Close #52: Use entire line as xref summary when possible

* eglot.el (eglot--xref-make): Make the item with summary being the
  whole line at given location, when possible
(eglot--locations-to-xrefs,eglot--path-xrefs,eglot--buf-xrefs):
Convert location list to xref list, trying to open each file just once.
(xref-backend-identifier-at-point):
(xref-backend-definitions):
(xref-backend-references): Pass the list of locations to
eglot--locations-to-xrefs.

eglot.el

@@ -1308,14 +1308,6 @@ DUMMY is ignored."
 (advice-add 'xref-find-definitions :after #'eglot--xref-reset-known-symbols)
 (advice-add 'xref-find-references :after #'eglot--xref-reset-known-symbols)
 
-(defun eglot--xref-make (name uri position)
-  "Like `xref-make' but with LSP's NAME, URI and POSITION."
-  (cl-destructuring-bind (&key line character) position
-    (xref-make name (xref-make-file-location
-                     (eglot--uri-to-path uri)
-                     ;; F!@(#*&#$)CKING OFF-BY-ONE again
-                     (1+ line) character))))
-
 (cl-defmethod xref-backend-identifier-completion-table ((_backend (eql eglot)))
   (when (eglot--server-capable :documentSymbolProvider)
     (let ((server (eglot--current-server-or-lose))
@@ -1346,6 +1338,58 @@ DUMMY is ignored."
                 :textDocumentPositionParams
                 (eglot--TextDocumentPositionParams))))
 
+(defun eglot--xref-make (identifier location)
+  "Make an `xref-item' for given IDENTIFIER and LSP LOCATION.
+If IDENTIFIER is nil, use the current buffer's contents at
+LOCATION as the summary."
+  (cl-destructuring-bind (&key uri range) location
+    (cl-destructuring-bind (&key start &allow-other-keys) range
+      (cl-destructuring-bind (&key line character) start
+        (xref-make
+         (or identifier
+             (let* ((erange (eglot--range-region range))
+                    (length (- (cdr erange) (car erange))))
+               (eglot--widening
+                (goto-char (car erange))
+                (let ((snippet
+                       (buffer-substring-no-properties (point-at-bol)
+                                                       (point-at-eol))))
+                  (add-face-text-property
+                   character (min (+ character length) (length snippet))
+                   'highlight t snippet)
+                  snippet))))
+         (xref-make-file-location (eglot--uri-to-path uri)
+                                  ;; F!@(#*&#$)CKING OFF-BY-ONE again
+                                  (1+ line) character))))))
+
+(defun eglot--buf-xrefs (identifier locations)
+  (mapcar (apply-partially #'eglot--xref-make identifier)
+          locations))
+
+(defun eglot--path-xrefs (identifier path-and-locations)
+  (cl-destructuring-bind (path . locations) path-and-locations
+    (let (already-visiting)
+      (cond
+       ((setq already-visiting (find-buffer-visiting path))
+        (with-current-buffer already-visiting
+          (eglot--buf-xrefs nil locations)))
+       ((file-readable-p path)
+        (with-temp-buffer
+          (insert-file-contents-literally path)
+          (eglot--buf-xrefs nil locations)))
+       (t
+        (eglot--buf-xrefs identifier locations))))))
+
+(defun eglot--locations-to-xrefs (identifier locations)
+  (let ((locations-by-path
+         (seq-group-by (lambda (location)
+                         (cl-destructuring-bind (&key uri range) location
+                           (eglot--uri-to-path uri)))
+                       locations)))
+    (apply #'append
+           (mapcar (apply-partially #'eglot--path-xrefs identifier)
+                   locations-by-path))))
+
 (cl-defmethod xref-backend-definitions ((_backend (eql eglot)) identifier)
   (let* ((rich-identifier
           (car (member identifier eglot--xref-known-symbols)))
@@ -1356,9 +1400,7 @@ DUMMY is ignored."
                              :textDocument/definition
                              (get-text-property
                               0 :textDocumentPositionParams identifier)))))
-    (mapcar (jsonrpc-lambda (&key uri range)
-              (eglot--xref-make identifier uri (plist-get range :start)))
-            location-or-locations)))
+    (eglot--locations-to-xrefs identifier location-or-locations)))
 
 (cl-defmethod xref-backend-references ((_backend (eql eglot)) identifier)
   (unless (eglot--server-capable :referencesProvider)
@@ -1369,9 +1411,8 @@ DUMMY is ignored."
                (and rich (get-text-property 0 :textDocumentPositionParams rich))))))
     (unless params
       (eglot--error "Don' know where %s is in the workspace!" identifier))
-    (mapcar
-     (jsonrpc-lambda (&key uri range)
-       (eglot--xref-make identifier uri (plist-get range :start)))
+    (eglot--locations-to-xrefs
+     identifier
      (jsonrpc-request (eglot--current-server-or-lose)
                       :textDocument/references
                       (append
@@ -1381,10 +1422,8 @@ DUMMY is ignored."
 
 (cl-defmethod xref-backend-apropos ((_backend (eql eglot)) pattern)
   (when (eglot--server-capable :workspaceSymbolProvider)
-    (mapcar
-     (jsonrpc-lambda (&key name location &allow-other-keys)
-       (cl-destructuring-bind (&key uri range) location
-         (eglot--xref-make name uri (plist-get range :start))))
+    (eglot--locations-to-xrefs
+     pattern
      (jsonrpc-request (eglot--current-server-or-lose)
                       :workspace/symbol
                       `(:query ,pattern)))))