Don't suggest &mut mut

[saleemjaffer]

Don't suggest &mut mut for &mut &T coerced to &T; fixes #57431

[rust-highfive]

r? @eddyb

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[saleemjaffer]

r? @spastorino

[cramertj]

Can you add a UI test including the testcase from the original issue?

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

travis_time:end:023c53b0:start=1554396556544060677,finish=1554396647813703033,duration=91269642356
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
Setting environment variables from .travis.yml
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---
[01:09:58] 
[01:09:58] ---- [ui] ui/nll/issue-47388.rs stdout ----
[01:09:58] diff of stderr:
[01:09:58] 
[01:09:58] 1 error[E0594]: cannot assign to `fancy_ref.num` which is behind a `&` reference
[01:09:58] 3    |
[01:09:58] 3    |
[01:09:58] - LL |     let fancy_ref = &(&mut fancy);
[01:09:58] -    |                     ------------- help: consider changing this to be a mutable reference: `&mut (&mut fancy)`
[01:09:58] 6 LL |     fancy_ref.num = 6;
[01:09:58] 7    |     ^^^^^^^^^^^^^^^^^ `fancy_ref` is a `&` reference, so the data it refers to cannot be written
[01:09:58] 
[01:09:58] 
[01:09:58] The actual stderr differed from the expected stderr.
[01:09:58] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/nll/issue-47388/issue-47388.stderr
[01:09:58] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/nll/issue-47388/issue-47388.stderr
[01:09:58] To update references, rerun the tests and pass the `--bless` flag
[01:09:58] To only update this specific test, also pass `--test-args nll/issue-47388.rs`
[01:09:58] error: 1 errors occurred comparing output.
[01:09:58] status: exit code: 1
[01:09:58] status: exit code: 1
[01:09:58] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/nll/issue-47388.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/nll/issue-47388/a" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/nll/issue-47388/auxiliary" "-A" "unused"
[01:09:58] ------------------------------------------
[01:09:58] 
[01:09:58] ------------------------------------------
[01:09:58] stderr:
[01:09:58] stderr:
[01:09:58] ------------------------------------------
[01:09:58] {"message":"cannot assign to `fancy_ref.num` which is behind a `&` reference","code":{"code":"E0594","explanation":null},"level":"error","spans":[{"file_name":"/checkout/src/test/ui/nll/issue-47388.rs","byte_start":142,"byte_end":159,"line_start":9,"line_end":9,"column_start":5,"column_end":22,"is_primary":true,"text":[{"text":"    fancy_ref.num = 6; //~ ERROR E0594","highlight_start":5,"highlight_end":22}],"label":"`fancy_ref` is a `&` reference, so the data it refers to cannot be written","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"error[E0594]: cannot assign to `fancy_ref.num` which is behind a `&` reference\n  --> /checkout/src/test/ui/nll/issue-47388.rs:9:5\n   |\nLL |     fancy_ref.num = 6; //~ ERROR E0594\n   |     ^^^^^^^^^^^^^^^^^ `fancy_ref` is a `&` reference, so the data it refers to cannot be written\n\n"}
[01:09:58] {"message":"For more information about this error, try `rustc --explain E0594`.","code":null,"level":"","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0594`.\n"}
[01:09:58] 
[01:09:58] ------------------------------------------
[01:09:58] 
---
[01:09:58] 
[01:09:58] thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:516:22
[01:09:58] 
[01:09:58] 
[01:09:58] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-6.0/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "6.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
[01:09:58] 
[01:09:58] 
[01:09:58] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:09:58] Build completed unsuccessfully in 0:04:27
[01:09:58] Build completed unsuccessfully in 0:04:27
[01:09:58] Makefile:48: recipe for target 'check' failed
[01:09:58] make: *** [check] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:25624344
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Thu Apr  4 18:00:55 UTC 2019
---
travis_time:end:2a1c7a40:start=1554400856799884046,finish=1554400856804891109,duration=5007063
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:036f04c8
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:07e30066
travis_time:start:07e30066
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:000211ab
$ dmesg | grep -i kill

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)

[meithecatte]

Comparing the code like this is a bad idea. Consider & mut - perfectly valid, if a bit unusual. Moreover, there's &'a mut. This should really be matched on some deeper level like the AST or MIR.

Take the above with a grain of salt, though, I'm not terribly familiar with this codebase.

[saleemjaffer]

@NieDzejkob Yeah you are right. This is where we are discussing how to solve this issue.

[pnkfelix]

I'm going to go ahead and close this PR. The issue itself even says don't use regexp-matching, which includes as (a special case) string-matching as was done here.