[elpa] externals/relint 41fad874a3 1/2: Find more bad string escapes: \8, \9, \x without following hex digit

[Mattias Engdegård]

branch: externals/relint
commit 41fad874a3968d5ae36fada9e4e4991f3f6ef39a
Author: Mattias Engdegård <mattiase@acm.org>
Commit: Mattias Engdegård <mattiase@acm.org>

    Find more bad string escapes: \8, \9, \x without following hex digit
    
    "\x" is particularly pernicious since it unexpectedly yields a NUL;
    this bug has been in Emacs for a very long time.
---
 README           | 51 +++++++++++++++++++++++++++++----------------------
 relint.el        | 14 +++++++++-----
 test/13.elisp    |  4 ++++
 test/13.expected |  4 ++++
 4 files changed, 46 insertions(+), 27 deletions(-)

diff --git a/README b/README
index 80500adc87..1db153a450 100644
--- a/README
+++ b/README
@@ -227,28 +227,6 @@ skip-syntax-forward and skip-syntax-backward.
     In general, A?, where A matches the empty string, can be
     simplified to just A.
 
-  - Ineffective string escape '\X'
-
-    A backslash precedes a character that does not need escaping in a
-    string literal (any string, not just regexps), like in "hot\-dog".
-
-    If the backslash should be part of the string then it probably
-    needs to be doubled; otherwise, it is pointless and should be
-    removed to avoid confusion.
-
-    In Emacs versions older than 27.1, a left round or square bracket,
-    '(' or '[', at the very start of a line in a multi-line string
-    could sometimes fool the Emacs-Lisp mode into believing it to be
-    the start of a function, thus people sometimes precede such
-    brackets with an otherwise unnecessary backslash. However, there
-    is usually no reason to put backslashes before brackets in strings
-    in general.
-
-  - Suspect range '+-X' or 'X-+'
-
-    A character range with '+' as one of its endpoints is more often an
-    incorrect attempt to include both '+' and '-' in the set.
-
   - Unnecessarily escaped 'X'
 
     A character is backslash-escaped in a skip set despite not being
@@ -324,6 +302,35 @@ skip-syntax-forward and skip-syntax-backward.
     A string argument to skip-syntax-forward or skip-syntax-backward
     is empty or "^", neither of which makes sense.
 
+  - Ineffective string escape '\X'
+
+    A backslash precedes a character that does not need escaping in a
+    string literal (any string, not just regexps), like in "hot\-dog".
+
+    If the backslash should be part of the string then it probably
+    needs to be doubled; otherwise, it is pointless and should be
+    removed to avoid confusion.
+
+    In Emacs versions older than 27.1, a left round or square bracket,
+    '(' or '[', at the very start of a line in a multi-line string
+    could sometimes fool the Emacs-Lisp mode into believing it to be
+    the start of a function, thus people sometimes precede such
+    brackets with an otherwise unnecessary backslash. However, there
+    is usually no reason to put backslashes before brackets in strings
+    in general.
+
+  - Character escape '\x' not followed by hex digit
+
+    In Emacs versions older than 30.1, a hex escape without any actual
+    hex digits, as in "\x", was silently accepted as a null byte which
+    is not what anyone would expect. If the backslash should be
+    included in the string, double it as usual.
+
+  - Suspect range '+-X' or 'X-+'
+
+    A character range with '+' as one of its endpoints is more often an
+    incorrect attempt to include both '+' and '-' in the set.
+
 
 * Suppressing diagnostics
 
diff --git a/relint.el b/relint.el
index fe27c17ba8..51a02aa6ef 100644
--- a/relint.el
+++ b/relint.el
@@ -2303,9 +2303,12 @@ STRING-START is the start of the string literal (first 
double quote)."
                 (and (memq c '(?\( ?\) ?\[ ?\] ?\'))
                      (relint--in-doc-string-p string-start)))
       (relint--warn (point) nil
-                    (format-message
-                     "Ineffective string escape `\\%s'"
-                     (relint--escape-string (char-to-string c) nil))))))
+                    (if (eq c ?x)
+                        (format-message
+                         "Character escape `\\x' not followed by hex digit")
+                      (format-message
+                       "Ineffective string escape `\\%s'"
+                       (relint--escape-string (char-to-string c) nil)))))))
 
 (defun relint--check-for-misplaced-backslashes ()
   "Check for misplaced backslashes in the current buffer."
@@ -2322,10 +2325,11 @@ STRING-START is the start of the string literal (first 
double quote)."
         (forward-char)
         (while (not (memq (char-after) '(?\" nil)))
           (when (looking-at
-                 (rx (1+ (or (seq ?\\ (any "0-9" "xuUN" "abfnrtv"
+                 (rx (1+ (or (seq ?\\ (or (any "0-7" "uUN" "abfnrtv"
                                            "des" "^" " "
                                            ?\\ ?\n ?\"
-                                           "CM"))
+                                           "CM")
+                                          (seq "x" xdigit)))
                              (not (any ?\\ ?\"))))))
             (goto-char (match-end 0)))
           (when (eq (following-char) ?\\)
diff --git a/test/13.elisp b/test/13.elisp
index 1f8cb5826f..d07f72fbd2 100644
--- a/test/13.elisp
+++ b/test/13.elisp
@@ -17,3 +17,7 @@ and \a \b \f \n \r \t \v \d \e \s \^P"))
 (defun f2 ()
   (re-search-forward "\$\.x\+\+")
   '("bracketed \q string"))
+
+(defun f3 ()
+  (print "a \7 \8 \9 b")
+  (print "c \xf \xg \x d"))
diff --git a/test/13.expected b/test/13.expected
index 53a4568aab..1bf32daa53 100644
--- a/test/13.expected
+++ b/test/13.expected
@@ -26,3 +26,7 @@
    ....^
 13.elisp:18:30: Ineffective string escape `\+'
 13.elisp:19:16: Ineffective string escape `\q'
+13.elisp:22:16: Ineffective string escape `\8'
+13.elisp:22:19: Ineffective string escape `\9'
+13.elisp:23:17: Character escape `\x' not followed by hex digit
+13.elisp:23:21: Character escape `\x' not followed by hex digit