bug#47488: Treatment of # in js.el

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Wed, 12 May 2021 15:32:01 +0200
> Cc: 47488@debbugs.gnu.org, Reuben Thomas <rrt@sc3d.org>
> 
> I've never heard of the #file directive, but:
> 
> ;; `cpp-font-lock-keywords-source-directives' is calculated from:
> ;;
> ;;        (regexp-opt
> ;;         '("define"  "elif" "else" "endif" "error" "file" "if" "ifdef"
> ;;           "ifndef" "import" "include" "line" "pragma" "undef" "warning"))
> 
> I spent 30 seconds googling, and I could find no list of directive that
> includes #file, so...  Bug #warning is used, and so is #line, so perhaps
> it's an old thing?

The #file directive is emitted by the preprocessor, so that later
compiling the preprocessed source would yield the correct source line
numbers.  Try preprocessing some .c file with "gcc -E" and you will
see these directives in the output.