bug#65137: 29.1; completion-substring-try-completion doesn't return the longest common substring

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Mon, 07 Aug 2023 19:24:15 -0400
> 
> 
> The substring completion style differs from the "basic" style by
> performing completion at the start of the input string.  So for example,
> both of these are valid completions for "bar":
> 
> (completion-substring-all-completions "bar" '("foobar" "foobarbaz") #'identity (length "bar"))
> -> ("foobarbaz" "foobar" . 0)
> 
> However, the substring completion style's try-completion implementation
> does not reflect this.  Since "foobar" is a prefix of all the valid
> completions, it should be returned by try-completion.  But it is not,
> regardless of the location of point:
> 
> (completion-substring-try-completion "bar" '("foobar" "foobarbaz") #'identity 0)
> -> ("bar" . 3)
> 
> (completion-substring-try-completion "bar" '("foobar" "foobarbaz") #'identity (length "bar"))
> -> ("bar" . 3)
> 
> This breaks completion when one completion candidate is a prefix of
> other completion candidates.  The recourse is moving point to the start
> of the input, so that the "basic" completion style takes over, which
> will correctly insert the common prefix:
> 
> (completion-basic-try-completion "bar" '("foobar" "foobarbaz") #'identity 0)
> -> ("foobar" . 6)
> 
> However, even this does not work in the project-file and xref-location
> completion categories, for which the "basic" style is not included in
> completion-category-defaults.  For such completion categories, there's
> simply no way to use completion to insert a common prefix.  This is bad,
> because a filename or identifier might easily be a prefix of another
> filename or identifier.
> 
> The solution is completion-substring-try-completion to be fixed to
> insert these common prefixes.  I'll try and fix this, although the code
> is a bit intimidating.

Adding Stefan, in case he has some comments or ideas.