Prevent rustc stderr/stdout from being duplicated.

[Mark-Simulacrum]

Please review carefully. I've not submitted patches to Cargo before, I think, so this may be flawed in some way I haven't detected yet. Tests are green locally, though.

Fixes #4223

[rust-highfive]

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[alexcrichton]

Looks good! Would it be possible to add a test for this?

[Mark-Simulacrum]

I'd guess it'd be somewhat hard -- I'd have to check for a lack of output. I worry that that would make the test too specific; but if you'd like I can look into adding one -- I believe I'd probably edit test/rustc.rs and add a test case?

[alexcrichton]

Oh yeah most Cargo test caes exhaustively match the output, so it shouldn't be too hard to test for a lack of output (just don't write it down but be sure to add some assertion). Anywhere's fine by me!

[Mark-Simulacrum]

Added a test, and confirmed it doesn't pass on master and does pass on this branch.

[alexcrichton]

@bors: r+

[bors]

📌 Commit f377ae2 has been approved by alexcrichton

[bors]

⌛ Testing commit f377ae2 with merge 854bc16...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 854bc16 to master...