When I’m looking at a diff in Emacs’s diff-mode, I usually use native functions like diff-apply-hunk to apply/revert or otherwise operate on hunks. Occasionally, though, I just want to kill (copy) part of the diff so I can do something else with it. When I do, I end up with leading - or + or ! characters that I have to strip manually. For example:

 @ -438,7 +438,7 @@ cc-other-file-alist. (ff-find-other-file evidently handles symbols specially.)"
 (setenv "ANDROID_HOME" "/Users/ryan/android-sdk")
-;; (setenv "ANDROID_SERIAL" "192.168.56.101:5555")
+(setenv "PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION" "cpp")
+(setenv "NDK_ABI" "armeabi-v7a")
+;; (setenv "NDK_ABI" "armeabi-v7a")

Rectangle operations help, but ideally I wouldn’t have to deal with this at all…and now I don’t! Here’s a piece of advice that automatically strips those characters when I kill inside a diff buffer:

(defadvice kill-new (before strip-leading-diff-chars activate)
  "When copying from a diff buffer, strip the leading -, +, ! characters."
  (if (eq major-mode 'diff-mode)
      (ad-set-arg 0 (replace-regexp-in-string "^." "" (ad-get-arg 0)))))