(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, 3 insertions, 0 deletions

diff --git a/doc/lispref/nonascii.texi b/doc/lispref/nonascii.texi
index b33082e2b24..1482becb9f5 100644
--- a/doc/lispref/nonascii.texi
+++ b/doc/lispref/nonascii.texi
@@ -1654,6 +1654,9 @@ argument, @var{size}, which tells it how many characters to look at,
 starting from point.  If the function succeeds in determining a coding
 system for the file, it should return that coding system.  Otherwise,
 it should return @code{nil}.
+Each function can also find the name of the file to which
+the buffer's content belong in the variable
+@code{auto-coding-file-name}.
 
 The functions in this list could be called either when the file is
 visited and Emacs wants to decode its contents, and/or when the file's