Fix checking if there have been new errors.

[ghost]

r? @nrc

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nrc (or someone else) soon.

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[nikomatsakis]

@bors r+ p=22

this is blocking compile-fail completely for me

[bors]

📌 Commit b3e30b5 has been approved by nikomatsakis

[nikomatsakis]

Also, score one for overflow checks! We really ought to be running these by default on homu in some configuration at least.

[bors]

⌛ Testing commit b3e30b5 with merge 552bf75...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt