Add +tools/lsp layer for lsp-mode & lsp-ui

This is a starting point to bring LSP ecosystem into spacemacs.

layers/+tools/lsp/packages.el

@@ -0,0 +1,64 @@
+(defconst lsp-packages
+  '(
+    company-lsp
+    (helm-xref :requires helm)
+    (ivy-xref :requires ivy)
+    lsp-mode
+    lsp-ui
+    markdown-mode  ;; depended by lsp-ui
+    ))
+
+(defun lsp/init-company-lsp ()
+  (use-package company-lsp
+    :after company
+    :init
+    ;; Language servers have better idea filtering and sorting,
+    ;; don't filter results on the client side.
+    (setq company-transformers nil
+          company-lsp-async t
+          company-lsp-cache-candidates nil)
+    ;; (spacemacs|add-company-backends :backends company-lsp :modes c-mode-common)
+    ))
+
+(defun lsp/init-helm-xref ()
+  (use-package helm-xref
+    :config
+    ;; This is required to make xref-find-references not give a prompt.
+    ;; xref-find-references asks the identifier (which has no text property) and then passes it to lsp-mode, which requires the text property at point to locate the references.
+    ;; https://debbugs.gnu.org/cgi/bugreport.cgi?bug=29619
+    (setq xref-prompt-for-identifier
+          '(not xref-find-definitions xref-find-definitions-other-window xref-find-definitions-other-frame xref-find-references spacemacs/jump-to-definition))
+
+    ;; Use helm-xref to display xref.el results.
+    (setq xref-show-xrefs-function 'helm-xref-show-xrefs)
+    ))
+
+(defun lsp/init-ivy-xref ()
+  (use-package ivy-xref
+    :config
+    (setq xref-prompt-for-identifier
+          '(not xref-find-definitions xref-find-definitions-other-window xref-find-definitions-other-frame xref-find-references spacemacs/jump-to-definition))
+
+    ;; Use ivy-xref to display xref.el results.
+    (setq xref-show-xrefs-function 'ivy-xref-show-xrefs)
+    ))
+
+(defun lsp/init-lsp-mode ()
+  (use-package lsp-mode
+    :config
+    (require 'lsp-imenu)
+    (add-hook 'lsp-after-open-hook #'lsp-enable-imenu)
+    ;; Disable lsp-flycheck.el in favor of lsp-ui-flycheck.el
+    (setq lsp-enable-flycheck nil)
+    ))
+
+(defun lsp/init-lsp-ui ()
+  (use-package lsp-ui
+    :after lsp-mode
+    :after markdown-mode
+    :config
+    (add-hook 'lsp-mode-hook #'lsp-ui-mode)
+    ))
+
+(defun lsp/init-markdown-mode ()
+  (use-package markdown-mode))