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

[Spencer Baugh]

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.



In GNU Emacs 29.1 (build 3, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2023-07-29 built on

Repository revision: cf24c7ac7608f41078fd2761c856892d5853b676
Repository branch: my-emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.8 (Green Obsidian)

Configured using:
 'configure --config-cache --with-x-toolkit=lucid
 --with-gif=ifavailable'

Configured features:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM
XINPUT2 XPM LUCID ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: ELisp/l

Memory information:
((conses 16 1210667 128287)
 (symbols 48 75558 3)
 (strings 32 305623 26170)
 (string-bytes 1 9620792)
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