port of pcwalton removal of #[unsafe_destructor] check.

Since the earlier commits impose rules on lifetimes that make
destructors safe, we no longer need the `#[unsafe_destructor]`
attribute nor its associated check.

----

Since this is removing a (somewhat common albeit unsafe) attribute,
this is a (wait for it)

[breaking-change]

Author: pnkfelix

src/librustc_typeck/check/wf.rs

@@ -206,22 +206,6 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
                                                         &fcx.inh.param_env.free_substs,
                                                         &trait_ref);
 
-            // There are special rules that apply to drop.
-            if
-                fcx.tcx().lang_items.drop_trait() == Some(trait_ref.def_id) &&
-                !attr::contains_name(item.attrs.as_slice(), "unsafe_destructor")
-            {
-                match self_ty.sty {
-                    ty::ty_struct(def_id, _) |
-                    ty::ty_enum(def_id, _) => {
-                        check_struct_safe_for_destructor(fcx, item.span, def_id);
-                    }
-                    _ => {
-                        // Coherence already reports an error in this case.
-                    }
-                }
-            }
-
             if fcx.tcx().lang_items.copy_trait() == Some(trait_ref.def_id) {
                 // This is checked in coherence.
                 return
@@ -595,22 +579,3 @@ fn filter_to_trait_obligations<'tcx>(bounds: ty::GenericBounds<'tcx>)
     }
     result
 }
-
-///////////////////////////////////////////////////////////////////////////
-// Special drop trait checking
-
-fn check_struct_safe_for_destructor<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
-                                              span: Span,
-                                              struct_did: ast::DefId) {
-    let struct_tpt = ty::lookup_item_type(fcx.tcx(), struct_did);
-    if struct_tpt.generics.has_type_params(subst::TypeSpace)
-        || struct_tpt.generics.has_region_params(subst::TypeSpace)
-    {
-        span_err!(fcx.tcx().sess, span, E0141,
-                  "cannot implement a destructor on a structure \
-                   with type parameters");
-        span_note!(fcx.tcx().sess, span,
-                   "use \"#[unsafe_destructor]\" on the implementation \
-                    to force the compiler to allow this");
-    }
-}

src/libsyntax/feature_gate.rs

@@ -300,9 +300,8 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                        "unsafe_destructor") {
                     self.gate_feature("unsafe_destructor",
                                       i.span,
-                                      "`#[unsafe_destructor]` allows too \
-                                       many unsafe patterns and may be \
-                                       removed in the future");
+                                      "`#[unsafe_destructor]` does nothing \
+                                       anymore")
                 }
 
                 if attr::contains_name(&i.attrs[],