`cargo fmt --message-format json` error is hard to understand (on stable).

[t-moe]

Problem

cargo fmt --message-format json shows an error that is hard to understand.

> cd /tmp && cargo new foo && cd foo/
> cargo fmt  --message-format json
Invalid value for `--emit` - using an unstable value without `--unstable-features`

Running the command in verbose mode reveals the invocation:

rustfmt --edition 2024 --emit json /tmp/foo/src/main.rs

cargo +nightly fmt --message-format json works as expected.

Note that the help output of cargo fmt --help explicitly lists JSON as an option (on stable).

Version

>  cargo version --verbose
cargo 1.90.0 (840b83a10 2025-07-30)
release: 1.90.0
commit-hash: 840b83a10fb0e039a83f4d70ad032892c287570a
commit-date: 2025-07-30
host: x86_64-unknown-linux-gnu
libgit2: 1.9.1 (sys:0.20.2 vendored)
libcurl: 8.14.1-DEV (sys:0.4.82+curl-8.14.1 vendored ssl:OpenSSL/3.5.0)
ssl: OpenSSL 3.5.0 8 Apr 2025
os: Ubuntu 24.4.0 (noble) [64-bit]
> rustfmt --version
rustfmt 1.8.0-stable (1159e78c47 2025-09-14)

[weihanglo]

@rustbot transfer rustfmt

[weihanglo]

Transfered as the cargo-fmt command is provided by https://github.com/rust-lang/rustfmt, as an official Cargo external command. The repository https://github.com/rust-lang/cargo doesn't own that command.