bug#1653: 23.0.60; encoding system nil != no-conversion, is it deliberately?

[poppyer]

In emacs23, setting a encoding system to nil doesn't behaver the same
as setting it to no-conversion. 
For example: (setq default-process-coding-system '(nil . utf-8))
and (setq default-process-coding-system '(no-conversion . utf-8))
have differenct effects on the following "call-process".
Is this deliberately? is there any good reason?

This setting will break quite a lot of 3rd party .el plugins
(emms/unicad etc), which usually set
xxxxxxxx-default-encoding-system's default value to nil.




In GNU Emacs 23.0.60.1 (i386-apple-darwin9.5.0, NS apple-appkit-949.35)
 of 2008-12-11 on neutron.local
Windowing system distributor `Apple', version 
97.112.112.108.101.45.97.112.112.107.105.116.45.57.52.57.46.51.53
configured using `configure  '--with-ns''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: zh_CN.UTF-8
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: nil
  value of $XMODIFIERS: @im=fcitx
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: Group

Minor modes in effect:
  erc-log-mode: t
  erc-list-mode: t
  erc-menu-mode: t
  erc-autojoin-mode: t
  erc-ring-mode: t
  erc-networks-mode: t
  erc-pcomplete-mode: t
  erc-track-mode: t
  erc-track-minor-mode: t
  erc-match-mode: t
  erc-button-mode: t
  erc-fill-mode: t
  erc-stamp-mode: t
  erc-netsplit-mode: t
  erc-irccontrols-mode: t
  erc-noncommands-mode: t
  erc-move-to-prompt-mode: t
  erc-readonly-mode: t
  gnus-undo-mode: t
  iswitchb-mode: t
  recentf-mode: t
  which-function-mode: t
  show-paren-mode: t
  mouse-sel-mode: t
  global-hl-line-mode: t
  pinbar-mode: t
  shell-dirtrack-mode: t
  tooltip-mode: t
  tool-bar-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
RET C-n q C-p C-p C-n C-n ESC g C-p C-p C-p C-p C-p 
C-n C-n C-n C-n C-n C-p C-p C-p C-n C-n ESC g C-p 9 
RET C-n SPC q C-p C-p C-p C-p C-p C-p C-p C-p ESC x 
d e s c r TAB ESC DEL r e DEL DEL b u TAB g TAB ESC 
DEL ESC DEL r e TAB p o TAB r TAB RET

Recent messages:
No more unread newsgroups
Making completion list... [3 times]
Checking spelling of DELIBERATELY...
DELIBERATELY is correct because of root DELIBERATE
Checking spelling of DELIBERATELY...
DELIBERATELY is correct because of root DELIBERATE
Checking spelling of DELIBERATELY...
DELIBERATELY is correct because of root DELIBERATE
Checking spelling of DELIBERATELY...
DELIBERATELY is correct because of root DELIBERATE