Custom whitespace-line-column for specific modes in whitespace-mode

Question

I want to force whitespace-mode highlight line after 99 character in rust-mode (cuz this length recommended by guidelines). For other modes i want to leave this variable with default value(80). How can i do this?

Answer 1

As erikstokes correctly points out, the variable that controls this behaviour is whitespace-line-column. If you unconditionally set this variable in your user-init-file, however, it will affect all modes.

The standard mechanism that Emacs provides for mode-specific customisations is hooks. So, to modify the variable in question for rust-mode only, you can write something like the following:

(add-hook 'rust-mode-hook
          (lambda ()
            (setq-local whitespace-line-column 99)))

or, in Emacs versions prior to 24.3:

(add-hook 'rust-mode-hook
          (lambda ()
            (set (make-local-variable 'whitespace-line-column) 99)))

Answer 2

Rather than whitespace-mode, try column-enforce-mode which can be installed from Emacs MELPA. See https://github.com/jordonbiondo/column-enforce-mode for more details.

Answer 3

The variable you want is whitespace-line-column:

Specify column beyond which the line is highlighted.

It must be an integer or nil. If nil, the ‘fill-column’ variable value is used.

Used when ‘whitespace-style’ includes ‘lines’ or ‘lines-tail’.

Add (setq whitespace-line-column 99) to your .emacs.