rust-format-buffer does not preserve point position

[Wilfred]

Suppose I have the following code, where | is the point:

fn foo(foo_bar_baz1: i32, foo_bar_baz2: i32, foo_bar_baz3: i32, foo_bar_baz4: i32, foo_bar_baz5: i32, foo_bar_baz6: i32) {

}

fn bar() {|}

Calling M-x rust-format-buffer naively moves to the same position after formatting, putting point in the arguments to foo. It then calls rust--after-revert-hook, which moves point to the start of the buffer.

Ideally, we'd use rustfmt's --write-mode=diff so emacs-rustfmt could just apply the diff. Emacs' markers are smart enough to move if text is inserted before them. This is how gofmt is integrated with Emacs.

A simple partial fix would be to move to the same line number and column number as before -- this would handle more cases correctly.

[tromey]

One possible fix might be to count non-whitespace characters before point, and then skip that many non-whitespace characters after restoring the buffer. The idea here is that presumably the reformat operation affects whitespace but nothing else.

[tromey]

The idea here is that presumably the reformat operation affects whitespace but nothing else.

This turns out to be incorrect; rustfmt can change comment styles.

[Dushistov]

@tromey

I also have problem with this, I setted rust-format-on-save to true, and this hurted me.
Time to time my cursor jump to begin of buffer. So I have to turn off rust-format-on-save for now, and run M-x cargo-process-fmt time to time by hands.

May be ask rustfmt team for feature: give to rustfmt position in file, and after formating rustfmt return new position? rustfmt can works with AST tree, and calculate cursor position against this AST tree, and recalculate based on changes in this AST tree.

[rofrol]

This is my solution based on #269 (comment)

;; -*- lexical-binding: t; -*-

(use-package rust-mode
    :mode ("\\.lalrpop\\'" . elm-mode)
    :straight t
    :config
    (setq rust-format-on-save nil)
    (defun rust-mode-format-buffer ()
      "format rust code without jumping to beginning when doing undo"
      (interactive)
      (shell-command
       (format "rustfmt %s"
	       (shell-quote-argument (buffer-file-name))))
      (revert-buffer t t t))

    (add-hook 'rust-mode-hook
	      (lambda ()
		(add-hook 'after-save-hook #'rust-mode-format-buffer nil 'make-it-local))))

[brotzeit]

This will be fixed with #269