In a mixed-editions workspace, cargo fmt with a given path, formats that path with both editions (which can fail)

[dfaure-kdab]

In a workspace where one package uses rust edition 2021 and another one uses rust edition 2024, cargo fmt -- <file> attempts to format that file with both editions. If the file uses a 2024-only feature like a let chain, this fails.
The reason this comes up in practice is that the pre-commit hook fmt calls cargo fmt -- <files changed by commit>, which seems like a reasonable thing to do.

$ cargo fmt --verbose -- newapp/src/main.rs
[bin (2024)] "/kdab/src/rust/fmt-bug/newapp/src/main.rs"
[bin (2021)] "/kdab/src/rust/fmt-bug/oldapp/src/main.rs"
rustfmt --edition 2021 newapp/src/main.rs /kdab/src/rust/fmt-bug/oldapp/src/main.rs
error: let chains are only allowed in Rust 2024 or later
 --> /kdab/src/rust/fmt-bug/newapp/src/main.rs:3:8
  |
3 |     if let Some(val) = opt
  |        ^^^^^^^^^^^^^^^^^^^

rustfmt --edition 2024 newapp/src/main.rs /kdab/src/rust/fmt-bug/newapp/src/main.rs

To reproduce this:

git clone https://github.com/dfaure-kdab/cargo-fmt-bug
cd cargo-fmt-bug
cargo fmt --verbose -- newapp/src/main.rs

run_rustfmt() just passes along the fmt_args to all calls to rustfmt, without realizing that if the argument is a filename then it should filter it based on the edition...

[ytmimi]

For reference, what version of cargo-fmt are you using?

[ytmimi]

@dfaure-kdab Also for reference, cargo-fmt doesn't support formatting individual files, so pre-commit hook fmt is fundamentally broken if it invokes cargo-fmt like that.

It's not that rustfmt is formatting the files twice with different editions. If you just run cargo fmt --verbose you'll see that cargo-fmt correctly identifies the edition for each workspace member and then invokes rustfmt on the root file for each package. In this case that's oldapp/src/main.rs and newapp/src/main.rs respectively.

The issues here is how cargo-fmt is getting invoked. Everything after the -- in cargo fmt --verbose -- newapp/src/main.rs is going to get passed to each of the underlying rustfmt calls that cargo-fmt makes. Because newapp/src/main.rs requires edition 2024 it will surely fail when cargo-fmt is in the process of formatting the oldapp package.

[dfaure-kdab]

Yes, you've described what I've described, so we agree that this doesn't work and why it doesn't work.

But then what should the pre-commit hook do, in order to format individual files correctly?

Either it calls rustfmt directly but then it's missing all the logic about which edition to use, or it calls cargo-fmt directly and you say that's not supported.
Wouldn't the best solution be for cargo-fmt to gain the feature of formatting individual files, for this purpose?