Add :retab command

See "Changing tabs" section in the Vim manual
https://vimhelp.org/change.txt.html#change.txt

evil-commands.el

@@ -5172,6 +5172,23 @@ event that triggered the execution of this command."
     (push event unread-command-events)))
 (evil-declare-ignore-repeat 'evil-exit-visual-and-repeat)
 
+(evil-define-command evil-retab (tabstop)
+  "Convert all tabs to spaces or the other way around.
+Replace all sequences of white-space containing a <Tab> with new
+strings of white-space using the new TABSTOP value given.
+If you do not specify a new TABSTOP size or it is zero, Evil uses the
+current value of `tab-width'."
+  (interactive "<a>")
+  (unless (or (not tabstop) (string-match-p "^[0-9]*$" tabstop))
+    (user-error "Invalid argument: %s" tabstop))
+  (let ((beg (if (use-region-p) (region-beginning) (point-min)))
+        (end (if (use-region-p) (region-end) (point-max)))
+        (retab (if indent-tabs-mode #'tabify #'untabify))
+        (tab-width (cond ((not tabstop) tab-width)
+                         ((equal tabstop "0") tab-width)
+                         (t (string-to-number tabstop)))))
+    (funcall retab beg end)))
+
 (provide 'evil-commands)
 
 ;;; evil-commands.el ends here

evil-maps.el

@@ -583,6 +583,7 @@ included in `evil-insert-state-bindings' by default."
 (evil-ex-define-cmd "nu[mber]" 'evil-ex-numbered-print)
 (evil-ex-define-cmd "#" 'evil-ex-numbered-print)
 (evil-ex-define-cmd "z" 'evil-ex-z)
+(evil-ex-define-cmd "ret[ab]" 'evil-retab)
 
 (when (featurep 'tab-bar)
   (evil-ex-define-cmd "tabnew" 'tab-bar-new-tab)

evil-tests.el

@@ -9809,6 +9809,49 @@ main(argc, argv) char **argv; {
            (should (equal evil-input-method "german-prefix"))))
       (remove-hook 'text-mode-hook #'use-german-input-method))))
 
+(ert-deftest evil-retab ()
+  "Test the :retab command"
+  :tags '(evil)
+  (ert-info ("From tabs to spaces")
+    (evil-test-buffer
+      "def foo():\n\twhile True:\n\t\treturn\n"
+      (setq indent-tabs-mode nil)
+      (setq tab-width 4)
+      (":retab" [return])
+      "def foo():\n    while True:\n        return\n"))
+  (ert-info ("From spaces to tabs")
+    (evil-test-buffer
+      "def foo():\n    while True:\n        return\n"
+      (setq indent-tabs-mode t)
+      (setq tab-width 4)
+      (":retab" [return])
+      "def foo():\n\twhile True:\n\t\treturn\n"))
+  (ert-info ("Specify tab width as an argument")
+    (evil-test-buffer
+      "def foo():\n\twhile True:\n\t\treturn\n"
+      (setq indent-tabs-mode nil)
+      (setq tab-width 4)
+      (":retab 2" [return])
+      "def foo():\n  while True:\n    return\n"))
+  (ert-info ("Invalid argument")
+    (evil-test-buffer
+      "def foo():\n\twhile True:\n\t\treturn\n"
+      (should-error (execute-kbd-macro ":retab foo" [return])))))
+
+(ert-deftest evil-retab-visual ()
+  "Test the :retab command on visual line selection.
+This test fails in the batch mode (and therefore in the GitHub CI)
+but it works fine in `M-x ert-run-tests-interactively'."
+  :tags '(evil visual)
+  (skip-unless (not noninteractive))
+  (ert-info ("Retab only selected lines")
+    (evil-test-buffer
+      "def foo():\n\twhile True:\n\t\treturn\n"
+      (setq indent-tabs-mode nil)
+      (setq tab-width 4)
+      ("Vj" ":retab" [return])
+      "def foo():\n    while True:\n\t\treturn\n")))
+
 (provide 'evil-tests)
 
 ;;; evil-tests.el ends here