LaTeX-mode does not automatically initialize when opening a .tex file

[variemai]

I confirm that...

• I have searched the issue tracker, documentation, FAQ, Discourse, and Google, in case this issue has already been reported/resolved.

• I have read "How to Debug Issues", and will use it to provide as much information about this issue as possible.

• The issue can be reproduced on the latest available commit of Doom.

• The issue can be reproduced on a stable release of Emacs, such as 27, 28, or 29. (Unstable versions end in .50, .60, or .9x)

Expected behavior

Opening a .tex file should initialize the AUCTEX major mode LaTeX-mode.

Current behavior

Opening a .tex file DOES NOT initialize the LaTeX-mode instead it defaults to the latex-mode.

This behavior can be fixed by explicitly setting in the doom config file:

(add-to-list 'auto-mode-alist '("\\.tex\\'" . LaTeX-mode))

However, it used to work without explicitly setting it.

Steps to reproduce

1. Run Emacs
2. Open a .tex file

System Information

https://pastebin.com/98CdELJv

[mwoitek]

I agree that this behavior is a little inconvenient. But this isn't a bug. I had this problem when working on a multi-file project. Some of my files contained mostly text and almost no LaTeX. In those cases, LaTeX-mode wasn't enabled automatically. So I looked into this.

It turns out that, by default, AUCTeX inspects your TeX files to decide which major mode should be enabled. For example, if your file contains \documentclass then LaTeX-mode is selected. If you don't want the mode to be inferred, you can set a default and then force AUCTeX to use it:

(setq TeX-default-mode #'LaTeX-mode ;; should already be the default mode
      TeX-force-default-mode t)

[lizhuohua]

I recently upgraded Doom emacs and encountered the same issue.

I tried the suggested two workarounds, but neither worked, and I am sure that my tex file contains \documentclass.

[mwoitek]

@lizhuohua It's hard to tell what's happening without examining your config carefully. Try adding the following comment to the very top of your files: % -*- mode: LaTeX; -*-. This should not fail. If it does, then the problem probably originates in your config. Anyway, I don't believe this is Doom's fault.

[lizhuohua]

@mwoitek Thank you. After some investigation, I found that this bug was introduced in commit da08aa2 and should only occur in Emacs version < 30.

This commit defines a new variable major-mode-remap-defaults introduced in Emacs 30, which makes AUCTeX mistakenly assume the current Emacs version is 30, and uses major-mode-remap-defaults for major mode redirection:

https://github.com/emacs-straight/auctex/blob/fc6ac87e5ed027e86b129a49d722897d127bbc50/tex-site.el#L127-L132

However, for Emacs version < 30, only major-mode-remap-alist is recognized, so my workaround is to add the following to my config:

(setq major-mode-remap-alist major-mode-remap-defaults)