auto merge of #13398 : nick29581/rust/unsized-enum, r=nikomatsakis

Now with proper checking of enums and allows unsized fields as the last field in a struct or variant. This PR only checks passing of unsized types and distinguishing them from sized ones. To be safe we also need to control storage.

Closes issues #12969 and #13121, supersedes #13375 (all the discussion there is valid here too).

src/librustdoc/visit_ast.rs

@@ -261,7 +261,7 @@ impl<'a> RustdocVisitor<'a> {
                 };
                 om.statics.push(s);
             },
-            ast::ItemTrait(ref gen, ref tr, ref met) => {
+            ast::ItemTrait(ref gen, _, ref tr, ref met) => {
                 let t = Trait {
                     name: item.ident,
                     methods: met.iter().map(|x| (*x).clone()).collect(),