(find-auto-coding): Provide filename to `auto-coding-functions`

Allow `auto-coding-functions` to know the file name.
Motivated by the needs of Editorconfig support.

* lisp/international/mule.el (auto-coding-file-name): New var.
(find-auto-coding): Let-bind it for `auto-coding-functions`.
Document the expectation that the arg be an absolute file name.

* doc/lispref/nonascii.texi (Default Coding Systems):
Mention `auto-coding-file-name`.

* test/lisp/international/mule-util-resources/test.utf-16le: New file.
* test/lisp/international/mule-tests.el (mule-tests--dir): New var.
(mule-tests--auto-coding): New fun.
(mule-tests--auto-coding-functions): New test.

1 files changed, 5 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index a7d17862242..779ff99d442 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2187,6 +2187,11 @@ completion candidate.
 * Lisp Changes in Emacs 30.1
 
 +++
+** 'auto-coding-functions' can know the name of the file.
+The functions on this hook can now find the name of the file to
+which the text belongs by consulting the variable 'auto-coding-file-name'.
+
++++
 ** New user option 'compilation-safety' to control safety of native code.
 It's now possible to control how safe is the code generated by native
 compilation, by customizing this user option.  It is also possible to