Rustup update warning printed to stdout with --message-format=json breaks json parsing #1721
Description
Checklist
- I've looked through the issues and pull requests for similar reports
Describe your issue
When I'm building with the --message-format=json flag, and cross detects that it needs to update the Rust version for the target, it prints a message to stdout in a human-readable format, and this breaks json parsing of the output. I'd expect the warning to be printed to stderr instead so I could still parse the json from stdout.
What target(s) are you cross-compiling for?
x86_64-unknown-linux-gnu
Which operating system is the host (e.g computer cross is on) running?
- macOS
- Windows
- Linux / BSD
- other OS (specify in description)
What architecture is the host?
- x86_64 / AMD64
- arm32
- arm64 (including Mac M1)
What container engine is cross using?
- docker
- podman
- other container engine (specify in description)
cross version
cross 0.2.5 (49cd054 2025-09-26)
Example
$ cross build --profile=dev --target=x86_64-unknown-linux-gnu --message-format=json
using newer rustc `1.91.0 (f8297e351 2025-10-28)` for the target. Current active rustc on the host is `rustc 1.89.0 (29483883e 2025-08-04)`.
> Update with `rustup update`
{"reason":"compiler-artifact","package_id":"registry+https://github.com/rust-lang/crates.io-index#proc-macro2@1.0.103","manifest_path":", ... <all the normal JSON emitted by cargo build>
Additional information / notes
No response