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, None)
 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,
 141                                 drop_hint: Option<cleanup::DropHintValue>)
 142                                 -> 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={} drop_hint={:?})", t, skip_dtor, drop_hint);
 145     let _icx = push_ctxt("drop_ty");
 146     let mut bcx = bcx;
 147     if bcx.fcx.type_needs_drop(t) {
 148         let ccx = bcx.ccx();
 149         let g = if skip_dtor {
 150             DropGlueKind::TyContents(t)
 151         } else {
 152             DropGlueKind::Ty(t)
 153         };
 154         let glue = get_drop_glue_core(ccx, g);
 155         let glue_type = get_drop_glue_type(ccx, t);
 156         let ptr = if glue_type != t {
 157             PointerCast(bcx, v, type_of(ccx, glue_type).ptr_to())
 158         } else {
 159             v
 160         };
 161
 162         match drop_hint {
 163             Some(drop_hint) => {
 164                 let hint_val = load_ty(bcx, drop_hint.value(), bcx.tcx().types.u8);
 165                 let moved_val =
 166                     C_integral(Type::i8(bcx.ccx()), adt::DTOR_MOVED_HINT as u64, false);
 167                 let may_need_drop =
 168                     ICmp(bcx, llvm::IntNE, hint_val, moved_val, DebugLoc::None);
 169                 bcx = with_cond(bcx, may_need_drop, |cx| {
 170                     Call(cx, glue, &[ptr], None, debug_loc);
 171                     cx
 172                 })
 173             }
 174             None => {
 175                 // No drop-hint ==> call standard drop glue
 176                 Call(bcx, glue, &[ptr], None, debug_loc);
 177             }
 178         }
 179     }
 180     bcx
 181 }
 182
 183 pub fn drop_ty_immediate<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 184                                      v: ValueRef,
 185                                      t: Ty<'tcx>,
 186                                      debug_loc: DebugLoc,
 187                                      skip_dtor: bool)
 188                                      -> Block<'blk, 'tcx> {
 189     let _icx = push_ctxt("drop_ty_immediate");
 190     let vp = alloca(bcx, type_of(bcx.ccx(), t), "");
 191     store_ty(bcx, v, vp, t);
 192     drop_ty_core(bcx, vp, t, debug_loc, skip_dtor, None)
 193 }
 194
 195 pub fn get_drop_glue<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, t: Ty<'tcx>) -> ValueRef {
 196     get_drop_glue_core(ccx, DropGlueKind::Ty(t))
 197 }
 198
 199 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 200 pub enum DropGlueKind<'tcx> {
 201     /// The normal path; runs the dtor, and then recurs on the contents
 202     Ty(Ty<'tcx>),
 203     /// Skips the dtor, if any, for ty; drops the contents directly.
 204     /// Note that the dtor is only skipped at the most *shallow*
 205     /// level, namely, an `impl Drop for Ty` itself. So, for example,
 206     /// if Ty is Newtype(S) then only the Drop impl for for Newtype
 207     /// itself will be skipped, while the Drop impl for S, if any,
 208     /// will be invoked.
 209     TyContents(Ty<'tcx>),
 210 }
 211
 212 impl<'tcx> DropGlueKind<'tcx> {
 213     fn ty(&self) -> Ty<'tcx> {
 214         match *self { DropGlueKind::Ty(t) | DropGlueKind::TyContents(t) => t }
 215     }
 216
 217     fn map_ty<F>(&self, mut f: F) -> DropGlueKind<'tcx> where F: FnMut(Ty<'tcx>) -> Ty<'tcx>
 218     {
 219         match *self {
 220             DropGlueKind::Ty(t) => DropGlueKind::Ty(f(t)),
 221             DropGlueKind::TyContents(t) => DropGlueKind::TyContents(f(t)),
 222         }
 223     }
 224 }
 225
 226 fn get_drop_glue_core<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 227                                 g: DropGlueKind<'tcx>) -> ValueRef {
 228     debug!("make drop glue for {:?}", g);
 229     let g = g.map_ty(|t| get_drop_glue_type(ccx, t));
 230     debug!("drop glue type {:?}", g);
 231     match ccx.drop_glues().borrow().get(&g) {
 232         Some(&glue) => return glue,
 233         _ => { }
 234     }
 235     let t = g.ty();
 236
 237     let llty = if type_is_sized(ccx.tcx(), t) {
 238         type_of(ccx, t).ptr_to()
 239     } else {
 240         type_of(ccx, ccx.tcx().mk_box(t)).ptr_to()
 241     };
 242
 243     let llfnty = Type::glue_fn(ccx, llty);
 244
 245     // To avoid infinite recursion, don't `make_drop_glue` until after we've
 246     // added the entry to the `drop_glues` cache.
 247     if let Some(old_sym) = ccx.available_drop_glues().borrow().get(&g) {
 248         let llfn = declare::declare_cfn(ccx, &old_sym, llfnty, ccx.tcx().mk_nil());
 249         ccx.drop_glues().borrow_mut().insert(g, llfn);
 250         return llfn;
 251     };
 252
 253     let fn_nm = mangle_internal_name_by_type_and_seq(ccx, t, "drop");
 254     let llfn = declare::define_cfn(ccx, &fn_nm, llfnty, ccx.tcx().mk_nil()).unwrap_or_else(||{
 255        ccx.sess().bug(&format!("symbol `{}` already defined", fn_nm));
 256     });
 257     ccx.available_drop_glues().borrow_mut().insert(g, fn_nm);
 258
 259     let _s = StatRecorder::new(ccx, format!("drop {:?}", t));
 260
 261     let empty_substs = ccx.tcx().mk_substs(Substs::trans_empty());
 262     let (arena, fcx): (TypedArena<_>, FunctionContext);
 263     arena = TypedArena::new();
 264     fcx = new_fn_ctxt(ccx, llfn, ast::DUMMY_NODE_ID, false,
 265                       ty::FnConverging(ccx.tcx().mk_nil()),
 266                       empty_substs, None, &arena);
 267
 268     let bcx = init_function(&fcx, false, ty::FnConverging(ccx.tcx().mk_nil()));
 269
 270     update_linkage(ccx, llfn, None, OriginalTranslation);
 271
 272     ccx.stats().n_glues_created.set(ccx.stats().n_glues_created.get() + 1);
 273     // All glue functions take values passed *by alias*; this is a
 274     // requirement since in many contexts glue is invoked indirectly and
 275     // the caller has no idea if it's dealing with something that can be
 276     // passed by value.
 277     //
 278     // llfn is expected be declared to take a parameter of the appropriate
 279     // type, so we don't need to explicitly cast the function parameter.
 280
 281     let llrawptr0 = get_param(llfn, fcx.arg_offset() as c_uint);
 282     let bcx = make_drop_glue(bcx, llrawptr0, g);
 283     finish_fn(&fcx, bcx, ty::FnConverging(ccx.tcx().mk_nil()), DebugLoc::None);
 284
 285     llfn
 286 }
 287
 288 fn trans_struct_drop_flag<'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
 289                                       t: Ty<'tcx>,
 290                                       struct_data: ValueRef,
 291                                       dtor_did: ast::DefId,
 292                                       class_did: ast::DefId,
 293                                       substs: &subst::Substs<'tcx>)
 294                                       -> Block<'blk, 'tcx> {
 295     assert!(type_is_sized(bcx.tcx(), t), "Precondition: caller must ensure t is sized");
 296
 297     let repr = adt::represent_type(bcx.ccx(), t);
 298     let drop_flag = unpack_datum!(bcx, adt::trans_drop_flag_ptr(bcx, &*repr, struct_data));
 299     let loaded = load_ty(bcx, drop_flag.val, bcx.tcx().dtor_type());
 300     let drop_flag_llty = type_of(bcx.fcx.ccx, bcx.tcx().dtor_type());
 301     let init_val = C_integral(drop_flag_llty, adt::DTOR_NEEDED as u64, false);
 302
 303     let bcx = if !bcx.ccx().check_drop_flag_for_sanity() {
 304         bcx
 305     } else {
 306         let drop_flag_llty = type_of(bcx.fcx.ccx, bcx.tcx().dtor_type());
 307         let done_val = C_integral(drop_flag_llty, adt::DTOR_DONE as u64, false);
 308         let not_init = ICmp(bcx, llvm::IntNE, loaded, init_val, DebugLoc::None);
 309         let not_done = ICmp(bcx, llvm::IntNE, loaded, done_val, DebugLoc::None);
 310         let drop_flag_neither_initialized_nor_cleared =
 311             And(bcx, not_init, not_done, DebugLoc::None);
 312         with_cond(bcx, drop_flag_neither_initialized_nor_cleared, |cx| {
 313             let llfn = cx.ccx().get_intrinsic(&("llvm.debugtrap"));
 314             Call(cx, llfn, &[], None, DebugLoc::None);
 315             cx
 316         })
 317     };
 318
 319     let drop_flag_dtor_needed = ICmp(bcx, llvm::IntEQ, loaded, init_val, DebugLoc::None);
 320     with_cond(bcx, drop_flag_dtor_needed, |cx| {
 321         trans_struct_drop(cx, t, struct_data, dtor_did, class_did, substs)
 322     })
 323 }
 324
 325 pub fn get_res_dtor<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 326                               did: ast::DefId,
 327                               t: Ty<'tcx>,
 328                               parent_id: ast::DefId,
 329                               substs: &Substs<'tcx>)
 330                               -> ValueRef {
 331     let _icx = push_ctxt("trans_res_dtor");
 332     let did = inline::maybe_instantiate_inline(ccx, did);
 333
 334     if !substs.types.is_empty() {
 335         assert_eq!(did.krate, ast::LOCAL_CRATE);
 336
 337         // Since we're in trans we don't care for any region parameters
 338         let substs = ccx.tcx().mk_substs(Substs::erased(substs.types.clone()));
 339
 340         let (val, _, _) = monomorphize::monomorphic_fn(ccx, did, substs, None);
 341
 342         val
 343     } else if did.krate == ast::LOCAL_CRATE {
 344         get_item_val(ccx, did.node)
 345     } else {
 346         let tcx = ccx.tcx();
 347         let name = csearch::get_symbol(&ccx.sess().cstore, did);
 348         let class_ty = tcx.lookup_item_type(parent_id).ty.subst(tcx, substs);
 349         let llty = type_of_dtor(ccx, class_ty);
 350         let dtor_ty = ccx.tcx().mk_ctor_fn(did,
 351                                            &[get_drop_glue_type(ccx, t)],
 352                                            ccx.tcx().mk_nil());
 353         foreign::get_extern_fn(ccx, &mut *ccx.externs().borrow_mut(), &name[..], llvm::CCallConv,
 354                                llty, dtor_ty)
 355     }
 356 }
 357
 358 fn trans_struct_drop<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 359                                  t: Ty<'tcx>,
 360                                  v0: ValueRef,
 361                                  dtor_did: ast::DefId,
 362                                  class_did: ast::DefId,
 363                                  substs: &subst::Substs<'tcx>)
 364                                  -> Block<'blk, 'tcx>
 365 {
 366     debug!("trans_struct_drop t: {}", t);
 367
 368     // Find and call the actual destructor
 369     let dtor_addr = get_res_dtor(bcx.ccx(), dtor_did, t, class_did, substs);
 370
 371     // Class dtors have no explicit args, so the params should
 372     // just consist of the environment (self).
 373     let params = unsafe {
 374         let ty = Type::from_ref(llvm::LLVMTypeOf(dtor_addr));
 375         ty.element_type().func_params()
 376     };
 377     assert_eq!(params.len(), if type_is_sized(bcx.tcx(), t) { 1 } else { 2 });
 378
 379     // Be sure to put the contents into a scope so we can use an invoke
 380     // instruction to call the user destructor but still call the field
 381     // destructors if the user destructor panics.
 382     //
 383     // FIXME (#14875) panic-in-drop semantics might be unsupported; we
 384     // might well consider changing below to more direct code.
 385     let contents_scope = bcx.fcx.push_custom_cleanup_scope();
 386
 387     // Issue #23611: schedule cleanup of contents, re-inspecting the
 388     // discriminant (if any) in case of variant swap in drop code.
 389     bcx.fcx.schedule_drop_adt_contents(cleanup::CustomScope(contents_scope), v0, t);
 390
 391     let glue_type = get_drop_glue_type(bcx.ccx(), t);
 392     let dtor_ty = bcx.tcx().mk_ctor_fn(class_did, &[glue_type], bcx.tcx().mk_nil());
 393     let (_, bcx) = if type_is_sized(bcx.tcx(), t) {
 394         invoke(bcx, dtor_addr, &[v0], dtor_ty, DebugLoc::None)
 395     } else {
 396         let args = [Load(bcx, expr::get_dataptr(bcx, v0)), Load(bcx, expr::get_len(bcx, v0))];
 397         invoke(bcx, dtor_addr, &args, dtor_ty, DebugLoc::None)
 398     };
 399
 400     bcx.fcx.pop_and_trans_custom_cleanup_scope(bcx, contents_scope)
 401 }
 402
 403 pub fn size_and_align_of_dst<'blk, 'tcx>(bcx: Block<'blk, 'tcx>, t: Ty<'tcx>, info: ValueRef)
 404                                          -> (ValueRef, ValueRef) {
 405     debug!("calculate size of DST: {}; with lost info: {}",
 406            t, bcx.val_to_string(info));
 407     if type_is_sized(bcx.tcx(), t) {
 408         let sizing_type = sizing_type_of(bcx.ccx(), t);
 409         let size = C_uint(bcx.ccx(), llsize_of_alloc(bcx.ccx(), sizing_type));
 410         let align = C_uint(bcx.ccx(), align_of(bcx.ccx(), t));
 411         return (size, align);
 412     }
 413     match t.sty {
 414         ty::TyStruct(id, substs) => {
 415             let ccx = bcx.ccx();
 416             // First get the size of all statically known fields.
 417             // Don't use type_of::sizing_type_of because that expects t to be sized.
 418             assert!(!t.is_simd(bcx.tcx()));
 419             let repr = adt::represent_type(ccx, t);
 420             let sizing_type = adt::sizing_type_of(ccx, &*repr, true);
 421             let sized_size = C_uint(ccx, llsize_of_alloc(ccx, sizing_type));
 422             let sized_align = C_uint(ccx, llalign_of_min(ccx, sizing_type));
 423
 424             // Recurse to get the size of the dynamically sized field (must be
 425             // the last field).
 426             let fields = bcx.tcx().struct_fields(id, substs);
 427             let last_field = fields[fields.len()-1];
 428             let field_ty = last_field.mt.ty;
 429             let (unsized_size, unsized_align) = size_and_align_of_dst(bcx, field_ty, info);
 430
 431             // Return the sum of sizes and max of aligns.
 432             let size = Add(bcx, sized_size, unsized_size, DebugLoc::None);
 433             let align = Select(bcx,
 434                                ICmp(bcx,
 435                                     llvm::IntULT,
 436                                     sized_align,
 437                                     unsized_align,
 438                                     DebugLoc::None),
 439                                sized_align,
 440                                unsized_align);
 441             (size, align)
 442         }
 443         ty::TyTrait(..) => {
 444             // info points to the vtable and the second entry in the vtable is the
 445             // dynamic size of the object.
 446             let info = PointerCast(bcx, info, Type::int(bcx.ccx()).ptr_to());
 447             let size_ptr = GEPi(bcx, info, &[1]);
 448             let align_ptr = GEPi(bcx, info, &[2]);
 449             (Load(bcx, size_ptr), Load(bcx, align_ptr))
 450         }
 451         ty::TySlice(_) | ty::TyStr => {
 452             let unit_ty = t.sequence_element_type(bcx.tcx());
 453             // The info in this case is the length of the str, so the size is that
 454             // times the unit size.
 455             let llunit_ty = sizing_type_of(bcx.ccx(), unit_ty);
 456             let unit_align = llalign_of_min(bcx.ccx(), llunit_ty);
 457             let unit_size = llsize_of_alloc(bcx.ccx(), llunit_ty);
 458             (Mul(bcx, info, C_uint(bcx.ccx(), unit_size), DebugLoc::None),
 459              C_uint(bcx.ccx(), unit_align))
 460         }
 461         _ => bcx.sess().bug(&format!("Unexpected unsized type, found {}", t))
 462     }
 463 }
 464
 465 fn make_drop_glue<'blk, 'tcx>(bcx: Block<'blk, 'tcx>, v0: ValueRef, g: DropGlueKind<'tcx>)
 466                               -> Block<'blk, 'tcx> {
 467     let t = g.ty();
 468     let skip_dtor = match g { DropGlueKind::Ty(_) => false, DropGlueKind::TyContents(_) => true };
 469     // NB: v0 is an *alias* of type t here, not a direct value.
 470     let _icx = push_ctxt("make_drop_glue");
 471
 472     // Only drop the value when it ... well, we used to check for
 473     // non-null, (and maybe we need to continue doing so), but we now
 474     // must definitely check for special bit-patterns corresponding to
 475     // the special dtor markings.
 476
 477     let inttype = Type::int(bcx.ccx());
 478     let dropped_pattern = C_integral(inttype, adt::dtor_done_usize(bcx.fcx.ccx) as u64, false);
 479
 480     match t.sty {
 481         ty::TyBox(content_ty) => {
 482             // Support for TyBox is built-in and its drop glue is
 483             // special. It may move to library and have Drop impl. As
 484             // a safe-guard, assert TyBox not used with TyContents.
 485             assert!(!skip_dtor);
 486             if !type_is_sized(bcx.tcx(), content_ty) {
 487                 let llval = GEPi(bcx, v0, &[0, abi::FAT_PTR_ADDR]);
 488                 let llbox = Load(bcx, llval);
 489                 let llbox_as_usize = PtrToInt(bcx, llbox, Type::int(bcx.ccx()));
 490                 let drop_flag_not_dropped_already =
 491                     ICmp(bcx, llvm::IntNE, llbox_as_usize, dropped_pattern, DebugLoc::None);
 492                 with_cond(bcx, drop_flag_not_dropped_already, |bcx| {
 493                     let bcx = drop_ty(bcx, v0, content_ty, DebugLoc::None);
 494                     let info = GEPi(bcx, v0, &[0, abi::FAT_PTR_EXTRA]);
 495                     let info = Load(bcx, info);
 496                     let (llsize, llalign) = size_and_align_of_dst(bcx, content_ty, info);
 497
 498                     // `Box<ZeroSizeType>` does not allocate.
 499                     let needs_free = ICmp(bcx,
 500                                           llvm::IntNE,
 501                                           llsize,
 502                                           C_uint(bcx.ccx(), 0u64),
 503                                           DebugLoc::None);
 504                     with_cond(bcx, needs_free, |bcx| {
 505                         trans_exchange_free_dyn(bcx, llbox, llsize, llalign, DebugLoc::None)
 506                     })
 507                 })
 508             } else {
 509                 let llval = v0;
 510                 let llbox = Load(bcx, llval);
 511                 let llbox_as_usize = PtrToInt(bcx, llbox, inttype);
 512                 let drop_flag_not_dropped_already =
 513                     ICmp(bcx, llvm::IntNE, llbox_as_usize, dropped_pattern, DebugLoc::None);
 514                 with_cond(bcx, drop_flag_not_dropped_already, |bcx| {
 515                     let bcx = drop_ty(bcx, llbox, content_ty, DebugLoc::None);
 516                     trans_exchange_free_ty(bcx, llbox, content_ty, DebugLoc::None)
 517                 })
 518             }
 519         }
 520         ty::TyStruct(did, substs) | ty::TyEnum(did, substs) => {
 521             let tcx = bcx.tcx();
 522             match (tcx.ty_dtor(did), skip_dtor) {
 523                 (ty::TraitDtor(dtor, true), false) => {
 524                     // FIXME(16758) Since the struct is unsized, it is hard to
 525                     // find the drop flag (which is at the end of the struct).
 526                     // Lets just ignore the flag and pretend everything will be
 527                     // OK.
 528                     if type_is_sized(bcx.tcx(), t) {
 529                         trans_struct_drop_flag(bcx, t, v0, dtor, did, substs)
 530                     } else {
 531                         // Give the user a heads up that we are doing something
 532                         // stupid and dangerous.
 533                         bcx.sess().warn(&format!("Ignoring drop flag in destructor for {}\
 534                                                  because the struct is unsized. See issue\
 535                                                  #16758", t));
 536                         trans_struct_drop(bcx, t, v0, dtor, did, substs)
 537                     }
 538                 }
 539                 (ty::TraitDtor(dtor, false), false) => {
 540                     trans_struct_drop(bcx, t, v0, dtor, did, substs)
 541                 }
 542                 (ty::NoDtor, _) | (_, true) => {
 543                     // No dtor? Just the default case
 544                     iter_structural_ty(bcx, v0, t, |bb, vv, tt| drop_ty(bb, vv, tt, DebugLoc::None))
 545                 }
 546             }
 547         }
 548         ty::TyTrait(..) => {
 549             // No support in vtable for distinguishing destroying with
 550             // versus without calling Drop::drop. Assert caller is
 551             // okay with always calling the Drop impl, if any.
 552             assert!(!skip_dtor);
 553             let data_ptr = GEPi(bcx, v0, &[0, abi::FAT_PTR_ADDR]);
 554             let vtable_ptr = Load(bcx, GEPi(bcx, v0, &[0, abi::FAT_PTR_EXTRA]));
 555             let dtor = Load(bcx, vtable_ptr);
 556             Call(bcx,
 557                  dtor,
 558                  &[PointerCast(bcx, Load(bcx, data_ptr), Type::i8p(bcx.ccx()))],
 559                  None,
 560                  DebugLoc::None);
 561             bcx
 562         }
 563         _ => {
 564             if bcx.fcx.type_needs_drop(t) {
 565                 iter_structural_ty(bcx,
 566                                    v0,
 567                                    t,
 568                                    |bb, vv, tt| drop_ty(bb, vv, tt, DebugLoc::None))
 569             } else {
 570                 bcx
 571             }
 572         }
 573     }
 574 }