Do not rewind parser and ignore following blocks

When encountering `pub ident`, attempt to identify the code that comes
afterwards, wether it is a brace block (assume it is a struct), a paren
list followed by a colon (assume struct) or a paren list followed by a
block (assume a fn). Consume those blocks to avoid any further parser
errors and return a `Placeholder` item in order to allow the parser to
continue. In the case of unenclosed blocks, the behavior is the same as
it is currently: no further errors are processed.

Author: estebank

src/librustc/hir/lowering.rs

@@ -1757,7 +1757,9 @@ impl<'a> LoweringContext<'a> {
                                bounds,
                                items)
             }
-            ItemKind::MacroDef(..) | ItemKind::Mac(..) => panic!("Shouldn't still be around"),
+            ItemKind::Placeholder | ItemKind::MacroDef(..) | ItemKind::Mac(..) => {
+                panic!("Shouldn't still be around")
+            }
         }
 
         // [1] `defaultness.has_value()` is never called for an `impl`, always `true` in order to

src/librustc/hir/map/def_collector.rs

@@ -113,6 +113,7 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
                 return visit::walk_item(self, i);
             }
             ItemKind::Mod(..) => DefPathData::Module(i.ident.name.as_str()),
+            ItemKind::Placeholder |
             ItemKind::Static(..) | ItemKind::Const(..) | ItemKind::Fn(..) =>
                 DefPathData::ValueNs(i.ident.name.as_str()),
             ItemKind::MacroDef(..) => DefPathData::MacroDef(i.ident.name.as_str()),

src/librustc_resolve/build_reduced_graph.rs

@@ -269,7 +269,7 @@ impl<'a> Resolver<'a> {
                 self.define(parent, ident, TypeNS, imported_binding);
             }
 
-            ItemKind::GlobalAsm(..) => {}
+            ItemKind::GlobalAsm(..) | ItemKind::Placeholder => {}
 
             ItemKind::Mod(..) if item.ident == keywords::Invalid.ident() => {} // Crate root
 

src/librustc_resolve/lib.rs

@@ -1947,7 +1947,8 @@ impl<'a> Resolver<'a> {
                 }
             }
 
-            ItemKind::ExternCrate(_) | ItemKind::MacroDef(..) | ItemKind::GlobalAsm(_)=> {
+            ItemKind::ExternCrate(_) | ItemKind::MacroDef(..) | ItemKind::GlobalAsm(_) |
+            ItemKind::Placeholder => {
                 // do nothing, these are just around to be encoded
             }
 

src/librustc_save_analysis/sig.rs

@@ -548,6 +548,7 @@ impl Sig for ast::Item {
             // FIXME should implement this (e.g., pub use).
             ast::ItemKind::Use(_) => Err("import"),
             ast::ItemKind::Mac(..) | ast::ItemKind::MacroDef(_) => Err("Macro"),
+            ast::ItemKind::Placeholder => Err("placeholder"),
         }
     }
 }

src/libsyntax/ast.rs

@@ -1977,6 +1977,7 @@ pub enum ItemKind {
 
     /// A macro definition.
     MacroDef(MacroDef),
+    Placeholder,
 }
 
 impl ItemKind {
@@ -1998,6 +1999,7 @@ impl ItemKind {
             ItemKind::Mac(..) |
             ItemKind::MacroDef(..) |
             ItemKind::Impl(..) |
+            ItemKind::Placeholder |
             ItemKind::AutoImpl(..) => "item"
         }
     }

src/libsyntax/fold.rs

@@ -935,6 +935,7 @@ pub fn noop_fold_item_kind<T: Folder>(i: ItemKind, folder: &mut T) -> ItemKind {
         ),
         ItemKind::Mac(m) => ItemKind::Mac(folder.fold_mac(m)),
         ItemKind::MacroDef(def) => ItemKind::MacroDef(folder.fold_macro_def(def)),
+        ItemKind::Placeholder => ItemKind::Placeholder,
     }
 }