Add lsp-scala and metals as a language backend for scala

CHANGELOG.develop

@@ -2430,6 +2430,8 @@ Other:
   - Evilify =ensime= search in insert/normal mode (thanks to Diego Alvarez)
 - Remove duplicated code (thanks to Tetsuro Takemoto)
 - Added ENSIME jump handlers (thanks to Joao Azevedo)
+- Added scala-lsp and metals as an additional backend for scala
+  (thanks to Rodolfo Hansen)
 **** Scheme
 - Added missing =parinfer= package declaration (thanks to Kalle Lindqvist)
 - Update install docs for Chicken 5 changes (thanks to Grant Shangreaux)

layers/+lang/scala/README.org

@@ -8,7 +8,9 @@
 - [[#description][Description]]
   - [[#features][Features:]]
 - [[#layer-installation][Layer Installation]]
-- [[#ensime][Ensime]]
+- [[#backends][Backends]]
+  - [[#ensime][Ensime]]
+  - [[#metals][Metals]]
 - [[#scalastyle][Scalastyle]]
   - [[#use-java-doc-style][Use Java doc-style]]
 - [[#automatically-show-the-type-of-the-symbol-under-the-cursor][Automatically show the type of the symbol under the cursor]]
@@ -22,6 +24,7 @@ This layer adds support for the Scala language to Spacemacs.
 
 ** Features:
 - Syntax highlighting
+- Support for language backend. Either using [[https://ensime.github.io/][ENSIME]] or [[https://scalameta.org/metals/][Metals]]
 - Auto-completion
 - Syntax-checking
 - Refactoring
@@ -31,7 +34,6 @@ This layer adds support for the Scala language to Spacemacs.
 - Eldoc integration
 - Test execution directly from Emacs
 - Automatic replacement of ASCII arrows with unicode ones
-- Automatic starting/stopping of [[https://ensime.github.io/][ENSIME]] IDE server
 
 * Layer Installation
 To use this configuration layer, add it to your =~/.spacemacs=. You will need to
@@ -46,7 +48,20 @@ version (Stable). Please add the following lines to =dotspacemacs/user-init=:
   (add-to-list 'package-pinned-packages '(ensime . "melpa-stable"))
 #+END_SRC
 
-* Ensime
+* Backends
+The currently supported language backends are:
+- scala-ensime (default)
+- scala-metals
+
+To set your choice of backend, configure the layer variable =scala-backend=.
+
+#+BEGIN_SRC emacs-lisp
+  (setq-default dotspacemacs-configuration-layers
+                '((scala :variables scala-backend 'scala-ensime))) ;or 'scala-metals
+#+END_SRC
+
+
+** Ensime
 [[https://ensime.github.io/][ENSIME]] provides IDE-like features, such as refactoring, incremental compilation
 and project-wide type-checking.
 
@@ -57,6 +72,22 @@ list) to generate these files.
 Installation instructions and usage can be found in the =Java= layer
 [[https://github.com/syl20bnr/spacemacs/tree/develop/layers/%2Blang/java/README.org#ensime][README.org]].
 
+** Metals
+Currently, you must manually install the metals server. It is possible to do so via coursier; the latest version can be built using the following commands:
+
+#+BEGIN_SRC bash
+./coursier bootstrap \
+  --java-opt -Xss4m \
+  --java-opt -Xms100m \
+  --java-opt -Dmetals.client=emacs \
+  org.scalameta:metals_2.12:0.5.1 \
+  -r bintray:scalacenter/releases \
+  -r sonatype:snapshots \
+  -o /usr/local/bin/metals-emacs -f
+#+END_SRC
+
+You will then have the common LSP keybindings; see http://develop.spacemacs.org/layers/+tools/lsp/README.html#key-bindings for more details.
+
 * Scalastyle
 [[http://www.scalastyle.org/][Scalastyle]] provides style-checking and linting. The Emacs functionality is
 provided by Flycheck.
@@ -128,12 +159,12 @@ the ascii arrows back.
 #+END_SRC
 
 * Auto-start
-If you prefer to have Ensime start when you load a scala file, you can enable it
+If you prefer to have the backend start when you load a scala file, you can enable it
 with
 
 #+BEGIN_SRC emacs-lisp
   (setq-default dotspacemacs-configuration-layers '(
-      (scala :variables scala-auto-start-ensime t)))
+      (scala :variables scala-auto-start-backend t)))
 #+END_SRC
 
 * Key bindings

layers/+lang/scala/config.el

@@ -20,5 +20,12 @@
 (defvar scala-use-unicode-arrows nil
   "If non-nil then `->`, `=>` and `<-` are replaced with unicode arrows.")
 
-(defvar scala-auto-start-ensime nil
-  "If non nil then ensime will be started when a scala file is opened.")
+(defconst scala-backends '(scala-ensime scala-metals)
+  "Backend server implementation to enable advanced IDE language features")
+
+(defvar scala-backend 'scala-ensime
+  "Backend used to trigger IDE language features.
+`scala-ensime' or `scala-metals' are currently supported")
+
+(defvar scala-auto-start-backend nil
+  "If non nil then ensime/metals will be started when a scala file is opened.")

layers/+lang/scala/funcs.el

@@ -14,9 +14,21 @@
   (spacemacs//java-setup-ensime)
   (add-to-list 'spacemacs-jump-handlers-scala-mode 'ensime-edit-definition))
 
+(defun spacemacs//scala-setup-metals ()
+  "Setup LSP metals for Scala."
+  (setq-local lsp-prefer-flymake nil))
+
 (defun spacemacs//scala-disable-flycheck-scala ()
   (push 'scala flycheck-disabled-checkers))
 
+(defun spacemacs//scala-backend-ensime-p ()
+  "Return true if the selected backend is ensime"
+  (eq scala-backend 'scala-ensime))
+
+(defun spacemacs//scala-backend-metals-p ()
+  "Return true if the selected backend is metals"
+  (eq scala-backend 'scala-metals))
+
 (defun spacemacs/scala-join-line ()
   "Adapt `scala-indent:join-line' to behave more like evil's line join.
 

layers/+lang/scala/layers.el

@@ -10,4 +10,4 @@
 ;;; License: GPLv3
 
 
-(configuration-layer/declare-layer 'java)
+(configuration-layer/declare-layers '(lsp java))

layers/+lang/scala/packages.el

@@ -11,10 +11,8 @@
 
 (setq scala-packages
       '(
-        eldoc
         ensime
-        flycheck
-        flyspell
+        (lsp-scala :requires lsp-mode)
         ggtags
         counsel-gtags
         helm-gtags
@@ -25,7 +23,7 @@
         ))
 
 (defun scala/post-init-eldoc ()
-  (when scala-enable-eldoc
+  (when (and scala-enable-eldoc (spacemacs//scala-backend-ensime-p))
     (add-hook 'scala-mode-hook #'spacemacs//java-setup-ensime-eldoc)))
 
 (defun scala/pre-init-ensime ()
@@ -35,10 +33,11 @@
 (defun scala/post-init-ensime ()
   (use-package ensime
     :defer t
+    :if (spacemacs//scala-backend-ensime-p)
     :init
     (progn
       (add-hook 'scala-mode-hook #'spacemacs//scala-setup-ensime)
-      (when scala-auto-start-ensime
+      (when scala-auto-start-backend
         (add-hook 'scala-mode-hook 'spacemacs//ensime-maybe-start)))
     :config
     (progn
@@ -52,7 +51,8 @@
   ;; Don't use scala checker if ensime mode is active, since it provides
   ;; better error checking.
   (with-eval-after-load 'flycheck
-    (add-hook 'ensime-mode-hook 'spacemacs//scala-disable-flycheck-scala)))
+    (add-hook 'ensime-mode-hook 'spacemacs//scala-disable-flycheck-scala)
+    (add-hook 'lsp-scala-)))
 
 (defun scala/post-init-flyspell ()
   (spell-checking/add-flyspell-hook 'scala-mode)
@@ -68,9 +68,20 @@
 (defun scala/init-sbt-mode ()
   (use-package sbt-mode
     :defer t
-    :init (spacemacs/set-leader-keys-for-major-mode 'scala-mode
-            "b." 'sbt-hydra
-            "bb" 'sbt-command)))
+    :config
+    ;; WORKAROUND: https://github.com/ensime/emacs-sbt-mode/issues/31
+    ;; allows for using SPACE in the minibuffer
+    (substitute-key-definition
+     'minibuffer-complete-word
+     'self-insert-command
+     minibuffer-local-completion-map)
+    :init
+    (progn
+      (spacemacs/declare-prefix-for-mode 'scala-mode "mb" "sbt")
+      (spacemacs/declare-prefix-for-mode 'scala-mode "mg" "goto")
+      (spacemacs/set-leader-keys-for-major-mode 'scala-mode
+        "b." 'sbt-hydra
+        "bb" 'sbt-command))))
 
 (defun scala/init-scala-mode ()
   (use-package scala-mode
@@ -81,6 +92,14 @@
         (add-to-list 'completion-ignored-extensions ext)))
     :config
     (progn
+      ;; Ensure only one of metals and ensime is loaded
+      (unless (spacemacs//scala-backend-ensime-p)
+        (progn
+          (fmakunbound 'ensime)
+          (remove-hook 'after-change-functions 'ensime-after-change-function)
+          (remove-hook 'window-configuration-change-hook
+                       'ensime-show-left-margin-hook)))
+
       ;; Automatically insert asterisk in a comment when enabled
       (defun scala/newline-and-indent-with-asterisk ()
         (interactive)
@@ -139,6 +158,15 @@ If it's part of a left arrow (`<-'),replace it with the unicode arrow."
             scala-indent:default-run-on-strategy
             scala-indent:operator-strategy))))
 
+(defun scala/init-lsp-scala ()
+  (use-package lsp-scala
+    :after scala-mode
+    :demand t
+    :if (spacemacs//scala-backend-metals-p)
+    :config
+    (add-hook 'scala-mode-local-vars-hook #'spacemacs//scala-setup-metals)
+    :hook ((scala-mode) . lsp)))
+
 (defun scala/post-init-ggtags ()
   (add-hook 'scala-mode-local-vars-hook #'spacemacs/ggtags-mode-enable))