make pretty source comparison check be fatal (fixes #52255)

[tinco]

This is not ready for merging because it reveals (at least) two regressions in the pretty suite. Should I attempt to fix those in this PR also?

[tinco]

This is now ready for merging, changes needed were trivial but indicative of the kind of regression this bug silently enabled.

[sanxiyn]

@bors r+

[bors]

📌 Commit b96deed has been approved by sanxiyn

[bors]

⌛ Testing commit b96deed with merge 7d73fffeab20294beb739dccca1ed53f374d4450...

[bors]

💔 Test failed - status-travis

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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#####################################################################     97.1%
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[00:01:07] extracting /checkout/obj/build/cache/2018-06-30/rust-std-beta-x86_64-unknown-linux-gnu.tar.gz
[00:01:07] downloading https://static.rust-lang.org/dist/2018-06-30/rustc-beta-x86_64-unknown-linux-gnu.tar.gz
No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
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The build has been terminated

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[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

#####################################################################     97.1%
######################################################################## 100.0%
[00:01:07] extracting /checkout/obj/build/cache/2018-06-30/rust-std-beta-x86_64-unknown-linux-gnu.tar.gz
[00:01:07] downloading https://static.rust-lang.org/dist/2018-06-30/rustc-beta-x86_64-unknown-linux-gnu.tar.gz
No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[kennytm]

@bors retry #40474

[bors]

⌛ Testing commit b96deed with merge 6cc42a4...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: sanxiyn
Pushing 6cc42a4 to master...