Add refactoring check suggesting to pass time unit to {Naive}DateTime…

….utc_now

Instead of calling DateTime.utc_now and then truncating the result with
DateTime.truncate/2, it's more efficient (and less code) to just pass a
time unit indicating the desired precision to DateTime.utc_now directly.

Dito for NaiveDateTime.

lib/credo/check/refactor/utc_now_truncate.ex

@@ -0,0 +1,186 @@
+defmodule Credo.Check.Refactor.UtcNowTruncate do
+  use Credo.Check,
+    id: "EX4032",
+    base_priority: :high,
+    explanations: [
+      check: """
+      Pass time unit to `{Naive}DateTime.utc_now` instead of composing with `{Naive}DateTime.truncate/2`.
+
+      This should be refactored:
+
+          DateTime.utc_now() |> DateTime.truncate(:second)
+
+      to look like this:
+
+          DateTime.utc_now(:second)
+
+      Dito for `NaiveDateTime`.
+
+      The reason for this is not just performance, because no separate function
+      call is required, but also brevity of the resulting code.
+      """
+    ]
+
+  @doc false
+  def run(source_file, params \\ []) do
+    issue_meta = IssueMeta.for(source_file, params)
+
+    Credo.Code.prewalk(source_file, &traverse(&1, &2, issue_meta))
+  end
+
+  # DateTime.truncate(DateTime.utc_now(), _)
+  # DateTime.truncate(DateTime.utc_now(_), _)
+  # DateTime.truncate(DateTime.utc_now(_, _), _)
+  defp traverse(
+         {{:., meta, [{:__aliases__, _, [:DateTime]}, :truncate]}, _,
+          [
+            {{:., _, [{:__aliases__, _, [:DateTime]}, :utc_now]}, _, _},
+            _
+          ]} =
+           ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  # DateTime.utc_now() |> DateTime.truncate(_)
+  # DateTime.utc_now(_) |> DateTime.truncate(_)
+  # DateTime.utc_now(_, _) |> DateTime.truncate(_)
+  defp traverse(
+         {:|>, meta,
+          [
+            {{:., _, [{:__aliases__, _, [:DateTime]}, :utc_now]}, _, _},
+            {{:., _, [{:__aliases__, _, [:DateTime]}, :truncate]}, _, [_]}
+          ]} = ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  # DateTime.truncate(_ |> DateTime.utc_now(), _)
+  # DateTime.truncate(_ |> DateTime.utc_now(_), _)
+  defp traverse(
+         {{:., meta, [{:__aliases__, _, [:DateTime]}, :truncate]}, _,
+          [
+            {:|>, _,
+             [
+               _,
+               {{:., _, [{:__aliases__, _, [:DateTime]}, :utc_now]}, _, _}
+             ]},
+            _
+          ]} = ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  # _ |> DateTime.utc_now() |> DateTime.truncate(_)
+  # _ |> DateTime.utc_now(_) |> DateTime.truncate(_)
+  defp traverse(
+         {:|>, meta,
+          [
+            {:|>, _,
+             [
+               _,
+               {{:., _, [{:__aliases__, _, [:DateTime]}, :utc_now]}, _, _}
+             ]},
+            {{:., _, [{:__aliases__, _, [:DateTime]}, :truncate]}, _, [_]}
+          ]} = ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  # NaiveDateTime.truncate(NaiveDateTime.utc_now(), _)
+  # NaiveDateTime.truncate(NaiveDateTime.utc_now(_), _)
+  # NaiveDateTime.truncate(NaiveDateTime.utc_now(_, _), _)
+  defp traverse(
+         {{:., meta, [{:__aliases__, _, [:NaiveDateTime]}, :truncate]}, _,
+          [
+            {{:., _, [{:__aliases__, _, [:NaiveDateTime]}, :utc_now]}, _, _},
+            _
+          ]} =
+           ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  # NaiveDateTime.utc_now() |> NaiveDateTime.truncate(_)
+  # NaiveDateTime.utc_now(_) |> NaiveDateTime.truncate(_)
+  # NaiveDateTime.utc_now(_, _) |> NaiveDateTime.truncate(_)
+  defp traverse(
+         {:|>, meta,
+          [
+            {{:., _, [{:__aliases__, _, [:NaiveDateTime]}, :utc_now]}, _, _},
+            {{:., _, [{:__aliases__, _, [:NaiveDateTime]}, :truncate]}, _, [_]}
+          ]} = ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  # NaiveDateTime.truncate(_ |> NaiveDateTime.utc_now(), _)
+  # NaiveDateTime.truncate(_ |> NaiveDateTime.utc_now(_), _)
+  defp traverse(
+         {{:., meta, [{:__aliases__, _, [:NaiveDateTime]}, :truncate]}, _,
+          [
+            {:|>, _,
+             [
+               _,
+               {{:., _, [{:__aliases__, _, [:NaiveDateTime]}, :utc_now]}, _, _}
+             ]},
+            _
+          ]} = ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  # _ |> NaiveDateTime.utc_now() |> NaiveDateTime.truncate(_)
+  # _ |> NaiveDateTime.utc_now(_) |> NaiveDateTime.truncate(_)
+  defp traverse(
+         {:|>, meta,
+          [
+            {:|>, _,
+             [
+               _,
+               {{:., _, [{:__aliases__, _, [:NaiveDateTime]}, :utc_now]}, _, _}
+             ]},
+            {{:., _, [{:__aliases__, _, [:NaiveDateTime]}, :truncate]}, _, [_]}
+          ]} = ast,
+         issues,
+         issue_meta
+       ) do
+    new_issue = issue_for(issue_meta, meta[:line], "utc_now_truncate")
+    {ast, issues ++ List.wrap(new_issue)}
+  end
+
+  defp traverse(ast, issues, _issue_meta) do
+    {ast, issues}
+  end
+
+  defp issue_for(issue_meta, line_no, trigger) do
+    format_issue(
+      issue_meta,
+      message:
+        "Pass time unit to `{Naive}DateTime.utc_now` instead of composing with `{Naive}DateTime.truncate/2`.",
+      trigger: trigger,
+      line_no: line_no
+    )
+  end
+end