Add support for the "splice operator" in Eshell

This allows splicing lists in-place in argument lists, which is
particularly important when defining aliases using the '$*' special
variable (bug#59960).

* lisp/eshell/esh-var.el (eshell-parse-variable): Add support for the
splice operator.
(eshell-interpolate-variable): Let 'eshell-parse-variable' handle
adding 'eshell-escape-arg'.
(eshell-complete-variable-reference): Handle the splice operator.

* lisp/eshell/esh-arg.el (eshell-concat-groups)
(eshell-prepare-splice): New functions...
(eshell-resolve-current-argument): ... use them.
(eshell-splice-args): New function.

* lisp/eshell/esh-cmd.el (eshell-rewrite-named-command): Handle
'eshell-splice-args'.

* lisp/eshell/esh-util.el (eshell-list-to-string): New function...
(eshell-flatten-and-stringify): ... use it.

* lisp/eshell/em-cmpl.el (eshell-complete-parse-arguments): Remove
'eshell-splice-args' sigils in Eshell command forms so that we can
perform completion on splice-expansions.

* lisp/eshell/em-unix.el (eshell-complete-host-reference): Don't try
to complete arguments containing "$@".

* test/lisp/eshell/esh-var-tets.el (esh-var-test/interp-list-var)
(esh-var-test/interp-list-var-concat, esh-var-test/interp-var-splice)
(esh-var-test/interp-var-splice-concat)
(esh-var-test/quoted-interp-list-var)
(esh-var-test/quoted-interp-list-var-concat)
(esh-var-test/quoted-interp-var-splice)
(esh-var-test/quoted-interp-var-splice-concat): New tests.

* test/lisp/eshell/em-alias-tests.el
(em-alias-test/alias-all-args-var-splice): New test.

* doc/misc/eshell.texi (Dollars Expansion): Explain the splice
operator.
(Aliases): Expand documentation and use '$@*'.
(Built-ins, Bugs and Ideas): Use '$@*' where appropriate.

* etc/NEWS: Announce this change.

1 files changed, 16 insertions, 10 deletions

diff --git a/lisp/eshell/em-cmpl.el b/lisp/eshell/em-cmpl.el
index ac82e3f225c..2c721eb9e31 100644
--- a/lisp/eshell/em-cmpl.el
+++ b/lisp/eshell/em-cmpl.el
@@ -342,17 +342,23 @@ to writing a completion function."
 	(setq pos (1+ pos))))
     (setq posns (cdr posns))
     (cl-assert (= (length args) (length posns)))
-    (let ((a args)
-	  (i 0)
-	  l)
+    (let ((a args) (i 0) new-start)
       (while a
-	(if (and (consp (car a))
-		 (eq (caar a) 'eshell-operator))
-	    (setq l i))
-	(setq a (cdr a) i (1+ i)))
-      (and l
-	   (setq args (nthcdr (1+ l) args)
-		 posns (nthcdr (1+ l) posns))))
+        ;; Remove any top-level `eshell-splice-args' sigils.  These
+        ;; are meant to be rewritten and can't actually be called.
+        (when (and (consp (car a))
+                   (eq (caar a) 'eshell-splice-args))
+          (setcar a (cadar a)))
+        ;; If there's an unreplaced `eshell-operator' sigil, consider
+        ;; the token after it the new start of our arguments.
+        (when (and (consp (car a))
+                   (eq (caar a) 'eshell-operator))
+          (setq new-start i))
+        (setq a (cdr a)
+              i (1+ i)))
+      (when new-start
+	(setq args (nthcdr (1+ new-start) args)
+	      posns (nthcdr (1+ new-start) posns))))
     (cl-assert (= (length args) (length posns)))
     (when (and args (eq (char-syntax (char-before end)) ? )
 	       (not (eq (char-before (1- end)) ?\\)))