bug#19031: 24.4; find-file in icomplete-mode shows completions with no input

[Andrii Kolomoiets <andreyk.mad@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Andrii Kolomoiets <andreyk.mad@gmail.com>
>> Date: Tue, 08 Dec 2020 12:43:21 +0200
>> Cc: Ole Laursen <olau@iola.dk>, Lars Ingebrigtsen <larsi@gnus.org>,
>>  19031@debbugs.gnu.org
>> 
>> 1. emacs -Q
>> 2. M-: (setq insert-default-directory nil)
>> 3. M-x icomplete-mode
>> 4. C-x C-f ~/
>> 
>> In this case everything works as described by the docstring: user input
>> is here so completions are shown.  But IMO Ole's issue is not
>> completely solved: bunch of uninteresting dotfiles are shown.
>
> Emacs never filters out the dotfiles, not by default anyway.  Try
> "C-x C-f TAB TAB", and you will see that.  IMO, it would be confusing
> if some completion packages did this and some didn't.

Yes.  It's not about filtering out dotfiles but about to make icomplete
to not show completions until user starts typing filename.  Completions
(including dotfiles) will be shown when user will type e.g. ".e" or when
the 'icomplete-show-matches-on-no-input' variable is t.

>> If the 'read-file-name-default' function can set the
>> 'minibuffer-default' variable to the substring of the minibuffer content
>> from (minibuffer-prompt-end) to the last occurence of the path
>> separator, then, in addition to the patched 'icomplete-exhibit', this
>> can give desired result: no completions will be show until some input
>> after path separator.
>
> But file-name input is not limited to absolute file names.  The user
> can legitimately enter a relative file name, in which case the
> separator may not be present at all.

If there are no separator in the input, 'minibuffer-default' will be
empty string and completions will be shown.

Example of desired behavior:
1. emacs -Q
2. M-x icomplete-mode
3. C-x C-f
   minibuffer content: ~/
   minibuffer-default is "~/"
   no completions are shown
4. Type ".em"
   minibuffer content: ~/.em
   minibuffer-default is "~/"
   completions are shown
5. Type "acs.d/"
   minibuffer content: ~/.emacs.d/
   minibuffer-default is "~/.emacs.d/"
   no completions are shown