Allow customizing partial-completion to be more like substring

The substring completion style completes "foo-bar" as "*foo-bar*".  The
partial-completion completion style completes "foo-bar" as "foo*bar*".

Previously, it was not possible to get completion of "foo-bar" to act as
"*foo*bar*", e.g. combining the partial-completion and substring styles.
This would be especially useful for things like project-find-file.

Now it is possible by customizing the completion-pcm-leading-wildcard variable
to a non-nil value.

Furthermore, it's convenient to be able to run
regular (completion-pcm-leading-wildcard=t, non-substring)
partial-completion before running completion-pcm-leading-wildcard=nil
partial-completion, since the former provides more narrowly targeted
completions.

It's possible to do this by customizing completion-styles.  Just add
'(partial-completion ((completion-pcm-leading-wildcard t))) and
'(partial-completion ((completion-pcm-leading-wildcard nil))) in that
order.  Then the completion machinery will first run partial-completion
with completion-pcm-leading-wildcard=t, and if that returns no
completions, run partial-completion with
completion-pcm-leading-wildcard=nil.

* lisp/minibuffer.el (completion--nth-completion): Allow an element of
completion-styles to contain a list of bindings.
(completion-styles): Document that.
(completion-pcm-leading-wildcard): Add.
(completion-pcm--string->pattern): Check completion-pcm-leading-wildcard.
(bug#70217)

doc/emacs/mini.texi

@@ -543,8 +543,14 @@ falls back on the next style.
 
 @vindex completion-styles
   The list variable @code{completion-styles} specifies the completion
-styles to use.  Each list element is the name of a completion style (a
-Lisp symbol).  The available style symbols are stored in the variable
+styles to use.  Each list element is either the name of a completion
+style (a Lisp symbol) or a list starting with the name of a completion
+style followed by @code{let}-style list of bindings which will be in
+effect for that completion style.  Multiple elements of
+@code{completion-styles} can name the same completion style with
+different variable bindings.
+
+The available style symbols are stored in the variable
 @code{completion-styles-alist} (@pxref{Completion Variables,,, elisp,
 The Emacs Lisp Reference Manual}).  The default completion styles are
 (in order):
@@ -569,6 +575,12 @@ Furthermore, a @samp{*} in the minibuffer text is treated as a
 @dfn{wildcard}---it matches any string of characters at the
 corresponding position in the completion alternative.
 
+@vindex completion-pcm-leading-wildcard
+If @code{completion-pcm-leading-wildcard} is set to @code{t}, this style
+always acts as if a @dfn{wildcard} is present at the start of the
+minibuffer text, similar to the @code{substring} style.  For example,
+@samp{l-m} will complete to @samp{emacs-lisp-mode}.
+
 @item emacs22
 @cindex @code{emacs22}, completion style
 This completion style is similar to @code{basic}, except that it

etc/NEWS

@@ -43,6 +43,28 @@ The 'find-function', 'find-library', 'find-face-definition', and
 'find-variable' commands now allow retrieving previous input using the
 usual minibuffer history commands.  Each command has a separate history.
 
+** Minibuffer and Completions
+
++++
+*** New user option 'completion-pcm-leading-wildcard'.
+This option configures how the partial-completion style does completion.
+It defaults to nil, which preserves the existing behavior.  When it is set
+to t, the partial-completion style behaves more like the substring
+style, in that a string being completed can match against a candidate
+anywhere in the candidate string.
+
++++
+*** 'completion-styles' now can contain lists of bindings.
+In addition to being a symbol naming a completion style, an element of
+'completion-styles' can now be a list of the form '(STYLE ((VARIABLE
+VALUE) ...))' where STYLE is a symbol naming a completion style.
+VARIABLE will be bound to VALUE (without evaluating it) while the style
+is executing.  This allows multiple references to the same style with
+different values for completion-affecting variables like
+'completion-pcm-leading-wildcard or 'completion-ignore-case'.  This also
+applies for the styles configuration in 'completion-category-overrides'
+and 'completion-category-defaults'.
+
 
 ** Windows
 

lisp/minibuffer.el

@@ -1141,7 +1141,15 @@ and DOC describes the way this style of completion works.")
     ;; and simply add "bar" to the end of the result.
     emacs22)
   "List of completion styles to use.
-The available styles are listed in `completion-styles-alist'.
+An element should be a symbol which is listed in
+`completion-styles-alist'.
+
+An element can also be a list of the form
+(STYLE ((VARIABLE VALUE) ...))
+STYLE must be a symbol listed in `completion-styles-alist', followed by
+a `let'-style list of variable/value pairs.  VARIABLE will be bound to
+VALUE (without evaluating it) while the style is handling completion.
+This allows repeating the same style with different configurations.
 
 Note that `completion-category-overrides' may override these
 styles for specific categories, such as files, buffers, etc."
@@ -1284,11 +1292,18 @@ overrides the default specified in `completion-category-defaults'."
          (result-and-style
           (seq-some
            (lambda (style)
+             (let (symbols values)
+               (when (consp style)
+                 (dolist (binding (cadr style))
+                   (push (car binding) symbols)
+                   (push (cadr binding) values))
+                 (setq style (car style)))
+               (cl-progv symbols values
                  (let ((probe (funcall
                                (or (nth n (assq style completion-styles-alist))
                                    (error "Invalid completion style %s" style))
                                string table pred point)))
-               (and probe (cons probe style))))
+                   (and probe (cons probe style))))))
            (completion--styles md)))
          (adjust-fn (get (cdr result-and-style) 'completion--adjust-metadata)))
     (when (and adjust-fn metadata)
@@ -3868,6 +3883,21 @@ the commands start with a \"-\" or a SPC."
 	     (setq trivial nil)))
 	 trivial)))
 
+(defcustom completion-pcm-leading-wildcard nil
+  "If non-nil, partial-completion completes as if there's a leading wildcard.
+
+If nil (the default), partial-completion requires a matching completion
+alternative to have the same beginning as the first \"word\" in the
+minibuffer text, where \"word\" is determined by
+`completion-pcm-word-delimiters'.
+
+If non-nil, partial-completion allows any string of characters to occur
+at the beginning of a completion alternative, as if a wildcard such as
+\"*\" was present at the beginning of the minibuffer text.  This makes
+partial-completion behave more like the substring completion style."
+  :version "30.1"
+  :type 'boolean)
+
 (defun completion-pcm--string->pattern (string &optional point)
   "Split STRING into a pattern.
 A pattern is a list where each element is either a string
@@ -3918,7 +3948,11 @@ or a symbol, see `completion-pcm--merge-completions'."
       (when (> (length string) p0)
         (if pending (push pending pattern))
         (push (substring string p0) pattern))
-      (nreverse pattern))))
+      (setq pattern (nreverse pattern))
+      (when completion-pcm-leading-wildcard
+        (when (stringp (car pattern))
+          (push 'prefix pattern)))
+      pattern)))
 
 (defun completion-pcm--optimize-pattern (p)
   ;; Remove empty strings in a separate phase since otherwise a ""