Add an option to not add a trailing comma to the last match arm

[dovreshef]

match_block_trailing_comma option only takes affect on block based match arms. A trailing comma is always added to the last non block based match arm.

Please add a new option to not add a trailing comma to the last match arm.

[dovreshef]

I'd like to take a stab at fixing this, but I'm not sure how.
I guess I'd need to add the arm_comma function a flag that will let it know if it is in the last arm?
In that case I'm not sure how to thread such a flag through the arm.rewrite trait function...

[nrc]

That sounds like a good way to implement this. I don't think you need to thread through the flag - the comma itself is added in the match rewrite (where you know whether it's the last arm or not). In the arm rewrite, arm_comma is only used to compute the length of the line, you can therefore always assume it is not the last line to be conservative.

[dovreshef]

It seems as though the arm_comma result is used in the arm rewrite. If I'm reading it right, here the comma is returned as part of the arm to the calling rewrite_match, which uses it here.

Another question, should the new config option takes priority over match_block_trailing_comma?
In block based match, where match_block_trailing_comma is true, and match_arm_trailing_comma is false, what should the behavior be for the last arm?

[nrc]

@dovreshef (sorry for the late reply, vacation). Your description is correct, so you can override the comma choice in rewrite_match.

what should the behavior be for the last arm?

no comma, i.e., match_arm_trailing_comma takes priority (and the config option should probably have the word 'last' in there some where).

[topecongiro]

Now rustfmt does not add trailing comma to the last match arm when trailing_comma = true.
Do we need more fine-grained option?

[nrc]

Seems fine to me (I'm generally in favour of coarser grained options).