Add auto-detection of completion system (#1602)

5 files changed, 57 insertions, 37 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 03b27ae..3348815 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,14 +5,15 @@
 ### New features
 
 * New functions `projectile-acquire-root` and `projectile-process-current-project-buffers-current`
-* New project commands `projectile-package-project`, `projectile-install-project`. 
+* New project commands `projectile-package-project`, `projectile-install-project`.
 * [#1539](https://github.com/bbatsov/projectile/pull/1539): New defcustom `projectile-auto-discover` controlling whether to automatically discover projects in the search path when `projectile-mode` activates.
 * Add [emacs-eldev](https://github.com/doublep/eldev) project type.
 * Add Dart project type.
-* [#1555](https://github.com/bbatsov/projectile/pull/1555) Add search with ripgrep. 
+* [#1555](https://github.com/bbatsov/projectile/pull/1555) Add search with ripgrep.
 * Add Python-poetry project type.
 * [#1576](https://github.com/bbatsov/projectile/pull/1576) Add OCaml [Dune](https://github.com/ocaml/dune) project type.
 * Add [Mill](http://www.lihaoyi.com/mill/) project type.
+* Auto-detect completion system, supporting `ido`, `ivy`, `helm` and the default completion system.
 
 ### Changes
 
diff --git a/README.md b/README.md
index 5cb2dba..3ab6d93 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,7 @@ it. Some of Projectile's features:
 * run make in a project with a single key chord
 * check for dirty repositories
 * toggle read-only mode for the entire project
-* support for multiple minibuffer completion/selection libraries (e.g. `ido`, `ivy` and `helm`)
+* support for multiple minibuffer completion/selection libraries (`ido`, `ivy`, `helm` and the default completion system)
 
 ## Projectile in Action
 
diff --git a/doc/modules/ROOT/pages/configuration.adoc b/doc/modules/ROOT/pages/configuration.adoc
index 00edfe8..4abdaec 100644
--- a/doc/modules/ROOT/pages/configuration.adoc
+++ b/doc/modules/ROOT/pages/configuration.adoc
@@ -287,10 +287,20 @@ top-level directory.
 
 == Completion Options
 
+=== Auto (default)
+
+By default Projectile detects the completion system in use, based
+on the mode variables `ido-mode`, `ivy-mode` and `helm-mode`.
+If none of those is activated, the `default` completion system is used.
+
 === Ido
 
-By default Projectile uses `ido` as its completion system. `ido` is
-extremely popular and it is built into Emacs.
+The `ido` completion system is extremely popular and it is built into Emacs.
+
+[source,elisp]
+----
+(setq projectile-completion-system 'ido)
+----
 
 TIP: As already noted above if you're going to use the `ido` completion it's
  **extremely highly** recommended that you install the optional
@@ -316,6 +326,9 @@ If you don't like `ido` and `ivy` you can use regular completion:
 ----
 
 You might want to combine default completion with `icomplete-mode` for optimum results.
+Emacs 27 added `fido-mode` to `icomplete`. If you are using `fido-mode`, projectile will
+use the `default` completion system. The same holds for `selectrum` which also relies
+on the `default` completion system.
 
 === Custom Completion Function
 
diff --git a/doc/modules/ROOT/pages/index.adoc b/doc/modules/ROOT/pages/index.adoc
index 8227a5a..da23e8e 100644
--- a/doc/modules/ROOT/pages/index.adoc
+++ b/doc/modules/ROOT/pages/index.adoc
@@ -47,7 +47,7 @@ Projectile's features:
 * visit project in dired
 * run make in a project with a single key chord
 * browse dirty version controlled projects
-* support for multiple minibuffer completion/selection libraries (e.g. `ido`, `ivy` and `helm`)
+* support for multiple minibuffer completion/selection libraries (`ido`, `ivy`, `helm` and the default completion system)
 
 == Projectile in Action
 
diff --git a/projectile.el b/projectile.el
index 1b7fdbb..5320407 100644
--- a/projectile.el
+++ b/projectile.el
@@ -47,6 +47,9 @@
   (require 'subr-x))
 
 (eval-when-compile
+  (defvar ido-mode)
+  (defvar ivy-mode)
+  (defvar helm-mode)
   (defvar ag-ignore-list)
   (defvar ggtags-completion-table)
   (defvar tags-completion-table)
@@ -199,10 +202,11 @@ When nil Projectile will consider the current directory the project root."
                  (const :tag "Yes" t)
                  (const :tag "Prompt for project" prompt)))
 
-(defcustom projectile-completion-system 'ido
+(defcustom projectile-completion-system 'auto
   "The completion system to be used by Projectile."
   :group 'projectile
   :type '(radio
+          (const :tag "Auto-detect" auto)
           (const :tag "Ido" ido)
           (const :tag "Helm" helm)
           (const :tag "Ivy" ivy)
@@ -1817,36 +1821,38 @@ project-root for every file."
   (let ((prompt (projectile-prepend-project-name prompt))
         res)
     (setq res
-          (cond
-           ((eq projectile-completion-system 'ido)
-            (ido-completing-read prompt choices nil nil initial-input))
-           ((eq projectile-completion-system 'default)
-            (completing-read prompt choices nil nil initial-input))
-           ((eq projectile-completion-system 'helm)
-            (if (and (fboundp 'helm)
-                     (fboundp 'helm-make-source))
-                (helm :sources
-                      (helm-make-source "Projectile" 'helm-source-sync
-                                        :candidates choices
-                                        :action (if action
-                                                    (prog1 action
-                                                      (setq action nil))
-                                                  #'identity))
-                      :prompt prompt
-                      :input initial-input
-                      :buffer "*helm-projectile*")
-              (user-error "Please install helm from \
-https://github.com/emacs-helm/helm")))
-           ((eq projectile-completion-system 'ivy)
-            (if (fboundp 'ivy-read)
-                (ivy-read prompt choices
-                          :initial-input initial-input
-                          :action (prog1 action
-                                    (setq action nil))
-                          :caller 'projectile-completing-read)
-              (user-error "Please install ivy from \
-https://github.com/abo-abo/swiper")))
-           (t (funcall projectile-completion-system prompt choices))))
+          (pcase (if (eq projectile-completion-system 'auto)
+                     (cond
+                      ((and (boundp 'ido-mode)  ido-mode)  'ido)
+                      ((and (boundp 'helm-mode) helm-mode) 'helm)
+                      ((and (boundp 'ivy-mode)  ivy-mode)  'ivy)
+                      (t 'default))
+                   projectile-completion-system)
+            ('default (completing-read prompt choices nil nil initial-input))
+            ('ido (ido-completing-read prompt choices nil nil initial-input))
+            ('helm
+             (if (and (fboundp 'helm)
+                      (fboundp 'helm-make-source))
+                 (helm :sources
+                       (helm-make-source "Projectile" 'helm-source-sync
+                                         :candidates choices
+                                         :action (if action
+                                                     (prog1 action
+                                                       (setq action nil))
+                                                   #'identity))
+                       :prompt prompt
+                       :input initial-input
+                       :buffer "*helm-projectile*")
+               (user-error "Please install helm from melpa")))
+            ('ivy
+             (if (fboundp 'ivy-read)
+                 (ivy-read prompt choices
+                           :initial-input initial-input
+                           :action (prog1 action
+                                     (setq action nil))
+                           :caller 'projectile-completing-read)
+               (user-error "Please install ivy from elpa")))
+            (_ (funcall projectile-completion-system prompt choices))))
     (if action
         (funcall action res)
       res)))