last-chance: new utility lib for dangling deterrence

* admin/last-chance.el: New file.

diff --git a/admin/last-chance.el b/admin/last-chance.el
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+++ b/admin/last-chance.el
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+;;; last-chance.el --- dangling deterrence     -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2000-2016 Free Software Foundation, Inc.
+
+;; Author: Thien-Thi Nguyen <ttn@gnu.org>
+;; Maintainer: emacs-devel@gnu.org
+;; Keywords: maint
+;; Package: emacs
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Late 2016.  In a recent build, the date in a ChangeLog file
+;; is not fontified.  Turns out the face ‘change-log-date-face’
+;; was declared obsolete since 22.1 and removed 2016-06-23.
+;; (compile "git show c430f7e23fc2c22f251ace4254e37dea1452dfc3")
+;;
+;; This library provides a command ‘last-chance’, which is a small
+;; combination of "git grep" and some regexp filtering.  For example,
+;; if point is on the symbol ‘change-log-date-face’ in the form:
+;;
+;;  (define-obsolete-face-alias 'change-log-date-face ...)
+;;
+;; then:
+;;
+;;  M-x last-chance RET
+;;
+;; will show you any references to ‘change-log-date-face’ in the
+;; *.el files in a new buffer (in Grep mode).  Hopefully you see
+;; only the obsolete declaration and can proceed w/ its removal.
+;; If not, please DTRT and refrain from the removal until those
+;; references are properly transitioned.
+;;
+;; [Insert "nobody reads ChangeLog files" lament, here.  --ttn]
+
+;;; Code:
+
+(require 'grep)
+(require 'thingatpt)
+
+(defvar last-chance-grep-command "git grep -n -H -F -e"
+  "Command that ends in \"-e\" to do the \"git grep\".
+This should include -n, -H, -F.")
+
+(defvar last-chance-uninteresting-regexps
+  '("ChangeLog[.0-9]*:"
+    ;; Add more ‘flush-lines’ args here.
+    )
+  "List of regexps that match uninteresting \"git grep\" hits.")
+
+(defvar-local last-chance-symbol nil
+  "Symbol set by ‘last-chance’ for ‘last-chance-cleanup’ to DTRT.")
+
+(defun last-chance-cleanup (buffer status)
+  "Filter lines in BUFFER; append STATUS and count of removed lines.
+If BUFFER is not seem to be one created by ‘last-chance’, do nothing.
+This function is intended to be added to ‘compilation-finish-functions’."
+  (let ((name (buffer-local-value 'last-chance-symbol buffer))
+        bef aft)
+    (when name
+      (with-current-buffer buffer
+        (setq bef (count-lines (point-min) (point-max)))
+        (goto-char (point-min))
+        (search-forward last-chance-grep-command)
+        (forward-line 1)
+        (let ((inhibit-read-only t))
+          (dolist (re last-chance-uninteresting-regexps)
+            (flush-lines re))
+          (keep-lines (format "\\_<%s\\_>" name)))
+        (setq aft (count-lines (point-min) (point-max)))
+        (goto-char (point-max))
+        (insert (format "(status: %s, lines removed: %d)"
+                        (car (split-string status "\n"))
+                        (- bef aft)))))))
+
+(defun last-chance (symbol)
+  "Grep the repo for SYMBOL, filtering the hits.
+This uses ‘last-chance-grep-command’ to do the grep and the
+regexps in ‘last-chance-uninteresting-regexps’ to filter the hits.
+Grepping is recursive starting under the dir that ‘vc-root-dir’
+finds (or the default directory if ‘vc-root-dir’ finds nothing).
+Output goes to the *grep* buffer.
+
+Interactively, Emacs queries for a symbol,
+defaulting to the one at point."
+  (interactive (list (read (let ((one (symbol-at-point)))
+                             (when one
+                               (setq one (symbol-name one)))
+                             (completing-read
+                              "Symbol: " obarray
+                              nil nil
+                              one nil one)))))
+  (let ((default-directory (or (vc-root-dir)
+                               default-directory)))
+    (grep (format "%s %s"
+                  last-chance-grep-command
+                  symbol)))
+  (setf (buffer-local-value 'last-chance-symbol
+                            (process-buffer
+                             (car compilation-in-progress)))
+        symbol))
+
+(add-to-list 'compilation-finish-functions
+             'last-chance-cleanup)
+
+(provide 'last-chance)
+
+;;; last-chance.el ends here