FR: `jj fix --no-index` to format stdin in a text editor

[cormacrelf]

Is your feature request related to a problem? Please describe.

I configure my formatters in jj config. Then I go and configure formatters in my editor, trying to make them match. These things never quite match. For example, say you have a jj fix script to add a header to every source file. You may have rustfmt configured, but it is much more complicated to get a script like that into your editor's format button. Plus, some of your rust projects might use rustfmt +nightly, some may not, etc.

JJ is good at configuring and running formatters, better (at what it handles, filesets) than almost any formatter in existence. But we can't use it as a formatter itself! It would be nice to just configure Vim to run a file through jj fix no matter the filetype. Then I could define formatting in one place, and get all the jj fix magic in the editor itself.

There is also an issue with testing jj fix configuration that this could solve. It's hard to do, you have to do a complicated dance with jj fix -s @, and a lot of jj undo and squashing. I've experienced writing a formatting wrapper script that matched itself, failed with exit code 0, and truncated itself.

Describe the solution you'd like

# for an editor configuration:
# format stdin as if it were a file at a given path in the repo, onto stdout
... | jj fix --no-index --stdin-filepath src/main.rs | ...

# test out your jj fix configuration, emits on stdout
jj fix --no-index src/main.rs | rg 'Copyright (C) 2025 The JJ Authors'

# or directly rewrite a list of paths in-place without updating @ commit
# should use filesets i think, because jj fix config has to match against
# filesets to determine tools to run
jj fix --no-index --in-place src/main.rs

--no-index would always imply --ignore-working-copy.

The command would be following the vein of git diff --no-index fileA fileB, which uses git config for diff tools but does not read the git index. That's exactly what this is, so I think it should have the same flag name.

For a text editor, you would configure it like this:

-- none-ls configuration for neovim

local h = require("null-ls.helpers")
local methods = require("null-ls.methods")

local FORMATTING = methods.internal.FORMATTING

return h.make_builtin({
   name = "jj_fix",
   meta = {
      url = "https://jj-vcs.github.io/jj/latest/config/#code-formatting-and-other-file-content-transformations",
      description = "Format with jj fix",
   },
   method = FORMATTING,
   filetypes = {},
   generator_opts = {
      command = "jj",
      args = { "fix", "--no-index", "--stdin-filepath", "$FILENAME" },
      to_stdin = true,
   },
   factory = h.formatter_factory,
})

(Side note it would also be nice to share the formatter configs with other JJ users in the same repo, can we do that with config #includes yet? Seems no)

Describe alternatives you've considered
Keep livin' in the past

Additional context
None

[scott2000]

I'm not sure about the naming of --no-index. It's a bit confusing because I believe that what Git calls the index, we call the working copy (and we also use the term "index" for something else entirely). Also, jj fix doesn't really have anything to do with the working copy since it can be used on commits other than @, so I don't think this naming matches the behavior anyway. I don't have any better naming ideas though.

Also, have you considered using treefmt? It provides similar functionality, and you could probably configure jj fix to always run treefmt --stdin $path, but I haven't tried it.

[cormacrelf]

Fair enough on the flag, two others that make some sense to me are:

• jj fix --standalone ...
• jj fix --pipe ...

I tried treefmt on your recommendation, and it is not very compatible with jj. The --stdin flag basically doesn't work, especially for rust but probably for most formatters, because it just makes a temp file and runs the formatter on that. Here you can see rustfmt is not actually running in bare stdin mode, which means it tries to walk the mod declarations and format files in the /tmp directory. This implementation of stdio formatting will also break any formatter that tries to discover config from the input file path, like biome-js. I guess you could improve treefmt. But in general most formatters these days support --stdin-filepath or some similar flag and JJ's stdin-first approach is really quite good.

; cat src/lib.rs
pub mod abc;

; treefmt --stdin src/lib.rs < src/lib.rs

ERRO formatter | rustfmt: failed to apply with options '[--edition=2021]': exit status 1
Error writing files: failed to resolve mod `abc`: /tmp/abc.rs does not exist

Edit; seems treefmt is working on making this better, numtide/treefmt#586. Seems like they're making a lot of demands on the shape of CLI args of various programs, I doubt that strategy will get very far. For example half of code formatters anywhere cannot even parse double-dash CLI flags. They should really just let you describe a formatter and whether it runs in stdin mode. I should be able to make rustfmt always run in stdin mode, because obviously its default recursive behaviour is undesirable in treefmt in the first place.

Even if treefmt gets better, it would still be extremely useful to be able to test jj fix without writing to disk, so I'd still like this feature.