bug#49624: compilation message end-column function off-by-one bug

["Mattias Engdegård" <mattiase@acm.org>]

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Compilation message patterns (in compilation-regexp-alist and -alist) can indicate starting and ending line and column numbers, either by supplying regexp match numbers or functions that return the respective line/column numbers when called. In other words, the integer N can be understood as a shorthand for the function

(lambda () (and (match-beginning N) (string-to-number (match-string N))))

except that isn't true for the ending column where there is a difference of 1; an END-COL function returning 13 means that the error's last column is 12.

There does not seem to be a good justification for this nor is it documented so it's probably just a bug; proposed patch attached below. Does it warrant a mention in etc/NEWS?


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From a2303f4651e225115207bad778024f913dc6dacb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <mattiase@acm.org>
Date: Sun, 18 Jul 2021 20:32:49 +0200
Subject: [PATCH] Fix off-by-one error in compilation rule end-column function

* lisp/progmodes/compile.el (compilation-error-properties):
When the end-column parameter of a compilation message rule is
a function, treat its return value the same way as textually
supplied values by adjusting both values in the same way.
---
 lisp/progmodes/compile.el            | 13 ++++++++-----
 test/lisp/progmodes/compile-tests.el | 27 +++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/lisp/progmodes/compile.el b/lisp/progmodes/compile.el
index e4363e11b8..02d1c58858 100644
--- a/lisp/progmodes/compile.el
+++ b/lisp/progmodes/compile.el
@@ -1248,11 +1248,14 @@ compilation-error-properties
                  (setq col (match-string-no-properties col))
                  (string-to-number col))))
     (setq end-col
-          (or (if (functionp end-col) (funcall end-col)
-                (and end-col
-                     (setq end-col (match-string-no-properties end-col))
-                     (- (string-to-number end-col) -1)))
-              (and end-line -1)))
+          (let ((ec (if (functionp end-col)
+                        (funcall end-col)
+                      (and end-col (match-beginning end-col)
+                           (string-to-number
+                            (match-string-no-properties end-col))))))
+            (if ec
+                (1+ ec)     ; Add one to get an exclusive upper bound.
+              (and end-line -1))))
     (if (consp type)            ; not a static type, check what it is.
 	(setq type (or (and (car type) (match-end (car type)) 1)
 		       (and (cdr type) (match-end (cdr type)) 0)
diff --git a/test/lisp/progmodes/compile-tests.el b/test/lisp/progmodes/compile-tests.el
index 0623cec528..2a3bb3dafa 100644
--- a/test/lisp/progmodes/compile-tests.el
+++ b/test/lisp/progmodes/compile-tests.el
@@ -515,4 +515,31 @@ compile-test-grep-regexps
       (compile--test-error-line testcase))
     (should (eq compilation-num-errors-found 8))))
 
+(ert-deftest compile-test-functions ()
+  "Test rules using functions instead of regexp group numbers."
+  (let* ((file-fun (lambda () '("my-file")))
+         (line-start-fun (lambda () 123))
+         (line-end-fun (lambda () 134))
+         (col-start-fun (lambda () 39))
+         (col-end-fun (lambda () 24))
+         (compilation-error-regexp-alist-alist
+         `((my-rule
+            ,(rx bol "My error message")
+            ,file-fun
+            (,line-start-fun . ,line-end-fun)
+            (,col-start-fun . ,col-end-fun))))
+         (compilation-error-regexp-alist '(my-rule)))
+  (with-temp-buffer
+    (font-lock-mode -1)
+    (let ((compilation-num-errors-found 0)
+          (compilation-num-warnings-found 0)
+          (compilation-num-infos-found 0))
+      (compile--test-error-line
+       '(my-rule
+         "My error message"
+         1 (39 . 24) (123 . 134) "my-file" 2))
+      (should (eq compilation-num-errors-found 1))
+      (should (eq compilation-num-warnings-found 0))
+      (should (eq compilation-num-infos-found 0))))))
+
 ;;; compile-tests.el ends here
-- 
2.21.1 (Apple Git-122.3)