rustfmt invoked without considering rustfmt.toml

[apiraino]

Hello from the Rust project 👋

I use Emacs 30.1 and rust-mode 20250609.1420 (0d5d54a) from MELPA. I was working on the triagebot and got a CI failure due to rustfmt behaving differently on my local workstation. The triagebot has a local rustfmt.toml formatting to --edition=2018. rust-mode now defaults to edition 2024 (was changed in #567).

I expected to see this happen: The local rustfmt.toml should have precedence over rust-mode default

Instead, this happened: the rustfmt.toml seems to be ignored

Here is a small sample that formats differently according to the edition.

// (setq rust-rustfmt-switches '("--edition" "2024"))
// (setq rust-rustfmt-switches '("--edition" "2018"))

use anyhow::{Context, anyhow};

Am I perhaps missing anything?

Thanks!

[nealsid]

It sounds like rustfmt overrides rustfmt.toml configuration with values from command line switches. I agree that rust-mode should not override rustfmt.toml by default. A workaround is that you can set rust-rustfmt-switches to nil.

I think that this change should be part of rust-mode (i.e. rust-mode should not set rust-rustfmt-switches). At the cost of a bit more complexity, there could be variable that indicates to only use command line switches when rustfmt.toml is not present. Something like rust-rustfmt-ignore-switches-when-rustfmt-toml-present (I cannot think of anything less verbose). That would allow developers working on their own projects who don't use rustfmt.toml to specify settings inside rust-rustfmt-switches while respecting the settings of other projects they work on.

[nealsid]

Actually, it seems that rustfmt falls back to a configuration file in the user's home directory if a project-specific file is not found:

https://rust-lang.github.io/rustfmt/?version=v1.8.0&search=

Might be simpler to just remove rust-rustfmt-switches and say that any configuration should happen by that method. It's not clear to me if there are switches that do not have corresponding settings in the toml file, in which case this proposal would prevent those options from being specified.

[psibi]

I agree that rust-mode should not override rustfmt.toml by default.

I agree.

A workaround is that you can set rust-rustfmt-switches to nil.

Yeah. I think this should be the default. PR for this is welcome.