src/librustc_trans/mir/constant.rs

   1 // Copyright 2012-2014 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 use llvm::{self, ValueRef};
  12 use rustc::middle::const_val::{ConstEvalErr, ConstVal, ErrKind};
  13 use rustc_const_math::ConstInt::*;
  14 use rustc_const_math::{ConstInt, ConstMathErr};
  15 use rustc::hir::def_id::DefId;
  16 use rustc::infer::TransNormalize;
  17 use rustc::mir;
  18 use rustc::mir::tcx::LvalueTy;
  19 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
  20 use rustc::ty::layout::{self, LayoutTyper};
  21 use rustc::ty::cast::{CastTy, IntTy};
  22 use rustc::ty::subst::{Kind, Substs, Subst};
  23 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
  24 use {adt, base, machine};
  25 use abi::{self, Abi};
  26 use callee;
  27 use builder::Builder;
  28 use common::{self, CrateContext, const_get_elt, val_ty};
  29 use common::{C_array, C_bool, C_bytes, C_integral, C_big_integral, C_u32, C_u64};
  30 use common::{C_null, C_struct, C_str_slice, C_undef, C_uint, C_vector, is_undef};
  31 use common::const_to_opt_u128;
  32 use consts;
  33 use monomorphize;
  34 use type_of;
  35 use type_::Type;
  36 use value::Value;
  37
  38 use syntax_pos::Span;
  39 use syntax::ast;
  40
  41 use std::fmt;
  42 use std::ptr;
  43
  44 use super::lvalue::Alignment;
  45 use super::operand::{OperandRef, OperandValue};
  46 use super::MirContext;
  47
  48 /// A sized constant rvalue.
  49 /// The LLVM type might not be the same for a single Rust type,
  50 /// e.g. each enum variant would have its own LLVM struct type.
  51 #[derive(Copy, Clone)]
  52 pub struct Const<'tcx> {
  53     pub llval: ValueRef,
  54     pub ty: Ty<'tcx>
  55 }
  56
  57 impl<'tcx> Const<'tcx> {
  58     pub fn new(llval: ValueRef, ty: Ty<'tcx>) -> Const<'tcx> {
  59         Const {
  60             llval,
  61             ty,
  62         }
  63     }
  64
  65     pub fn from_constint<'a>(ccx: &CrateContext<'a, 'tcx>, ci: &ConstInt)
  66     -> Const<'tcx> {
  67         let tcx = ccx.tcx();
  68         let (llval, ty) = match *ci {
  69             I8(v) => (C_integral(Type::i8(ccx), v as u64, true), tcx.types.i8),
  70             I16(v) => (C_integral(Type::i16(ccx), v as u64, true), tcx.types.i16),
  71             I32(v) => (C_integral(Type::i32(ccx), v as u64, true), tcx.types.i32),
  72             I64(v) => (C_integral(Type::i64(ccx), v as u64, true), tcx.types.i64),
  73             I128(v) => (C_big_integral(Type::i128(ccx), v as u128), tcx.types.i128),
  74             Isize(v) => {
  75                 let i = v.as_i64(ccx.tcx().sess.target.int_type);
  76                 (C_integral(Type::int(ccx), i as u64, true), tcx.types.isize)
  77             },
  78             U8(v) => (C_integral(Type::i8(ccx), v as u64, false), tcx.types.u8),
  79             U16(v) => (C_integral(Type::i16(ccx), v as u64, false), tcx.types.u16),
  80             U32(v) => (C_integral(Type::i32(ccx), v as u64, false), tcx.types.u32),
  81             U64(v) => (C_integral(Type::i64(ccx), v, false), tcx.types.u64),
  82             U128(v) => (C_big_integral(Type::i128(ccx), v), tcx.types.u128),
  83             Usize(v) => {
  84                 let u = v.as_u64(ccx.tcx().sess.target.uint_type);
  85                 (C_integral(Type::int(ccx), u, false), tcx.types.usize)
  86             },
  87         };
  88         Const { llval: llval, ty: ty }
  89     }
  90
  91     /// Translate ConstVal into a LLVM constant value.
  92     pub fn from_constval<'a>(ccx: &CrateContext<'a, 'tcx>,
  93                              cv: ConstVal,
  94                              ty: Ty<'tcx>)
  95                              -> Const<'tcx> {
  96         let llty = type_of::type_of(ccx, ty);
  97         let val = match cv {
  98             ConstVal::Float(v) => {
  99                 let bits = match v.ty {
 100                     ast::FloatTy::F32 => C_u32(ccx, v.bits as u32),
 101                     ast::FloatTy::F64 => C_u64(ccx, v.bits as u64)
 102                 };
 103                 consts::bitcast(bits, llty)
 104             }
 105             ConstVal::Bool(v) => C_bool(ccx, v),
 106             ConstVal::Integral(ref i) => return Const::from_constint(ccx, i),
 107             ConstVal::Str(ref v) => C_str_slice(ccx, v.clone()),
 108             ConstVal::ByteStr(ref v) => consts::addr_of(ccx, C_bytes(ccx, v), 1, "byte_str"),
 109             ConstVal::Char(c) => C_integral(Type::char(ccx), c as u64, false),
 110             ConstVal::Function(..) => C_null(type_of::type_of(ccx, ty)),
 111             ConstVal::Variant(_) |
 112             ConstVal::Struct(_) | ConstVal::Tuple(_) |
 113             ConstVal::Array(..) | ConstVal::Repeat(..) => {
 114                 bug!("MIR must not use `{:?}` (aggregates are expanded to MIR rvalues)", cv)
 115             }
 116         };
 117
 118         assert!(!ty.has_erasable_regions());
 119
 120         Const::new(val, ty)
 121     }
 122
 123     fn get_pair(&self) -> (ValueRef, ValueRef) {
 124         (const_get_elt(self.llval, &[0]),
 125          const_get_elt(self.llval, &[1]))
 126     }
 127
 128     fn get_fat_ptr(&self) -> (ValueRef, ValueRef) {
 129         assert_eq!(abi::FAT_PTR_ADDR, 0);
 130         assert_eq!(abi::FAT_PTR_EXTRA, 1);
 131         self.get_pair()
 132     }
 133
 134     fn as_lvalue(&self) -> ConstLvalue<'tcx> {
 135         ConstLvalue {
 136             base: Base::Value(self.llval),
 137             llextra: ptr::null_mut(),
 138             ty: self.ty
 139         }
 140     }
 141
 142     pub fn to_operand<'a>(&self, ccx: &CrateContext<'a, 'tcx>) -> OperandRef<'tcx> {
 143         let llty = type_of::immediate_type_of(ccx, self.ty);
 144         let llvalty = val_ty(self.llval);
 145
 146         let val = if llty == llvalty && common::type_is_imm_pair(ccx, self.ty) {
 147             let (a, b) = self.get_pair();
 148             OperandValue::Pair(a, b)
 149         } else if llty == llvalty && common::type_is_immediate(ccx, self.ty) {
 150             // If the types match, we can use the value directly.
 151             OperandValue::Immediate(self.llval)
 152         } else {
 153             // Otherwise, or if the value is not immediate, we create
 154             // a constant LLVM global and cast its address if necessary.
 155             let align = ccx.align_of(self.ty);
 156             let ptr = consts::addr_of(ccx, self.llval, align, "const");
 157             OperandValue::Ref(consts::ptrcast(ptr, llty.ptr_to()), Alignment::AbiAligned)
 158         };
 159
 160         OperandRef {
 161             val,
 162             ty: self.ty
 163         }
 164     }
 165 }
 166
 167 impl<'tcx> fmt::Debug for Const<'tcx> {
 168     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 169         write!(f, "Const({:?}: {:?})", Value(self.llval), self.ty)
 170     }
 171 }
 172
 173 #[derive(Copy, Clone)]
 174 enum Base {
 175     /// A constant value without an unique address.
 176     Value(ValueRef),
 177
 178     /// String literal base pointer (cast from array).
 179     Str(ValueRef),
 180
 181     /// The address of a static.
 182     Static(ValueRef)
 183 }
 184
 185 /// An lvalue as seen from a constant.
 186 #[derive(Copy, Clone)]
 187 struct ConstLvalue<'tcx> {
 188     base: Base,
 189     llextra: ValueRef,
 190     ty: Ty<'tcx>
 191 }
 192
 193 impl<'tcx> ConstLvalue<'tcx> {
 194     fn to_const(&self, span: Span) -> Const<'tcx> {
 195         match self.base {
 196             Base::Value(val) => Const::new(val, self.ty),
 197             Base::Str(ptr) => {
 198                 span_bug!(span, "loading from `str` ({:?}) in constant",
 199                           Value(ptr))
 200             }
 201             Base::Static(val) => {
 202                 span_bug!(span, "loading from `static` ({:?}) in constant",
 203                           Value(val))
 204             }
 205         }
 206     }
 207
 208     pub fn len<'a>(&self, ccx: &CrateContext<'a, 'tcx>) -> ValueRef {
 209         match self.ty.sty {
 210             ty::TyArray(_, n) => C_uint(ccx, n),
 211             ty::TySlice(_) | ty::TyStr => {
 212                 assert!(self.llextra != ptr::null_mut());
 213                 self.llextra
 214             }
 215             _ => bug!("unexpected type `{}` in ConstLvalue::len", self.ty)
 216         }
 217     }
 218 }
 219
 220 /// Machinery for translating a constant's MIR to LLVM values.
 221 /// FIXME(eddyb) use miri and lower its allocations to LLVM.
 222 struct MirConstContext<'a, 'tcx: 'a> {
 223     ccx: &'a CrateContext<'a, 'tcx>,
 224     mir: &'a mir::Mir<'tcx>,
 225
 226     /// Type parameters for const fn and associated constants.
 227     substs: &'tcx Substs<'tcx>,
 228
 229     /// Values of locals in a constant or const fn.
 230     locals: IndexVec<mir::Local, Option<Result<Const<'tcx>, ConstEvalErr<'tcx>>>>
 231 }
 232
 233 fn add_err<'tcx, U, V>(failure: &mut Result<U, ConstEvalErr<'tcx>>,
 234                        value: &Result<V, ConstEvalErr<'tcx>>)
 235 {
 236     if let &Err(ref err) = value {
 237         if failure.is_ok() {
 238             *failure = Err(err.clone());
 239         }
 240     }
 241 }
 242
 243 impl<'a, 'tcx> MirConstContext<'a, 'tcx> {
 244     fn new(ccx: &'a CrateContext<'a, 'tcx>,
 245            mir: &'a mir::Mir<'tcx>,
 246            substs: &'tcx Substs<'tcx>,
 247            args: IndexVec<mir::Local, Result<Const<'tcx>, ConstEvalErr<'tcx>>>)
 248            -> MirConstContext<'a, 'tcx> {
 249         let mut context = MirConstContext {
 250             ccx,
 251             mir,
 252             substs,
 253             locals: (0..mir.local_decls.len()).map(|_| None).collect(),
 254         };
 255         for (i, arg) in args.into_iter().enumerate() {
 256             // Locals after local 0 are the function arguments
 257             let index = mir::Local::new(i + 1);
 258             context.locals[index] = Some(arg);
 259         }
 260         context
 261     }
 262
 263     fn trans_def(ccx: &'a CrateContext<'a, 'tcx>,
 264                  def_id: DefId,
 265                  substs: &'tcx Substs<'tcx>,
 266                  args: IndexVec<mir::Local, Result<Const<'tcx>, ConstEvalErr<'tcx>>>)
 267                  -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 268         let instance = monomorphize::resolve(ccx.shared(), def_id, substs);
 269         let mir = ccx.tcx().instance_mir(instance.def);
 270         MirConstContext::new(ccx, &mir, instance.substs, args).trans()
 271     }
 272
 273     fn monomorphize<T>(&self, value: &T) -> T
 274         where T: TransNormalize<'tcx>
 275     {
 276         self.ccx.tcx().trans_apply_param_substs(self.substs, value)
 277     }
 278
 279     fn trans(&mut self) -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 280         let tcx = self.ccx.tcx();
 281         let mut bb = mir::START_BLOCK;
 282
 283         // Make sure to evaluate all statemenets to
 284         // report as many errors as we possibly can.
 285         let mut failure = Ok(());
 286
 287         loop {
 288             let data = &self.mir[bb];
 289             for statement in &data.statements {
 290                 let span = statement.source_info.span;
 291                 match statement.kind {
 292                     mir::StatementKind::Assign(ref dest, ref rvalue) => {
 293                         let ty = dest.ty(self.mir, tcx);
 294                         let ty = self.monomorphize(&ty).to_ty(tcx);
 295                         let value = self.const_rvalue(rvalue, ty, span);
 296                         add_err(&mut failure, &value);
 297                         self.store(dest, value, span);
 298                     }
 299                     mir::StatementKind::StorageLive(_) |
 300                     mir::StatementKind::StorageDead(_) |
 301                     mir::StatementKind::Validate(..) |
 302                     mir::StatementKind::EndRegion(_) |
 303                     mir::StatementKind::Nop => {}
 304                     mir::StatementKind::InlineAsm { .. } |
 305                     mir::StatementKind::SetDiscriminant{ .. } => {
 306                         span_bug!(span, "{:?} should not appear in constants?", statement.kind);
 307                     }
 308                 }
 309             }
 310
 311             let terminator = data.terminator();
 312             let span = terminator.source_info.span;
 313             bb = match terminator.kind {
 314                 mir::TerminatorKind::Drop { target, .. } | // No dropping.
 315                 mir::TerminatorKind::Goto { target } => target,
 316                 mir::TerminatorKind::Return => {
 317                     failure?;
 318                     return self.locals[mir::RETURN_POINTER].clone().unwrap_or_else(|| {
 319                         span_bug!(span, "no returned value in constant");
 320                     });
 321                 }
 322
 323                 mir::TerminatorKind::Assert { ref cond, expected, ref msg, target, .. } => {
 324                     let cond = self.const_operand(cond, span)?;
 325                     let cond_bool = common::const_to_uint(cond.llval) != 0;
 326                     if cond_bool != expected {
 327                         let err = match *msg {
 328                             mir::AssertMessage::BoundsCheck { ref len, ref index } => {
 329                                 let len = self.const_operand(len, span)?;
 330                                 let index = self.const_operand(index, span)?;
 331                                 ErrKind::IndexOutOfBounds {
 332                                     len: common::const_to_uint(len.llval),
 333                                     index: common::const_to_uint(index.llval)
 334                                 }
 335                             }
 336                             mir::AssertMessage::Math(ref err) => {
 337                                 ErrKind::Math(err.clone())
 338                             }
 339                         };
 340
 341                         let err = ConstEvalErr { span: span, kind: err };
 342                         err.report(tcx, span, "expression");
 343                         failure = Err(err);
 344                     }
 345                     target
 346                 }
 347
 348                 mir::TerminatorKind::Call { ref func, ref args, ref destination, .. } => {
 349                     let fn_ty = func.ty(self.mir, tcx);
 350                     let fn_ty = self.monomorphize(&fn_ty);
 351                     let (def_id, substs) = match fn_ty.sty {
 352                         ty::TyFnDef(def_id, substs) => (def_id, substs),
 353                         _ => span_bug!(span, "calling {:?} (of type {}) in constant",
 354                                        func, fn_ty)
 355                     };
 356
 357                     let mut arg_vals = IndexVec::with_capacity(args.len());
 358                     for arg in args {
 359                         let arg_val = self.const_operand(arg, span);
 360                         add_err(&mut failure, &arg_val);
 361                         arg_vals.push(arg_val);
 362                     }
 363                     if let Some((ref dest, target)) = *destination {
 364                         let result = if fn_ty.fn_sig(tcx).abi() == Abi::RustIntrinsic {
 365                             match &tcx.item_name(def_id).as_str()[..] {
 366                                 "size_of" => {
 367                                     let llval = C_uint(self.ccx,
 368                                         self.ccx.size_of(substs.type_at(0)));
 369                                     Ok(Const::new(llval, tcx.types.usize))
 370                                 }
 371                                 "min_align_of" => {
 372                                     let llval = C_uint(self.ccx,
 373                                         self.ccx.align_of(substs.type_at(0)));
 374                                     Ok(Const::new(llval, tcx.types.usize))
 375                                 }
 376                                 _ => span_bug!(span, "{:?} in constant", terminator.kind)
 377                             }
 378                         } else {
 379                             MirConstContext::trans_def(self.ccx, def_id, substs, arg_vals)
 380                         };
 381                         add_err(&mut failure, &result);
 382                         self.store(dest, result, span);
 383                         target
 384                     } else {
 385                         span_bug!(span, "diverging {:?} in constant", terminator.kind);
 386                     }
 387                 }
 388                 _ => span_bug!(span, "{:?} in constant", terminator.kind)
 389             };
 390         }
 391     }
 392
 393     fn store(&mut self,
 394              dest: &mir::Lvalue<'tcx>,
 395              value: Result<Const<'tcx>, ConstEvalErr<'tcx>>,
 396              span: Span) {
 397         if let mir::Lvalue::Local(index) = *dest {
 398             self.locals[index] = Some(value);
 399         } else {
 400             span_bug!(span, "assignment to {:?} in constant", dest);
 401         }
 402     }
 403
 404     fn const_lvalue(&self, lvalue: &mir::Lvalue<'tcx>, span: Span)
 405                     -> Result<ConstLvalue<'tcx>, ConstEvalErr<'tcx>> {
 406         let tcx = self.ccx.tcx();
 407
 408         if let mir::Lvalue::Local(index) = *lvalue {
 409             return self.locals[index].clone().unwrap_or_else(|| {
 410                 span_bug!(span, "{:?} not initialized", lvalue)
 411             }).map(|v| v.as_lvalue());
 412         }
 413
 414         let lvalue = match *lvalue {
 415             mir::Lvalue::Local(_)  => bug!(), // handled above
 416             mir::Lvalue::Static(box mir::Static { def_id, ty }) => {
 417                 ConstLvalue {
 418                     base: Base::Static(consts::get_static(self.ccx, def_id)),
 419                     llextra: ptr::null_mut(),
 420                     ty: self.monomorphize(&ty),
 421                 }
 422             }
 423             mir::Lvalue::Projection(ref projection) => {
 424                 let tr_base = self.const_lvalue(&projection.base, span)?;
 425                 let projected_ty = LvalueTy::Ty { ty: tr_base.ty }
 426                     .projection_ty(tcx, &projection.elem);
 427                 let base = tr_base.to_const(span);
 428                 let projected_ty = self.monomorphize(&projected_ty).to_ty(tcx);
 429                 let is_sized = self.ccx.shared().type_is_sized(projected_ty);
 430
 431                 let (projected, llextra) = match projection.elem {
 432                     mir::ProjectionElem::Deref => {
 433                         let (base, extra) = if is_sized {
 434                             (base.llval, ptr::null_mut())
 435                         } else {
 436                             base.get_fat_ptr()
 437                         };
 438                         if self.ccx.statics().borrow().contains_key(&base) {
 439                             (Base::Static(base), extra)
 440                         } else if let ty::TyStr = projected_ty.sty {
 441                             (Base::Str(base), extra)
 442                         } else {
 443                             let v = base;
 444                             let v = self.ccx.const_unsized().borrow().get(&v).map_or(v, |&v| v);
 445                             let mut val = unsafe { llvm::LLVMGetInitializer(v) };
 446                             if val.is_null() {
 447                                 span_bug!(span, "dereference of non-constant pointer `{:?}`",
 448                                           Value(base));
 449                             }
 450                             if projected_ty.is_bool() {
 451                                 let i1_type = Type::i1(self.ccx);
 452                                 if val_ty(val) != i1_type {
 453                                     unsafe {
 454                                         val = llvm::LLVMConstTrunc(val, i1_type.to_ref());
 455                                     }
 456                                 }
 457                             }
 458                             (Base::Value(val), extra)
 459                         }
 460                     }
 461                     mir::ProjectionElem::Field(ref field, _) => {
 462                         let llprojected = adt::const_get_field(self.ccx, tr_base.ty, base.llval,
 463                                                                field.index());
 464                         let llextra = if is_sized {
 465                             ptr::null_mut()
 466                         } else {
 467                             tr_base.llextra
 468                         };
 469                         (Base::Value(llprojected), llextra)
 470                     }
 471                     mir::ProjectionElem::Index(ref index) => {
 472                         let llindex = self.const_operand(index, span)?.llval;
 473
 474                         let iv = if let Some(iv) = common::const_to_opt_u128(llindex, false) {
 475                             iv
 476                         } else {
 477                             span_bug!(span, "index is not an integer-constant expression")
 478                         };
 479
 480                         // Produce an undef instead of a LLVM assertion on OOB.
 481                         let len = common::const_to_uint(tr_base.len(self.ccx));
 482                         let llelem = if iv < len as u128 {
 483                             const_get_elt(base.llval, &[iv as u32])
 484                         } else {
 485                             C_undef(type_of::type_of(self.ccx, projected_ty))
 486                         };
 487
 488                         (Base::Value(llelem), ptr::null_mut())
 489                     }
 490                     _ => span_bug!(span, "{:?} in constant", projection.elem)
 491                 };
 492                 ConstLvalue {
 493                     base: projected,
 494                     llextra,
 495                     ty: projected_ty
 496                 }
 497             }
 498         };
 499         Ok(lvalue)
 500     }
 501
 502     fn const_operand(&self, operand: &mir::Operand<'tcx>, span: Span)
 503                      -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 504         debug!("const_operand({:?} @ {:?})", operand, span);
 505         let result = match *operand {
 506             mir::Operand::Consume(ref lvalue) => {
 507                 Ok(self.const_lvalue(lvalue, span)?.to_const(span))
 508             }
 509
 510             mir::Operand::Constant(ref constant) => {
 511                 let ty = self.monomorphize(&constant.ty);
 512                 match constant.literal.clone() {
 513                     mir::Literal::Item { def_id, substs } => {
 514                         let substs = self.monomorphize(&substs);
 515                         MirConstContext::trans_def(self.ccx, def_id, substs, IndexVec::new())
 516                     }
 517                     mir::Literal::Promoted { index } => {
 518                         let mir = &self.mir.promoted[index];
 519                         MirConstContext::new(self.ccx, mir, self.substs, IndexVec::new()).trans()
 520                     }
 521                     mir::Literal::Value { value } => {
 522                         Ok(Const::from_constval(self.ccx, value, ty))
 523                     }
 524                 }
 525             }
 526         };
 527         debug!("const_operand({:?} @ {:?}) = {:?}", operand, span,
 528                result.as_ref().ok());
 529         result
 530     }
 531
 532     fn const_array(&self, array_ty: Ty<'tcx>, fields: &[ValueRef])
 533                    -> Const<'tcx>
 534     {
 535         let elem_ty = array_ty.builtin_index().unwrap_or_else(|| {
 536             bug!("bad array type {:?}", array_ty)
 537         });
 538         let llunitty = type_of::type_of(self.ccx, elem_ty);
 539         // If the array contains enums, an LLVM array won't work.
 540         let val = if fields.iter().all(|&f| val_ty(f) == llunitty) {
 541             C_array(llunitty, fields)
 542         } else {
 543             C_struct(self.ccx, fields, false)
 544         };
 545         Const::new(val, array_ty)
 546     }
 547
 548     fn const_rvalue(&self, rvalue: &mir::Rvalue<'tcx>,
 549                     dest_ty: Ty<'tcx>, span: Span)
 550                     -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 551         let tcx = self.ccx.tcx();
 552         debug!("const_rvalue({:?}: {:?} @ {:?})", rvalue, dest_ty, span);
 553         let val = match *rvalue {
 554             mir::Rvalue::Use(ref operand) => self.const_operand(operand, span)?,
 555
 556             mir::Rvalue::Repeat(ref elem, ref count) => {
 557                 let elem = self.const_operand(elem, span)?;
 558                 let size = count.as_u64(tcx.sess.target.uint_type);
 559                 let fields = vec![elem.llval; size as usize];
 560                 self.const_array(dest_ty, &fields)
 561             }
 562
 563             mir::Rvalue::Aggregate(ref kind, ref operands) => {
 564                 // Make sure to evaluate all operands to
 565                 // report as many errors as we possibly can.
 566                 let mut fields = Vec::with_capacity(operands.len());
 567                 let mut failure = Ok(());
 568                 for operand in operands {
 569                     match self.const_operand(operand, span) {
 570                         Ok(val) => fields.push(val.llval),
 571                         Err(err) => if failure.is_ok() { failure = Err(err); }
 572                     }
 573                 }
 574                 failure?;
 575
 576                 match **kind {
 577                     mir::AggregateKind::Array(_) => {
 578                         self.const_array(dest_ty, &fields)
 579                     }
 580                     mir::AggregateKind::Adt(..) |
 581                     mir::AggregateKind::Closure(..) |
 582                     mir::AggregateKind::Tuple => {
 583                         Const::new(trans_const(self.ccx, dest_ty, kind, &fields), dest_ty)
 584                     }
 585                 }
 586             }
 587
 588             mir::Rvalue::Cast(ref kind, ref source, cast_ty) => {
 589                 let operand = self.const_operand(source, span)?;
 590                 let cast_ty = self.monomorphize(&cast_ty);
 591
 592                 let val = match *kind {
 593                     mir::CastKind::ReifyFnPointer => {
 594                         match operand.ty.sty {
 595                             ty::TyFnDef(def_id, substs) => {
 596                                 callee::resolve_and_get_fn(self.ccx, def_id, substs)
 597                             }
 598                             _ => {
 599                                 span_bug!(span, "{} cannot be reified to a fn ptr",
 600                                           operand.ty)
 601                             }
 602                         }
 603                     }
 604                     mir::CastKind::ClosureFnPointer => {
 605                         match operand.ty.sty {
 606                             ty::TyClosure(def_id, substs) => {
 607                                 // Get the def_id for FnOnce::call_once
 608                                 let fn_once = tcx.lang_items.fn_once_trait().unwrap();
 609                                 let call_once = tcx
 610                                     .global_tcx().associated_items(fn_once)
 611                                     .find(|it| it.kind == ty::AssociatedKind::Method)
 612                                     .unwrap().def_id;
 613                                 // Now create its substs [Closure, Tuple]
 614                                 let input = tcx.fn_sig(def_id)
 615                                     .subst(tcx, substs.substs).input(0);
 616                                 let input = tcx.erase_late_bound_regions_and_normalize(&input);
 617                                 let substs = tcx.mk_substs([operand.ty, input]
 618                                     .iter().cloned().map(Kind::from));
 619                                 callee::resolve_and_get_fn(self.ccx, call_once, substs)
 620                             }
 621                             _ => {
 622                                 bug!("{} cannot be cast to a fn ptr", operand.ty)
 623                             }
 624                         }
 625                     }
 626                     mir::CastKind::UnsafeFnPointer => {
 627                         // this is a no-op at the LLVM level
 628                         operand.llval
 629                     }
 630                     mir::CastKind::Unsize => {
 631                         // unsize targets other than to a fat pointer currently
 632                         // can't be in constants.
 633                         assert!(common::type_is_fat_ptr(self.ccx, cast_ty));
 634
 635                         let pointee_ty = operand.ty.builtin_deref(true, ty::NoPreference)
 636                             .expect("consts: unsizing got non-pointer type").ty;
 637                         let (base, old_info) = if !self.ccx.shared().type_is_sized(pointee_ty) {
 638                             // Normally, the source is a thin pointer and we are
 639                             // adding extra info to make a fat pointer. The exception
 640                             // is when we are upcasting an existing object fat pointer
 641                             // to use a different vtable. In that case, we want to
 642                             // load out the original data pointer so we can repackage
 643                             // it.
 644                             let (base, extra) = operand.get_fat_ptr();
 645                             (base, Some(extra))
 646                         } else {
 647                             (operand.llval, None)
 648                         };
 649
 650                         let unsized_ty = cast_ty.builtin_deref(true, ty::NoPreference)
 651                             .expect("consts: unsizing got non-pointer target type").ty;
 652                         let ptr_ty = type_of::in_memory_type_of(self.ccx, unsized_ty).ptr_to();
 653                         let base = consts::ptrcast(base, ptr_ty);
 654                         let info = base::unsized_info(self.ccx, pointee_ty,
 655                                                       unsized_ty, old_info);
 656
 657                         if old_info.is_none() {
 658                             let prev_const = self.ccx.const_unsized().borrow_mut()
 659                                                      .insert(base, operand.llval);
 660                             assert!(prev_const.is_none() || prev_const == Some(operand.llval));
 661                         }
 662                         assert_eq!(abi::FAT_PTR_ADDR, 0);
 663                         assert_eq!(abi::FAT_PTR_EXTRA, 1);
 664                         C_struct(self.ccx, &[base, info], false)
 665                     }
 666                     mir::CastKind::Misc if common::type_is_immediate(self.ccx, operand.ty) => {
 667                         debug_assert!(common::type_is_immediate(self.ccx, cast_ty));
 668                         let r_t_in = CastTy::from_ty(operand.ty).expect("bad input type for cast");
 669                         let r_t_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 670                         let ll_t_out = type_of::immediate_type_of(self.ccx, cast_ty);
 671                         let llval = operand.llval;
 672                         let signed = if let CastTy::Int(IntTy::CEnum) = r_t_in {
 673                             let l = self.ccx.layout_of(operand.ty);
 674                             adt::is_discr_signed(&l)
 675                         } else {
 676                             operand.ty.is_signed()
 677                         };
 678
 679                         unsafe {
 680                             match (r_t_in, r_t_out) {
 681                                 (CastTy::Int(_), CastTy::Int(_)) => {
 682                                     let s = signed as llvm::Bool;
 683                                     llvm::LLVMConstIntCast(llval, ll_t_out.to_ref(), s)
 684                                 }
 685                                 (CastTy::Int(_), CastTy::Float) => {
 686                                     if signed {
 687                                         llvm::LLVMConstSIToFP(llval, ll_t_out.to_ref())
 688                                     } else {
 689                                         llvm::LLVMConstUIToFP(llval, ll_t_out.to_ref())
 690                                     }
 691                                 }
 692                                 (CastTy::Float, CastTy::Float) => {
 693                                     llvm::LLVMConstFPCast(llval, ll_t_out.to_ref())
 694                                 }
 695                                 (CastTy::Float, CastTy::Int(IntTy::I)) => {
 696                                     llvm::LLVMConstFPToSI(llval, ll_t_out.to_ref())
 697                                 }
 698                                 (CastTy::Float, CastTy::Int(_)) => {
 699                                     llvm::LLVMConstFPToUI(llval, ll_t_out.to_ref())
 700                                 }
 701                                 (CastTy::Ptr(_), CastTy::Ptr(_)) |
 702                                 (CastTy::FnPtr, CastTy::Ptr(_)) |
 703                                 (CastTy::RPtr(_), CastTy::Ptr(_)) => {
 704                                     consts::ptrcast(llval, ll_t_out)
 705                                 }
 706                                 (CastTy::Int(_), CastTy::Ptr(_)) => {
 707                                     llvm::LLVMConstIntToPtr(llval, ll_t_out.to_ref())
 708                                 }
 709                                 (CastTy::Ptr(_), CastTy::Int(_)) |
 710                                 (CastTy::FnPtr, CastTy::Int(_)) => {
 711                                     llvm::LLVMConstPtrToInt(llval, ll_t_out.to_ref())
 712                                 }
 713                                 _ => bug!("unsupported cast: {:?} to {:?}", operand.ty, cast_ty)
 714                             }
 715                         }
 716                     }
 717                     mir::CastKind::Misc => { // Casts from a fat-ptr.
 718                         let ll_cast_ty = type_of::immediate_type_of(self.ccx, cast_ty);
 719                         let ll_from_ty = type_of::immediate_type_of(self.ccx, operand.ty);
 720                         if common::type_is_fat_ptr(self.ccx, operand.ty) {
 721                             let (data_ptr, meta_ptr) = operand.get_fat_ptr();
 722                             if common::type_is_fat_ptr(self.ccx, cast_ty) {
 723                                 let ll_cft = ll_cast_ty.field_types();
 724                                 let ll_fft = ll_from_ty.field_types();
 725                                 let data_cast = consts::ptrcast(data_ptr, ll_cft[0]);
 726                                 assert_eq!(ll_cft[1].kind(), ll_fft[1].kind());
 727                                 C_struct(self.ccx, &[data_cast, meta_ptr], false)
 728                             } else { // cast to thin-ptr
 729                                 // Cast of fat-ptr to thin-ptr is an extraction of data-ptr and
 730                                 // pointer-cast of that pointer to desired pointer type.
 731                                 consts::ptrcast(data_ptr, ll_cast_ty)
 732                             }
 733                         } else {
 734                             bug!("Unexpected non-fat-pointer operand")
 735                         }
 736                     }
 737                 };
 738                 Const::new(val, cast_ty)
 739             }
 740
 741             mir::Rvalue::Ref(_, bk, ref lvalue) => {
 742                 let tr_lvalue = self.const_lvalue(lvalue, span)?;
 743
 744                 let ty = tr_lvalue.ty;
 745                 let ref_ty = tcx.mk_ref(tcx.types.re_erased,
 746                     ty::TypeAndMut { ty: ty, mutbl: bk.to_mutbl_lossy() });
 747
 748                 let base = match tr_lvalue.base {
 749                     Base::Value(llval) => {
 750                         // FIXME: may be wrong for &*(&simd_vec as &fmt::Debug)
 751                         let align = if self.ccx.shared().type_is_sized(ty) {
 752                             self.ccx.align_of(ty)
 753                         } else {
 754                             self.ccx.tcx().data_layout.pointer_align.abi() as machine::llalign
 755                         };
 756                         if bk == mir::BorrowKind::Mut {
 757                             consts::addr_of_mut(self.ccx, llval, align, "ref_mut")
 758                         } else {
 759                             consts::addr_of(self.ccx, llval, align, "ref")
 760                         }
 761                     }
 762                     Base::Str(llval) |
 763                     Base::Static(llval) => llval
 764                 };
 765
 766                 let ptr = if self.ccx.shared().type_is_sized(ty) {
 767                     base
 768                 } else {
 769                     C_struct(self.ccx, &[base, tr_lvalue.llextra], false)
 770                 };
 771                 Const::new(ptr, ref_ty)
 772             }
 773
 774             mir::Rvalue::Len(ref lvalue) => {
 775                 let tr_lvalue = self.const_lvalue(lvalue, span)?;
 776                 Const::new(tr_lvalue.len(self.ccx), tcx.types.usize)
 777             }
 778
 779             mir::Rvalue::BinaryOp(op, ref lhs, ref rhs) => {
 780                 let lhs = self.const_operand(lhs, span)?;
 781                 let rhs = self.const_operand(rhs, span)?;
 782                 let ty = lhs.ty;
 783                 let binop_ty = op.ty(tcx, lhs.ty, rhs.ty);
 784                 let (lhs, rhs) = (lhs.llval, rhs.llval);
 785                 Const::new(const_scalar_binop(op, lhs, rhs, ty), binop_ty)
 786             }
 787
 788             mir::Rvalue::CheckedBinaryOp(op, ref lhs, ref rhs) => {
 789                 let lhs = self.const_operand(lhs, span)?;
 790                 let rhs = self.const_operand(rhs, span)?;
 791                 let ty = lhs.ty;
 792                 let val_ty = op.ty(tcx, lhs.ty, rhs.ty);
 793                 let binop_ty = tcx.intern_tup(&[val_ty, tcx.types.bool], false);
 794                 let (lhs, rhs) = (lhs.llval, rhs.llval);
 795                 assert!(!ty.is_fp());
 796
 797                 match const_scalar_checked_binop(tcx, op, lhs, rhs, ty) {
 798                     Some((llval, of)) => {
 799                         let llof = C_bool(self.ccx, of);
 800                         Const::new(C_struct(self.ccx, &[llval, llof], false), binop_ty)
 801                     }
 802                     None => {
 803                         span_bug!(span, "{:?} got non-integer operands: {:?} and {:?}",
 804                                   rvalue, Value(lhs), Value(rhs));
 805                     }
 806                 }
 807             }
 808
 809             mir::Rvalue::UnaryOp(op, ref operand) => {
 810                 let operand = self.const_operand(operand, span)?;
 811                 let lloperand = operand.llval;
 812                 let llval = match op {
 813                     mir::UnOp::Not => {
 814                         unsafe {
 815                             llvm::LLVMConstNot(lloperand)
 816                         }
 817                     }
 818                     mir::UnOp::Neg => {
 819                         let is_float = operand.ty.is_fp();
 820                         unsafe {
 821                             if is_float {
 822                                 llvm::LLVMConstFNeg(lloperand)
 823                             } else {
 824                                 llvm::LLVMConstNeg(lloperand)
 825                             }
 826                         }
 827                     }
 828                 };
 829                 Const::new(llval, operand.ty)
 830             }
 831
 832             mir::Rvalue::NullaryOp(mir::NullOp::SizeOf, ty) => {
 833                 assert!(self.ccx.shared().type_is_sized(ty));
 834                 let llval = C_uint(self.ccx, self.ccx.size_of(ty));
 835                 Const::new(llval, tcx.types.usize)
 836             }
 837
 838             _ => span_bug!(span, "{:?} in constant", rvalue)
 839         };
 840
 841         debug!("const_rvalue({:?}: {:?} @ {:?}) = {:?}", rvalue, dest_ty, span, val);
 842
 843         Ok(val)
 844     }
 845
 846 }
 847
 848 fn to_const_int(value: ValueRef, t: Ty, tcx: TyCtxt) -> Option<ConstInt> {
 849     match t.sty {
 850         ty::TyInt(int_type) => const_to_opt_u128(value, true)
 851             .and_then(|input| ConstInt::new_signed(input as i128, int_type,
 852                                                    tcx.sess.target.int_type)),
 853         ty::TyUint(uint_type) => const_to_opt_u128(value, false)
 854             .and_then(|input| ConstInt::new_unsigned(input, uint_type,
 855                                                      tcx.sess.target.uint_type)),
 856         _ => None
 857
 858     }
 859 }
 860
 861 pub fn const_scalar_binop(op: mir::BinOp,
 862                           lhs: ValueRef,
 863                           rhs: ValueRef,
 864                           input_ty: Ty) -> ValueRef {
 865     assert!(!input_ty.is_simd());
 866     let is_float = input_ty.is_fp();
 867     let signed = input_ty.is_signed();
 868
 869     unsafe {
 870         match op {
 871             mir::BinOp::Add if is_float => llvm::LLVMConstFAdd(lhs, rhs),
 872             mir::BinOp::Add             => llvm::LLVMConstAdd(lhs, rhs),
 873
 874             mir::BinOp::Sub if is_float => llvm::LLVMConstFSub(lhs, rhs),
 875             mir::BinOp::Sub             => llvm::LLVMConstSub(lhs, rhs),
 876
 877             mir::BinOp::Mul if is_float => llvm::LLVMConstFMul(lhs, rhs),
 878             mir::BinOp::Mul             => llvm::LLVMConstMul(lhs, rhs),
 879
 880             mir::BinOp::Div if is_float => llvm::LLVMConstFDiv(lhs, rhs),
 881             mir::BinOp::Div if signed   => llvm::LLVMConstSDiv(lhs, rhs),
 882             mir::BinOp::Div             => llvm::LLVMConstUDiv(lhs, rhs),
 883
 884             mir::BinOp::Rem if is_float => llvm::LLVMConstFRem(lhs, rhs),
 885             mir::BinOp::Rem if signed   => llvm::LLVMConstSRem(lhs, rhs),
 886             mir::BinOp::Rem             => llvm::LLVMConstURem(lhs, rhs),
 887
 888             mir::BinOp::BitXor => llvm::LLVMConstXor(lhs, rhs),
 889             mir::BinOp::BitAnd => llvm::LLVMConstAnd(lhs, rhs),
 890             mir::BinOp::BitOr  => llvm::LLVMConstOr(lhs, rhs),
 891             mir::BinOp::Shl    => {
 892                 let rhs = base::cast_shift_const_rhs(op.to_hir_binop(), lhs, rhs);
 893                 llvm::LLVMConstShl(lhs, rhs)
 894             }
 895             mir::BinOp::Shr    => {
 896                 let rhs = base::cast_shift_const_rhs(op.to_hir_binop(), lhs, rhs);
 897                 if signed { llvm::LLVMConstAShr(lhs, rhs) }
 898                 else      { llvm::LLVMConstLShr(lhs, rhs) }
 899             }
 900             mir::BinOp::Eq | mir::BinOp::Ne |
 901             mir::BinOp::Lt | mir::BinOp::Le |
 902             mir::BinOp::Gt | mir::BinOp::Ge => {
 903                 if is_float {
 904                     let cmp = base::bin_op_to_fcmp_predicate(op.to_hir_binop());
 905                     llvm::LLVMConstFCmp(cmp, lhs, rhs)
 906                 } else {
 907                     let cmp = base::bin_op_to_icmp_predicate(op.to_hir_binop(),
 908                                                                 signed);
 909                     llvm::LLVMConstICmp(cmp, lhs, rhs)
 910                 }
 911             }
 912             mir::BinOp::Offset => unreachable!("BinOp::Offset in const-eval!")
 913         }
 914     }
 915 }
 916
 917 pub fn const_scalar_checked_binop<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 918                                             op: mir::BinOp,
 919                                             lllhs: ValueRef,
 920                                             llrhs: ValueRef,
 921                                             input_ty: Ty<'tcx>)
 922                                             -> Option<(ValueRef, bool)> {
 923     if let (Some(lhs), Some(rhs)) = (to_const_int(lllhs, input_ty, tcx),
 924                                      to_const_int(llrhs, input_ty, tcx)) {
 925         let result = match op {
 926             mir::BinOp::Add => lhs + rhs,
 927             mir::BinOp::Sub => lhs - rhs,
 928             mir::BinOp::Mul => lhs * rhs,
 929             mir::BinOp::Shl => lhs << rhs,
 930             mir::BinOp::Shr => lhs >> rhs,
 931             _ => {
 932                 bug!("Operator `{:?}` is not a checkable operator", op)
 933             }
 934         };
 935
 936         let of = match result {
 937             Ok(_) => false,
 938             Err(ConstMathErr::Overflow(_)) |
 939             Err(ConstMathErr::ShiftNegative) => true,
 940             Err(err) => {
 941                 bug!("Operator `{:?}` on `{:?}` and `{:?}` errored: {}",
 942                      op, lhs, rhs, err.description());
 943             }
 944         };
 945
 946         Some((const_scalar_binop(op, lllhs, llrhs, input_ty), of))
 947     } else {
 948         None
 949     }
 950 }
 951
 952 impl<'a, 'tcx> MirContext<'a, 'tcx> {
 953     pub fn trans_constant(&mut self,
 954                           bcx: &Builder<'a, 'tcx>,
 955                           constant: &mir::Constant<'tcx>)
 956                           -> Const<'tcx>
 957     {
 958         debug!("trans_constant({:?})", constant);
 959         let ty = self.monomorphize(&constant.ty);
 960         let result = match constant.literal.clone() {
 961             mir::Literal::Item { def_id, substs } => {
 962                 let substs = self.monomorphize(&substs);
 963                 MirConstContext::trans_def(bcx.ccx, def_id, substs, IndexVec::new())
 964             }
 965             mir::Literal::Promoted { index } => {
 966                 let mir = &self.mir.promoted[index];
 967                 MirConstContext::new(bcx.ccx, mir, self.param_substs, IndexVec::new()).trans()
 968             }
 969             mir::Literal::Value { value } => {
 970                 Ok(Const::from_constval(bcx.ccx, value, ty))
 971             }
 972         };
 973
 974         let result = result.unwrap_or_else(|_| {
 975             // We've errored, so we don't have to produce working code.
 976             let llty = type_of::type_of(bcx.ccx, ty);
 977             Const::new(C_undef(llty), ty)
 978         });
 979
 980         debug!("trans_constant({:?}) = {:?}", constant, result);
 981         result
 982     }
 983 }
 984
 985
 986 pub fn trans_static_initializer<'a, 'tcx>(
 987     ccx: &CrateContext<'a, 'tcx>,
 988     def_id: DefId)
 989     -> Result<ValueRef, ConstEvalErr<'tcx>>
 990 {
 991     MirConstContext::trans_def(ccx, def_id, Substs::empty(), IndexVec::new())
 992         .map(|c| c.llval)
 993 }
 994
 995 /// Construct a constant value, suitable for initializing a
 996 /// GlobalVariable, given a case and constant values for its fields.
 997 /// Note that this may have a different LLVM type (and different
 998 /// alignment!) from the representation's `type_of`, so it needs a
 999 /// pointer cast before use.
1000 ///
1001 /// The LLVM type system does not directly support unions, and only
1002 /// pointers can be bitcast, so a constant (and, by extension, the
1003 /// GlobalVariable initialized by it) will have a type that can vary
1004 /// depending on which case of an enum it is.
1005 ///
1006 /// To understand the alignment situation, consider `enum E { V64(u64),
1007 /// V32(u32, u32) }` on Windows.  The type has 8-byte alignment to
1008 /// accommodate the u64, but `V32(x, y)` would have LLVM type `{i32,
1009 /// i32, i32}`, which is 4-byte aligned.
1010 ///
1011 /// Currently the returned value has the same size as the type, but
1012 /// this could be changed in the future to avoid allocating unnecessary
1013 /// space after values of shorter-than-maximum cases.
1014 fn trans_const<'a, 'tcx>(
1015     ccx: &CrateContext<'a, 'tcx>,
1016     t: Ty<'tcx>,
1017     kind: &mir::AggregateKind,
1018     vals: &[ValueRef]
1019 ) -> ValueRef {
1020     let l = ccx.layout_of(t);
1021     let variant_index = match *kind {
1022         mir::AggregateKind::Adt(_, index, _, _) => index,
1023         _ => 0,
1024     };
1025     match *l {
1026         layout::CEnum { discr: d, min, max, .. } => {
1027             let discr = match *kind {
1028                 mir::AggregateKind::Adt(adt_def, _, _, _) => {
1029                     adt_def.discriminant_for_variant(ccx.tcx(), variant_index)
1030                            .to_u128_unchecked() as u64
1031                 },
1032                 _ => 0,
1033             };
1034             assert_eq!(vals.len(), 0);
1035             adt::assert_discr_in_range(min, max, discr);
1036             C_integral(Type::from_integer(ccx, d), discr, true)
1037         }
1038         layout::General { discr: d, ref variants, .. } => {
1039             let variant = &variants[variant_index];
1040             let lldiscr = C_integral(Type::from_integer(ccx, d), variant_index as u64, true);
1041             let mut vals_with_discr = vec![lldiscr];
1042             vals_with_discr.extend_from_slice(vals);
1043             let mut contents = build_const_struct(ccx, &variant, &vals_with_discr[..]);
1044             let needed_padding = l.size(ccx).bytes() - variant.stride().bytes();
1045             if needed_padding > 0 {
1046                 contents.push(padding(ccx, needed_padding));
1047             }
1048             C_struct(ccx, &contents[..], false)
1049         }
1050         layout::UntaggedUnion { ref variants, .. }=> {
1051             assert_eq!(variant_index, 0);
1052             let contents = build_const_union(ccx, variants, vals[0]);
1053             C_struct(ccx, &contents, variants.packed)
1054         }
1055         layout::Univariant { ref variant, .. } => {
1056             assert_eq!(variant_index, 0);
1057             let contents = build_const_struct(ccx, &variant, vals);
1058             C_struct(ccx, &contents[..], variant.packed)
1059         }
1060         layout::Vector { .. } => {
1061             C_vector(vals)
1062         }
1063         layout::RawNullablePointer { nndiscr, .. } => {
1064             if variant_index as u64 == nndiscr {
1065                 assert_eq!(vals.len(), 1);
1066                 vals[0]
1067             } else {
1068                 C_null(type_of::type_of(ccx, t))
1069             }
1070         }
1071         layout::StructWrappedNullablePointer { ref nonnull, nndiscr, .. } => {
1072             if variant_index as u64 == nndiscr {
1073                 C_struct(ccx, &build_const_struct(ccx, &nonnull, vals), false)
1074             } else {
1075                 // Always use null even if it's not the `discrfield`th
1076                 // field; see #8506.
1077                 C_null(type_of::type_of(ccx, t))
1078             }
1079         }
1080         _ => bug!("trans_const: cannot handle type {} repreented as {:#?}", t, l)
1081     }
1082 }
1083
1084 /// Building structs is a little complicated, because we might need to
1085 /// insert padding if a field's value is less aligned than its type.
1086 ///
1087 /// Continuing the example from `trans_const`, a value of type `(u32,
1088 /// E)` should have the `E` at offset 8, but if that field's
1089 /// initializer is 4-byte aligned then simply translating the tuple as
1090 /// a two-element struct will locate it at offset 4, and accesses to it
1091 /// will read the wrong memory.
1092 fn build_const_struct<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
1093                                 st: &layout::Struct,
1094                                 vals: &[ValueRef])
1095                                 -> Vec<ValueRef> {
1096     assert_eq!(vals.len(), st.offsets.len());
1097
1098     if vals.len() == 0 {
1099         return Vec::new();
1100     }
1101
1102     // offset of current value
1103     let mut offset = 0;
1104     let mut cfields = Vec::new();
1105     cfields.reserve(st.offsets.len()*2);
1106
1107     let parts = st.field_index_by_increasing_offset().map(|i| {
1108         (&vals[i], st.offsets[i].bytes())
1109     });
1110     for (&val, target_offset) in parts {
1111         if offset < target_offset {
1112             cfields.push(padding(ccx, target_offset - offset));
1113             offset = target_offset;
1114         }
1115         assert!(!is_undef(val));
1116         cfields.push(val);
1117         offset += machine::llsize_of_alloc(ccx, val_ty(val));
1118     }
1119
1120     if offset < st.stride().bytes() {
1121         cfields.push(padding(ccx, st.stride().bytes() - offset));
1122     }
1123
1124     cfields
1125 }
1126
1127 fn build_const_union<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
1128                                un: &layout::Union,
1129                                field_val: ValueRef)
1130                                -> Vec<ValueRef> {
1131     let mut cfields = vec![field_val];
1132
1133     let offset = machine::llsize_of_alloc(ccx, val_ty(field_val));
1134     let size = un.stride().bytes();
1135     if offset != size {
1136         cfields.push(padding(ccx, size - offset));
1137     }
1138
1139     cfields
1140 }
1141
1142 fn padding(ccx: &CrateContext, size: u64) -> ValueRef {
1143     C_undef(Type::array(&Type::i8(ccx), size))
1144 }