See #17478

Do not use for, unless you know exactly why.

See #17478

See #17478

See #17478

No spaces before commas.

See #17478

See #17478

See #17478

Use Array.new with a block instead of `.times.collect` / `.times.map`.

See #17478

Favor `unless` over `if` for negative conditions
(or control flow ||).

See #17478

Checks of literals used in conditions.

See #17478

Do not use then for multi-line if.

See #7478.

Use def with parentheses when there are parameters.

See #17478

See #17478

See #17478

Use Unix-style line endings. See #17478.

See #17478

[ci skip]

Closes #17406

Also removes the note from the development/testing.md guide

[ci skip]

Almost all cops are starting as disabled until we can fix their
violations.

This enables multiple Rubocop cops that already conform to our codebase
and do not require fixes.

Also fix one use of `gsub` that would be faster as `delete`.

Use `tr` instead of `gsub` when you are replacing the same number of
characters. Use `delete` instead of `gsub` when you are deleting
characters.

Also fixes the errors caused by enabling the cop. casecmp is more performant than `.downcase` and `==`.

This reports uses of `File.exists?` and `Dir.exists?`, which were both
deprecated in Ruby and will eventually be removed in favor of
`.exist?`. Also fixes all existing uses of the deprecated methods.

This requires no code changes since it doesn't actually change anything
in the codebase, just preventative.

To 60.

Discussed in #14233.

See [their releases](https://github.com/bbatsov/rubocop/releases) for
more info.

Changes:
- Enable DisplayCopNames for lint output.
- Default behavior for `Alias` changed, set to enforce `prefer_alias_method`.
- Enabling Rails cops changed to new syntax.
- Remove StyleGuides and move Descriptions to comments.
- Add missing cops.
- Add TODOs for cops that should be enabled in the future.
- Set TargetRubyVersion to 2.1.