Eglot: improve caching in eglot-completion-at-point

When answering the :textDocument/completion request, LSP servers
provide a :isIncomplete flag in the response, which allows Eglot to
know if "further typing should result in recomputing [the completions]
list.

If :isIncomplete is false (i.e. the full set was returned), Eglot
caches the response in a global variable eglot--capf-cache that
persists for the duration of the "completion session", taken to be the
interval between two calls to completion-in-region-mode.

If the cache has been set, and Eglot detects that "further typing" has
happened, it is safe to use the cache instead of making a request to
the server.

Thus eglot--capf-cache-flush, added to completion-in-region-mode-hook,
is used to flush this cache.  Since the popular Company completion
package doesn't use completion-in-region-mode, eglot--capf-cache-flush
is also added to its company-after-completion-hook.

* lisp/progmodes/eglot.el (eglot--managed-mode): Set
'completion-in-region-mode-hook and company-after-completion-hook.
(eglot--capf-cache): New variable.
(eglot--capf-cache-flush): New function.
(eglot-completion-at-point): Rework.

* etc/EGLOT-NEWS: Update.

1 files changed, 22 insertions, 0 deletions

diff --git a/etc/EGLOT-NEWS b/etc/EGLOT-NEWS
index dd04e677285..09772a1e71a 100644
--- a/etc/EGLOT-NEWS
+++ b/etc/EGLOT-NEWS
@@ -18,6 +18,28 @@ to look up issue github#1234, go to
 https://github.com/joaotavora/eglot/issues/1234.
 
 
+* Changes in upcoming Eglot 1.14
+
+** Faster, more responsive completion
+
+Eglot takes advantage of LSP's "isIncomplete" flag in responses to
+completion requests to drive new completion-caching mechanism for the
+duration of each completion session.  Once a full set of completions
+is obtained for a given position, the server needn't be contacted in
+many scenarios, resulting in significantly less communication
+overhead.  This works with the popular Company package and stock
+completion-at-point interfaces.
+
+A variable 'eglot-cache-session-completions', t by default, controls
+this.  The mechanism was tested with ccls, jdtls, pylsp, golsp and
+clangd.  Notably, the C/C++ language server Clangd version 15 has a
+bug in its "isIcomplete" flag (it is fixed in later versions).  If you
+run into problems, disable this mechanism like so:
+
+(add-hook 'c-common-mode-hook
+   (lambda () (setq-local eglot-cache-session-completions nil)))
+
+
 * Changes in Eglot 1.13 (15/03/2023)
 
 ** ELPA installations on Emacs 26.3 are supported again.