bug#50690: Avoid double argument evaluation in vc-call

[Basil L. Contovounesios]

Severity: wishlist
Tags: patch

Is there any reason why the macro vc-call should continue to evaluate
its second argument twice?  Both 'make bootstrap' and 'make check' pass
with the attached patch:

>From 5cdd24e4eb3e1e79857d76c2b780fe7f245ad9f6 Mon Sep 17 00:00:00 2001
From: "Basil L. Contovounesios" <contovob@tcd.ie>
Date: Sun, 19 Sep 2021 22:39:20 +0100
Subject: [PATCH 2/2] * lisp/vc/vc-hooks.el (vc-call): Eval arg once.

---
 lisp/vc/vc-hooks.el | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lisp/vc/vc-hooks.el b/lisp/vc/vc-hooks.el
index 0612310640..b7760e3bba 100644
--- a/lisp/vc/vc-hooks.el
+++ b/lisp/vc/vc-hooks.el
@@ -256,9 +256,9 @@ vc-call-backend
 (defmacro vc-call (fun file &rest args)
   "A convenience macro for calling VC backend functions.
 Functions called by this macro must accept FILE as the first argument.
-ARGS specifies any additional arguments.  FUN should be unquoted.
-BEWARE!! FILE is evaluated twice!!"
-  `(vc-call-backend (vc-backend ,file) ',fun ,file ,@args))
+ARGS specifies any additional arguments.  FUN should be unquoted."
+  (macroexp-let2 nil file file
+    `(vc-call-backend (vc-backend ,file) ',fun ,file ,@args)))
 
 (defsubst vc-parse-buffer (pattern i)
   "Find PATTERN in the current buffer and return its Ith submatch."
-- 
2.33.0

Thanks,

-- 
Basil

In GNU Emacs 28.0.50 (build 2, x86_64-pc-linux-gnu, X toolkit, cairo version 
1.16.0, Xaw3d scroll bars)
 of 2021-09-20 built on tia
Repository revision: 3c513435369826e44a53dcb12ecc14657e03c73d
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-Og -ggdb' --config-cache
 --prefix=/home/blc/.local --enable-checking=structs
 --with-x-toolkit=lucid --with-file-notification=yes --with-x'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
X11 XAW3D XDBE XIM XPM LUCID ZLIB

Important settings:
  value of $LANG: en_IE.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix