rustc: Allow trailing comma in record fields

[lht]

Proposal at https://mail.mozilla.org/pipermail/rust-dev/2011-November/000976.html

[nikomatsakis]

Do we want the trailing commas to be preserved? This patch always inserts them in the pretty printer, whether or not they were present in the original source.

[brson]

Yeah, agreed that to do this the pretty printer needs to remember whether the comma was used or not. We'll need an additional pp-exact test case that ensures commas aren't added to records without them.

[nikomatsakis]

It will be a rather uglier patch, though, as we will have to store in the AST whether there was a trailing comma, which will lead to modifications trickling around...and there are a lot of cases to consider. pat_rec, call arguments, and so forth. To what extent does the pretty printer preserve your formatting today? Maybe it should just never or always emit trailing commas and so forth.

[brson]

The main place the pretty-printer attempts to preserve formatting now is with comments and whitespace. It's fair for the pp to just be opinionated, so maybe this is fine as-is and not worth the mess of tacking on a flag to the AST.

[lht]

To make the result less "ugly", maybe the pp should only preserve the trailing comma if the record is not wrapped in a single line. I haven't read how pp works, but I guess no flag is needed because we can check if the line number is advanced.

[brson]

I don't believe there is a general way for the pp to know that what it's printing is going to end up causing a line break.

[brson]

I pulled this as-is