Rework empty abbrev table omitting

There were two problems with the original implementation:

1. It changed the behavior of insert-abbrev-table-description when
READABLE is nil to sometimes insert one Emacs Lisp expression and
sometimes insert nothing.
2. It broke the tests.

This commit reworks this so that insert-abbrev-table-description
always inserts an expressions even if no abbrevs need to be saved and
making only write-abbrev-file check that a table has any abbrevs to
save before calling insert-abbrev-table-description.  This duplicates
the work of filtering the table for savable abbrevs, but the benefit
of keeping the API is worth it.

* doc/lispref/abbrevs.texi (Abbrev Tables): Update documentation.
* lisp/abbrev.el (write-abbrev-file): Skip tables without user abbrevs
(insert-abbrev-table-description): Always insert the define
expression.
(abbrev--table-symbols): New function.
* test/lisp/abbrev-tests.el (abbrev--table-symbols-test):
Add test for abbrev--table-symbols.

doc/lispref/abbrevs.texi

@@ -122,9 +122,7 @@ System abbrevs are listed and identified as such.  Otherwise the
 description is a Lisp expression---a call to @code{define-abbrev-table}
 that would define @var{name} as it is currently defined, but without
 the system abbrevs.  (The mode or package using @var{name} is supposed
-to add these to @var{name} separately.)  If the Lisp expression would
-not define any abbrevs (i.e.@: it defines an empty abbrev table), this
-function inserts nothing.
+to add these to @var{name} separately.)
 @end defun
 
 @node Defining Abbrevs
@@ -234,7 +232,8 @@ Emacs commands to offer to save your abbrevs.
 Save all abbrev definitions (except system abbrevs), for all abbrev
 tables listed in @code{abbrev-table-name-list}, in the file
 @var{filename}, in the form of a Lisp program that when loaded will
-define the same abbrevs.  If @var{filename} is @code{nil} or omitted,
+define the same abbrevs.  Tables that do not have any abbrevs to save
+are omitted.  If @var{filename} is @code{nil} or omitted,
 @code{abbrev-file-name} is used.  This function returns @code{nil}.
 @end deffn
 

etc/NEWS

@@ -258,10 +258,9 @@ case does not match.
 for abbrevs that have them.
 
 +++
-** 'insert-abbrev-table-description' skips empty tables.
-'insert-abbrev-table-description' skips inserting empty tables when
-inserting non-readable tables.  By extension, this makes
-'write-abbrev-file' skip writing empty tables.
+** 'write-abbrev-file' skips empty tables.
+'write-abbrev-file' now skips inserting a 'define-abbrev-table' form for
+tables which do not have any non-system abbrevs to save.
 
 +++
 ** The new functions and commands 'text-property-search-forward' and

lisp/abbrev.el

@@ -251,7 +251,8 @@ have been saved."
 		     (lambda (s1 s2)
 		       (string< (symbol-name s1)
 				(symbol-name s2)))))
-	(insert-abbrev-table-description table nil))
+	(if (abbrev--table-symbols table)
+            (insert-abbrev-table-description table nil)))
       (when (unencodable-char-position (point-min) (point-max) 'utf-8)
 	(setq coding-system-for-write
 	      (if (> emacs-major-version 24)
@@ -937,33 +938,38 @@ is inserted.
 If READABLE is nil, an expression is inserted.  The expression is
 a call to `define-abbrev-table' that when evaluated will define
 the abbrev table NAME exactly as it is currently defined.
-Abbrevs marked as \"system abbrevs\" are ignored.  If the
-resulting expression would not define any abbrevs, nothing is
-inserted."
+Abbrevs marked as \"system abbrevs\" are ignored."
+  (let ((table (symbol-value name))
+        (symbols (abbrev--table-symbols name readable)))
+    (setq symbols (sort symbols 'string-lessp))
+    (let ((standard-output (current-buffer)))
+      (if readable
+          (progn
+            (insert "(")
+            (prin1 name)
+            (insert ")\n\n")
+            (mapc 'abbrev--describe symbols)
+            (insert "\n\n"))
+        (insert "(define-abbrev-table '")
+        (prin1 name)
+        (if (null symbols)
+            (insert " '())\n\n")
+          (insert "\n  '(\n")
+          (mapc 'abbrev--write symbols)
+          (insert "   ))\n\n")))
+      nil)))
+
+(defun abbrev--table-symbols (name &optional system)
+  "Return the user abbrev symbols in the abbrev table named NAME.
+NAME is a symbol whose value is an abbrev table.  System abbrevs
+are omitted unless SYSTEM is non-nil."
   (let ((table (symbol-value name))
         (symbols ()))
     (mapatoms (lambda (sym)
-                (if (and (symbol-value sym) (or readable (not (abbrev-get sym :system))))
+                (if (and (symbol-value sym) (or system (not (abbrev-get sym :system))))
                     (push sym symbols)))
               table)
-    (when symbols
-      (setq symbols (sort symbols 'string-lessp))
-      (let ((standard-output (current-buffer)))
-        (if readable
-            (progn
-              (insert "(")
-              (prin1 name)
-              (insert ")\n\n")
-              (mapc 'abbrev--describe symbols)
-              (insert "\n\n"))
-          (insert "(define-abbrev-table '")
-          (prin1 name)
-          (if (null symbols)
-              (insert " '())\n\n")
-            (insert "\n  '(\n")
-            (mapc 'abbrev--write symbols)
-            (insert "   ))\n\n")))
-        nil))))
+    symbols))
 
 (defun define-abbrev-table (tablename definitions
                                       &optional docstring &rest props)

test/lisp/abbrev-tests.el

@@ -64,6 +64,14 @@
     (should (= (length table) obarray-default-size))
     (should (eq (abbrev-table-get table 'foo) 'bar))))
 
+(ert-deftest abbrev--table-symbols-test ()
+  (let ((ert-test-abbrevs (setup-test-abbrev-table)))
+    (define-abbrev ert-test-abbrevs "sys" "system abbrev" nil :system t)
+    (should (equal (mapcar #'symbol-name (abbrev--table-symbols 'ert-test-abbrevs))
+                   '("a-e-t")))
+    (should (equal (mapcar #'symbol-name (abbrev--table-symbols 'ert-test-abbrevs t))
+                   '("a-e-t" "sys")))))
+
 (ert-deftest abbrev-table-get-put-test ()
   (let ((table (make-abbrev-table)))
     (should-not (abbrev-table-get table 'foo))