fix dropck overflow error message

Perf numbers:

Before this patchset:
572.70user 5.52system 7:33.21elapsed 127%CPU (0avgtext+0avgdata 1173368maxresident)k
llvm-time: 385.858

After this patch:
557.84user 5.73system 7:22.10elapsed 127%CPU (0avgtext+0avgdata 1142848maxresident)k
llvm-time: 385.834

nice 2.5% perf improvement

src/librustc_typeck/check/dropck.rs

@@ -18,6 +18,7 @@ use util::nodemap::FnvHashSet;
 
 use syntax::ast;
 use syntax::codemap::{self, Span};
+use syntax::parse::token::special_idents;
 
 /// check_drop_impl confirms that the Drop implementation identfied by
 /// `drop_impl_did` is not any more specialized than the type it is
@@ -286,27 +287,26 @@ pub fn check_safety_of_destructor_if_necessary<'a, 'tcx>(rcx: &mut Rcx<'a, 'tcx>
  // was somehow on the root.
  }
  TypeContext::ADT { def_id, variant, field, field_index } => {
- // FIXME (pnkfelix): eventually lookup arg_name
- // for the given index on struct variants.
- // TODO: be saner
- if let ty::ADTKind::Enum = tcx.lookup_adt_def(def_id).adt_kind() {
- span_note!(
- rcx.tcx().sess,
- span,
- "overflowed on enum {} variant {} argument {} type: {}",
- tcx.item_path_str(def_id),
- variant,
- field_index,
- detected_on_typ);
+ let adt = tcx.lookup_adt_def(def_id);
+ let variant_name = match adt.adt_kind() {
+ ty::ADTKind::Enum => format!("enum {} variant {}",
+ tcx.item_path_str(def_id),
+ variant),
+ ty::ADTKind::Struct => format!("struct {}",
+ tcx.item_path_str(def_id))
+ };
+ let field_name = if field == special_idents::unnamed_field.name {
+ format!("#{}", field_index)
  } else {
- span_note!(
- rcx.tcx().sess,
- span,
- "overflowed on struct {} field {} type: {}",
- tcx.item_path_str(def_id),
- field,
- detected_on_typ);
- }
+ format!("`{}`", field)
+ };
+ span_note!(
+ rcx.tcx().sess,
+ span,
+ "overflowed on {} field {} type: {}",
+ variant_name,
+ field_name,
+ detected_on_typ);
  }
  }
  }