fix some clippy warnings in librustc_errors

[llogiq]

Again, those are small changes to improve readability.

[rust-highfive]

r? @arielb1

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[arielb1]

@bors r+ rollup

[bors]

📌 Commit fc51386 has been approved by arielb1

[bors]

⌛ Testing commit fc51386 with merge b3cf1a8...

[bors]

💔 Test failed - status-travis

[Mark-Simulacrum]

Presumably a spurious failure, though of course we don't actually have the build.log it mentions as far as I know:

[00:26:22] ERROR: An error was encountered with the build.
[INFO ]  Performing some trivial sanity checks
[INFO ]  Build started 20170520.061733
[INFO ]  Building environment variables
[INFO ]  =================================================================
[INFO ]  Retrieving needed toolchain components' tarballs
[ERROR]
[ERROR]  >>
[ERROR]  >>  Build failed in step 'Retrieving needed toolchain components' tarballs'
[ERROR]  >>        called in step '(top-level)'
[ERROR]  >>
[ERROR]  >>  Error happened in: do_isl_get[scripts/build/companion_libs/121-isl.sh@741]
[ERROR]  >>        called from: do_companion_libs_get[scripts/build/companion_libs.sh@15]
[ERROR]  >>        called from: main[scripts/crosstool-NG.sh@591]
[ERROR]  >>
[ERROR]  >>  For more info on this error, look at the file: 'build.log'
[ERROR]  >>  There is a list of known issues, some with workarounds, in:
[ERROR]  >>      '/usr/local/share/doc/crosstool-ng/crosstool-ng-1.22.0/B - Known issues.txt'
[ERROR]
[ERROR]  (elapsed: 4:53.35)
[04:54] / make: *** [build] Error 1
[00:26:22] The command '/bin/sh -c ./build-toolchains.sh' returned a non-zero code: 1
[00:26:22] The command has failed after 5 attempts.

@bors retry

• Tracking issue for spurious network failures on bots #40474

Edit: looks like a network failure actually

[llogiq]

Yeah, doesn't look related.