n = 1 n += 1 n **= 1 n &&= 1 will be a ruby expression, not a shorthand of next command.

(ruby) n = 1
1
(ruby) n += 100
101
(rdbg) p n    # command
=> 101
(rdbg) n    # next command

Background

IRB's command recognization is changed in ruby/irb#961 (merged but not released yet) to fix some reported issues

irb(main):001>measure = Measure.first
=> error
irb(main):002> jobs = Job.where(id: ids)
deprecation message of Multi-irb

The rules are:

• Multiline input is not a command
• Assign-like expression is not a command, checked by a simple regexp and String#start_with?
• If method with the same name as command is defined, follow the method override policy of each command
• If local variable with the same name as command is defined, it is not a command

This pull request will change debug's command recognizing rule similar to IRB style.

Implementation difference between IRB

Debug does not have the third rule: method override policy. The fourth rule of defined local variables is also dropped.
Command check is called for every keystroke to colorize input and to calculate prompt when used with Reline. Local variable check is too costly in remote debug session for this usage.

# IRB
irb(main):001> help = 1
=> 1
irb(main):002> help + 1
=> 2
irb(main):003> irb_help # help command

# Debug
(ruby) n = 1
1
(rdbg) n + 1 # this will be next command
Unknown option: + 1
(rdbg) p n + 1 # workaround
=> 2