bug#74879: 30.0.92; trusted-content-p and trusted-files cannot be used for non-file buffers

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Thank you for the recent addition of `trusted-content-p'. Is there a
> possibility to use `trusted-content-p' in buffers which are not backed
> by a file? I use Flymake in *scratch* or similar buffers and it seems
> that this won't continue to work given that `trusted-content-p' needs a
> `buffer-file-truename'.

Good question.
We don't really have a good answer yet, AFAIK, in large part because we
don't have enough experience with it.
Off the top of my head, here are some elements relevant to this
discussion, in random order:

- The current setup is a kind of "minimal" change for Emacs-30 because
  it's late in the pretest, so as much as possible we should separate
  the discussion into what's a simple enough solution for Emacs-30 and
  what we should use in the longer term.

- You should be able to get fully-featured Flymake in *scratch*
  with (setq-local trusted-files :all).
  Maybe we should do that when we setup *scratch*?
  Which other non-file buffers would need that?  The minibuffer?

- Trust sucks, so we really should work on better solutions where we
  don't need to rely on trust, such as running code in `bwrap` or other
  kinds of sandboxes.

- I think we do want some kind of hook, with which we can have (for
  instance) `emacs-lisp-mode` tell Emacs to trust the user init file,
  the early-init file, the custom-file, and all the files in
  `load-path`.

- There is overlap with `safe-local-variable-directories`,
  `enable-local-variables` and it would be nice to consolidate (which
  can require delicate timing if we want the major mode to inform which
  content to trust).


- Stefan