Port of pcwalton removal of #[unsafe_destructor] check.

Earlier commits impose rules on lifetimes that make generic
destructors safe; thus we no longer need the `#[unsafe_destructor]`
attribute nor its associated check.

----

So remove the check for the unsafe_destructor attribute.

And remove outdated compile-fail tests from when lifetime-parameteric
dtors were disallowed/unsafe.

In addition, when one uses the attribute without the associated
feature, report that the attribute is deprecated.

However, I do not think this is a breaking-change, because the
attribute and feature are still currently accepted by the compiler.
(After the next snapshot that has this commit, we can remove the
feature itself and the attribute as well.)

----

I consider this to:

Fix #22196

(techincally there is still the post snapshot work of removing the
last remants of the feature and the attribute, but the ticket can
still be closed in my opinion).

Author: pnkfelix

src/librustc_typeck/check/wf.rs

@@ -23,7 +23,6 @@ use util::ppaux::{Repr, UserString};
 use std::collections::HashSet;
 use syntax::ast;
 use syntax::ast_util::local_def;
-use syntax::attr;
 use syntax::codemap::Span;
 use syntax::parse::token::{self, special_idents};
 use syntax::visit;
@@ -250,22 +249,6 @@ 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, "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
@@ -761,22 +744,3 @@ fn filter_to_trait_obligations<'tcx>(bounds: ty::InstantiatedPredicates<'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

@@ -58,7 +58,6 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
     ("log_syntax", "1.0.0", Active),
     ("trace_macros", "1.0.0", Active),
     ("concat_idents", "1.0.0", Active),
-    ("unsafe_destructor", "1.0.0", Active),
     ("intrinsics", "1.0.0", Active),
     ("lang_items", "1.0.0", Active),
 
@@ -92,6 +91,10 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
     ("start", "1.0.0", Active),
     ("main", "1.0.0", Active),
 
+    // Deprecate after snapshot
+    // SNAP a923278
+    ("unsafe_destructor", "1.0.0", Active),
+
     // A temporary feature gate used to enable parser extensions needed
     // to bootstrap fix for #5723.
     ("issue_5723_bootstrap", "1.0.0", Accepted),
@@ -193,7 +196,6 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
     ("repr", Normal),
     ("path", Normal),
     ("abi", Normal),
-    ("unsafe_destructor", Normal),
     ("automatically_derived", Normal),
     ("no_mangle", Normal),
     ("no_link", Normal),
@@ -205,7 +207,8 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
     ("link_args", Normal),
     ("macro_escape", Normal),
 
-
+    ("unsafe_destructor", Gated("unsafe_destructor",
+                                "`#[unsafe_destructor]` does nothing anymore")),
     ("staged_api", Gated("staged_api",
                          "staged_api is for use by rustc only")),
     ("plugin", Gated("plugin",
@@ -571,15 +574,6 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                     _ => {}
                 }
 
-                if attr::contains_name(&i.attrs,
-                                       "unsafe_destructor") {
-                    self.gate_feature("unsafe_destructor",
-                                      i.span,
-                                      "`#[unsafe_destructor]` allows too \
-                                       many unsafe patterns and may be \
-                                       removed in the future");
-                }
-
                 if attr::contains_name(&i.attrs[..],
                                        "old_orphan_check") {
                     self.gate_feature(

src/test/compile-fail/gated-unsafe-destructor.rs

@@ -10,14 +10,18 @@
 
 // Test that `#[unsafe_destructor]` attribute is gated by `unsafe_destructor`
 // feature gate.
+//
+// (This test can be removed entirely when we remove the
+// `unsafe_destructor` feature itself.)
 
 struct D<'a>(&'a u32);
 
 #[unsafe_destructor]
+//~^ ERROR `#[unsafe_destructor]` does nothing anymore
+//~| HELP: add #![feature(unsafe_destructor)] to the crate attributes to enable
+// (but of couse there is no point in doing so)
 impl<'a> Drop for D<'a> {
-    //~^ ERROR `#[unsafe_destructor]` allows too many unsafe patterns
     fn drop(&mut self) { }
 }
-//~^ HELP: add #![feature(unsafe_destructor)] to the crate attributes to enable
 
 pub fn main() { }