Maybe Style/StringConcatenation should be conservative by default

[jaynetics]

as per https://docs.rubocop.org/rubocop/cops_style.html#stylestringconcatenation, aggressive mode is currently the default.

this is not ideal because it triggers false positives and people have to research it and change the mode.

• we hit a case with URI#+, which returns a URI and prevents double slashes
  • e.g. uri + '/some_path' always returns a valid URI
• Pathname#+ behaves similarly and thus yields similar false positives
• custom implementations of #+ are also affected

[koic]

This configuration was introduced at #9153 with aggressive as the default. There are #9144 arguments, but I think I can accept this proposal to default to conservative. This is because I prefer to be as safe as possible, but others may think differently, like some of #9144.
@rubocop/rubocop-core What do you think about it?

[bbatsov]

I still stand by my original opinion. That cop is next to useless with the conservative config. Likely we'll just have it disabled by default in RuboCop 2.0, as one of the main goals is to streamline the default configuration (profile) and have be more conservative in general.

[bbatsov]

See also #10726

[noraj]

rubocop -A broke my code when it's supposed to be safe.

A minimal example of how this breaks filepath:

# frozen_string_literal: true

require 'pathname'

FILENAME = 'mime.types'

pn = Pathname.new('/etc')
fp = pn + 'nginx' + FILENAME

puts fp # /etc/nginx/mime.types

➜ rubocop -A pathname.rb
Inspecting 1 file
C

Offenses:

pathname.rb:8:6: C: [Corrected] Style/StringConcatenation: Prefer string interpolation to string concatenation.
fp = pn + 'nginx' + FILENAME
     ^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

# frozen_string_literal: true

require 'pathname'

FILENAME = 'mime.types'

pn = Pathname.new('/etc')
fp = "#{pn}nginx#{FILENAME}"

puts fp # /etcnginxmime.types

[bquorning]

rubocop -A broke my code when it's supposed to be safe.

-A isn’t safe:

❯ rubocop --help | grep \\-A
    -A, --autocorrect-all            Autocorrect offenses (safe and unsafe).

[noraj]

Right, I should have used -a