest_ancestor_sha)])

      files =
        diff
        |> String.split("\n")
        |> Enum.filter(&(&1 =~ ~r/(lib|test).*\.exs?/))
        |> Enum.flat_map(&["--files-included", &1])

      unless files == [] do
        Mix.Task.run("credo", ["-C", config_name | files])
      end
    end
  end
end

I noticed @hauleth s comment and I decided to figure out how to do it locally.

My workflow would be to run credo on my current changes not staged for committing. To only get reports on the current unstaged changes is done as follows.

mix credo --strict --format=flycheck | reviewdog -efm="%f:%l:%c: %t: %m" -efm="%f:%l: %t: %m" -name="credo" -diff="git diff"

The key here is to play around with the -diff command. That’s the output reviewdog will use to determine which violations (according to credo) are relevant to show you.

Say you want to get only new issues since you created your feature branch off of the develop branch, this will do too:

mix credo --strict --format=flycheck | reviewdog -efm="%f:%l:%c: %t: %m" -efm="%f:%l: %t: %m" -name="credo" -diff="git diff HEAD $(git merge-base HEAD develop) "