bug#43725: 28.0.50; Include feature/native-comp into master

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

Eli Zaretskii <eliz@gnu.org> writes:

> Andrea, can you please explain why there are so many changes in Lisp
> files on the branch?
>
> For example, what are changes like this one about:
>
>   +(declare-function subr-native-lambda-list "data.c")

`subr-native-lambda-list' returns the lambda list for a dynamic native
compiled function or t if lexically scoped.

This is available only building "--with-nativecomp", this is to remove a
bytecompiler warning for vanilla build.  

> Or this:
>
>   +   ((and (featurep 'nativecomp)
>   +         (subrp def)
>   +         (listp (subr-native-lambda-list def)))
>   +    (subr-native-lambda-list def))

This is to retrive the lambda list of dynamic scoped native compiled
lisp function so that `help-function-arglist' can return it.

> Or this:
>
>   +  ;; Never native compile to allow cc-defs.el:2345 hack.
>   +  (declare (speed -1))

cc-defs does a dirty hack where is capturing the bytecode object of
`cc-bytecomp-compiling-or-loading'.

From cc-defs.el:
========
;; Ugly hack to pull in the definition of `cc-bytecomp-compiling-or-loading'
;; from cc-bytecomp to make it available at loadtime.  This is the same
;; mechanism used in cc-mode.el for `c-populate-syntax-table'.
(defalias 'cc-bytecomp-compiling-or-loading
  (cc-eval-when-compile
    (let ((f (symbol-function 'cc-bytecomp-compiling-or-loading)))
      (if (byte-code-function-p f) f (byte-compile f)))))
========

Native code cannot go through the read print loop so this hack can't
work native compiling the function, speed -1 ensure the function is
present in the eln file but in form of bytecode.

> This will not work on MS-Windows, you need to use path-separator to do
> it portably:
>
>   +    (when (featurep 'nativecomp)
>   +      (defvar comp-eln-load-path)
>   +      (let ((path-env (getenv "EMACSNATIVELOADPATH")))
>   +        (when path-env
>   +          (dolist (path (split-string path-env ":")) <<<<<<<<<<<<<
>   +            (unless (string= "" path)
>   +              (push path comp-eln-load-path)))))
>   +      (push (concat user-emacs-directory "eln-cache/") comp-eln-load-path))

Fixed by 31416495ad.

I'll keep on working through your points the coming week, sincere thanks
for reviewing.

  Andrea