Indicate disabled checkers in verification

Fix a long-standing confusion about verification results which would
show all great even though the checker isn’t used because it was
disabled, probably even without the user being aware of it.

Closes GH-749

CHANGES.md

@@ -39,6 +39,7 @@ now for questions and discussions about development. [GH-820]
   * Explicitly display errors at point with `C-c ! h` [GH-834]
   * Merge message and checker columns in the error list to remove redundant
     ellipsis [GH-828]
+  * Indicate disabled checkers in verification buffers [GH-749]
 
 * Bug fixes:
 

flycheck.el

@@ -1935,6 +1935,8 @@ into the verification results."
   (insert-button (symbol-name checker)
                  'type 'help-flycheck-checker-doc
                  'help-args (list checker))
+  (when (with-current-buffer buffer (flycheck-disabled-checker-p checker))
+    (insert (propertize " (disabled)" 'face '(bold error))))
   (princ "\n")
   (let ((results (with-current-buffer buffer
                    (flycheck-verify-generic-checker checker))))
@@ -1988,6 +1990,24 @@ buffer."
                    'help-args (list mode)))
   (princ ":\n\n"))
 
+(defun flycheck--verify-print-footer (buffer)
+  "Print a footer for BUFFER in the current buffer.
+
+BUFFER is the buffer being verified."
+  (princ "Flycheck Mode is ")
+  (let ((enabled (buffer-local-value 'flycheck-mode buffer)))
+    (insert (propertize (if enabled "enabled" "disabled")
+                        'face (if enabled 'success '(warning bold)))))
+  (princ
+   (with-current-buffer buffer
+     ;; Use key binding state in the verified buffer to print the help.
+     (substitute-command-keys
+      ".  Use \\[universal-argument] \\[flycheck-disable-checker] to enable disabled checkers.")))
+  (save-excursion
+    (let ((end (point)))
+      (backward-paragraph)
+      (fill-region-as-paragraph (point) end))))
+
 (defun flycheck-verify-checker (checker)
   "Check whether a CHECKER can be used in this buffer.
 
@@ -2011,10 +2031,12 @@ is applicable from Emacs Lisp code.  Use
         (flycheck--verify-print-header "Syntax checker in buffer " buffer)
         (flycheck--verify-princ-checker checker buffer 'with-mm)
         (if (with-current-buffer buffer (flycheck-may-use-checker checker))
-            (insert (propertize "Flycheck can use this syntax checker for this buffer."
+            (insert (propertize "Flycheck can use this syntax checker for this buffer.\n"
                                 'face 'success))
-          (insert (propertize "Flycheck cannot use this syntax checker for this buffer."
-                              'face 'error)))))))
+          (insert (propertize "Flycheck cannot use this syntax checker for this buffer.\n"
+                              'face 'error)))
+        (insert "\n")
+        (flycheck--verify-print-footer buffer)))))
 
 (defun flycheck-verify-setup ()
   "Check whether Flycheck can be used in this buffer.
@@ -2039,14 +2061,8 @@ possible problems are shown."
                               'face '(bold error))))
         (dolist (checker checkers)
           (flycheck--verify-princ-checker checker buffer))
-        (princ "Flycheck Mode is ")
-        (let ((enabled (buffer-local-value 'flycheck-mode buffer)))
-          (insert (propertize (if enabled "enabled" "disabled")
-                              'face (if enabled 'success '(warning bold)))))
-        (princ ".\n")
 
         (-when-let (selected-checker (buffer-local-value 'flycheck-checker buffer))
-          (princ "\n")
           (insert (propertize "The following checker is explicitly selected for this buffer:\n\n"
                               'face 'bold))
           (flycheck--verify-princ-checker selected-checker buffer 'with-mm))
@@ -2060,7 +2076,9 @@ possible problems are shown."
               (princ "  - ")
               (princ checker)
               (princ "\n"))
-            (princ "\nTry adding these syntax checkers to `flycheck-checkers'.")))))))
+            (princ "\nTry adding these syntax checkers to `flycheck-checkers'.\n")))
+
+        (flycheck--verify-print-footer buffer)))))
 
 
 ;;; Predicates for generic syntax checkers