auto merge of #9535 : alexcrichton/rust/no-format-default, r=thestinger

As mentioned in #9456, the format! syntax extension would previously consider an
empty format as a 'Unknown' format which could then also get coerced into a
different style of format on another argument.

This is unusual behavior because `{}` is a very common format and if you have
`{0} {0:?}` you wouldn't expect them both to be coereced to the `Poly`
formatter. This commit removes this coercion, but still retains the requirement
that each argument has exactly one format specified for it (an empty format now
counts as well).

Perhaps at a later date we can add support for multiple formats of one argument,
but this puts us in at least a backwards-compatible situation if we decide to do
that.

src/librustdoc/html/render.rs

@@ -1159,11 +1159,12 @@ impl<'self> fmt::Default for Sidebar<'self> {
                 write!(w, "<a class='{ty} {class}' href='{curty, select,
                                 mod{../}
                                 other{}
-                           }{ty, select,
+                           }{tysel, select,
                                 mod{{name}/index.html}
                                 other{#.{name}.html}
                            }'>{name}</a><br/>",
                        ty = short,
+                       tysel = short,
                        class = class,
                        curty = shortty(cur),
                        name = item.as_slice());