Fix --message-format JSON when rustc emits non-JSON warnings

[fmdkdd]

The --message-format JSON flag parses all the stderr output of rustc to JSON,
but rustc can emit non-JSON lines to stderr (e.g., for warning about the
unstable -Z flag on the stable channel), causing cargo to fail reporting
compilation errors when using --message-format JSON.

This commit adds a check to look for lines beginning with { to only parse
these lines as JSON. Other lines from rustc are forwarded to the stderr of
cargo.

Fixes #3390.

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[fmdkdd]

@matklad To forward the output to stderr, using cx.config.shell().err() as you suggested caused a bunch of errors of this sort:

260 |     return Ok(Work::new(move |state| {
    |               ^^^^^^^^^ trait `std::cell::RefCell<core::shell::MultiShell>: std::marker::Sync` not satisfied
    |
    = note: `std::cell::RefCell<core::shell::MultiShell>` cannot be shared between threads safely
    = note: required because it appears within the type `util::config::Config`
   [...]

I used io::stderr() directly, which works (errors from rustc are correctly forwarded). But I'm not sure if it's the right solution for that one.

Please let me know if I should change it (and hint at the correct way to do it, since I haven't looked much at dealing with threads in Rust).

[alexcrichton]

This looks good to me, thanks! Could you also add a test for this?

[fmdkdd]

@alexcrichton Done. I've added to make sure that JSON errors are emitted, and that stderr is forwarded.

The test is skipped with a nightly rustc, since the warning for -Z won't appear there, and I didn't know how to trigger a warning for all versions.

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 3e32503 has been approved by alexcrichton

[bors]

⌛ Testing commit 3e32503 with merge c07ecb0...

[bors]

💔 Test failed - status-appveyor

[alexcrichton]

Oh that looks like rust-lang/rust#38177 which was fixed in rust-lang/rust#38357 which should be in the next nightly, I'll r+ then.

[alexcrichton]

@bors: retry

[bors]

⌛ Testing commit 3e32503 with merge 109d9c5...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 109d9c5 to master...