Improve heuristics for match arm body placement

[marcusklaas]

Implement some extremely basic heuristic for the placement of the match body arm. We place it on the same line if it fits and contains no more line breaks compared to putting it on the next lines.

This should close https://github.com/nrc/rustfmt/issues/280.

Oh and it also moves some match tests from the tests/**/expr.rs to tests/**/match.rs.

[marcusklaas]

Rebased.

[nrc]

What bug is the second commit addressing?

[marcusklaas]

This one: https://github.com/nrc/rustfmt/issues/228

That particular case was fixed earlier, but the bug was still present. The indentation would be off for multi-line bodies that started on the same line as the pattern.

[nrc]

The bool seems only to indicate if we took the second option, rather than actually checking for line breaks. It would be nice if we didn't need it tbh.

[marcusklaas]

It's a very vague comment. What I meant to say is that it indicates whether the result should be placed on a new line.

[nrc]

Could you add the test case from #280 please? Also a match arm like this:

            ast::ItemConst(ref typ, ref expr) =>
                self.process_static_or_const_item(item, &typ, &expr),

[nrc]

r+ with that

[nrc]

@marcusklaas So, having played with this, I wonder if this was a mistake (sorry). I think that perhaps what we should do is force a block if the arm body contains a new line. E.g.,

ast::ItemConst(ref typ, ref expr) =>
    self.process_static_or_const_item(item, &typ, &expr),

would be rewritten to

ast::ItemConst(ref typ, ref expr) => {
    self.process_static_or_const_item(item, &typ, &expr),
}

Then it doesn't matter if we screw up formatting in a tiny space, any newline forces a block, and a block always has maximal space. That saves us using a heuristic, and gives (arguably) better formatting, at the expense of an extra line per match arm (which doesn't seem too bad to me). What do you think?

[marcusklaas]

Fully agreed. This is actually what I had in mind with this issue: https://github.com/nrc/rustfmt/issues/390. Let's do it!