src/librustc_trans/trans/glue.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //!
  12 //
  13 // Code relating to drop glue.
  14
  15
  16 use back::abi;
  17 use back::link::*;
  18 use llvm;
  19 use llvm::{ValueRef, get_param};
  20 use metadata::csearch;
  21 use middle::lang_items::ExchangeFreeFnLangItem;
  22 use middle::subst;
  23 use middle::subst::{Subst, Substs};
  24 use middle::ty::{self, Ty};
  25 use trans::adt;
  26 use trans::adt::GetDtorType; // for tcx.dtor_type()
  27 use trans::base::*;
  28 use trans::build::*;
  29 use trans::callee;
  30 use trans::cleanup;
  31 use trans::cleanup::CleanupMethods;
  32 use trans::common::*;
  33 use trans::debuginfo::DebugLoc;
  34 use trans::declare;
  35 use trans::expr;
  36 use trans::foreign;
  37 use trans::inline;
  38 use trans::machine::*;
  39 use trans::monomorphize;
  40 use trans::type_of::{type_of, type_of_dtor, sizing_type_of, align_of};
  41 use trans::type_::Type;
  42
  43 use arena::TypedArena;
  44 use libc::c_uint;
  45 use syntax::ast;
  46
  47 pub fn trans_exchange_free_dyn<'blk, 'tcx>(cx: Block<'blk, 'tcx>,
  48                                            v: ValueRef,
  49                                            size: ValueRef,
  50                                            align: ValueRef,
  51                                            debug_loc: DebugLoc)
  52                                            -> Block<'blk, 'tcx> {
  53     let _icx = push_ctxt("trans_exchange_free");
  54     let ccx = cx.ccx();
  55     callee::trans_lang_call(cx,
  56         langcall(cx, None, "", ExchangeFreeFnLangItem),
  57         &[PointerCast(cx, v, Type::i8p(ccx)), size, align],
  58         Some(expr::Ignore),
  59         debug_loc).bcx
  60 }
  61
  62 pub fn trans_exchange_free<'blk, 'tcx>(cx: Block<'blk, 'tcx>,
  63                                        v: ValueRef,
  64                                        size: u64,
  65                                        align: u32,
  66                                        debug_loc: DebugLoc)
  67                                        -> Block<'blk, 'tcx> {
  68     trans_exchange_free_dyn(cx,
  69                             v,
  70                             C_uint(cx.ccx(), size),
  71                             C_uint(cx.ccx(), align),
  72                             debug_loc)
  73 }
  74
  75 pub fn trans_exchange_free_ty<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
  76                                           ptr: ValueRef,
  77                                           content_ty: Ty<'tcx>,
  78                                           debug_loc: DebugLoc)
  79                                           -> Block<'blk, 'tcx> {
  80     assert!(type_is_sized(bcx.ccx().tcx(), content_ty));
  81     let sizing_type = sizing_type_of(bcx.ccx(), content_ty);
  82     let content_size = llsize_of_alloc(bcx.ccx(), sizing_type);
  83
  84     // `Box<ZeroSizeType>` does not allocate.
  85     if content_size != 0 {
  86         let content_align = align_of(bcx.ccx(), content_ty);
  87         trans_exchange_free(bcx, ptr, content_size, content_align, debug_loc)
  88     } else {
  89         bcx
  90     }
  91 }
  92
  93 pub fn get_drop_glue_type<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
  94                                     t: Ty<'tcx>) -> Ty<'tcx> {
  95     let tcx = ccx.tcx();
  96     // Even if there is no dtor for t, there might be one deeper down and we
  97     // might need to pass in the vtable ptr.
  98     if !type_is_sized(tcx, t) {
  99         return t
 100     }
 101
 102     // FIXME (#22815): note that type_needs_drop conservatively
 103     // approximates in some cases and may say a type expression
 104     // requires drop glue when it actually does not.
 105     //
 106     // (In this case it is not clear whether any harm is done, i.e.
 107     // erroneously returning `t` in some cases where we could have
 108     // returned `tcx.types.i8` does not appear unsound. The impact on
 109     // code quality is unknown at this time.)
 110
 111     if !type_needs_drop(tcx, t) {
 112         return tcx.types.i8;
 113     }
 114     match t.sty {
 115         ty::TyBox(typ) if !type_needs_drop(tcx, typ)
 116                          && type_is_sized(tcx, typ) => {
 117             let llty = sizing_type_of(ccx, typ);
 118             // `Box<ZeroSizeType>` does not allocate.
 119             if llsize_of_alloc(ccx, llty) == 0 {
 120                 tcx.types.i8
 121             } else {
 122                 t
 123             }
 124         }
 125         _ => t
 126     }
 127 }
 128
 129 pub fn drop_ty<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 130                            v: ValueRef,
 131                            t: Ty<'tcx>,
 132                            debug_loc: DebugLoc) -> Block<'blk, 'tcx> {
 133     drop_ty_core(bcx, v, t, debug_loc, false)
 134 }
 135
 136 pub fn drop_ty_core<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 137                                 v: ValueRef,
 138                                 t: Ty<'tcx>,
 139                                 debug_loc: DebugLoc,
 140                                 skip_dtor: bool) -> Block<'blk, 'tcx> {
 141     // NB: v is an *alias* of type t here, not a direct value.
 142     debug!("drop_ty_core(t={:?}, skip_dtor={})", t, skip_dtor);
 143     let _icx = push_ctxt("drop_ty");
 144     if bcx.fcx.type_needs_drop(t) {
 145         let ccx = bcx.ccx();
 146         let g = if skip_dtor {
 147             DropGlueKind::TyContents(t)
 148         } else {
 149             DropGlueKind::Ty(t)
 150         };
 151         let glue = get_drop_glue_core(ccx, g);
 152         let glue_type = get_drop_glue_type(ccx, t);
 153         let ptr = if glue_type != t {
 154             PointerCast(bcx, v, type_of(ccx, glue_type).ptr_to())
 155         } else {
 156             v
 157         };
 158
 159         Call(bcx, glue, &[ptr], None, debug_loc);
 160     }
 161     bcx
 162 }
 163
 164 pub fn drop_ty_immediate<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 165                                      v: ValueRef,
 166                                      t: Ty<'tcx>,
 167                                      debug_loc: DebugLoc,
 168                                      skip_dtor: bool)
 169                                      -> Block<'blk, 'tcx> {
 170     let _icx = push_ctxt("drop_ty_immediate");
 171     let vp = alloca(bcx, type_of(bcx.ccx(), t), "");
 172     store_ty(bcx, v, vp, t);
 173     drop_ty_core(bcx, vp, t, debug_loc, skip_dtor)
 174 }
 175
 176 pub fn get_drop_glue<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, t: Ty<'tcx>) -> ValueRef {
 177     get_drop_glue_core(ccx, DropGlueKind::Ty(t))
 178 }
 179
 180 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 181 pub enum DropGlueKind<'tcx> {
 182     /// The normal path; runs the dtor, and then recurs on the contents
 183     Ty(Ty<'tcx>),
 184     /// Skips the dtor, if any, for ty; drops the contents directly.
 185     /// Note that the dtor is only skipped at the most *shallow*
 186     /// level, namely, an `impl Drop for Ty` itself. So, for example,
 187     /// if Ty is Newtype(S) then only the Drop impl for for Newtype
 188     /// itself will be skipped, while the Drop impl for S, if any,
 189     /// will be invoked.
 190     TyContents(Ty<'tcx>),
 191 }
 192
 193 impl<'tcx> DropGlueKind<'tcx> {
 194     fn ty(&self) -> Ty<'tcx> {
 195         match *self { DropGlueKind::Ty(t) | DropGlueKind::TyContents(t) => t }
 196     }
 197
 198     fn map_ty<F>(&self, mut f: F) -> DropGlueKind<'tcx> where F: FnMut(Ty<'tcx>) -> Ty<'tcx>
 199     {
 200         match *self {
 201             DropGlueKind::Ty(t) => DropGlueKind::Ty(f(t)),
 202             DropGlueKind::TyContents(t) => DropGlueKind::TyContents(f(t)),
 203         }
 204     }
 205 }
 206
 207 fn get_drop_glue_core<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 208                                 g: DropGlueKind<'tcx>) -> ValueRef {
 209     debug!("make drop glue for {:?}", g);
 210     let g = g.map_ty(|t| get_drop_glue_type(ccx, t));
 211     debug!("drop glue type {:?}", g);
 212     match ccx.drop_glues().borrow().get(&g) {
 213         Some(&glue) => return glue,
 214         _ => { }
 215     }
 216     let t = g.ty();
 217
 218     let llty = if type_is_sized(ccx.tcx(), t) {
 219         type_of(ccx, t).ptr_to()
 220     } else {
 221         type_of(ccx, ty::mk_uniq(ccx.tcx(), t)).ptr_to()
 222     };
 223
 224     let llfnty = Type::glue_fn(ccx, llty);
 225
 226     // To avoid infinite recursion, don't `make_drop_glue` until after we've
 227     // added the entry to the `drop_glues` cache.
 228     if let Some(old_sym) = ccx.available_drop_glues().borrow().get(&g) {
 229         let llfn = declare::declare_cfn(ccx, &old_sym, llfnty, ty::mk_nil(ccx.tcx()));
 230         ccx.drop_glues().borrow_mut().insert(g, llfn);
 231         return llfn;
 232     };
 233
 234     let fn_nm = mangle_internal_name_by_type_and_seq(ccx, t, "drop");
 235     let llfn = declare::define_cfn(ccx, &fn_nm, llfnty, ty::mk_nil(ccx.tcx())).unwrap_or_else(||{
 236        ccx.sess().bug(&format!("symbol `{}` already defined", fn_nm));
 237     });
 238     ccx.available_drop_glues().borrow_mut().insert(g, fn_nm);
 239
 240     let _s = StatRecorder::new(ccx, format!("drop {:?}", t));
 241
 242     let empty_substs = ccx.tcx().mk_substs(Substs::trans_empty());
 243     let (arena, fcx): (TypedArena<_>, FunctionContext);
 244     arena = TypedArena::new();
 245     fcx = new_fn_ctxt(ccx, llfn, ast::DUMMY_NODE_ID, false,
 246                       ty::FnConverging(ty::mk_nil(ccx.tcx())),
 247                       empty_substs, None, &arena);
 248
 249     let bcx = init_function(&fcx, false, ty::FnConverging(ty::mk_nil(ccx.tcx())));
 250
 251     update_linkage(ccx, llfn, None, OriginalTranslation);
 252
 253     ccx.stats().n_glues_created.set(ccx.stats().n_glues_created.get() + 1);
 254     // All glue functions take values passed *by alias*; this is a
 255     // requirement since in many contexts glue is invoked indirectly and
 256     // the caller has no idea if it's dealing with something that can be
 257     // passed by value.
 258     //
 259     // llfn is expected be declared to take a parameter of the appropriate
 260     // type, so we don't need to explicitly cast the function parameter.
 261
 262     let llrawptr0 = get_param(llfn, fcx.arg_offset() as c_uint);
 263     let bcx = make_drop_glue(bcx, llrawptr0, g);
 264     finish_fn(&fcx, bcx, ty::FnConverging(ty::mk_nil(ccx.tcx())), DebugLoc::None);
 265
 266     llfn
 267 }
 268
 269 fn trans_struct_drop_flag<'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
 270                                       t: Ty<'tcx>,
 271                                       struct_data: ValueRef,
 272                                       dtor_did: ast::DefId,
 273                                       class_did: ast::DefId,
 274                                       substs: &subst::Substs<'tcx>)
 275                                       -> Block<'blk, 'tcx> {
 276     assert!(type_is_sized(bcx.tcx(), t), "Precondition: caller must ensure t is sized");
 277
 278     let repr = adt::represent_type(bcx.ccx(), t);
 279     let drop_flag = unpack_datum!(bcx, adt::trans_drop_flag_ptr(bcx, &*repr, struct_data));
 280     let loaded = load_ty(bcx, drop_flag.val, bcx.tcx().dtor_type());
 281     let drop_flag_llty = type_of(bcx.fcx.ccx, bcx.tcx().dtor_type());
 282     let init_val = C_integral(drop_flag_llty, adt::DTOR_NEEDED as u64, false);
 283
 284     let bcx = if !bcx.ccx().check_drop_flag_for_sanity() {
 285         bcx
 286     } else {
 287         let drop_flag_llty = type_of(bcx.fcx.ccx, bcx.tcx().dtor_type());
 288         let done_val = C_integral(drop_flag_llty, adt::DTOR_DONE as u64, false);
 289         let not_init = ICmp(bcx, llvm::IntNE, loaded, init_val, DebugLoc::None);
 290         let not_done = ICmp(bcx, llvm::IntNE, loaded, done_val, DebugLoc::None);
 291         let drop_flag_neither_initialized_nor_cleared =
 292             And(bcx, not_init, not_done, DebugLoc::None);
 293         with_cond(bcx, drop_flag_neither_initialized_nor_cleared, |cx| {
 294             let llfn = cx.ccx().get_intrinsic(&("llvm.debugtrap"));
 295             Call(cx, llfn, &[], None, DebugLoc::None);
 296             cx
 297         })
 298     };
 299
 300     let drop_flag_dtor_needed = ICmp(bcx, llvm::IntEQ, loaded, init_val, DebugLoc::None);
 301     with_cond(bcx, drop_flag_dtor_needed, |cx| {
 302         trans_struct_drop(cx, t, struct_data, dtor_did, class_did, substs)
 303     })
 304 }
 305
 306 pub fn get_res_dtor<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 307                               did: ast::DefId,
 308                               t: Ty<'tcx>,
 309                               parent_id: ast::DefId,
 310                               substs: &Substs<'tcx>)
 311                               -> ValueRef {
 312     let _icx = push_ctxt("trans_res_dtor");
 313     let did = inline::maybe_instantiate_inline(ccx, did);
 314
 315     if !substs.types.is_empty() {
 316         assert_eq!(did.krate, ast::LOCAL_CRATE);
 317
 318         // Since we're in trans we don't care for any region parameters
 319         let substs = ccx.tcx().mk_substs(Substs::erased(substs.types.clone()));
 320
 321         let (val, _, _) = monomorphize::monomorphic_fn(ccx, did, substs, None);
 322
 323         val
 324     } else if did.krate == ast::LOCAL_CRATE {
 325         get_item_val(ccx, did.node)
 326     } else {
 327         let tcx = ccx.tcx();
 328         let name = csearch::get_symbol(&ccx.sess().cstore, did);
 329         let class_ty = ty::lookup_item_type(tcx, parent_id).ty.subst(tcx, substs);
 330         let llty = type_of_dtor(ccx, class_ty);
 331         let dtor_ty = ty::mk_ctor_fn(ccx.tcx(),
 332                                      did,
 333                                      &[get_drop_glue_type(ccx, t)],
 334                                      ty::mk_nil(ccx.tcx()));
 335         foreign::get_extern_fn(ccx, &mut *ccx.externs().borrow_mut(), &name[..], llvm::CCallConv,
 336                                llty, dtor_ty)
 337     }
 338 }
 339
 340 fn trans_struct_drop<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 341                                  t: Ty<'tcx>,
 342                                  v0: ValueRef,
 343                                  dtor_did: ast::DefId,
 344                                  class_did: ast::DefId,
 345                                  substs: &subst::Substs<'tcx>)
 346                                  -> Block<'blk, 'tcx>
 347 {
 348     debug!("trans_struct_drop t: {}", t);
 349
 350     // Find and call the actual destructor
 351     let dtor_addr = get_res_dtor(bcx.ccx(), dtor_did, t, class_did, substs);
 352
 353     // Class dtors have no explicit args, so the params should
 354     // just consist of the environment (self).
 355     let params = unsafe {
 356         let ty = Type::from_ref(llvm::LLVMTypeOf(dtor_addr));
 357         ty.element_type().func_params()
 358     };
 359     assert_eq!(params.len(), 1);
 360
 361     // Be sure to put the contents into a scope so we can use an invoke
 362     // instruction to call the user destructor but still call the field
 363     // destructors if the user destructor panics.
 364     //
 365     // FIXME (#14875) panic-in-drop semantics might be unsupported; we
 366     // might well consider changing below to more direct code.
 367     let contents_scope = bcx.fcx.push_custom_cleanup_scope();
 368
 369     // Issue #23611: schedule cleanup of contents, re-inspecting the
 370     // discriminant (if any) in case of variant swap in drop code.
 371     bcx.fcx.schedule_drop_adt_contents(cleanup::CustomScope(contents_scope), v0, t);
 372
 373     let glue_type = get_drop_glue_type(bcx.ccx(), t);
 374     let dtor_ty = ty::mk_ctor_fn(bcx.tcx(), class_did, &[glue_type], ty::mk_nil(bcx.tcx()));
 375     let (_, bcx) = invoke(bcx, dtor_addr, &[v0], dtor_ty, DebugLoc::None);
 376
 377     bcx.fcx.pop_and_trans_custom_cleanup_scope(bcx, contents_scope)
 378 }
 379
 380 pub fn size_and_align_of_dst<'blk, 'tcx>(bcx: Block<'blk, 'tcx>, t: Ty<'tcx>, info: ValueRef)
 381                                          -> (ValueRef, ValueRef) {
 382     debug!("calculate size of DST: {}; with lost info: {}",
 383            t, bcx.val_to_string(info));
 384     if type_is_sized(bcx.tcx(), t) {
 385         let sizing_type = sizing_type_of(bcx.ccx(), t);
 386         let size = C_uint(bcx.ccx(), llsize_of_alloc(bcx.ccx(), sizing_type));
 387         let align = C_uint(bcx.ccx(), align_of(bcx.ccx(), t));
 388         return (size, align);
 389     }
 390     match t.sty {
 391         ty::TyStruct(id, substs) => {
 392             let ccx = bcx.ccx();
 393             // First get the size of all statically known fields.
 394             // Don't use type_of::sizing_type_of because that expects t to be sized.
 395             assert!(!ty::type_is_simd(bcx.tcx(), t));
 396             let repr = adt::represent_type(ccx, t);
 397             let sizing_type = adt::sizing_type_of(ccx, &*repr, true);
 398             let sized_size = C_uint(ccx, llsize_of_alloc(ccx, sizing_type));
 399             let sized_align = C_uint(ccx, llalign_of_min(ccx, sizing_type));
 400
 401             // Recurse to get the size of the dynamically sized field (must be
 402             // the last field).
 403             let fields = ty::struct_fields(bcx.tcx(), id, substs);
 404             let last_field = fields[fields.len()-1];
 405             let field_ty = last_field.mt.ty;
 406             let (unsized_size, unsized_align) = size_and_align_of_dst(bcx, field_ty, info);
 407
 408             // Return the sum of sizes and max of aligns.
 409             let size = Add(bcx, sized_size, unsized_size, DebugLoc::None);
 410             let align = Select(bcx,
 411                                ICmp(bcx,
 412                                     llvm::IntULT,
 413                                     sized_align,
 414                                     unsized_align,
 415                                     DebugLoc::None),
 416                                sized_align,
 417                                unsized_align);
 418             (size, align)
 419         }
 420         ty::TyTrait(..) => {
 421             // info points to the vtable and the second entry in the vtable is the
 422             // dynamic size of the object.
 423             let info = PointerCast(bcx, info, Type::int(bcx.ccx()).ptr_to());
 424             let size_ptr = GEPi(bcx, info, &[1]);
 425             let align_ptr = GEPi(bcx, info, &[2]);
 426             (Load(bcx, size_ptr), Load(bcx, align_ptr))
 427         }
 428         ty::TySlice(_) | ty::TyStr => {
 429             let unit_ty = ty::sequence_element_type(bcx.tcx(), t);
 430             // The info in this case is the length of the str, so the size is that
 431             // times the unit size.
 432             let llunit_ty = sizing_type_of(bcx.ccx(), unit_ty);
 433             let unit_align = llalign_of_min(bcx.ccx(), llunit_ty);
 434             let unit_size = llsize_of_alloc(bcx.ccx(), llunit_ty);
 435             (Mul(bcx, info, C_uint(bcx.ccx(), unit_size), DebugLoc::None),
 436              C_uint(bcx.ccx(), unit_align))
 437         }
 438         _ => bcx.sess().bug(&format!("Unexpected unsized type, found {}", t))
 439     }
 440 }
 441
 442 fn make_drop_glue<'blk, 'tcx>(bcx: Block<'blk, 'tcx>, v0: ValueRef, g: DropGlueKind<'tcx>)
 443                               -> Block<'blk, 'tcx> {
 444     let t = g.ty();
 445     let skip_dtor = match g { DropGlueKind::Ty(_) => false, DropGlueKind::TyContents(_) => true };
 446     // NB: v0 is an *alias* of type t here, not a direct value.
 447     let _icx = push_ctxt("make_drop_glue");
 448
 449     // Only drop the value when it ... well, we used to check for
 450     // non-null, (and maybe we need to continue doing so), but we now
 451     // must definitely check for special bit-patterns corresponding to
 452     // the special dtor markings.
 453
 454     let inttype = Type::int(bcx.ccx());
 455     let dropped_pattern = C_integral(inttype, adt::dtor_done_usize(bcx.fcx.ccx) as u64, false);
 456
 457     match t.sty {
 458         ty::TyBox(content_ty) => {
 459             // Support for TyBox is built-in and its drop glue is
 460             // special. It may move to library and have Drop impl. As
 461             // a safe-guard, assert TyBox not used with TyContents.
 462             assert!(!skip_dtor);
 463             if !type_is_sized(bcx.tcx(), content_ty) {
 464                 let llval = GEPi(bcx, v0, &[0, abi::FAT_PTR_ADDR]);
 465                 let llbox = Load(bcx, llval);
 466                 let llbox_as_usize = PtrToInt(bcx, llbox, Type::int(bcx.ccx()));
 467                 let drop_flag_not_dropped_already =
 468                     ICmp(bcx, llvm::IntNE, llbox_as_usize, dropped_pattern, DebugLoc::None);
 469                 with_cond(bcx, drop_flag_not_dropped_already, |bcx| {
 470                     let bcx = drop_ty(bcx, v0, content_ty, DebugLoc::None);
 471                     let info = GEPi(bcx, v0, &[0, abi::FAT_PTR_EXTRA]);
 472                     let info = Load(bcx, info);
 473                     let (llsize, llalign) = size_and_align_of_dst(bcx, content_ty, info);
 474
 475                     // `Box<ZeroSizeType>` does not allocate.
 476                     let needs_free = ICmp(bcx,
 477                                           llvm::IntNE,
 478                                           llsize,
 479                                           C_uint(bcx.ccx(), 0u64),
 480                                           DebugLoc::None);
 481                     with_cond(bcx, needs_free, |bcx| {
 482                         trans_exchange_free_dyn(bcx, llbox, llsize, llalign, DebugLoc::None)
 483                     })
 484                 })
 485             } else {
 486                 let llval = v0;
 487                 let llbox = Load(bcx, llval);
 488                 let llbox_as_usize = PtrToInt(bcx, llbox, inttype);
 489                 let drop_flag_not_dropped_already =
 490                     ICmp(bcx, llvm::IntNE, llbox_as_usize, dropped_pattern, DebugLoc::None);
 491                 with_cond(bcx, drop_flag_not_dropped_already, |bcx| {
 492                     let bcx = drop_ty(bcx, llbox, content_ty, DebugLoc::None);
 493                     trans_exchange_free_ty(bcx, llbox, content_ty, DebugLoc::None)
 494                 })
 495             }
 496         }
 497         ty::TyStruct(did, substs) | ty::TyEnum(did, substs) => {
 498             let tcx = bcx.tcx();
 499             match (ty::ty_dtor(tcx, did), skip_dtor) {
 500                 (ty::TraitDtor(dtor, true), false) => {
 501                     // FIXME(16758) Since the struct is unsized, it is hard to
 502                     // find the drop flag (which is at the end of the struct).
 503                     // Lets just ignore the flag and pretend everything will be
 504                     // OK.
 505                     if type_is_sized(bcx.tcx(), t) {
 506                         trans_struct_drop_flag(bcx, t, v0, dtor, did, substs)
 507                     } else {
 508                         // Give the user a heads up that we are doing something
 509                         // stupid and dangerous.
 510                         bcx.sess().warn(&format!("Ignoring drop flag in destructor for {}\
 511                                                  because the struct is unsized. See issue\
 512                                                  #16758", t));
 513                         trans_struct_drop(bcx, t, v0, dtor, did, substs)
 514                     }
 515                 }
 516                 (ty::TraitDtor(dtor, false), false) => {
 517                     trans_struct_drop(bcx, t, v0, dtor, did, substs)
 518                 }
 519                 (ty::NoDtor, _) | (_, true) => {
 520                     // No dtor? Just the default case
 521                     iter_structural_ty(bcx, v0, t, |bb, vv, tt| drop_ty(bb, vv, tt, DebugLoc::None))
 522                 }
 523             }
 524         }
 525         ty::TyTrait(..) => {
 526             // No support in vtable for distinguishing destroying with
 527             // versus without calling Drop::drop. Assert caller is
 528             // okay with always calling the Drop impl, if any.
 529             assert!(!skip_dtor);
 530             let data_ptr = GEPi(bcx, v0, &[0, abi::FAT_PTR_ADDR]);
 531             let vtable_ptr = Load(bcx, GEPi(bcx, v0, &[0, abi::FAT_PTR_EXTRA]));
 532             let dtor = Load(bcx, vtable_ptr);
 533             Call(bcx,
 534                  dtor,
 535                  &[PointerCast(bcx, Load(bcx, data_ptr), Type::i8p(bcx.ccx()))],
 536                  None,
 537                  DebugLoc::None);
 538             bcx
 539         }
 540         _ => {
 541             if bcx.fcx.type_needs_drop(t) {
 542                 iter_structural_ty(bcx,
 543                                    v0,
 544                                    t,
 545                                    |bb, vv, tt| drop_ty(bb, vv, tt, DebugLoc::None))
 546             } else {
 547                 bcx
 548             }
 549         }
 550     }
 551 }