Allow ref and mut modifiers for short form field patterns

[sfackler]

Previously, if you wanted to bind a field mutably or by ref, you had to
do something like Foo { x: ref mut x }. You can now just do
Foo { ref mut x }.

Closes #6137

[huonw]

Could the test include Foo { mut x } and Foo { ref x }?

[alexcrichton]

This is amazing! I've always wanted this.

If you expand the test coverage I'd r+. In addition to what @huonw mentioned, it'd be nice to see some coverage of making sure that 'ref mut' requires the binding to mutable, 'ref' doesn't move, things like that.

[huonw]

Also, probably worth checking multiple fields Foo { ref x, ref y }, and combinations like Foo { x: whatever, ref y } and Foo { mut y, .. } work.

[sfackler]

Will do

[brson]

Since this is a change to the language we should probably discuss it in a meeting.

[pcwalton]

+1