* lisp/external-completion.el (external-completion-table): Tweak docstring.
Explicitly handle `lambda`.

* lisp/progmodes/eglot.el (xref-backend-identifier-completion-table):
Rename category to `eglot-xref`.

* lisp/progmodes/eglot.el
(xref-backend-identifier-completion-table): Update.

* lisp/external-completion.el (cl-lib): Require cl-lib.
(external-completion-table): Rework for caching.

* lisp/external-completion.el (Commentary): Update.
(completion-styles-alist): Name the style 'external'.
(external-completion-table): Change order of arguments.  Use
double-dashed symbols.
(external-completion--try-completion)
(external-completion--all-completions): Use double-dashed symbols.

"External completion" is a much better name.  There are already a
thousand in-Emacs meanings for "backend" and this new style is really
meant to be used for tools _outside_ of Emacs.

Fido mode's mantra is to use the 'flex' completion style wherever
possible, but this clashes with some completion table categories that
can't use that style, such as the 'backend-completion' category, which
uses the backend-completion-backend-style style.

* lisp/icomplete.el (icomplete--fido-mode-setup): Don't override
completion-category-defaults.

This completion style is meant to be used with a "programmable
completion" table that interfaces with an external providing
completions, such as a shell utility, an inferior process, an http
server.  The table and external tool are tasked to do the matching of
the pattern string to the potential candidates of completion, and as
such it fully controls the style.

When this completion style is in use, the usual styles configured
by the user or other in `completion-styles' are completely
overriden.  This can be seen as a drawback, but, on the other hand,
the regular the full data set to be available in Emacs' addressing
space, which is often not feasible.

* lisp/backend-completion.el: New file.

* lisp/progmodes/eglot.el (Package-Requires): Require
backend-completion.
(completion-styles-alist): Don't set it.
(eglot--lsp-backend-style-call)
(eglot--lsp-backend-style-try-completion)
(eglot--lsp-backend-style-all-completions): Delete.

bug#59338

* lisp/progmodes/eglot.el (eglot--path-to-uri): Check for windows path

project.el is a GNU ELPA :core package, so this kind of trick isn't
needed.

* lisp/progmodes/eglot.el (eglot--connect): Don't fboundp
project-name.