auto merge of #14696 : jakub-/rust/dead-struct-fields, r=alexcrichton

This uncovered some dead code, most notably in middle/liveness.rs, which I think suggests there must be something fishy with that part of the code.

The #[allow(dead_code)] annotations on some of the fields I am not super happy about but as I understand, marker type may disappear at some point.

src/libsyntax/ast.rs

@@ -1053,6 +1053,15 @@ pub struct StructField_ {
     pub attrs: Vec<Attribute>,
 }
 
+impl StructField_ {
+    pub fn ident(&self) -> Option<Ident> {
+        match self.kind {
+            NamedField(ref ident, _) => Some(ident.clone()),
+            UnnamedField(_) => None
+        }
+    }
+}
+
 pub type StructField = Spanned<StructField_>;
 
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash)]