How to stop Emacs from deleting trailing whitespace?

Question

By default, Emacs quietly deletes trailing whitespaces once the user moves on to the next line. To reproduce the issue in plain Emacs, do this:

1. Start plain Emacs: emacs --no-init-file --no-site-file
2. Insert trailing tabs or spaces on the first line (e.g. five spaces).
3. Press enter to go to the second line.
4. Now go back to the first line. The spaces we inserted in step 2 have all disappeared!

This is not good. I want to preserve the trailing whitespace I have typed. Default Emacs seems to make it impossible for me to enter trailing whitespace.

Questions:

1. What is the cause or rationale for this default behavior?
2. How can I preserve the trailing whitespace I have entered?

Answer 1

Doing C-h k RET (describe-key RET) yields:

RET (translated from <return>) runs the command newline [...]

[...]

If ‘electric-indent-mode’ is enabled, this indents the final new line that it adds, and reindents the preceding line. To just insert a newline, use M-x electric-indent-just-newline.

So it seems to be a feature of indentation (command newline-and-indent does that also) that is here invoked automatically by elecric-indent-mode (which is active by default). You could avoid indentation by either:

• disabling electric-indent-mode

• binding RET to electric-indent-just-newline.

If you want to still be able to indent, without removing trailing whitespace, I suspect you can change the definition of newline-and-indent. Defined in simple.el, its code is:

(defun newline-and-indent ()
  "Insert a newline, then indent according to major mode.
Indentation is done using the value of `indent-line-function'.
In programming language modes, this is the same as TAB.
In some text modes, where TAB inserts a tab, this command indents to the
column specified by the function `current-left-margin'."
  (interactive "*")
  (delete-horizontal-space t)
  (newline nil t)
  (indent-according-to-mode))

Redefining without the delete-horizontal-space call seems to partially do the trick (but electric-mode seems to perform some more trailing whitespace removeal, I haven't studied this in detail).