* lisp/gnus/mm-encode.el (mm-default-buffer-type): New defun.

* lisp/gnus/mml.el (mml-attach-buffer): Use it.

* lisp/gnus/gnus-agent.el (gnus-agent-load-local)
(gnus-agent-expire-group-1, gnus-agent-update-files-total-fetched-for):
* lisp/gnus/gnus-art.el (gnus-article-header-rank):
* lisp/gnus/gnus-async.el (gnus-async-wait-for-article):
* lisp/gnus/gnus-cache.el (gnus-cache-update-file-total-fetched-for):
* lisp/gnus/gnus-registry.el (gnus-registry-import-eld):
* lisp/gnus/gnus-salt.el (gnus-pick-line-number, gnus-tree-minimize)
(gnus-generate-vertical-tree):
* lisp/gnus/gnus-spec.el (gnus-correct-substring):
* lisp/gnus/gnus-start.el (gnus-ask-server-for-new-groups, gnus-method-rank):
* lisp/gnus/gnus-sum.el (gnus-data-update-list)
(gnus-summary-number-of-articles-in-thread, gnus-build-sparse-threads)
(gnus-summary-prepare-threads, gnus-invisible-cut-children)
(gnus-summary-limit-children, gnus-expunge-thread)
(gnus-summary-refer-references, gnus-summary-update-mark)
(gnus-summary-hide-all-threads, gnus-summary-save-parts-1):
* lisp/gnus/gnus-topic.el (gnus-topic-articles-in-topic)
(gnus-topic-prepare-topic, gnus-topic-display-missing-topic)
(gnus-topic-update-topic-line):
* lisp/gnus/gnus-uu.el (gnus-uu-post-encoded):
* lisp/gnus/gnus-win.el (gnus-configure-frame):
* lisp/gnus/mail-source.el (mail-source-fetch-directory)
(mail-source-fetch-maildir, mail-source-fetch-imap):
* lisp/gnus/message.el (message-remove-header)
(message-remove-first-header, message-make-caesar-translation-table):
* lisp/gnus/mm-encode.el (mm-qp-or-base64):
* lisp/gnus/mml.el (mml-compute-boundary, mml-compute-boundary-1):
* lisp/gnus/nnatom.el (nnatom--read-links):
* lisp/gnus/nnbabyl.el (nnbabyl-retrieve-headers):
* lisp/gnus/nndoc.el (nndoc-oe-dbx-dissection, nndoc-dissect-buffer)
(nndoc-dissect-mime-parts-sub):
* lisp/gnus/nneething.el (nneething-retrieve-headers):
* lisp/gnus/nnheader.el (nnheader-generate-fake-message-id)
(nnheader-insert-head, nnheader-translate-file-chars):
* lisp/gnus/nnimap.el (nnimap-find-wanted-parts-1)
(nnimap-retrieve-group-data-early, nnimap-send-command):
* lisp/gnus/nnmail.el (nnmail-process-babyl-mail-format)
(nnmail-process-unix-mail-format, nnmail-process-mmdf-mail-format)
(nnmail-get-new-mail-1):
* lisp/gnus/nnmaildir.el (nnmaildir--scan):
* lisp/gnus/nnrss.el (nnrss-check-group):
* lisp/gnus/nnselect.el (nnselect-request-thread):
* lisp/gnus/nnspool.el (nnspool-retrieve-headers):
* lisp/gnus/nntp.el (nntp-retrieve-headers)
(nntp-finish-retrieve-group-infos, nntp-retrieve-groups)
(nntp-retrieve-articles, nntp-retrieve-headers-with-xover):
* lisp/gnus/nnvirtual.el (nnvirtual-create-mapping):
* lisp/gnus/nnweb.el (nnweb-google-parse-1, nnweb-google-create-mapping)
(nnweb-gmane-create-mapping):
* lisp/gnus/spam.el (spam-resolve-registrations-routine): Prefer incf to
cl-incf.

Run "TZ=UTC0 admin/update-copyright".

* lisp/gnus/gnus-group.el (features): Use `dlet`.
(gnus-tmp-level, gnus-tmp-marked, gnus-tmp-group): Declare vars.
(gnus-group-insert-group-line): Bind dynbound vars via `let` rather
than as formal args.  Bind `number` as dynbound.
(gnus-visual, gnus-score-find-score-files-function)
(gnus-home-score-file, gnus-apply-kill-hook)
(gnus-summary-expunge-below): Declare vars.
(gnus-group-restart, gnus-group-list-plus): Fix `interactive` spec
since the arg is unused.

* lisp/gnus/mail-source.el (mail-source-bind, mail-source-bind-common):
Use `dlet` and suppress the warnings about the non-prefixed dynbound vars.
(mail-source-set-1): Remove unused var `auth-info`.
(mail-source-call-script): Remove unused var `background`.
(mail-source-fetch-pop, mail-source-check-pop): Bind pop3 vars with `dlet`.

* lisp/gnus/gnus-int.el (mail-source-plugged, gnus-inhibit-demon):
Declare vars.
(gnus-server-opened, gnus-status-message)
(gnus-open-server, gnus-close-server, gnus-request-list)
(gnus-finish-retrieve-group-infos, gnus-retrieve-group-data-early)
(gnus-request-list-newsgroups, gnus-request-newgroups)
(gnus-request-regenerate, gnus-request-compact, gnus-request-group)
(gnus-retrieve-groups, gnus-request-post, gnus-request-expunge-group)
(gnus-request-scan, gnus-request-update-info, gnus-request-marks)
(gnus-request-accept-article, gnus-request-create-group)
(gnus-asynchronous-p, gnus-remove-denial):
Bind `gnus-command-method` via `let` rather than as formal args.

* lisp/gnus/gnus-topic.el (gnus-topic-insert-topic-line):
Pass documented vars to eval for `gnus-topic-line-format-spec`.

* lisp/gnus/message.el (message-yank-original): Use `cl-progv` rather
than `eval` to bind the vars from `message-cite-style`.

* lisp/gnus/mml.el (mml-parse-1): Use `apply` instead of `eval`.
(gnus-newsgroup-name, gnus-displaying-mime, gnus-newsgroup-name)
(gnus-article-prepare-hook, gnus-newsgroup-charset)
(gnus-original-article-buffer, gnus-message-buffer)
(message-this-is-news, message-this-is-mail): Declare vars.

* lisp/gnus/deuglify.el (gnus-outlook-rearrange-article): Remove unused
var `cite-marks`.
* lisp/gnus/gnus-art.el (ansi-color-context-region): Declare var.
(gnus-mime-display-attachment-buttons-in-header): Move declaration
before first use.
(gnus-mime-display-alternative): Remove unused var `from`.
* lisp/gnus/gnus-bookmark.el (gnus-bookmark-bmenu-list): Remove unused
var `start` `end`.
* lisp/gnus/gnus-cache.el (gnus-article-decode-hook)
(nnml-generate-active-function): Declare var.
* lisp/gnus/gnus-cite.el (gnus-message-citation-mode): Remove unused
var `keywords`.
* lisp/gnus/gnus-cloud.el (gnus-cloud-encode-data): Remove unused var
`cipher`.
(gnus-cloud-ensure-cloud-group): Remove unused var `method`.
* lisp/gnus/gnus-delay.el (gnus-delay-article): Remove unused var `days`.
* lisp/gnus/gnus-html.el (gnus-html-wash-images): Remove unused vars
`tag`, `string`, and `images`.
(gnus-html-wash-tags): Remove unused vars `string` and `images`.
* lisp/gnus/gnus-msg.el (gnus-msg-mail): Remove unused var `group-name`.
(gnus-group-mail, gnus-group-news, gnus-summary-mail-other-window)
(gnus-summary-news-other-window):
Remove unused vars `group` and `buffer`.
(gnus-configure-posting-styles): Remove unused vars `style` and `attribute`.
* lisp/gnus/gnus-picon.el (gnus-picon-find-face): Remove unused vars
`database`, `directory`, and `instance`.
(gnus-picon-transform-newsgroups): Remove unused var `point`.
* lisp/gnus/gnus-range.el (gnus-range-difference): Remove unused var `safe`.
* lisp/gnus/gnus-score.el (gnus-score-load-file): Remove unused var
`score-fn`.
* lisp/gnus/gnus-sum.el (message-options-set-recipient): Declare var.
* lisp/gnus/gnus-undo.el (gnus-undo): Fix docstring lie.
* lisp/gnus/gnus-util.el (print-string-length)
(iswitchb-make-buflist-hook): Declare vars.
(gnus-emacs-version): Remove unused var `codename`.
(gnus-rename-file): Remove unused vars `old-name` and `new-name`.
* lisp/gnus/gnus-uu.el (gnus-uu-yenc-article): Remove unused var
`start-char`.
(gnus-asynchronous): Declare var.
* lisp/gnus/mm-partial.el (gnus-displaying-mime): Declare var.
(mm-inline-partial): Remove unused var `buffer`.
* lisp/gnus/mm-view.el (w3m-force-redisplay, w3m-safe-url-regexp)
(gnus-displaying-mime, gnus-original-article-buffer)
(gnus-article-prepare-hook): Declare vars.
* lisp/gnus/mml-smime.el (mml-smime-epg-encrypt): Remove unused var
`boundary`.
(mml-smime-epg-verify): Remove unused vars `plain` and `signature-file`.
* lisp/gnus/mml1991.el (pgg-text-mode): Declare var.
* lisp/gnus/mml2015.el (pgg-text-mode): Declare var.
(mml2015-pgg-decrypt): Remove unused var `result`.
(mml2015-epg-key-image-to-string): Remove unused var `error`.
(mml2015-epg-decrypt): Remove unused var `result`.
(mml2015-epg-verify): Remove unused vars `plain` and `signature-file`.
* lisp/gnus/nnbabyl.el (nnml-current-directory): Declare var.
* lisp/gnus/nndiary.el (nndiary-files): Move declaration before first use.
* lisp/gnus/nnfolder.el (nnfolder-request-accept-article):
Remove unused var `buf`.
* lisp/gnus/nnmail.el (nnmail-parse-active): Remove unused var `err`.
* lisp/gnus/nnmairix.el (nnmairix-request-group): Remove unused var `args`.
(nnmairix-request-create-group): Remove unused var `info`.
(nnmairix-request-list): Remove unused var `folder`.
(nnmairix-request-set-mark): Remove unused var `propto`.
(nnmairix-request-set-mark): Remove unused vars `number` and `method`.
(nnmairix-close-group): Remove unused var `method`.
(nnmairix-create-search-group-from-message): Remove unused var `cq`.
(nnmairix-create-server-and-default-group): Remove unused var `create`.
(nnmairix-purge-old-groups): Remove unused var `folder`.
(nnmairix-remove-tick-mark-original-article, nnmairix-get-valid-servers):
Remove unused var `cur`.
(nnmairix-replace-group-and-numbers): Remove unused var `header`.
(nnmairix-goto-original-article): Remove unused var `rval`.
(nnmairix-widget-create-query): Remove unused var `allwidgets`.
* lisp/gnus/nnmbox.el (nnml-current-directory): Declare var.
* lisp/gnus/nnmh.el (nnmh-toplev): Move declaration before first use.
(nnmh-request-list-1): Remove unused var `rdir`.
* lisp/gnus/nnml.el (nnml-generate-nov-file): Remove unused var `file`.
* lisp/gnus/nnrss.el (nnrss-request-article): Remove unused var `post`.
(nnrss-request-article): Remove unused var `fn`.
(nnrss-check-group): Remove unused var `rdf-ns`.
* lisp/gnus/nnweb.el (nnweb-request-article): Remove unused var `active`.
(nnweb-google-parse-1): Remove unused var `Score`.
* lisp/gnus/spam-stat.el (spam-stat-error-holder): Remove var.
(spam-stat-buffer-words-with-scores): Remove unused var `word`.
(spam-stat-score-buffer): Remove unused var `spam-stat-error-holder`.
(spam-stat-split-fancy): Use `err` instead of `spam-stat-error-holder`.
* lisp/gnus/spam-wash.el (spam-wash): Remove unused var `handle`.
* lisp/gnus/spam.el (spam-copy-or-move-routine): Remove unused vars
`article` and `mark`.
(spam-register-routine): Remove unused var `article`.
(spam-log-undo-registration): Remove unused var `found`.
(spam-ifile-register-with-ifile): Remove unused var `parameters`.
(spam-check-stat): Remove unused vars `category` and `return`.
(spam-parse-list): Remove unused var `found`.
(spam-filelist-register-routine): Remove unused var `from`.