Skip checking for unused mutable locals that have no name

[KiChjang]

Fixes #50343.

[rust-highfive]

r? @michaelwoerister

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[KiChjang]

r? @nikomatsakis

[rust-highfive]

The job x86_64-gnu-llvm-3.9 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.

[00:04:57] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:57] tidy error: /checkout/src/test/compile-fail/nll/unused-mut-issue-50343.rs:15: line longer than 100 chars
[00:04:59] some tidy checks failed
[00:04:59] 
[00:04:59] 
[00:04:59] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:59] 
[00:04:59] 
[00:04:59] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:59] Build completed unsuccessfully in 0:01:59
[00:04:59] Build completed unsuccessfully in 0:01:59
[00:04:59] Makefile:79: recipe for target 'tidy' failed
[00:04:59] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:02b0f5fa
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

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)

[nikomatsakis]

@bors r+

[bors]

📌 Commit 4bc4848 has been approved by nikomatsakis

[bors]

⌛ Testing commit 4bc4848 with merge 22a41e4...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 22a41e4 to master...