regression: Cargo not rendering json output

[theemathas]

The following three crater outputs contain json that rustc output as-is. They are not properly formatted like in other crater outputs.

https://crater-reports.s3.amazonaws.com/beta-1.94-2/beta-2026-01-20/reg/atelier_openapi-0.1.0/log.txt
https://crater-reports.s3.amazonaws.com/beta-1.94-2/beta-2026-01-20/reg/cargo_atelier-0.2.7/log.txt
https://crater-reports.s3.amazonaws.com/beta-1.94-2/beta-2026-01-20/reg/weld-bin-0.1.4/log.txt

Discovered in #151865

[Mark-Simulacrum]

I suspect this might be a Cargo regression. Unfortunately it's hard to say since the warnings/errors here are new in 1.94, but I've not seen this before. AFAIK, Cargo is responsible for converting from JSON to human readable messages though.

Tagging as such, probably needs some investigation locally.

[Skgland]

I think this might be a crater problem. Crater uses --message-format=json and tries to parse a Message from each line as json¹.
If parsing fails it is printed as is (this appears to be happening here).
If parsing succeeds and it is a comiler-message it is replaced with the rendered message², otherwise it is removed³.

Attempting to parse one of the json messages with serde_json and cargo_metadata@0.22 gives me Err(Error("recursion limit exceeded", line: 1, column: 56627))

Details

// Cargo.toml
// [package]
// name = "issue151606"
// version = "0.1.0"
// edition = "2024"
// 
// [dependencies]
// cargo_metadata = "0.22"
// serde_json = "1.0.149"

let msg: Result<cargo_metadata::Message, _> =
        serde_json::from_str(include_str!("../example.json"));
    println!("{msg:?}");

Footnotes

1. https://github.com/rust-lang/crater/blob/1a2e31fb1abee11df8c42246e7159bb1fbd442e3/src/runner/test.rs#L155-L161 ↩

2. https://github.com/rust-lang/crater/blob/1a2e31fb1abee11df8c42246e7159bb1fbd442e3/src/runner/test.rs#L161-L188 ↩

3. https://github.com/rust-lang/crater/blob/1a2e31fb1abee11df8c42246e7159bb1fbd442e3/src/runner/test.rs#L190 ↩

[ehuss]

There are a few options:

• Enable the unbounded_depth option on serde_json (and the disable_recursion_limit method).
• Use the serde_stacker crate to handle the deep stack.
• Don't use cargo_metdata, and instead define your own Message struct that does not include the macro expansion (which you don't need). There's a ton of fields that crater doesn't need, so the struct should be very small.

[ehuss]

There's maybe a larger question of what the rest of the Rust community is supposed to do if they want to ingest JSON in similar situations. Maybe cargo_metdata could provide a type that does not include the expansion field?

[Mark-Simulacrum]

How does Cargo parse this JSON (which approach does it use)?

[ehuss]

It defines structs with the minimal fields that it needs: https://github.com/rust-lang/cargo/blob/8099cf76c0a5fb30fecdcbe7aaa4055a031631de/src/cargo/core/compiler/mod.rs#L2172-L2208

EDIT: And there's a comment there discussing the stack overflow problem.

[theemathas]

Is this a regression, or is this just the first time we've seen an error with json that's nested this deep?