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 use util::ppaux;
  43 use util::ppaux::{ty_to_short_str, Repr};
  44
  45 use arena::TypedArena;
  46 use libc::c_uint;
  47 use syntax::ast;
  48
  49 pub fn trans_exchange_free_dyn<'blk, 'tcx>(cx: Block<'blk, 'tcx>,
  50                                            v: ValueRef,
  51                                            size: ValueRef,
  52                                            align: ValueRef,
  53                                            debug_loc: DebugLoc)
  54                                            -> Block<'blk, 'tcx> {
  55     let _icx = push_ctxt("trans_exchange_free");
  56     let ccx = cx.ccx();
  57     callee::trans_lang_call(cx,
  58         langcall(cx, None, "", ExchangeFreeFnLangItem),
  59         &[PointerCast(cx, v, Type::i8p(ccx)), size, align],
  60         Some(expr::Ignore),
  61         debug_loc).bcx
  62 }
  63
  64 pub fn trans_exchange_free<'blk, 'tcx>(cx: Block<'blk, 'tcx>,
  65                                        v: ValueRef,
  66                                        size: u64,
  67                                        align: u32,
  68                                        debug_loc: DebugLoc)
  69                                        -> Block<'blk, 'tcx> {
  70     trans_exchange_free_dyn(cx,
  71                             v,
  72                             C_uint(cx.ccx(), size),
  73                             C_uint(cx.ccx(), align),
  74                             debug_loc)
  75 }
  76
  77 pub fn trans_exchange_free_ty<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
  78                                           ptr: ValueRef,
  79                                           content_ty: Ty<'tcx>,
  80                                           debug_loc: DebugLoc)
  81                                           -> Block<'blk, 'tcx> {
  82     assert!(type_is_sized(bcx.ccx().tcx(), content_ty));
  83     let sizing_type = sizing_type_of(bcx.ccx(), content_ty);
  84     let content_size = llsize_of_alloc(bcx.ccx(), sizing_type);
  85
  86     // `Box<ZeroSizeType>` does not allocate.
  87     if content_size != 0 {
  88         let content_align = align_of(bcx.ccx(), content_ty);
  89         trans_exchange_free(bcx, ptr, content_size, content_align, debug_loc)
  90     } else {
  91         bcx
  92     }
  93 }
  94
  95 pub fn get_drop_glue_type<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
  96                                     t: Ty<'tcx>) -> Ty<'tcx> {
  97     let tcx = ccx.tcx();
  98     // Even if there is no dtor for t, there might be one deeper down and we
  99     // might need to pass in the vtable ptr.
 100     if !type_is_sized(tcx, t) {
 101         return t
 102     }
 103
 104     // FIXME (#22815): note that type_needs_drop conservatively
 105     // approximates in some cases and may say a type expression
 106     // requires drop glue when it actually does not.
 107     //
 108     // (In this case it is not clear whether any harm is done, i.e.
 109     // erroneously returning `t` in some cases where we could have
 110     // returned `tcx.types.i8` does not appear unsound. The impact on
 111     // code quality is unknown at this time.)
 112
 113     if !type_needs_drop(tcx, t) {
 114         return tcx.types.i8;
 115     }
 116     match t.sty {
 117         ty::ty_uniq(typ) if !type_needs_drop(tcx, typ)
 118                          && type_is_sized(tcx, typ) => {
 119             let llty = sizing_type_of(ccx, typ);
 120             // `Box<ZeroSizeType>` does not allocate.
 121             if llsize_of_alloc(ccx, llty) == 0 {
 122                 tcx.types.i8
 123             } else {
 124                 t
 125             }
 126         }
 127         _ => t
 128     }
 129 }
 130
 131 pub fn drop_ty<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 132                            v: ValueRef,
 133                            t: Ty<'tcx>,
 134                            debug_loc: DebugLoc) -> Block<'blk, 'tcx> {
 135     drop_ty_core(bcx, v, t, debug_loc, false)
 136 }
 137
 138 pub fn drop_ty_core<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 139                                 v: ValueRef,
 140                                 t: Ty<'tcx>,
 141                                 debug_loc: DebugLoc,
 142                                 skip_dtor: bool) -> Block<'blk, 'tcx> {
 143     // NB: v is an *alias* of type t here, not a direct value.
 144     debug!("drop_ty_core(t={}, skip_dtor={})", t.repr(bcx.tcx()), skip_dtor);
 145     let _icx = push_ctxt("drop_ty");
 146     if bcx.fcx.type_needs_drop(t) {
 147         let ccx = bcx.ccx();
 148         let g = if skip_dtor {
 149             DropGlueKind::TyContents(t)
 150         } else {
 151             DropGlueKind::Ty(t)
 152         };
 153         let glue = get_drop_glue_core(ccx, g);
 154         let glue_type = get_drop_glue_type(ccx, t);
 155         let ptr = if glue_type != t {
 156             PointerCast(bcx, v, type_of(ccx, glue_type).ptr_to())
 157         } else {
 158             v
 159         };
 160
 161         Call(bcx, glue, &[ptr], None, debug_loc);
 162     }
 163     bcx
 164 }
 165
 166 pub fn drop_ty_immediate<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 167                                      v: ValueRef,
 168                                      t: Ty<'tcx>,
 169                                      debug_loc: DebugLoc,
 170                                      skip_dtor: bool)
 171                                      -> Block<'blk, 'tcx> {
 172     let _icx = push_ctxt("drop_ty_immediate");
 173     let vp = alloca(bcx, type_of(bcx.ccx(), t), "");
 174     store_ty(bcx, v, vp, t);
 175     drop_ty_core(bcx, vp, t, debug_loc, skip_dtor)
 176 }
 177
 178 pub fn get_drop_glue<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, t: Ty<'tcx>) -> ValueRef {
 179     get_drop_glue_core(ccx, DropGlueKind::Ty(t))
 180 }
 181
 182 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 183 pub enum DropGlueKind<'tcx> {
 184     /// The normal path; runs the dtor, and then recurs on the contents
 185     Ty(Ty<'tcx>),
 186     /// Skips the dtor, if any, for ty; drops the contents directly.
 187     /// Note that the dtor is only skipped at the most *shallow*
 188     /// level, namely, an `impl Drop for Ty` itself. So, for example,
 189     /// if Ty is Newtype(S) then only the Drop impl for for Newtype
 190     /// itself will be skipped, while the Drop impl for S, if any,
 191     /// will be invoked.
 192     TyContents(Ty<'tcx>),
 193 }
 194
 195 impl<'tcx> DropGlueKind<'tcx> {
 196     fn ty(&self) -> Ty<'tcx> {
 197         match *self { DropGlueKind::Ty(t) | DropGlueKind::TyContents(t) => t }
 198     }
 199
 200     fn map_ty<F>(&self, mut f: F) -> DropGlueKind<'tcx> where F: FnMut(Ty<'tcx>) -> Ty<'tcx>
 201     {
 202         match *self {
 203             DropGlueKind::Ty(t) => DropGlueKind::Ty(f(t)),
 204             DropGlueKind::TyContents(t) => DropGlueKind::TyContents(f(t)),
 205         }
 206     }
 207
 208     fn to_string<'a>(&self, ccx: &CrateContext<'a, 'tcx>) -> String {
 209         let t_str = ppaux::ty_to_string(ccx.tcx(), self.ty());
 210         match *self {
 211             DropGlueKind::Ty(_) => format!("DropGlueKind::Ty({})", t_str),
 212             DropGlueKind::TyContents(_) => format!("DropGlueKind::TyContents({})", t_str),
 213         }
 214     }
 215 }
 216
 217 fn get_drop_glue_core<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 218                                 g: DropGlueKind<'tcx>) -> ValueRef {
 219     debug!("make drop glue for {}", g.to_string(ccx));
 220     let g = g.map_ty(|t| get_drop_glue_type(ccx, t));
 221     debug!("drop glue type {}", g.to_string(ccx));
 222     match ccx.drop_glues().borrow().get(&g) {
 223         Some(&glue) => return glue,
 224         _ => { }
 225     }
 226     let t = g.ty();
 227
 228     let llty = if type_is_sized(ccx.tcx(), t) {
 229         type_of(ccx, t).ptr_to()
 230     } else {
 231         type_of(ccx, ty::mk_uniq(ccx.tcx(), t)).ptr_to()
 232     };
 233
 234     let llfnty = Type::glue_fn(ccx, llty);
 235
 236     // To avoid infinite recursion, don't `make_drop_glue` until after we've
 237     // added the entry to the `drop_glues` cache.
 238     if let Some(old_sym) = ccx.available_drop_glues().borrow().get(&g) {
 239         let llfn = declare::declare_cfn(ccx, &old_sym, llfnty, ty::mk_nil(ccx.tcx()));
 240         ccx.drop_glues().borrow_mut().insert(g, llfn);
 241         return llfn;
 242     };
 243
 244     let fn_nm = mangle_internal_name_by_type_and_seq(ccx, t, "drop");
 245     let llfn = declare::define_cfn(ccx, &fn_nm, llfnty, ty::mk_nil(ccx.tcx())).unwrap_or_else(||{
 246        ccx.sess().bug(&format!("symbol `{}` already defined", fn_nm));
 247     });
 248     ccx.available_drop_glues().borrow_mut().insert(g, fn_nm);
 249
 250     let _s = StatRecorder::new(ccx, format!("drop {}", ty_to_short_str(ccx.tcx(), t)));
 251
 252     let empty_substs = ccx.tcx().mk_substs(Substs::trans_empty());
 253     let (arena, fcx): (TypedArena<_>, FunctionContext);
 254     arena = TypedArena::new();
 255     fcx = new_fn_ctxt(ccx, llfn, ast::DUMMY_NODE_ID, false,
 256                       ty::FnConverging(ty::mk_nil(ccx.tcx())),
 257                       empty_substs, None, &arena);
 258
 259     let bcx = init_function(&fcx, false, ty::FnConverging(ty::mk_nil(ccx.tcx())));
 260
 261     update_linkage(ccx, llfn, None, OriginalTranslation);
 262
 263     ccx.stats().n_glues_created.set(ccx.stats().n_glues_created.get() + 1);
 264     // All glue functions take values passed *by alias*; this is a
 265     // requirement since in many contexts glue is invoked indirectly and
 266     // the caller has no idea if it's dealing with something that can be
 267     // passed by value.
 268     //
 269     // llfn is expected be declared to take a parameter of the appropriate
 270     // type, so we don't need to explicitly cast the function parameter.
 271
 272     let llrawptr0 = get_param(llfn, fcx.arg_pos(0) as c_uint);
 273     let bcx = make_drop_glue(bcx, llrawptr0, g);
 274     finish_fn(&fcx, bcx, ty::FnConverging(ty::mk_nil(ccx.tcx())), DebugLoc::None);
 275
 276     llfn
 277 }
 278
 279 fn trans_struct_drop_flag<'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
 280                                       t: Ty<'tcx>,
 281                                       struct_data: ValueRef,
 282                                       dtor_did: ast::DefId,
 283                                       class_did: ast::DefId,
 284                                       substs: &subst::Substs<'tcx>)
 285                                       -> Block<'blk, 'tcx> {
 286     assert!(type_is_sized(bcx.tcx(), t), "Precondition: caller must ensure t is sized");
 287
 288     let repr = adt::represent_type(bcx.ccx(), t);
 289     let drop_flag = unpack_datum!(bcx, adt::trans_drop_flag_ptr(bcx, &*repr, struct_data));
 290     let loaded = load_ty(bcx, drop_flag.val, bcx.tcx().dtor_type());
 291     let drop_flag_llty = type_of(bcx.fcx.ccx, bcx.tcx().dtor_type());
 292     let init_val = C_integral(drop_flag_llty, adt::DTOR_NEEDED as u64, false);
 293
 294     let bcx = if !bcx.ccx().check_drop_flag_for_sanity() {
 295         bcx
 296     } else {
 297         let drop_flag_llty = type_of(bcx.fcx.ccx, bcx.tcx().dtor_type());
 298         let done_val = C_integral(drop_flag_llty, adt::DTOR_DONE as u64, false);
 299         let not_init = ICmp(bcx, llvm::IntNE, loaded, init_val, DebugLoc::None);
 300         let not_done = ICmp(bcx, llvm::IntNE, loaded, done_val, DebugLoc::None);
 301         let drop_flag_neither_initialized_nor_cleared =
 302             And(bcx, not_init, not_done, DebugLoc::None);
 303         with_cond(bcx, drop_flag_neither_initialized_nor_cleared, |cx| {
 304             let llfn = cx.ccx().get_intrinsic(&("llvm.debugtrap"));
 305             Call(cx, llfn, &[], None, DebugLoc::None);
 306             cx
 307         })
 308     };
 309
 310     let drop_flag_dtor_needed = ICmp(bcx, llvm::IntEQ, loaded, init_val, DebugLoc::None);
 311     with_cond(bcx, drop_flag_dtor_needed, |cx| {
 312         trans_struct_drop(cx, t, struct_data, dtor_did, class_did, substs)
 313     })
 314 }
 315
 316 pub fn get_res_dtor<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 317                               did: ast::DefId,
 318                               t: Ty<'tcx>,
 319                               parent_id: ast::DefId,
 320                               substs: &Substs<'tcx>)
 321                               -> ValueRef {
 322     let _icx = push_ctxt("trans_res_dtor");
 323     let did = inline::maybe_instantiate_inline(ccx, did);
 324
 325     if !substs.types.is_empty() {
 326         assert_eq!(did.krate, ast::LOCAL_CRATE);
 327
 328         // Since we're in trans we don't care for any region parameters
 329         let substs = ccx.tcx().mk_substs(Substs::erased(substs.types.clone()));
 330
 331         let (val, _, _) = monomorphize::monomorphic_fn(ccx, did, substs, None);
 332
 333         val
 334     } else if did.krate == ast::LOCAL_CRATE {
 335         get_item_val(ccx, did.node)
 336     } else {
 337         let tcx = ccx.tcx();
 338         let name = csearch::get_symbol(&ccx.sess().cstore, did);
 339         let class_ty = ty::lookup_item_type(tcx, parent_id).ty.subst(tcx, substs);
 340         let llty = type_of_dtor(ccx, class_ty);
 341         let dtor_ty = ty::mk_ctor_fn(ccx.tcx(),
 342                                      did,
 343                                      &[get_drop_glue_type(ccx, t)],
 344                                      ty::mk_nil(ccx.tcx()));
 345         foreign::get_extern_fn(ccx, &mut *ccx.externs().borrow_mut(), &name[..], llvm::CCallConv,
 346                                llty, dtor_ty)
 347     }
 348 }
 349
 350 fn trans_struct_drop<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 351                                  t: Ty<'tcx>,
 352                                  v0: ValueRef,
 353                                  dtor_did: ast::DefId,
 354                                  class_did: ast::DefId,
 355                                  substs: &subst::Substs<'tcx>)
 356                                  -> Block<'blk, 'tcx>
 357 {
 358     debug!("trans_struct_drop t: {}", bcx.ty_to_string(t));
 359
 360     // Find and call the actual destructor
 361     let dtor_addr = get_res_dtor(bcx.ccx(), dtor_did, t, class_did, substs);
 362
 363     // Class dtors have no explicit args, so the params should
 364     // just consist of the environment (self).
 365     let params = unsafe {
 366         let ty = Type::from_ref(llvm::LLVMTypeOf(dtor_addr));
 367         ty.element_type().func_params()
 368     };
 369     assert_eq!(params.len(), 1);
 370
 371     // Be sure to put the contents into a scope so we can use an invoke
 372     // instruction to call the user destructor but still call the field
 373     // destructors if the user destructor panics.
 374     //
 375     // FIXME (#14875) panic-in-drop semantics might be unsupported; we
 376     // might well consider changing below to more direct code.
 377     let contents_scope = bcx.fcx.push_custom_cleanup_scope();
 378
 379     // Issue #23611: schedule cleanup of contents, re-inspecting the
 380     // discriminant (if any) in case of variant swap in drop code.
 381     bcx.fcx.schedule_drop_adt_contents(cleanup::CustomScope(contents_scope), v0, t);
 382
 383     let glue_type = get_drop_glue_type(bcx.ccx(), t);
 384     let dtor_ty = ty::mk_ctor_fn(bcx.tcx(), class_did, &[glue_type], ty::mk_nil(bcx.tcx()));
 385     let (_, bcx) = invoke(bcx, dtor_addr, &[v0], dtor_ty, DebugLoc::None);
 386
 387     bcx.fcx.pop_and_trans_custom_cleanup_scope(bcx, contents_scope)
 388 }
 389
 390 pub fn size_and_align_of_dst<'blk, 'tcx>(bcx: Block<'blk, 'tcx>, t: Ty<'tcx>, info: ValueRef)
 391                                          -> (ValueRef, ValueRef) {
 392     debug!("calculate size of DST: {}; with lost info: {}",
 393            bcx.ty_to_string(t), bcx.val_to_string(info));
 394     if type_is_sized(bcx.tcx(), t) {
 395         let sizing_type = sizing_type_of(bcx.ccx(), t);
 396         let size = C_uint(bcx.ccx(), llsize_of_alloc(bcx.ccx(), sizing_type));
 397         let align = C_uint(bcx.ccx(), align_of(bcx.ccx(), t));
 398         return (size, align);
 399     }
 400     match t.sty {
 401         ty::ty_struct(id, substs) => {
 402             let ccx = bcx.ccx();
 403             // First get the size of all statically known fields.
 404             // Don't use type_of::sizing_type_of because that expects t to be sized.
 405             assert!(!ty::type_is_simd(bcx.tcx(), t));
 406             let repr = adt::represent_type(ccx, t);
 407             let sizing_type = adt::sizing_type_of(ccx, &*repr, true);
 408             let sized_size = C_uint(ccx, llsize_of_alloc(ccx, sizing_type));
 409             let sized_align = C_uint(ccx, llalign_of_min(ccx, sizing_type));
 410
 411             // Recurse to get the size of the dynamically sized field (must be
 412             // the last field).
 413             let fields = ty::struct_fields(bcx.tcx(), id, substs);
 414             let last_field = fields[fields.len()-1];
 415             let field_ty = last_field.mt.ty;
 416             let (unsized_size, unsized_align) = size_and_align_of_dst(bcx, field_ty, info);
 417
 418             // Return the sum of sizes and max of aligns.
 419             let size = Add(bcx, sized_size, unsized_size, DebugLoc::None);
 420             let align = Select(bcx,
 421                                ICmp(bcx,
 422                                     llvm::IntULT,
 423                                     sized_align,
 424                                     unsized_align,
 425                                     DebugLoc::None),
 426                                sized_align,
 427                                unsized_align);
 428             (size, align)
 429         }
 430         ty::ty_trait(..) => {
 431             // info points to the vtable and the second entry in the vtable is the
 432             // dynamic size of the object.
 433             let info = PointerCast(bcx, info, Type::int(bcx.ccx()).ptr_to());
 434             let size_ptr = GEPi(bcx, info, &[1]);
 435             let align_ptr = GEPi(bcx, info, &[2]);
 436             (Load(bcx, size_ptr), Load(bcx, align_ptr))
 437         }
 438         ty::ty_vec(_, None) | ty::ty_str => {
 439             let unit_ty = ty::sequence_element_type(bcx.tcx(), t);
 440             // The info in this case is the length of the str, so the size is that
 441             // times the unit size.
 442             let llunit_ty = sizing_type_of(bcx.ccx(), unit_ty);
 443             let unit_align = llalign_of_min(bcx.ccx(), llunit_ty);
 444             let unit_size = llsize_of_alloc(bcx.ccx(), llunit_ty);
 445             (Mul(bcx, info, C_uint(bcx.ccx(), unit_size), DebugLoc::None),
 446              C_uint(bcx.ccx(), unit_align))
 447         }
 448         _ => bcx.sess().bug(&format!("Unexpected unsized type, found {}",
 449                                     bcx.ty_to_string(t)))
 450     }
 451 }
 452
 453 fn make_drop_glue<'blk, 'tcx>(bcx: Block<'blk, 'tcx>, v0: ValueRef, g: DropGlueKind<'tcx>)
 454                               -> Block<'blk, 'tcx> {
 455     let t = g.ty();
 456     let skip_dtor = match g { DropGlueKind::Ty(_) => false, DropGlueKind::TyContents(_) => true };
 457     // NB: v0 is an *alias* of type t here, not a direct value.
 458     let _icx = push_ctxt("make_drop_glue");
 459
 460     // Only drop the value when it ... well, we used to check for
 461     // non-null, (and maybe we need to continue doing so), but we now
 462     // must definitely check for special bit-patterns corresponding to
 463     // the special dtor markings.
 464
 465     let inttype = Type::int(bcx.ccx());
 466     let dropped_pattern = C_integral(inttype, adt::dtor_done_usize(bcx.fcx.ccx) as u64, false);
 467
 468     match t.sty {
 469         ty::ty_uniq(content_ty) => {
 470             // Support for ty_uniq is built-in and its drop glue is
 471             // special. It may move to library and have Drop impl. As
 472             // a safe-guard, assert ty_uniq not used with TyContents.
 473             assert!(!skip_dtor);
 474             if !type_is_sized(bcx.tcx(), content_ty) {
 475                 let llval = GEPi(bcx, v0, &[0, abi::FAT_PTR_ADDR]);
 476                 let llbox = Load(bcx, llval);
 477                 let llbox_as_usize = PtrToInt(bcx, llbox, Type::int(bcx.ccx()));
 478                 let drop_flag_not_dropped_already =
 479                     ICmp(bcx, llvm::IntNE, llbox_as_usize, dropped_pattern, DebugLoc::None);
 480                 with_cond(bcx, drop_flag_not_dropped_already, |bcx| {
 481                     let bcx = drop_ty(bcx, v0, content_ty, DebugLoc::None);
 482                     let info = GEPi(bcx, v0, &[0, abi::FAT_PTR_EXTRA]);
 483                     let info = Load(bcx, info);
 484                     let (llsize, llalign) = size_and_align_of_dst(bcx, content_ty, info);
 485
 486                     // `Box<ZeroSizeType>` does not allocate.
 487                     let needs_free = ICmp(bcx,
 488                                           llvm::IntNE,
 489                                           llsize,
 490                                           C_uint(bcx.ccx(), 0u64),
 491                                           DebugLoc::None);
 492                     with_cond(bcx, needs_free, |bcx| {
 493                         trans_exchange_free_dyn(bcx, llbox, llsize, llalign, DebugLoc::None)
 494                     })
 495                 })
 496             } else {
 497                 let llval = v0;
 498                 let llbox = Load(bcx, llval);
 499                 let llbox_as_usize = PtrToInt(bcx, llbox, inttype);
 500                 let drop_flag_not_dropped_already =
 501                     ICmp(bcx, llvm::IntNE, llbox_as_usize, dropped_pattern, DebugLoc::None);
 502                 with_cond(bcx, drop_flag_not_dropped_already, |bcx| {
 503                     let bcx = drop_ty(bcx, llbox, content_ty, DebugLoc::None);
 504                     trans_exchange_free_ty(bcx, llbox, content_ty, DebugLoc::None)
 505                 })
 506             }
 507         }
 508         ty::ty_struct(did, substs) | ty::ty_enum(did, substs) => {
 509             let tcx = bcx.tcx();
 510             match (ty::ty_dtor(tcx, did), skip_dtor) {
 511                 (ty::TraitDtor(dtor, true), false) => {
 512                     // FIXME(16758) Since the struct is unsized, it is hard to
 513                     // find the drop flag (which is at the end of the struct).
 514                     // Lets just ignore the flag and pretend everything will be
 515                     // OK.
 516                     if type_is_sized(bcx.tcx(), t) {
 517                         trans_struct_drop_flag(bcx, t, v0, dtor, did, substs)
 518                     } else {
 519                         // Give the user a heads up that we are doing something
 520                         // stupid and dangerous.
 521                         bcx.sess().warn(&format!("Ignoring drop flag in destructor for {}\
 522                                                  because the struct is unsized. See issue\
 523                                                  #16758",
 524                                                 bcx.ty_to_string(t)));
 525                         trans_struct_drop(bcx, t, v0, dtor, did, substs)
 526                     }
 527                 }
 528                 (ty::TraitDtor(dtor, false), false) => {
 529                     trans_struct_drop(bcx, t, v0, dtor, did, substs)
 530                 }
 531                 (ty::NoDtor, _) | (_, true) => {
 532                     // No dtor? Just the default case
 533                     iter_structural_ty(bcx, v0, t, |bb, vv, tt| drop_ty(bb, vv, tt, DebugLoc::None))
 534                 }
 535             }
 536         }
 537         ty::ty_trait(..) => {
 538             // No support in vtable for distinguishing destroying with
 539             // versus without calling Drop::drop. Assert caller is
 540             // okay with always calling the Drop impl, if any.
 541             assert!(!skip_dtor);
 542             let data_ptr = GEPi(bcx, v0, &[0, abi::FAT_PTR_ADDR]);
 543             let vtable_ptr = Load(bcx, GEPi(bcx, v0, &[0, abi::FAT_PTR_EXTRA]));
 544             let dtor = Load(bcx, vtable_ptr);
 545             Call(bcx,
 546                  dtor,
 547                  &[PointerCast(bcx, Load(bcx, data_ptr), Type::i8p(bcx.ccx()))],
 548                  None,
 549                  DebugLoc::None);
 550             bcx
 551         }
 552         _ => {
 553             if bcx.fcx.type_needs_drop(t) {
 554                 iter_structural_ty(bcx,
 555                                    v0,
 556                                    t,
 557                                    |bb, vv, tt| drop_ty(bb, vv, tt, DebugLoc::None))
 558             } else {
 559                 bcx
 560             }
 561         }
 562     }
 563 }