Rollup of 19 pull requests

[kennytm]

Successful merges:

• Rustdoc remove old style files #57929 (Rustdoc remove old style files)
• Fix #57730 #57981 (Fix Merge visitors in AST validation #57730)
• Stabilize slice_sort_by_cached_key #58074 (Stabilize slice_sort_by_cached_key)
• Add specific feature gate error for const-unstable features #58196 (Add specific feature gate error for const-unstable features)
• Remove code for updating copyright years in generate-deriving-span-tests #58293 (Remove code for updating copyright years in generate-deriving-span-tests)
• Don't default on std crate when manipulating browser history #58306 (Don't default on std crate when manipulating browser history)
• librustc_mir: use ? in impl_snapshot_for! macro #58359 (librustc_mir: use ? in impl_snapshot_for! macro)
• Instant::checked_duration_since #58395 (Instant::checked_duration_since)
• fix Box::into_unique effecitvely transmuting to a raw ptr #58429 (fix Box::into_unique effecitvely transmuting to a raw ptr)
• Update which libcore/liballoc tests Miri ignores, and document why #58433 (Update which libcore/liballoc tests Miri ignores, and document why)
• Use posix_spawn_file_actions_addchdir_np when possible #58438 (Use posix_spawn_file_actions_addchdir_np when possible)
• Whitelist the ARM v6 target-feature #58440 (Whitelist the ARM v6 target-feature)
• rustdoc: mask compiler_builtins docs #58448 (rustdoc: mask compiler_builtins docs)
• split MaybeUninit into several features, expand docs a bit #58468 (split MaybeUninit into several features, expand docs a bit)
• compile-pass test for #53606 #58479 (compile-pass test for NLL promotes arbitrary code to be evaluated by miri if the result is unused #53606)
• Fix runtime error in generate-keyword-tests #58489 (Fix runtime error in generate-keyword-tests)
• Fix documentation for std::path::PathBuf::pop #58496 (Fix documentation for std::path::PathBuf::pop)
• Notify myself when Clippy toolstate changes #58509 (Notify myself when Clippy toolstate changes)
• Fix tracking issue for error iterators #58521 (Fix tracking issue for error iterators)

[kennytm]

@bors retry

[kennytm]

@bors r+

Removed #58477 (already testing)
Added #58521.

[bors]

📌 Commit c65a82a has been approved by kennytm

[bors]

⌛ Testing commit c65a82a with merge 8af675a...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: kennytm
Pushing 8af675a to master...

[rust-highfive]

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_fold:end:services

travis_fold:start:git.checkout
travis_time:start:26403f80
$ git clone --depth=2 --branch=master https://github.com/rust-lang/rust.git rust-lang/rust
---
  fi
travis_time:end:0002623e:start=1550398500132386472,finish=1550398500139270972,duration=6884500
travis_fold:end:before_script.3
travis_time:start:0ba89b50
$ MESSAGE_FILE=$(mktemp -t msg.XXXXXX); . src/ci/docker/x86_64-gnu-tools/repo.sh; commit_toolstate_change "$MESSAGE_FILE" "$TRAVIS_BUILD_DIR/src/tools/publish_toolstate.py" "$(git rev-parse HEAD)" "$(git log --format=%s -n1 HEAD)" "$MESSAGE_FILE" "$TOOLSTATE_REPO_ACCESS_TOKEN";

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