compiler/rustc_middle/src/mir/interpret/mod.rs

   1 //! An interpreter for MIR used in CTFE and by miri.
   2
   3 #[macro_export]
   4 macro_rules! err_unsup {
   5     ($($tt:tt)*) => {
   6         $crate::mir::interpret::InterpError::Unsupported(
   7             $crate::mir::interpret::UnsupportedOpInfo::$($tt)*
   8         )
   9     };
  10 }
  11
  12 #[macro_export]
  13 macro_rules! err_unsup_format {
  14     ($($tt:tt)*) => { err_unsup!(Unsupported(format!($($tt)*))) };
  15 }
  16
  17 #[macro_export]
  18 macro_rules! err_inval {
  19     ($($tt:tt)*) => {
  20         $crate::mir::interpret::InterpError::InvalidProgram(
  21             $crate::mir::interpret::InvalidProgramInfo::$($tt)*
  22         )
  23     };
  24 }
  25
  26 #[macro_export]
  27 macro_rules! err_ub {
  28     ($($tt:tt)*) => {
  29         $crate::mir::interpret::InterpError::UndefinedBehavior(
  30             $crate::mir::interpret::UndefinedBehaviorInfo::$($tt)*
  31         )
  32     };
  33 }
  34
  35 #[macro_export]
  36 macro_rules! err_ub_format {
  37     ($($tt:tt)*) => { err_ub!(Ub(format!($($tt)*))) };
  38 }
  39
  40 #[macro_export]
  41 macro_rules! err_exhaust {
  42     ($($tt:tt)*) => {
  43         $crate::mir::interpret::InterpError::ResourceExhaustion(
  44             $crate::mir::interpret::ResourceExhaustionInfo::$($tt)*
  45         )
  46     };
  47 }
  48
  49 #[macro_export]
  50 macro_rules! err_machine_stop {
  51     ($($tt:tt)*) => {
  52         $crate::mir::interpret::InterpError::MachineStop(Box::new($($tt)*))
  53     };
  54 }
  55
  56 // In the `throw_*` macros, avoid `return` to make them work with `try {}`.
  57 #[macro_export]
  58 macro_rules! throw_unsup {
  59     ($($tt:tt)*) => { Err::<!, _>(err_unsup!($($tt)*))? };
  60 }
  61
  62 #[macro_export]
  63 macro_rules! throw_unsup_format {
  64     ($($tt:tt)*) => { throw_unsup!(Unsupported(format!($($tt)*))) };
  65 }
  66
  67 #[macro_export]
  68 macro_rules! throw_inval {
  69     ($($tt:tt)*) => { Err::<!, _>(err_inval!($($tt)*))? };
  70 }
  71
  72 #[macro_export]
  73 macro_rules! throw_ub {
  74     ($($tt:tt)*) => { Err::<!, _>(err_ub!($($tt)*))? };
  75 }
  76
  77 #[macro_export]
  78 macro_rules! throw_ub_format {
  79     ($($tt:tt)*) => { throw_ub!(Ub(format!($($tt)*))) };
  80 }
  81
  82 #[macro_export]
  83 macro_rules! throw_exhaust {
  84     ($($tt:tt)*) => { Err::<!, _>(err_exhaust!($($tt)*))? };
  85 }
  86
  87 #[macro_export]
  88 macro_rules! throw_machine_stop {
  89     ($($tt:tt)*) => { Err::<!, _>(err_machine_stop!($($tt)*))? };
  90 }
  91
  92 mod allocation;
  93 mod error;
  94 mod pointer;
  95 mod queries;
  96 mod value;
  97
  98 use std::convert::TryFrom;
  99 use std::fmt;
 100 use std::io;
 101 use std::io::{Read, Write};
 102 use std::num::NonZeroU32;
 103 use std::sync::atomic::{AtomicU32, Ordering};
 104
 105 use rustc_ast::LitKind;
 106 use rustc_data_structures::fx::FxHashMap;
 107 use rustc_data_structures::sync::{HashMapExt, Lock};
 108 use rustc_data_structures::tiny_list::TinyList;
 109 use rustc_hir::def_id::DefId;
 110 use rustc_macros::HashStable;
 111 use rustc_middle::ty::print::with_no_trimmed_paths;
 112 use rustc_serialize::{Decodable, Encodable};
 113 use rustc_target::abi::Endian;
 114
 115 use crate::mir;
 116 use crate::ty::codec::{TyDecoder, TyEncoder};
 117 use crate::ty::subst::GenericArgKind;
 118 use crate::ty::{self, Instance, Ty, TyCtxt};
 119
 120 pub use self::error::{
 121     struct_error, CheckInAllocMsg, ErrorHandled, EvalToAllocationRawResult, EvalToConstValueResult,
 122     InterpError, InterpErrorInfo, InterpResult, InvalidProgramInfo, MachineStopType,
 123     ResourceExhaustionInfo, UndefinedBehaviorInfo, UninitBytesAccess, UnsupportedOpInfo,
 124 };
 125
 126 pub use self::value::{get_slice_bytes, ConstAlloc, ConstValue, Scalar, ScalarMaybeUninit};
 127
 128 pub use self::allocation::{Allocation, AllocationExtra, InitMask, Relocations};
 129
 130 pub use self::pointer::{Pointer, PointerArithmetic};
 131
 132 /// Uniquely identifies one of the following:
 133 /// - A constant
 134 /// - A static
 135 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, TyEncodable, TyDecodable)]
 136 #[derive(HashStable, Lift)]
 137 pub struct GlobalId<'tcx> {
 138     /// For a constant or static, the `Instance` of the item itself.
 139     /// For a promoted global, the `Instance` of the function they belong to.
 140     pub instance: ty::Instance<'tcx>,
 141
 142     /// The index for promoted globals within their function's `mir::Body`.
 143     pub promoted: Option<mir::Promoted>,
 144 }
 145
 146 impl GlobalId<'tcx> {
 147     pub fn display(self, tcx: TyCtxt<'tcx>) -> String {
 148         let instance_name = with_no_trimmed_paths(|| tcx.def_path_str(self.instance.def.def_id()));
 149         if let Some(promoted) = self.promoted {
 150             format!("{}::{:?}", instance_name, promoted)
 151         } else {
 152             instance_name
 153         }
 154     }
 155 }
 156
 157 /// Input argument for `tcx.lit_to_const`.
 158 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, HashStable)]
 159 pub struct LitToConstInput<'tcx> {
 160     /// The absolute value of the resultant constant.
 161     pub lit: &'tcx LitKind,
 162     /// The type of the constant.
 163     pub ty: Ty<'tcx>,
 164     /// If the constant is negative.
 165     pub neg: bool,
 166 }
 167
 168 /// Error type for `tcx.lit_to_const`.
 169 #[derive(Copy, Clone, Debug, Eq, PartialEq, HashStable)]
 170 pub enum LitToConstError {
 171     /// The literal's inferred type did not match the expected `ty` in the input.
 172     /// This is used for graceful error handling (`delay_span_bug`) in
 173     /// type checking (`Const::from_anon_const`).
 174     TypeError,
 175     UnparseableFloat,
 176     Reported,
 177 }
 178
 179 #[derive(Copy, Clone, Eq, Hash, Ord, PartialEq, PartialOrd)]
 180 pub struct AllocId(pub u64);
 181
 182 // We want the `Debug` output to be readable as it is used by `derive(Debug)` for
 183 // all the Miri types.
 184 impl fmt::Debug for AllocId {
 185     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 186         if f.alternate() { write!(f, "a{}", self.0) } else { write!(f, "alloc{}", self.0) }
 187     }
 188 }
 189
 190 impl fmt::Display for AllocId {
 191     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 192         fmt::Debug::fmt(self, f)
 193     }
 194 }
 195
 196 #[derive(TyDecodable, TyEncodable)]
 197 enum AllocDiscriminant {
 198     Alloc,
 199     Fn,
 200     Static,
 201 }
 202
 203 pub fn specialized_encode_alloc_id<'tcx, E: TyEncoder<'tcx>>(
 204     encoder: &mut E,
 205     tcx: TyCtxt<'tcx>,
 206     alloc_id: AllocId,
 207 ) -> Result<(), E::Error> {
 208     match tcx.global_alloc(alloc_id) {
 209         GlobalAlloc::Memory(alloc) => {
 210             trace!("encoding {:?} with {:#?}", alloc_id, alloc);
 211             AllocDiscriminant::Alloc.encode(encoder)?;
 212             alloc.encode(encoder)?;
 213         }
 214         GlobalAlloc::Function(fn_instance) => {
 215             trace!("encoding {:?} with {:#?}", alloc_id, fn_instance);
 216             AllocDiscriminant::Fn.encode(encoder)?;
 217             fn_instance.encode(encoder)?;
 218         }
 219         GlobalAlloc::Static(did) => {
 220             assert!(!tcx.is_thread_local_static(did));
 221             // References to statics doesn't need to know about their allocations,
 222             // just about its `DefId`.
 223             AllocDiscriminant::Static.encode(encoder)?;
 224             did.encode(encoder)?;
 225         }
 226     }
 227     Ok(())
 228 }
 229
 230 // Used to avoid infinite recursion when decoding cyclic allocations.
 231 type DecodingSessionId = NonZeroU32;
 232
 233 #[derive(Clone)]
 234 enum State {
 235     Empty,
 236     InProgressNonAlloc(TinyList<DecodingSessionId>),
 237     InProgress(TinyList<DecodingSessionId>, AllocId),
 238     Done(AllocId),
 239 }
 240
 241 pub struct AllocDecodingState {
 242     // For each `AllocId`, we keep track of which decoding state it's currently in.
 243     decoding_state: Vec<Lock<State>>,
 244     // The offsets of each allocation in the data stream.
 245     data_offsets: Vec<u32>,
 246 }
 247
 248 impl AllocDecodingState {
 249     pub fn new_decoding_session(&self) -> AllocDecodingSession<'_> {
 250         static DECODER_SESSION_ID: AtomicU32 = AtomicU32::new(0);
 251         let counter = DECODER_SESSION_ID.fetch_add(1, Ordering::SeqCst);
 252
 253         // Make sure this is never zero.
 254         let session_id = DecodingSessionId::new((counter & 0x7FFFFFFF) + 1).unwrap();
 255
 256         AllocDecodingSession { state: self, session_id }
 257     }
 258
 259     pub fn new(data_offsets: Vec<u32>) -> Self {
 260         let decoding_state = vec![Lock::new(State::Empty); data_offsets.len()];
 261
 262         Self { decoding_state, data_offsets }
 263     }
 264 }
 265
 266 #[derive(Copy, Clone)]
 267 pub struct AllocDecodingSession<'s> {
 268     state: &'s AllocDecodingState,
 269     session_id: DecodingSessionId,
 270 }
 271
 272 impl<'s> AllocDecodingSession<'s> {
 273     /// Decodes an `AllocId` in a thread-safe way.
 274     pub fn decode_alloc_id<D>(&self, decoder: &mut D) -> Result<AllocId, D::Error>
 275     where
 276         D: TyDecoder<'tcx>,
 277     {
 278         // Read the index of the allocation.
 279         let idx = usize::try_from(decoder.read_u32()?).unwrap();
 280         let pos = usize::try_from(self.state.data_offsets[idx]).unwrap();
 281
 282         // Decode the `AllocDiscriminant` now so that we know if we have to reserve an
 283         // `AllocId`.
 284         let (alloc_kind, pos) = decoder.with_position(pos, |decoder| {
 285             let alloc_kind = AllocDiscriminant::decode(decoder)?;
 286             Ok((alloc_kind, decoder.position()))
 287         })?;
 288
 289         // Check the decoding state to see if it's already decoded or if we should
 290         // decode it here.
 291         let alloc_id = {
 292             let mut entry = self.state.decoding_state[idx].lock();
 293
 294             match *entry {
 295                 State::Done(alloc_id) => {
 296                     return Ok(alloc_id);
 297                 }
 298                 ref mut entry @ State::Empty => {
 299                     // We are allowed to decode.
 300                     match alloc_kind {
 301                         AllocDiscriminant::Alloc => {
 302                             // If this is an allocation, we need to reserve an
 303                             // `AllocId` so we can decode cyclic graphs.
 304                             let alloc_id = decoder.tcx().reserve_alloc_id();
 305                             *entry =
 306                                 State::InProgress(TinyList::new_single(self.session_id), alloc_id);
 307                             Some(alloc_id)
 308                         }
 309                         AllocDiscriminant::Fn | AllocDiscriminant::Static => {
 310                             // Fns and statics cannot be cyclic, and their `AllocId`
 311                             // is determined later by interning.
 312                             *entry =
 313                                 State::InProgressNonAlloc(TinyList::new_single(self.session_id));
 314                             None
 315                         }
 316                     }
 317                 }
 318                 State::InProgressNonAlloc(ref mut sessions) => {
 319                     if sessions.contains(&self.session_id) {
 320                         bug!("this should be unreachable");
 321                     } else {
 322                         // Start decoding concurrently.
 323                         sessions.insert(self.session_id);
 324                         None
 325                     }
 326                 }
 327                 State::InProgress(ref mut sessions, alloc_id) => {
 328                     if sessions.contains(&self.session_id) {
 329                         // Don't recurse.
 330                         return Ok(alloc_id);
 331                     } else {
 332                         // Start decoding concurrently.
 333                         sessions.insert(self.session_id);
 334                         Some(alloc_id)
 335                     }
 336                 }
 337             }
 338         };
 339
 340         // Now decode the actual data.
 341         let alloc_id = decoder.with_position(pos, |decoder| {
 342             match alloc_kind {
 343                 AllocDiscriminant::Alloc => {
 344                     let alloc = <&'tcx Allocation as Decodable<_>>::decode(decoder)?;
 345                     // We already have a reserved `AllocId`.
 346                     let alloc_id = alloc_id.unwrap();
 347                     trace!("decoded alloc {:?}: {:#?}", alloc_id, alloc);
 348                     decoder.tcx().set_alloc_id_same_memory(alloc_id, alloc);
 349                     Ok(alloc_id)
 350                 }
 351                 AllocDiscriminant::Fn => {
 352                     assert!(alloc_id.is_none());
 353                     trace!("creating fn alloc ID");
 354                     let instance = ty::Instance::decode(decoder)?;
 355                     trace!("decoded fn alloc instance: {:?}", instance);
 356                     let alloc_id = decoder.tcx().create_fn_alloc(instance);
 357                     Ok(alloc_id)
 358                 }
 359                 AllocDiscriminant::Static => {
 360                     assert!(alloc_id.is_none());
 361                     trace!("creating extern static alloc ID");
 362                     let did = <DefId as Decodable<D>>::decode(decoder)?;
 363                     trace!("decoded static def-ID: {:?}", did);
 364                     let alloc_id = decoder.tcx().create_static_alloc(did);
 365                     Ok(alloc_id)
 366                 }
 367             }
 368         })?;
 369
 370         self.state.decoding_state[idx].with_lock(|entry| {
 371             *entry = State::Done(alloc_id);
 372         });
 373
 374         Ok(alloc_id)
 375     }
 376 }
 377
 378 /// An allocation in the global (tcx-managed) memory can be either a function pointer,
 379 /// a static, or a "real" allocation with some data in it.
 380 #[derive(Debug, Clone, Eq, PartialEq, Hash, TyDecodable, TyEncodable, HashStable)]
 381 pub enum GlobalAlloc<'tcx> {
 382     /// The alloc ID is used as a function pointer.
 383     Function(Instance<'tcx>),
 384     /// The alloc ID points to a "lazy" static variable that did not get computed (yet).
 385     /// This is also used to break the cycle in recursive statics.
 386     Static(DefId),
 387     /// The alloc ID points to memory.
 388     Memory(&'tcx Allocation),
 389 }
 390
 391 impl GlobalAlloc<'tcx> {
 392     /// Panics if the `GlobalAlloc` does not refer to an `GlobalAlloc::Memory`
 393     #[track_caller]
 394     #[inline]
 395     pub fn unwrap_memory(&self) -> &'tcx Allocation {
 396         match *self {
 397             GlobalAlloc::Memory(mem) => mem,
 398             _ => bug!("expected memory, got {:?}", self),
 399         }
 400     }
 401
 402     /// Panics if the `GlobalAlloc` is not `GlobalAlloc::Function`
 403     #[track_caller]
 404     #[inline]
 405     pub fn unwrap_fn(&self) -> Instance<'tcx> {
 406         match *self {
 407             GlobalAlloc::Function(instance) => instance,
 408             _ => bug!("expected function, got {:?}", self),
 409         }
 410     }
 411 }
 412
 413 crate struct AllocMap<'tcx> {
 414     /// Maps `AllocId`s to their corresponding allocations.
 415     alloc_map: FxHashMap<AllocId, GlobalAlloc<'tcx>>,
 416
 417     /// Used to ensure that statics and functions only get one associated `AllocId`.
 418     /// Should never contain a `GlobalAlloc::Memory`!
 419     //
 420     // FIXME: Should we just have two separate dedup maps for statics and functions each?
 421     dedup: FxHashMap<GlobalAlloc<'tcx>, AllocId>,
 422
 423     /// The `AllocId` to assign to the next requested ID.
 424     /// Always incremented; never gets smaller.
 425     next_id: AllocId,
 426 }
 427
 428 impl<'tcx> AllocMap<'tcx> {
 429     crate fn new() -> Self {
 430         AllocMap { alloc_map: Default::default(), dedup: Default::default(), next_id: AllocId(0) }
 431     }
 432     fn reserve(&mut self) -> AllocId {
 433         let next = self.next_id;
 434         self.next_id.0 = self.next_id.0.checked_add(1).expect(
 435             "You overflowed a u64 by incrementing by 1... \
 436              You've just earned yourself a free drink if we ever meet. \
 437              Seriously, how did you do that?!",
 438         );
 439         next
 440     }
 441 }
 442
 443 impl<'tcx> TyCtxt<'tcx> {
 444     /// Obtains a new allocation ID that can be referenced but does not
 445     /// yet have an allocation backing it.
 446     ///
 447     /// Make sure to call `set_alloc_id_memory` or `set_alloc_id_same_memory` before returning such
 448     /// an `AllocId` from a query.
 449     pub fn reserve_alloc_id(self) -> AllocId {
 450         self.alloc_map.lock().reserve()
 451     }
 452
 453     /// Reserves a new ID *if* this allocation has not been dedup-reserved before.
 454     /// Should only be used for function pointers and statics, we don't want
 455     /// to dedup IDs for "real" memory!
 456     fn reserve_and_set_dedup(self, alloc: GlobalAlloc<'tcx>) -> AllocId {
 457         let mut alloc_map = self.alloc_map.lock();
 458         match alloc {
 459             GlobalAlloc::Function(..) | GlobalAlloc::Static(..) => {}
 460             GlobalAlloc::Memory(..) => bug!("Trying to dedup-reserve memory with real data!"),
 461         }
 462         if let Some(&alloc_id) = alloc_map.dedup.get(&alloc) {
 463             return alloc_id;
 464         }
 465         let id = alloc_map.reserve();
 466         debug!("creating alloc {:?} with id {}", alloc, id);
 467         alloc_map.alloc_map.insert(id, alloc.clone());
 468         alloc_map.dedup.insert(alloc, id);
 469         id
 470     }
 471
 472     /// Generates an `AllocId` for a static or return a cached one in case this function has been
 473     /// called on the same static before.
 474     pub fn create_static_alloc(self, static_id: DefId) -> AllocId {
 475         self.reserve_and_set_dedup(GlobalAlloc::Static(static_id))
 476     }
 477
 478     /// Generates an `AllocId` for a function.  Depending on the function type,
 479     /// this might get deduplicated or assigned a new ID each time.
 480     pub fn create_fn_alloc(self, instance: Instance<'tcx>) -> AllocId {
 481         // Functions cannot be identified by pointers, as asm-equal functions can get deduplicated
 482         // by the linker (we set the "unnamed_addr" attribute for LLVM) and functions can be
 483         // duplicated across crates.
 484         // We thus generate a new `AllocId` for every mention of a function. This means that
 485         // `main as fn() == main as fn()` is false, while `let x = main as fn(); x == x` is true.
 486         // However, formatting code relies on function identity (see #58320), so we only do
 487         // this for generic functions.  Lifetime parameters are ignored.
 488         let is_generic = instance
 489             .substs
 490             .into_iter()
 491             .any(|kind| !matches!(kind.unpack(), GenericArgKind::Lifetime(_)));
 492         if is_generic {
 493             // Get a fresh ID.
 494             let mut alloc_map = self.alloc_map.lock();
 495             let id = alloc_map.reserve();
 496             alloc_map.alloc_map.insert(id, GlobalAlloc::Function(instance));
 497             id
 498         } else {
 499             // Deduplicate.
 500             self.reserve_and_set_dedup(GlobalAlloc::Function(instance))
 501         }
 502     }
 503
 504     /// Interns the `Allocation` and return a new `AllocId`, even if there's already an identical
 505     /// `Allocation` with a different `AllocId`.
 506     /// Statics with identical content will still point to the same `Allocation`, i.e.,
 507     /// their data will be deduplicated through `Allocation` interning -- but they
 508     /// are different places in memory and as such need different IDs.
 509     pub fn create_memory_alloc(self, mem: &'tcx Allocation) -> AllocId {
 510         let id = self.reserve_alloc_id();
 511         self.set_alloc_id_memory(id, mem);
 512         id
 513     }
 514
 515     /// Returns `None` in case the `AllocId` is dangling. An `InterpretCx` can still have a
 516     /// local `Allocation` for that `AllocId`, but having such an `AllocId` in a constant is
 517     /// illegal and will likely ICE.
 518     /// This function exists to allow const eval to detect the difference between evaluation-
 519     /// local dangling pointers and allocations in constants/statics.
 520     #[inline]
 521     pub fn get_global_alloc(self, id: AllocId) -> Option<GlobalAlloc<'tcx>> {
 522         self.alloc_map.lock().alloc_map.get(&id).cloned()
 523     }
 524
 525     #[inline]
 526     #[track_caller]
 527     /// Panics in case the `AllocId` is dangling. Since that is impossible for `AllocId`s in
 528     /// constants (as all constants must pass interning and validation that check for dangling
 529     /// ids), this function is frequently used throughout rustc, but should not be used within
 530     /// the miri engine.
 531     pub fn global_alloc(self, id: AllocId) -> GlobalAlloc<'tcx> {
 532         match self.get_global_alloc(id) {
 533             Some(alloc) => alloc,
 534             None => bug!("could not find allocation for {}", id),
 535         }
 536     }
 537
 538     /// Freezes an `AllocId` created with `reserve` by pointing it at an `Allocation`. Trying to
 539     /// call this function twice, even with the same `Allocation` will ICE the compiler.
 540     pub fn set_alloc_id_memory(self, id: AllocId, mem: &'tcx Allocation) {
 541         if let Some(old) = self.alloc_map.lock().alloc_map.insert(id, GlobalAlloc::Memory(mem)) {
 542             bug!("tried to set allocation ID {}, but it was already existing as {:#?}", id, old);
 543         }
 544     }
 545
 546     /// Freezes an `AllocId` created with `reserve` by pointing it at an `Allocation`. May be called
 547     /// twice for the same `(AllocId, Allocation)` pair.
 548     fn set_alloc_id_same_memory(self, id: AllocId, mem: &'tcx Allocation) {
 549         self.alloc_map.lock().alloc_map.insert_same(id, GlobalAlloc::Memory(mem));
 550     }
 551 }
 552
 553 ////////////////////////////////////////////////////////////////////////////////
 554 // Methods to access integers in the target endianness
 555 ////////////////////////////////////////////////////////////////////////////////
 556
 557 #[inline]
 558 pub fn write_target_uint(
 559     endianness: Endian,
 560     mut target: &mut [u8],
 561     data: u128,
 562 ) -> Result<(), io::Error> {
 563     // This u128 holds an "any-size uint" (since smaller uints can fits in it)
 564     // So we do not write all bytes of the u128, just the "payload".
 565     match endianness {
 566         Endian::Little => target.write(&data.to_le_bytes())?,
 567         Endian::Big => target.write(&data.to_be_bytes()[16 - target.len()..])?,
 568     };
 569     debug_assert!(target.len() == 0); // We should have filled the target buffer.
 570     Ok(())
 571 }
 572
 573 #[inline]
 574 pub fn read_target_uint(endianness: Endian, mut source: &[u8]) -> Result<u128, io::Error> {
 575     // This u128 holds an "any-size uint" (since smaller uints can fits in it)
 576     let mut buf = [0u8; std::mem::size_of::<u128>()];
 577     // So we do not read exactly 16 bytes into the u128, just the "payload".
 578     let uint = match endianness {
 579         Endian::Little => {
 580             source.read(&mut buf)?;
 581             Ok(u128::from_le_bytes(buf))
 582         }
 583         Endian::Big => {
 584             source.read(&mut buf[16 - source.len()..])?;
 585             Ok(u128::from_be_bytes(buf))
 586         }
 587     };
 588     debug_assert!(source.len() == 0); // We should have consumed the source buffer.
 589     uint
 590 }
 591
 592 /// Computes the unsigned absolute value without wrapping or panicking.
 593 #[inline]
 594 pub fn uabs(value: i64) -> u64 {
 595     // The only tricky part here is if value == i64::MIN. In that case,
 596     // wrapping_abs() returns i64::MIN == -2^63. Casting this value to a u64
 597     // gives 2^63, the correct value.
 598     value.wrapping_abs() as u64
 599 }