Added error-format flag to x.py.

[ghost]

Fixes #48475

r? @Mark-Simulacrum

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[Mark-Simulacrum]

This looks good, thank you!

@bors r+ rollup

[bors]

📌 Commit 264a921 has been approved by Mark-Simulacrum

[bors]

🌲 The tree is currently closed for pull requests below priority 100, this pull request will be tested once the tree is reopened

[Manishearth]

This isn't enough, actually. The reason RUSTFLAGS doesn't just work here is because we ask cargo for --message-format=json (and then strip out the flag it passes down to rustc).

Then we strip out json from the output and read it, only passing through stuff that isn't json.

We'll need to make that pass smarter, where it detects which JSON messages come from cargo and only strips those out.

[Mark-Simulacrum]

Hm, yeah. Ideally we'd tell rustc somehow that it's messages should go to a different file descriptor (which might be possible as we wrap rustc anyway...). Easiest thing to do for now though is probably to print out the line we received here: https://github.com/penpalperson/rust/blob/264a92182e037c46cdcf37890e3142eaf634444d/src/bootstrap/compile.rs#L1011, since that should be enough to catch all cargo-related messages. @penpalperson Are you interested in making that change?