### New features

* [#2620](https://github.com/rubocop/rubocop/pull/2620): New cop `Style/ZeroLengthPredicate` checks for `object.size == 0` and variants, and suggests replacing them with an appropriate `empty?` predicate. ([@drenmi][])
* [#2657](https://github.com/rubocop/rubocop/pull/2657): Floating headers in HTML output. ([@mattparlane][])
* Add new `Style/SpaceAroundKeyword` cop. ([@lumeet][])
* [#2745](https://github.com/rubocop/rubocop/pull/2745): New cop `Style/MultilineHashBraceLayout` checks that the closing brace in a hash literal is symmetrical with respect to the opening brace and the hash elements. ([@panthomakos][])
* [#2761](https://github.com/rubocop/rubocop/pull/2761): New cop `Style/MultilineMethodDefinitionBraceLayout` checks that the closing brace in a method definition is symmetrical with respect to the opening brace and the method parameters. ([@panthomakos][])
* [#2699](https://github.com/rubocop/rubocop/pull/2699): `Performance/Casecmp` can register offenses when `str.downcase` or `str.upcase` are passed to an equality method. ([@rrosenblum][])
* [#2766](https://github.com/rubocop/rubocop/pull/2766): New cop `Style/MultilineMethodCallBraceLayout` checks that the closing brace in a method call is symmetrical with respect to the opening brace and the method arguments. ([@panthomakos][])

### Bug fixes

* [#2723](https://github.com/rubocop/rubocop/issues/2723): Fix NoMethodError in Style/GuardClause. ([@drenmi][])
* [#2674](https://github.com/rubocop/rubocop/issues/2674): Also check for Hash#update alias in `Performance/RedundantMerge`. ([@drenmi][])
* [#2630](https://github.com/rubocop/rubocop/issues/2630): Take frozen string literals into account in `Style/MutableConstant`. ([@segiddins][])
* [#2642](https://github.com/rubocop/rubocop/issues/2642): Support assignment via `||=` in `Style/MutableConstant`. ([@segiddins][])
* [#2646](https://github.com/rubocop/rubocop/issues/2646): Fix auto-correcting assignment to a constant in `Style/ConditionalAssignment`. ([@segiddins][])
* [#2614](https://github.com/rubocop/rubocop/issues/2614): Check for zero return value from `casecmp` in `Performance/casecmp`. ([@segiddins][])
* [#2647](https://github.com/rubocop/rubocop/issues/2647): Allow `xstr` interpolations in `Lint/LiteralInInterpolation`. ([@segiddins][])
* Report a violation when `freeze` is called on a frozen string literal in `Style/RedundantFreeze`. ([@segiddins][])
* [#2641](https://github.com/rubocop/rubocop/issues/2641): Fix crashing on empty methods with block args in `Performance/RedundantBlockCall`. ([@segiddins][])
* `Lint/DuplicateMethods` doesn't crash when `class_eval` is used with an implicit receiver. ([@lumeet][])
* [#2654](https://github.com/rubocop/rubocop/issues/2654): Fix handling of unary operations in `Style/RedundantParentheses`. ([@lumeet][])
* [#2661](https://github.com/rubocop/rubocop/issues/2661): `Style/Next` doesn't crash when auto-correcting modifier `if/unless`. ([@lumeet][])
* [#2665](https://github.com/rubocop/rubocop/pull/2665): Make specs pass when running on Windows. ([@jonas054][])
* [#2691](https://github.com/rubocop/rubocop/pull/2691): Do not register an offense in `Performance/TimesMap` for calling `map` or `collect` on a variable named `times`. ([@rrosenblum][])
* [#2689](https://github.com/rubocop/rubocop/pull/2689): Change `Performance/RedundantBlockCall` to respect parentheses usage. ([@rrosenblum][])
* [#2694](https://github.com/rubocop/rubocop/issues/2694): Fix caching when using a different JSON gem such as Oj. ([@stormbreakerbg][])
* [#2707](https://github.com/rubocop/rubocop/pull/2707): Change `Lint/NestedMethodDefinition` to respect `Class.new` and `Module.new`. ([@owst][])
* [#2701](https://github.com/rubocop/rubocop/pull/2701): Do not consider assignments to the same variable as useless if later assignments are within a loop. ([@owst][])
* [#2696](https://github.com/rubocop/rubocop/issues/2696): `Style/NestedModifier` adds parentheses around a condition when needed. ([@lumeet][])
* [#2666](https://github.com/rubocop/rubocop/issues/2666): Fix bug when auto-correcting symbol literals in `Lint/LiteralInInterpolation`. ([@lumeet][])
* [#2664](https://github.com/rubocop/rubocop/issues/2664): `Performance/Casecmp` can auto-correct case comparison to variables and method calls without error. ([@rrosenblum][])
* [#2729](https://github.com/rubocop/rubocop/issues/2729): Fix handling of hash literal as the first argument in `Style/RedundantParentheses`. ([@lumeet][])
* [#2703](https://github.com/rubocop/rubocop/issues/2703): Handle byte order mark in `Style/IndentationWidth`, `Style/ElseAlignment`, `Lint/EndAlignment`, and `Lint/DefEndAlignment`. ([@jonas054][])
* [#2710](https://github.com/rubocop/rubocop/pull/2710): Fix handling of fullwidth characters in some cops. ([@seikichi][])
* [#2690](https://github.com/rubocop/rubocop/issues/2690): Fix alignment of operands that are part of an assignment in `Style/MultilineOperationIndentation`. ([@jonas054][])
* [#2228](https://github.com/rubocop/rubocop/issues/2228): Use the config of a related cop whether it's enabled or not. ([@madwort][])
* [#2721](https://github.com/rubocop/rubocop/issues/2721): Do not register an offense for constants wrapped in parentheses passed to `rescue` in `Style/RedundantParentheses`. ([@rrosenblum][])
* [#2742](https://github.com/rubocop/rubocop/issues/2742): Fix `Style/TrailingCommaInArguments` & `Style/TrailingCommaInLiteral` for inline single element arrays. ([@annih][])
* [#2768](https://github.com/rubocop/rubocop/issues/2768): Allow parentheses after keyword `not` in `Style/MethodCallParentheses`. ([@lumeet][])
* [#2758](https://github.com/rubocop/rubocop/issues/2758): Allow leading underscores in camel case variable names. ([@mmcguinn][])

### Changes

* Remove `Style/SpaceAfterControlKeyword` and `Style/SpaceBeforeModifierKeyword` as the more generic `Style/SpaceAroundKeyword` handles the same cases. ([@lumeet][])
* Handle comparisons with `!=` in `Performance/casecmp`. ([@segiddins][])
* [#2684](https://github.com/rubocop/rubocop/pull/2684): Do not base `Style/FrozenStringLiteralComment` on the version of Ruby that is running. ([@rrosenblum][])
* [#2732](https://github.com/rubocop/rubocop/issues/2732): Change the default style of `Style/SignalException` to `only_raise`. ([@bbatsov][])

[@bbatsov]: https://github.com/bbatsov
[@jonas054]: https://github.com/jonas054
[@yujinakayama]: https://github.com/yujinakayama
[@dblock]: https://github.com/dblock
[@nevir]: https://github.com/nevir
[@daviddavis]: https://github.com/daviddavis
[@sds]: https://github.com/sds
[@fancyremarker]: https://github.com/fancyremarker
[@sinisterchipmunk]: https://github.com/sinisterchipmunk
[@vonTronje]: https://github.com/vonTronje
[@agrimm]: https://github.com/agrimm
[@pmenglund]: https://github.com/pmenglund
[@chulkilee]: https://github.com/chulkilee
[@codez]: https://github.com/codez
[@emou]: https://github.com/emou
[@skanev]: http://github.com/skanev
[@claco]: http://github.com/claco
[@rifraf]: http://github.com/rifraf
[@scottmatthewman]: https://github.com/scottmatthewman
[@ma2gedev]: http://github.com/ma2gedev
[@jeremyolliver]: https://github.com/jeremyolliver
[@hannestyden]: https://github.com/hannestyden
[@geniou]: https://github.com/geniou
[@jkogara]: https://github.com/jkogara
[@tmorris-fiksu]: https://github.com/tmorris-fiksu
[@mockdeep]: https://github.com/mockdeep
[@hiroponz]: https://github.com/hiroponz
[@tamird]: https://github.com/tamird
[@fshowalter]: https://github.com/fshowalter
[@cschramm]: https://github.com/cschramm
[@bquorning]: https://github.com/bquorning
[@bcobb]: https://github.com/bcobb
[@irrationalfab]: https://github.com/irrationalfab
[@tommeier]: https://github.com/tommeier
[@sfeldon]: https://github.com/sfeldon
[@biinari]: https://github.com/biinari
[@barunio]: https://github.com/barunio
[@molawson]: https://github.com/molawson
[@wndhydrnt]: https://github.com/wndhydrnt
[@ggilder]: https://github.com/ggilder
[@salbertson]: https://github.com/salbertson
[@camilleldn]: https://github.com/camilleldn
[@mcls]: https://github.com/mcls
[@yous]: https://github.com/yous
[@vrthra]: https://github.com/vrthra
[@SkuliOskarsson]: https://github.com/SkuliOskarsson
[@jspanjers]: https://github.com/jspanjers
[@sch1zo]: https://github.com/sch1zo
[@smangelsdorf]: https://github.com/smangelsdorf
[@mvz]: https://github.com/mvz
[@jfelchner]: https://github.com/jfelchner
[@janraasch]: https://github.com/janraasch
[@jcarbo]: https://github.com/jcarbo
[@oneamtu]: https://github.com/oneamtu
[@toy]: https://github.com/toy
[@Koronen]: https://github.com/Koronen
[@blainesch]: https://github.com/blainesch
[@marxarelli]: https://github.com/marxarelli
[@katieschilling]: https://github.com/katieschilling
[@kakutani]: https://github.com/kakutani
[@rrosenblum]: https://github.com/rrosenblum
[@mattjmcnaughton]: https://github.com/mattjmcnaughton
[@huerlisi]: https://github.com/huerlisi
[@volkert]: https://github.com/volkert
[@lumeet]: https://github.com/lumeet
[@mmozuras]: https://github.com/mmozuras
[@d4rk5eed]: https://github.com/d4rk5eed
[@cshaffer]: https://github.com/cshaffer
[@eitoball]: https://github.com/eitoball
[@iainbeeston]: https://github.com/iainbeeston
[@pimterry]: https://github.com/pimterry
[@palkan]: https://github.com/palkan
[@jdoconnor]: https://github.com/jdoconnor
[@meganemura]: https://github.com/meganemura
[@zvkemp]: https://github.com/zvkemp
[@vassilevsky]: https://github.com/vassilevsky
[@gerry3]: https://github.com/gerry3
[@ypresto]: https://github.com/ypresto
[@clowder]: https://github.com/clowder
[@mudge]: https://github.com/mudge
[@mzp]: https://github.com/mzp
[@bankair]: https://github.com/bankair
[@crimsonknave]: https://github.com/crimsonknave
[@renuo]: https://github.com/renuo
[@sdeframond]: https://github.com/sdeframond
[@til]: https://github.com/til
[@carhartl]: https://github.com/carhartl
[@dylandavidson]: https://github.com/dylandavidson
[@tmr08c]: https://github.com/tmr08c
[@hbd225]: https://github.com/hbd225
[@l8nite]: https://github.com/l8nite
[@sumeet]: https://github.com/sumeet
[@ojab]: https://github.com/ojab
[@chastell]: https://github.com/chastell
[@glasnt]: https://github.com/glasnt
[@crazydog115]: https://github.com/crazydog115
[@RGBD]: https://github.com/RGBD
[@panthomakos]: https://github.com/panthomakos
[@matugm]: https://github.com/matugm
[@m1foley]: https://github.com/m1foley
[@tejasbubane]: https://github.com/tejasbubane
[@bmorrall]: https://github.com/bmorrall
[@fphilipe]: https://github.com/fphilipe
[@gotrevor]: https://github.com/gotrevor
[@awwaiid]: https://github.com/awwaiid
[@segiddins]: https://github.com/segiddins
[@urbanautomaton]: https://github.com/urbanautomaton.com
[@unmanbearpig]: https://github.com/unmanbearpig
[@maxjacobson]: https://github.com/maxjacobson
[@sliuu]: https://github.com/sliuu
[@edmz]: https://github.com/edmz
[@syndbg]: https://github.com/syndbg
[@wli]: https://github.com/wli
[@caseywebdev]: https://github.com/caseywebdev
[@MGerrior]: https://github.com/MGerrior
[@imtayadeway]: https://github.com/imtayadeway
[@mrfoto]: https://github.com/mrfoto
[@karreiro]: https://github.com/karreiro
[@dreyks]: https://github.com/dreyks
[@hmadison]: https://github.com/hmadison
[@miquella]: https://github.com/miquella
[@jhansche]: https://github.com/jhansche
[@cornelius]: https://github.com/cornelius
[@eagletmt]: https://github.com/eagletmt
[@apiology]: https://github.com/apiology
[@alexdowad]: https://github.com/alexdowad
[@minustehbare]: https://github.com/minustehbare
[@tansaku]: https://github.com/tansaku
[@ptrippett]: https://github.com/ptrippett
[@br3nda]: https://github.com/br3nda
[@jujugrrr]: https://github.com/jujugrrr
[@sometimesfood]: https://github.com/sometimesfood
[@cgriego]: https://github.com/cgriego
[@savef]: https://github.com/savef
[@volmer]: https://github.com/volmer
[@domcleal]: https://github.com/domcleal
[@codebeige]: https://github.com/codebeige
[@weh]: https://github.com/weh
[@bfontaine]: https://github.com/bfontaine
[@jawshooah]: https://github.com/jawshooah
[@DNNX]: https://github.com/DNNX
[@mvidner]: https://github.com/mvidner
[@mattparlane]: https://github.com/mattparlane
[@drenmi]: https://github.com/drenmi
[@stormbreakerbg]: https://github.com/stormbreakerbg
[@owst]: https://github.com/owst
[@seikichi]: https://github.com/seikichi
[@madwort]: https://github.com/madwort
[@annih]: https://github.com/annih
[@mmcguinn]: https://github.com/mmcguinn
