bug#59960: 30.0.50; Eshell's $* variable (used to define aliases) doesn't work as expected

[Jim Porter]

This bug report originally comes from a Reddit post[1] where a user 
found that aliasing the Eshell built-in "mv" and "cp" commands didn't 
work properly. I'll show a slightly simpler example though. Starting 
from "emacs -Q -f eshell":

  ~ $ alias my-echo "echo A $* Z"
  ~ $ my-echo b c d
  ("A"
   ("b" "c" "d")
   "Z")

  ~ $ echo A b c d Z
  ("A" "b" "c" "d" "Z")

As you can see above, the "$*" special variable inserts the list of 
arguments passed to the alias *as a sublist*. That might be what you 
want sometimes, but more likely, you'd want to insert the arguments 
in-place, like with ",@" in Emacs Lisp backquote forms. (Note: for 
external processes, this isn't an issue because Eshell flattens all the 
arguments to external commands.)

Attached is a draft patch to fix this. It adds a new "$@expr" variable 
expansion to Eshell which lets you splice a list in-place:

  ~ $ alias my-echo 'echo A $@* Z'
  ~ $ my-echo b c d
  ("A" "b" "c" "d" "Z")

Currently, this only works in simple situations like the above. It 
should work for any variable, but likely won't work with Eshell for 
loops, inside double quotes, etc. It would be nice to get this working 
in a wider variety of places in Eshell command forms before merging. 
Still, even what's here is already pretty useful, I think. I'm posting 
this now mainly to get feedback on whether this is the right direction 
to go in the first place.

[1] 
https://old.reddit.com/r/emacs/comments/xs2ofo/eshell_aliases_for_mv_and_cp_using_their_elisp/

0001-Support-completion-of-variables-with-the-length-oper.patch
Description: Text document

0002-Add-support-for-the-splice-operator-in-Eshell.patch
Description: Text document