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.

2 files changed, 83 insertions, 0 deletions

diff --git a/test/lisp/eshell/em-alias-tests.el b/test/lisp/eshell/em-alias-tests.el
index aca622220e3..0a26e8d2011 100644
--- a/test/lisp/eshell/em-alias-tests.el
+++ b/test/lisp/eshell/em-alias-tests.el
@@ -72,6 +72,15 @@
    (eshell-match-command-output "show-all-args a" "a\n")
    (eshell-match-command-output "show-all-args a b c" "a\nb\nc\n")))
 
+(ert-deftest em-alias-test/alias-all-args-var-splice ()
+  "Test alias with splicing the $* variable"
+  (with-temp-eshell
+   (eshell-insert-command "alias show-all-args 'echo args: $@*'")
+   (eshell-match-command-output "show-all-args" "args:\n")
+   (eshell-match-command-output "show-all-args a" "(\"args:\" \"a\")\n")
+   (eshell-match-command-output "show-all-args a b c"
+                                "(\"args:\" \"a\" \"b\" \"c\")\n")))
+
 (ert-deftest em-alias-test/alias-all-args-var-indices ()
   "Test alias with the $* variable using indices"
   (with-temp-eshell
diff --git a/test/lisp/eshell/esh-var-tests.el b/test/lisp/eshell/esh-var-tests.el
index 96fde026a54..d95669fdaf8 100644
--- a/test/lisp/eshell/esh-var-tests.el
+++ b/test/lisp/eshell/esh-var-tests.el
@@ -60,6 +60,18 @@
   (eshell-command-result-equal "echo $\"user-login-name\"-foo"
                                (concat user-login-name "-foo")))
 
+(ert-deftest esh-var-test/interp-list-var ()
+  "Interpolate list variable"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo $eshell-test-value"
+                                 '(1 2 3))))
+
+(ert-deftest esh-var-test/interp-list-var-concat ()
+  "Interpolate and concat list variable"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo a$'eshell-test-value'z"
+                                 '("a1" 2 "3z"))))
+
 (ert-deftest esh-var-test/interp-var-indices ()
   "Interpolate list variable with indices"
   (let ((eshell-test-value '("zero" "one" "two" "three" "four")))
@@ -131,6 +143,26 @@
     (eshell-command-result-equal "echo $#eshell-test-value" 1)
     (eshell-command-result-equal "echo $#eshell-test-value[foo]" 3)))
 
+(ert-deftest esh-var-test/interp-var-splice ()
+  "Splice-interpolate list variable"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo a $@eshell-test-value z"
+                                 '("a" 1 2 3 "z"))))
+
+(ert-deftest esh-var-test/interp-var-splice-concat ()
+  "Splice-interpolate and concat list variable"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo it is a$@'eshell-test-value'z"
+                                 '("it" "is" "a1" 2 "3z"))
+    ;; This is a tricky case.  We're concatenating a spliced list and
+    ;; a non-spliced list.  The general rule is that splicing should
+    ;; work as though the user typed "$X[0] $X[1] ... $X[N]".  That
+    ;; means that the last value of our splice should get concatenated
+    ;; into the first value of the non-spliced list.
+    (eshell-command-result-equal
+     "echo it is $@'eshell-test-value'$eshell-test-value"
+     '("it" "is" 1 2 (31 2 3)))))
+
 (ert-deftest esh-var-test/interp-lisp ()
   "Interpolate Lisp form evaluation"
   (eshell-command-result-equal "+ $(+ 1 2) 3" 6))
@@ -197,6 +229,9 @@
    (eshell-match-command-output "echo ${echo hi}-${*echo there}"
                                 "hi-there\n")))
 
+
+;; Quoted variable interpolation
+
 (ert-deftest esh-var-test/quoted-interp-var ()
   "Interpolate variable inside double-quotes"
   (eshell-command-result-equal "echo \"$user-login-name\""
@@ -209,6 +244,18 @@
   (eshell-command-result-equal "echo \"hi, $\\\"user-login-name\\\"\""
                                (concat "hi, " user-login-name)))
 
+(ert-deftest esh-var-test/quoted-interp-list-var ()
+  "Interpolate list variable inside double-quotes"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo \"$eshell-test-value\""
+                                 "(1 2 3)")))
+
+(ert-deftest esh-var-test/quoted-interp-list-var-concat ()
+  "Interpolate and concat list variable inside double-quotes"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo \"a$'eshell-test-value'z\""
+                                 "a(1 2 3)z")))
+
 (ert-deftest esh-var-test/quoted-interp-var-indices ()
   "Interpolate string variable with indices inside double-quotes"
   (let ((eshell-test-value '("zero" "one" "two" "three" "four")))
@@ -291,6 +338,18 @@ inside double-quotes"
     (eshell-command-result-equal "echo \"$#eshell-test-value[foo]\""
                                  "3")))
 
+(ert-deftest esh-var-test/quoted-interp-var-splice ()
+  "Splice-interpolate list variable inside double-quotes"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo a \"$@eshell-test-value\" z"
+                                 '("a" "1 2 3" "z"))))
+
+(ert-deftest esh-var-test/quoted-interp-var-splice-concat ()
+  "Splice-interpolate and concat list variable inside double-quotes"
+  (let ((eshell-test-value '(1 2 3)))
+    (eshell-command-result-equal "echo \"a$@'eshell-test-value'z\""
+                                 "a1 2 3z")))
+
 (ert-deftest esh-var-test/quoted-interp-lisp ()
   "Interpolate Lisp form evaluation inside double-quotes"
   (eshell-command-result-equal "echo \"hi $(concat \\\"the\\\" \\\"re\\\")\""
@@ -325,6 +384,21 @@ inside double-quotes"
                                "foo\nbar baz"))
 
 
+;; Interpolating commands
+
+(ert-deftest esh-var-test/command-interp ()
+  "Interpolate a variable as a command name"
+  (let ((eshell-test-value "printnl"))
+    (eshell-command-result-equal "$eshell-test-value hello there"
+                                 "hello\nthere\n")))
+
+(ert-deftest esh-var-test/command-interp-splice ()
+  "Interpolate a splice variable as a command name with arguments"
+  (let ((eshell-test-value '("printnl" "hello" "there")))
+    (eshell-command-result-equal "$@eshell-test-value"
+                                 "hello\nthere\n")))
+
+
 ;; Interpolated variable conversion
 
 (ert-deftest esh-var-test/interp-convert-var-number ()