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

[poppyer@gmail.com]

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

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