[unstable option] `match_arm_indent`

[ytmimi]

Tracking issue for unstable option: match_arm_indent.
This option was added by #6525 and requested in #2937

Option documentation.

See Processes.md, "Stabilising an Option":

• Is the default value correct?
• The design and implementation of the option are sound and clean.
• The option is well tested, both in unit tests and, optimally, in real usage.
• There is no open bug about the option that prevents its use.

Also see:

• Configuration Option Stabilization
• How can I help get a config option stabilized?

[kaathewisegit]

I think that "Is the default value correct?" can be safely checked off, as by default it preserves the current formatting choices.

Design issues:

• Add match_arm_indent option #6525 (comment)