Avoid writing empty abbrev tables

Fixes bug#29923

'insert-abbrev-table-description' with a non-nil READABLE inserts Lisp
forms suitable for evaluation to restore the defined abbrevs.  We
don't have to insert a form for tables that do not have any abbrevs.

To implement this, we need to filter out system abbrevs before
checking if a table is empty, because system abbrevs were previously
skipped in the 'abbrev--write' call, at which point we would already
have started inserting the beginning of a table definition form.

* lisp/abbrev.el (insert-abbrev-table-description):
Skip inserting empty tables when READABLE is non-nil.
Clarify behavior in documentation string.
(abbrev--write): Remove system abbrev check.

* doc/lispref/abbrevs.texi (Abbrev Tables): Document behavior
with empty tables.

* etc/NEWS: Mention the change in behavior of
'insert-abbrev-table-description'.

1 files changed, 3 insertions, 1 deletions

diff --git a/doc/lispref/abbrevs.texi b/doc/lispref/abbrevs.texi
index 087e6945203..4c9e653cb19 100644
--- a/doc/lispref/abbrevs.texi
+++ b/doc/lispref/abbrevs.texi
@@ -122,7 +122,9 @@ 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.)
+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.
 @end defun
 
 @node Defining Abbrevs