How to inline a variable in Emacs?

Question

I'm writing Rust using rust-analyzer and lsp-mode

Suppose I have code:

let x = 10;
let y = 20 + x;

I want to put my cursor over x and hit some shortcut (or whatever) and the code becomes:

let y = 20 + 10;

From searching the internet it seems I'm the first person in the world who ever wanted to inline a variable in Emacs⁈

I don't know about emacs specifically, but I'd think it needs to be supported by rust-analyzer/clippy to first have it a s a lint suggestion and offer a replacement code. Then, I believe, your editor should already have a shortcut to apply the suggestion. — Eugene Sh.
– Eugene Sh., Commented May 26 at 15:03
@EugeneSh. Looks like rust-analyzer in VSCode can do it - image. So I figure it should be possible, but I also am not familiar with emacs. — kmdreko
– kmdreko, Commented May 26 at 16:17
@kmdreko Interesting. Looks like it is a part of the Rust extension rather than rust-analyzer? — Eugene Sh.
– Eugene Sh., Commented May 26 at 16:19
@EugeneSh. no idea why you would think that, the rust-analyzer assist is inline_local_variable — kmdreko
– kmdreko, Commented May 26 at 16:26
Yeah, just checked it, sure still depends on rust-analyzer, which absolutely makes sense. — Eugene Sh.
– Eugene Sh., Commented May 26 at 16:27

Sibi · Accepted Answer · 2025-05-27 04:35:48Z

1

Since rust-analyzer supports that code action, you should be able to use it using lsp-mode's support of rust-analyzer. I did a quick testing using my Emacs instance and can confirm that the code action works fine via lsp-mode.

edited May 27 at 4:35

answered May 27 at 4:28

Sibi

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samthebest · Accepted Answer · 2025-05-28 12:42:59Z

So after a lot of digging around I'm pretty sure there isn't a shortcut to do this, or even a specific command that can be mapped to a shortcut. Rather one must find the action from a list. So I had to write a function to automate that as follows:

;; Inline variable
(defun lsp-rust-inline-action ()
  "Execute the Rust Analyzer inline refactor code action."
  (interactive)
  (lsp-request-async
   "textDocument/codeAction"
   (lsp--text-document-code-action-params)
   (lambda (actions)
     (let ((inline-action
            (seq-find (lambda (action)
                        (string= (gethash "kind" action) "refactor.inline"))
                      actions)))
       (if inline-action
           (lsp-execute-code-action inline-action)
         (message "No inline action available here"))))
   :error-handler #'lsp--error
   :mode 'detached))

(with-eval-after-load 'rust-mode
  (define-key rust-mode-map (kbd "M-v") #'lsp-rust-inline-action))

So after adding this to my ~/.emacs.d/init.el I can inline variables with M-v (ALT + v).

NOTE: It doesn't seem inlining is always available for all variables, it seems the rust-analyzer only supports a subclass of situations - this can make things very confusing.

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