src/librustc_middle/ty/codec.rs - third_party/rust - Git at Google

 // This module contains some shared code for encoding and decoding various
 // things from the `ty` module, and in particular implements support for
 // "shorthands" which allow to have pointers back into the already encoded
 // stream instead of re-encoding the same thing twice.
 //
 // The functionality in here is shared between persisting to crate metadata and
 // persisting to incr. comp. caches.

 use crate::arena::ArenaAllocatable;
 use crate::infer::canonical::{CanonicalVarInfo, CanonicalVarInfos};
 use crate::mir::{self, interpret::Allocation};
 use crate::ty::subst::SubstsRef;
 use crate::ty::{self, List, Ty, TyCtxt};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def_id::{CrateNum, DefId};
 use rustc_serialize::{opaque, Decodable, Decoder, Encodable, Encoder};
 use rustc_span::Span;
 use std::convert::{TryFrom, TryInto};
 use std::hash::Hash;
 use std::intrinsics;
 use std::marker::DiscriminantKind;

 /// The shorthand encoding uses an enum's variant index `usize`
 /// and is offset by this value so it never matches a real variant.
 /// This offset is also chosen so that the first byte is never < 0x80.
 pub const SHORTHAND_OFFSET: usize = 0x80;

 pub trait EncodableWithShorthand: Clone + Eq + Hash {
     type Variant: Encodable;
     fn variant(&self) -> &Self::Variant;
 }

 #[allow(rustc::usage_of_ty_tykind)]
 impl<'tcx> EncodableWithShorthand for Ty<'tcx> {
     type Variant = ty::TyKind<'tcx>;
     fn variant(&self) -> &Self::Variant {
         &self.kind
     }
 }

 impl<'tcx> EncodableWithShorthand for ty::Predicate<'tcx> {
     type Variant = ty::PredicateKind<'tcx>;
     fn variant(&self) -> &Self::Variant {
         self.kind()
     }
 }

 pub trait TyEncoder: Encoder {
     fn position(&self) -> usize;
 }

 impl TyEncoder for opaque::Encoder {
     #[inline]
     fn position(&self) -> usize {
         self.position()
     }
 }

 /// Encode the given value or a previously cached shorthand.
 pub fn encode_with_shorthand<E, T, M>(encoder: &mut E, value: &T, cache: M) -> Result<(), E::Error>
 where
     E: TyEncoder,
     M: for<'b> Fn(&'b mut E) -> &'b mut FxHashMap<T, usize>,
     T: EncodableWithShorthand,
     <T::Variant as DiscriminantKind>::Discriminant: Ord + TryFrom<usize>,
 {
     let existing_shorthand = cache(encoder).get(value).cloned();
     if let Some(shorthand) = existing_shorthand {
         return encoder.emit_usize(shorthand);
     }

     let variant = value.variant();

     let start = encoder.position();
     variant.encode(encoder)?;
     let len = encoder.position() - start;

     // The shorthand encoding uses the same usize as the
     // discriminant, with an offset so they can't conflict.
     let discriminant = intrinsics::discriminant_value(variant);
     assert!(discriminant < SHORTHAND_OFFSET.try_into().ok().unwrap());

     let shorthand = start + SHORTHAND_OFFSET;

     // Get the number of bits that leb128 could fit
     // in the same space as the fully encoded type.
     let leb128_bits = len * 7;

     // Check that the shorthand is a not longer than the
     // full encoding itself, i.e., it's an obvious win.
     if leb128_bits >= 64 || (shorthand as u64) < (1 << leb128_bits) {
         cache(encoder).insert(value.clone(), shorthand);
     }

     Ok(())
 }

 pub trait TyDecoder<'tcx>: Decoder {
     fn tcx(&self) -> TyCtxt<'tcx>;

     fn peek_byte(&self) -> u8;

     fn position(&self) -> usize;

     fn cached_ty_for_shorthand<F>(
         &mut self,
         shorthand: usize,
         or_insert_with: F,
     ) -> Result<Ty<'tcx>, Self::Error>
     where
         F: FnOnce(&mut Self) -> Result<Ty<'tcx>, Self::Error>;

     fn cached_predicate_for_shorthand<F>(
         &mut self,
         shorthand: usize,
         or_insert_with: F,
     ) -> Result<ty::Predicate<'tcx>, Self::Error>
     where
         F: FnOnce(&mut Self) -> Result<ty::Predicate<'tcx>, Self::Error>;

     fn with_position<F, R>(&mut self, pos: usize, f: F) -> R
     where
         F: FnOnce(&mut Self) -> R;

     fn map_encoded_cnum_to_current(&self, cnum: CrateNum) -> CrateNum;

     fn positioned_at_shorthand(&self) -> bool {
         (self.peek_byte() & (SHORTHAND_OFFSET as u8)) != 0
     }
 }

 #[inline]
 pub fn decode_arena_allocable<'tcx, D, T: ArenaAllocatable<'tcx> + Decodable>(
     decoder: &mut D,
 ) -> Result<&'tcx T, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     Ok(decoder.tcx().arena.alloc(Decodable::decode(decoder)?))
 }

 #[inline]
 pub fn decode_arena_allocable_slice<'tcx, D, T: ArenaAllocatable<'tcx> + Decodable>(
     decoder: &mut D,
 ) -> Result<&'tcx [T], D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     Ok(decoder.tcx().arena.alloc_from_iter(<Vec<T> as Decodable>::decode(decoder)?))
 }

 #[inline]
 pub fn decode_cnum<D>(decoder: &mut D) -> Result<CrateNum, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let cnum = CrateNum::from_u32(u32::decode(decoder)?);
     Ok(decoder.map_encoded_cnum_to_current(cnum))
 }

 #[allow(rustc::usage_of_ty_tykind)]
 #[inline]
 pub fn decode_ty<D>(decoder: &mut D) -> Result<Ty<'tcx>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     // Handle shorthands first, if we have an usize > 0x80.
     if decoder.positioned_at_shorthand() {
         let pos = decoder.read_usize()?;
         assert!(pos >= SHORTHAND_OFFSET);
         let shorthand = pos - SHORTHAND_OFFSET;

         decoder.cached_ty_for_shorthand(shorthand, |decoder| {
             decoder.with_position(shorthand, Ty::decode)
         })
     } else {
         let tcx = decoder.tcx();
         Ok(tcx.mk_ty(ty::TyKind::decode(decoder)?))
     }
 }

 #[inline]
 pub fn decode_predicate<D>(decoder: &mut D) -> Result<ty::Predicate<'tcx>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     // Handle shorthands first, if we have an usize > 0x80.
     if decoder.positioned_at_shorthand() {
         let pos = decoder.read_usize()?;
         assert!(pos >= SHORTHAND_OFFSET);
         let shorthand = pos - SHORTHAND_OFFSET;

         decoder.cached_predicate_for_shorthand(shorthand, |decoder| {
             decoder.with_position(shorthand, ty::Predicate::decode)
         })
     } else {
         let tcx = decoder.tcx();
         Ok(tcx.mk_predicate(ty::PredicateKind::decode(decoder)?))
     }
 }

 #[inline]
 pub fn decode_spanned_predicates<D>(
     decoder: &mut D,
 ) -> Result<&'tcx [(ty::Predicate<'tcx>, Span)], D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let tcx = decoder.tcx();
     Ok(tcx.arena.alloc_from_iter(
         (0..decoder.read_usize()?)
             .map(|_| Decodable::decode(decoder))
             .collect::<Result<Vec<_>, _>>()?,
     ))
 }

 #[inline]
 pub fn decode_substs<D>(decoder: &mut D) -> Result<SubstsRef<'tcx>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let len = decoder.read_usize()?;
     let tcx = decoder.tcx();
     Ok(tcx.mk_substs((0..len).map(|_| Decodable::decode(decoder)))?)
 }

 #[inline]
 pub fn decode_place<D>(decoder: &mut D) -> Result<mir::Place<'tcx>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let local: mir::Local = Decodable::decode(decoder)?;
     let len = decoder.read_usize()?;
     let projection: &'tcx List<mir::PlaceElem<'tcx>> =
         decoder.tcx().mk_place_elems((0..len).map(|_| Decodable::decode(decoder)))?;
     Ok(mir::Place { local, projection })
 }

 #[inline]
 pub fn decode_region<D>(decoder: &mut D) -> Result<ty::Region<'tcx>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     Ok(decoder.tcx().mk_region(Decodable::decode(decoder)?))
 }

 #[inline]
 pub fn decode_ty_slice<D>(decoder: &mut D) -> Result<&'tcx ty::List<Ty<'tcx>>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let len = decoder.read_usize()?;
     Ok(decoder.tcx().mk_type_list((0..len).map(|_| Decodable::decode(decoder)))?)
 }

 #[inline]
 pub fn decode_adt_def<D>(decoder: &mut D) -> Result<&'tcx ty::AdtDef, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let def_id = DefId::decode(decoder)?;
     Ok(decoder.tcx().adt_def(def_id))
 }

 #[inline]
 pub fn decode_existential_predicate_slice<D>(
     decoder: &mut D,
 ) -> Result<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let len = decoder.read_usize()?;
     Ok(decoder.tcx().mk_existential_predicates((0..len).map(|_| Decodable::decode(decoder)))?)
 }

 #[inline]
 pub fn decode_canonical_var_infos<D>(decoder: &mut D) -> Result<CanonicalVarInfos<'tcx>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     let len = decoder.read_usize()?;
     let interned: Result<Vec<CanonicalVarInfo>, _> =
         (0..len).map(|_| Decodable::decode(decoder)).collect();
     Ok(decoder.tcx().intern_canonical_var_infos(interned?.as_slice()))
 }

 #[inline]
 pub fn decode_const<D>(decoder: &mut D) -> Result<&'tcx ty::Const<'tcx>, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     Ok(decoder.tcx().mk_const(Decodable::decode(decoder)?))
 }

 #[inline]
 pub fn decode_allocation<D>(decoder: &mut D) -> Result<&'tcx Allocation, D::Error>
 where
     D: TyDecoder<'tcx>,
 {
     Ok(decoder.tcx().intern_const_alloc(Decodable::decode(decoder)?))
 }

 #[macro_export]
 macro_rules! __impl_decoder_methods {
     ($($name:ident -> $ty:ty;)*) => {
         $(
             #[inline]
             fn $name(&mut self) -> Result<$ty, Self::Error> {
                 self.opaque.$name()
             }
         )*
     }
 }

 #[macro_export]
 macro_rules! impl_arena_allocatable_decoder {
     ([]$args:tt) => {};
     ([decode $(, $attrs:ident)*]
      [[$DecoderName:ident [$($typaram:tt),*]], [$name:ident: $ty:ty, $gen_ty:ty], $tcx:lifetime]) => {
          // FIXME(#36588): These impls are horribly unsound as they allow
          // the caller to pick any lifetime for `'tcx`, including `'static`.
         #[allow(unused_lifetimes)]
         impl<'_x, '_y, '_z, '_w, '_a, $($typaram),*> SpecializedDecoder<&'_a $gen_ty>
         for $DecoderName<$($typaram),*>
         where &'_a $gen_ty: UseSpecializedDecodable
         {
             #[inline]
             fn specialized_decode(&mut self) -> Result<&'_a $gen_ty, Self::Error> {
                 unsafe {
                     std::mem::transmute::<
                         Result<&$tcx $ty, Self::Error>,
                         Result<&'_a $gen_ty, Self::Error>,
                     >(decode_arena_allocable(self))
                 }
             }
         }

         #[allow(unused_lifetimes)]
         impl<'_x, '_y, '_z, '_w, '_a, $($typaram),*> SpecializedDecoder<&'_a [$gen_ty]>
         for $DecoderName<$($typaram),*>
         where &'_a [$gen_ty]: UseSpecializedDecodable
         {
             #[inline]
             fn specialized_decode(&mut self) -> Result<&'_a [$gen_ty], Self::Error> {
                 unsafe {
                     std::mem::transmute::<
                         Result<&$tcx [$ty], Self::Error>,
                         Result<&'_a [$gen_ty], Self::Error>,
                     >(decode_arena_allocable_slice(self))
                 }
             }
         }
     };
     ([$ignore:ident $(, $attrs:ident)*]$args:tt) => {
         impl_arena_allocatable_decoder!([$($attrs),*]$args);
     };
 }

 #[macro_export]
 macro_rules! impl_arena_allocatable_decoders {
     ($args:tt, [$($a:tt $name:ident: $ty:ty, $gen_ty:ty;)*], $tcx:lifetime) => {
         $(
             impl_arena_allocatable_decoder!($a [$args, [$name: $ty, $gen_ty], $tcx]);
         )*
     }
 }

 #[macro_export]
 macro_rules! implement_ty_decoder {
     ($DecoderName:ident <$($typaram:tt),*>) => {
         mod __ty_decoder_impl {
             use std::borrow::Cow;
             use std::mem::transmute;

             use rustc_serialize::{Decoder, SpecializedDecoder, UseSpecializedDecodable};

             use $crate::infer::canonical::CanonicalVarInfos;
             use $crate::ty;
             use $crate::ty::codec::*;
             use $crate::ty::subst::InternalSubsts;
             use rustc_hir::def_id::CrateNum;

             use rustc_span::Span;

             use super::$DecoderName;

             impl<$($typaram ),*> Decoder for $DecoderName<$($typaram),*> {
                 type Error = String;

                 __impl_decoder_methods! {
                     read_nil -> ();

                     read_u128 -> u128;
                     read_u64 -> u64;
                     read_u32 -> u32;
                     read_u16 -> u16;
                     read_u8 -> u8;
                     read_usize -> usize;

                     read_i128 -> i128;
                     read_i64 -> i64;
                     read_i32 -> i32;
                     read_i16 -> i16;
                     read_i8 -> i8;
                     read_isize -> isize;

                     read_bool -> bool;
                     read_f64 -> f64;
                     read_f32 -> f32;
                     read_char -> char;
                     read_str -> Cow<'_, str>;
                 }

                 fn error(&mut self, err: &str) -> Self::Error {
                     self.opaque.error(err)
                 }
             }

             // FIXME(#36588): These impls are horribly unsound as they allow
             // the caller to pick any lifetime for `'tcx`, including `'static`.

             arena_types!(impl_arena_allocatable_decoders, [$DecoderName [$($typaram),*]], 'tcx);

             impl<$($typaram),*> SpecializedDecoder<CrateNum>
             for $DecoderName<$($typaram),*> {
                 fn specialized_decode(&mut self) -> Result<CrateNum, Self::Error> {
                     decode_cnum(self)
                 }
             }

             impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x ty::TyS<'_y>>
             for $DecoderName<$($typaram),*>
             where &'_x ty::TyS<'_y>: UseSpecializedDecodable
             {
                 fn specialized_decode(&mut self) -> Result<&'_x ty::TyS<'_y>, Self::Error> {
                     unsafe {
                         transmute::<
                             Result<ty::Ty<'tcx>, Self::Error>,
                             Result<&'_x ty::TyS<'_y>, Self::Error>,
                         >(decode_ty(self))
                     }
                 }
             }

             impl<'_x, $($typaram),*> SpecializedDecoder<ty::Predicate<'_x>>
             for $DecoderName<$($typaram),*> {
                 fn specialized_decode(&mut self) -> Result<ty::Predicate<'_x>, Self::Error> {
                     unsafe {
                         transmute::<
                             Result<ty::Predicate<'tcx>, Self::Error>,
                             Result<ty::Predicate<'_x>, Self::Error>,
                         >(decode_predicate(self))
                     }
                 }
             }

             impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x [(ty::Predicate<'_y>, Span)]>
             for $DecoderName<$($typaram),*>
             where &'_x [(ty::Predicate<'_y>, Span)]: UseSpecializedDecodable {
                 fn specialized_decode(&mut self)
                                       -> Result<&'_x [(ty::Predicate<'_y>, Span)], Self::Error>
                 {
                     unsafe { transmute(decode_spanned_predicates(self)) }
                 }
             }

             impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x InternalSubsts<'_y>>
             for $DecoderName<$($typaram),*>
             where &'_x InternalSubsts<'_y>: UseSpecializedDecodable {
                 fn specialized_decode(&mut self) -> Result<&'_x InternalSubsts<'_y>, Self::Error> {
                     unsafe { transmute(decode_substs(self)) }
                 }
             }

             impl<'_x, $($typaram),*> SpecializedDecoder<$crate::mir::Place<'_x>>
             for $DecoderName<$($typaram),*> {
                 fn specialized_decode(
                     &mut self
                 ) -> Result<$crate::mir::Place<'_x>, Self::Error> {
                     unsafe { transmute(decode_place(self)) }
                 }
             }

             impl<'_x, $($typaram),*> SpecializedDecoder<ty::Region<'_x>>
             for $DecoderName<$($typaram),*> {
                 fn specialized_decode(&mut self) -> Result<ty::Region<'_x>, Self::Error> {
                     unsafe { transmute(decode_region(self)) }
                 }
             }

             impl<'_x, '_y, '_z, $($typaram),*> SpecializedDecoder<&'_x ty::List<&'_y ty::TyS<'_z>>>
             for $DecoderName<$($typaram),*>
             where &'_x ty::List<&'_y ty::TyS<'_z>>: UseSpecializedDecodable {
                 fn specialized_decode(&mut self)
                                       -> Result<&'_x ty::List<&'_y ty::TyS<'_z>>, Self::Error> {
                     unsafe { transmute(decode_ty_slice(self)) }
                 }
             }

             impl<'_x, $($typaram),*> SpecializedDecoder<&'_x ty::AdtDef>
             for $DecoderName<$($typaram),*> {
                 fn specialized_decode(&mut self) -> Result<&'_x ty::AdtDef, Self::Error> {
                     unsafe { transmute(decode_adt_def(self)) }
                 }
             }

             impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x ty::List<ty::ExistentialPredicate<'_y>>>
             for $DecoderName<$($typaram),*>
             where &'_x ty::List<ty::ExistentialPredicate<'_y>>: UseSpecializedDecodable {
                 fn specialized_decode(&mut self)
                     -> Result<&'_x ty::List<ty::ExistentialPredicate<'_y>>, Self::Error> {
                         unsafe { transmute(decode_existential_predicate_slice(self)) }
                 }
             }

             impl<'_x, $($typaram),*> SpecializedDecoder<CanonicalVarInfos<'_x>>
                 for $DecoderName<$($typaram),*> {
                 fn specialized_decode(&mut self)
                     -> Result<CanonicalVarInfos<'_x>, Self::Error> {
                         unsafe { transmute(decode_canonical_var_infos(self)) }
                 }
             }

             impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x $crate::ty::Const<'_y>>
             for $DecoderName<$($typaram),*>
             where &'_x $crate::ty::Const<'_y>: UseSpecializedDecodable {
                 fn specialized_decode(&mut self) -> Result<&'_x ty::Const<'_y>, Self::Error> {
                     unsafe { transmute(decode_const(self)) }
                 }
             }

             impl<'_x, $($typaram),*> SpecializedDecoder<&'_x $crate::mir::interpret::Allocation>
             for $DecoderName<$($typaram),*> {
                 fn specialized_decode(
                     &mut self
                 ) -> Result<&'_x $crate::mir::interpret::Allocation, Self::Error> {
                     unsafe { transmute(decode_allocation(self)) }
                 }
             }
         }
     };
 }
	// This module contains some shared code for encoding and decoding various
	// things from the `ty` module, and in particular implements support for
	// "shorthands" which allow to have pointers back into the already encoded
	// stream instead of re-encoding the same thing twice.
	//
	// The functionality in here is shared between persisting to crate metadata and
	// persisting to incr. comp. caches.

	use crate::arena::ArenaAllocatable;
	use crate::infer::canonical::{CanonicalVarInfo, CanonicalVarInfos};
	use crate::mir::{self, interpret::Allocation};
	use crate::ty::subst::SubstsRef;
	use crate::ty::{self, List, Ty, TyCtxt};
	use rustc_data_structures::fx::FxHashMap;
	use rustc_hir::def_id::{CrateNum, DefId};
	use rustc_serialize::{opaque, Decodable, Decoder, Encodable, Encoder};
	use rustc_span::Span;
	use std::convert::{TryFrom, TryInto};
	use std::hash::Hash;
	use std::intrinsics;
	use std::marker::DiscriminantKind;

	/// The shorthand encoding uses an enum's variant index `usize`
	/// and is offset by this value so it never matches a real variant.
	/// This offset is also chosen so that the first byte is never < 0x80.
	pub const SHORTHAND_OFFSET: usize = 0x80;

	pub trait EncodableWithShorthand: Clone + Eq + Hash {
	type Variant: Encodable;
	fn variant(&self) -> &Self::Variant;
	}

	#[allow(rustc::usage_of_ty_tykind)]
	impl<'tcx> EncodableWithShorthand for Ty<'tcx> {
	type Variant = ty::TyKind<'tcx>;
	fn variant(&self) -> &Self::Variant {
	&self.kind
	}
	}

	impl<'tcx> EncodableWithShorthand for ty::Predicate<'tcx> {
	type Variant = ty::PredicateKind<'tcx>;
	fn variant(&self) -> &Self::Variant {
	self.kind()
	}
	}

	pub trait TyEncoder: Encoder {
	fn position(&self) -> usize;
	}

	impl TyEncoder for opaque::Encoder {
	#[inline]
	fn position(&self) -> usize {
	self.position()
	}
	}

	/// Encode the given value or a previously cached shorthand.
	pub fn encode_with_shorthand<E, T, M>(encoder: &mut E, value: &T, cache: M) -> Result<(), E::Error>
	where
	E: TyEncoder,
	M: for<'b> Fn(&'b mut E) -> &'b mut FxHashMap<T, usize>,
	T: EncodableWithShorthand,
	<T::Variant as DiscriminantKind>::Discriminant: Ord + TryFrom<usize>,
	{
	let existing_shorthand = cache(encoder).get(value).cloned();
	if let Some(shorthand) = existing_shorthand {
	return encoder.emit_usize(shorthand);
	}

	let variant = value.variant();

	let start = encoder.position();
	variant.encode(encoder)?;
	let len = encoder.position() - start;

	// The shorthand encoding uses the same usize as the
	// discriminant, with an offset so they can't conflict.
	let discriminant = intrinsics::discriminant_value(variant);
	assert!(discriminant < SHORTHAND_OFFSET.try_into().ok().unwrap());

	let shorthand = start + SHORTHAND_OFFSET;

	// Get the number of bits that leb128 could fit
	// in the same space as the fully encoded type.
	let leb128_bits = len * 7;

	// Check that the shorthand is a not longer than the
	// full encoding itself, i.e., it's an obvious win.
	if leb128_bits >= 64 \|\| (shorthand as u64) < (1 << leb128_bits) {
	cache(encoder).insert(value.clone(), shorthand);
	}

	Ok(())
	}

	pub trait TyDecoder<'tcx>: Decoder {
	fn tcx(&self) -> TyCtxt<'tcx>;

	fn peek_byte(&self) -> u8;

	fn position(&self) -> usize;

	fn cached_ty_for_shorthand<F>(
	&mut self,
	shorthand: usize,
	or_insert_with: F,
	) -> Result<Ty<'tcx>, Self::Error>
	where
	F: FnOnce(&mut Self) -> Result<Ty<'tcx>, Self::Error>;

	fn cached_predicate_for_shorthand<F>(
	&mut self,
	shorthand: usize,
	or_insert_with: F,
	) -> Result<ty::Predicate<'tcx>, Self::Error>
	where
	F: FnOnce(&mut Self) -> Result<ty::Predicate<'tcx>, Self::Error>;

	fn with_position<F, R>(&mut self, pos: usize, f: F) -> R
	where
	F: FnOnce(&mut Self) -> R;

	fn map_encoded_cnum_to_current(&self, cnum: CrateNum) -> CrateNum;

	fn positioned_at_shorthand(&self) -> bool {
	(self.peek_byte() & (SHORTHAND_OFFSET as u8)) != 0
	}
	}

	#[inline]
	pub fn decode_arena_allocable<'tcx, D, T: ArenaAllocatable<'tcx> + Decodable>(
	decoder: &mut D,
	) -> Result<&'tcx T, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	Ok(decoder.tcx().arena.alloc(Decodable::decode(decoder)?))
	}

	#[inline]
	pub fn decode_arena_allocable_slice<'tcx, D, T: ArenaAllocatable<'tcx> + Decodable>(
	decoder: &mut D,
	) -> Result<&'tcx [T], D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	Ok(decoder.tcx().arena.alloc_from_iter(<Vec<T> as Decodable>::decode(decoder)?))
	}

	#[inline]
	pub fn decode_cnum<D>(decoder: &mut D) -> Result<CrateNum, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let cnum = CrateNum::from_u32(u32::decode(decoder)?);
	Ok(decoder.map_encoded_cnum_to_current(cnum))
	}

	#[allow(rustc::usage_of_ty_tykind)]
	#[inline]
	pub fn decode_ty<D>(decoder: &mut D) -> Result<Ty<'tcx>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	// Handle shorthands first, if we have an usize > 0x80.
	if decoder.positioned_at_shorthand() {
	let pos = decoder.read_usize()?;
	assert!(pos >= SHORTHAND_OFFSET);
	let shorthand = pos - SHORTHAND_OFFSET;

	decoder.cached_ty_for_shorthand(shorthand, \|decoder\| {
	decoder.with_position(shorthand, Ty::decode)
	})
	} else {
	let tcx = decoder.tcx();
	Ok(tcx.mk_ty(ty::TyKind::decode(decoder)?))
	}
	}

	#[inline]
	pub fn decode_predicate<D>(decoder: &mut D) -> Result<ty::Predicate<'tcx>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	// Handle shorthands first, if we have an usize > 0x80.
	if decoder.positioned_at_shorthand() {
	let pos = decoder.read_usize()?;
	assert!(pos >= SHORTHAND_OFFSET);
	let shorthand = pos - SHORTHAND_OFFSET;

	decoder.cached_predicate_for_shorthand(shorthand, \|decoder\| {
	decoder.with_position(shorthand, ty::Predicate::decode)
	})
	} else {
	let tcx = decoder.tcx();
	Ok(tcx.mk_predicate(ty::PredicateKind::decode(decoder)?))
	}
	}

	#[inline]
	pub fn decode_spanned_predicates<D>(
	decoder: &mut D,
	) -> Result<&'tcx [(ty::Predicate<'tcx>, Span)], D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let tcx = decoder.tcx();
	Ok(tcx.arena.alloc_from_iter(
	(0..decoder.read_usize()?)
	.map(\|_\| Decodable::decode(decoder))
	.collect::<Result<Vec<_>, _>>()?,
	))
	}

	#[inline]
	pub fn decode_substs<D>(decoder: &mut D) -> Result<SubstsRef<'tcx>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let len = decoder.read_usize()?;
	let tcx = decoder.tcx();
	Ok(tcx.mk_substs((0..len).map(\|_\| Decodable::decode(decoder)))?)
	}

	#[inline]
	pub fn decode_place<D>(decoder: &mut D) -> Result<mir::Place<'tcx>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let local: mir::Local = Decodable::decode(decoder)?;
	let len = decoder.read_usize()?;
	let projection: &'tcx List<mir::PlaceElem<'tcx>> =
	decoder.tcx().mk_place_elems((0..len).map(\|_\| Decodable::decode(decoder)))?;
	Ok(mir::Place { local, projection })
	}

	#[inline]
	pub fn decode_region<D>(decoder: &mut D) -> Result<ty::Region<'tcx>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	Ok(decoder.tcx().mk_region(Decodable::decode(decoder)?))
	}

	#[inline]
	pub fn decode_ty_slice<D>(decoder: &mut D) -> Result<&'tcx ty::List<Ty<'tcx>>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let len = decoder.read_usize()?;
	Ok(decoder.tcx().mk_type_list((0..len).map(\|_\| Decodable::decode(decoder)))?)
	}

	#[inline]
	pub fn decode_adt_def<D>(decoder: &mut D) -> Result<&'tcx ty::AdtDef, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let def_id = DefId::decode(decoder)?;
	Ok(decoder.tcx().adt_def(def_id))
	}

	#[inline]
	pub fn decode_existential_predicate_slice<D>(
	decoder: &mut D,
	) -> Result<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let len = decoder.read_usize()?;
	Ok(decoder.tcx().mk_existential_predicates((0..len).map(\|_\| Decodable::decode(decoder)))?)
	}

	#[inline]
	pub fn decode_canonical_var_infos<D>(decoder: &mut D) -> Result<CanonicalVarInfos<'tcx>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	let len = decoder.read_usize()?;
	let interned: Result<Vec<CanonicalVarInfo>, _> =
	(0..len).map(\|_\| Decodable::decode(decoder)).collect();
	Ok(decoder.tcx().intern_canonical_var_infos(interned?.as_slice()))
	}

	#[inline]
	pub fn decode_const<D>(decoder: &mut D) -> Result<&'tcx ty::Const<'tcx>, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	Ok(decoder.tcx().mk_const(Decodable::decode(decoder)?))
	}

	#[inline]
	pub fn decode_allocation<D>(decoder: &mut D) -> Result<&'tcx Allocation, D::Error>
	where
	D: TyDecoder<'tcx>,
	{
	Ok(decoder.tcx().intern_const_alloc(Decodable::decode(decoder)?))
	}

	#[macro_export]
	macro_rules! __impl_decoder_methods {
	($($name:ident -> $ty:ty;)*) => {
	$(
	#[inline]
	fn $name(&mut self) -> Result<$ty, Self::Error> {
	self.opaque.$name()
	}
	)*
	}
	}

	#[macro_export]
	macro_rules! impl_arena_allocatable_decoder {
	([]$args:tt) => {};
	([decode $(, $attrs:ident)*]
	[[$DecoderName:ident [$($typaram:tt),*]], [$name:ident: $ty:ty, $gen_ty:ty], $tcx:lifetime]) => {
	// FIXME(#36588): These impls are horribly unsound as they allow
	// the caller to pick any lifetime for `'tcx`, including `'static`.
	#[allow(unused_lifetimes)]
	impl<'_x, '_y, '_z, '_w, '_a, $($typaram),*> SpecializedDecoder<&'_a $gen_ty>
	for $DecoderName<$($typaram),*>
	where &'_a $gen_ty: UseSpecializedDecodable
	{
	#[inline]
	fn specialized_decode(&mut self) -> Result<&'_a $gen_ty, Self::Error> {
	unsafe {
	std::mem::transmute::<
	Result<&$tcx $ty, Self::Error>,
	Result<&'_a $gen_ty, Self::Error>,
	>(decode_arena_allocable(self))
	}
	}
	}

	#[allow(unused_lifetimes)]
	impl<'_x, '_y, '_z, '_w, '_a, $($typaram),*> SpecializedDecoder<&'_a [$gen_ty]>
	for $DecoderName<$($typaram),*>
	where &'_a [$gen_ty]: UseSpecializedDecodable
	{
	#[inline]
	fn specialized_decode(&mut self) -> Result<&'_a [$gen_ty], Self::Error> {
	unsafe {
	std::mem::transmute::<
	Result<&$tcx [$ty], Self::Error>,
	Result<&'_a [$gen_ty], Self::Error>,
	>(decode_arena_allocable_slice(self))
	}
	}
	}
	};
	([$ignore:ident $(, $attrs:ident)*]$args:tt) => {
	impl_arena_allocatable_decoder!([$($attrs),*]$args);
	};
	}

	#[macro_export]
	macro_rules! impl_arena_allocatable_decoders {
	($args:tt, [$($a:tt $name:ident: $ty:ty, $gen_ty:ty;)*], $tcx:lifetime) => {
	$(
	impl_arena_allocatable_decoder!($a [$args, [$name: $ty, $gen_ty], $tcx]);
	)*
	}
	}

	#[macro_export]
	macro_rules! implement_ty_decoder {
	($DecoderName:ident <$($typaram:tt),*>) => {
	mod __ty_decoder_impl {
	use std::borrow::Cow;
	use std::mem::transmute;

	use rustc_serialize::{Decoder, SpecializedDecoder, UseSpecializedDecodable};

	use $crate::infer::canonical::CanonicalVarInfos;
	use $crate::ty;
	use $crate::ty::codec::*;
	use $crate::ty::subst::InternalSubsts;
	use rustc_hir::def_id::CrateNum;

	use rustc_span::Span;

	use super::$DecoderName;

	impl<$($typaram ),> Decoder for $DecoderName<$($typaram),> {
	type Error = String;

	__impl_decoder_methods! {
	read_nil -> ();

	read_u128 -> u128;
	read_u64 -> u64;
	read_u32 -> u32;
	read_u16 -> u16;
	read_u8 -> u8;
	read_usize -> usize;

	read_i128 -> i128;
	read_i64 -> i64;
	read_i32 -> i32;
	read_i16 -> i16;
	read_i8 -> i8;
	read_isize -> isize;

	read_bool -> bool;
	read_f64 -> f64;
	read_f32 -> f32;
	read_char -> char;
	read_str -> Cow<'_, str>;
	}

	fn error(&mut self, err: &str) -> Self::Error {
	self.opaque.error(err)
	}
	}

	// FIXME(#36588): These impls are horribly unsound as they allow
	// the caller to pick any lifetime for `'tcx`, including `'static`.

	arena_types!(impl_arena_allocatable_decoders, [$DecoderName [$($typaram),*]], 'tcx);

	impl<$($typaram),*> SpecializedDecoder<CrateNum>
	for $DecoderName<$($typaram),*> {
	fn specialized_decode(&mut self) -> Result<CrateNum, Self::Error> {
	decode_cnum(self)
	}
	}

	impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x ty::TyS<'_y>>
	for $DecoderName<$($typaram),*>
	where &'_x ty::TyS<'_y>: UseSpecializedDecodable
	{
	fn specialized_decode(&mut self) -> Result<&'_x ty::TyS<'_y>, Self::Error> {
	unsafe {
	transmute::<
	Result<ty::Ty<'tcx>, Self::Error>,
	Result<&'_x ty::TyS<'_y>, Self::Error>,
	>(decode_ty(self))
	}
	}
	}

	impl<'_x, $($typaram),*> SpecializedDecoder<ty::Predicate<'_x>>
	for $DecoderName<$($typaram),*> {
	fn specialized_decode(&mut self) -> Result<ty::Predicate<'_x>, Self::Error> {
	unsafe {
	transmute::<
	Result<ty::Predicate<'tcx>, Self::Error>,
	Result<ty::Predicate<'_x>, Self::Error>,
	>(decode_predicate(self))
	}
	}
	}

	impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x [(ty::Predicate<'_y>, Span)]>
	for $DecoderName<$($typaram),*>
	where &'_x [(ty::Predicate<'_y>, Span)]: UseSpecializedDecodable {
	fn specialized_decode(&mut self)
	-> Result<&'_x [(ty::Predicate<'_y>, Span)], Self::Error>
	{
	unsafe { transmute(decode_spanned_predicates(self)) }
	}
	}

	impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x InternalSubsts<'_y>>
	for $DecoderName<$($typaram),*>
	where &'_x InternalSubsts<'_y>: UseSpecializedDecodable {
	fn specialized_decode(&mut self) -> Result<&'_x InternalSubsts<'_y>, Self::Error> {
	unsafe { transmute(decode_substs(self)) }
	}
	}

	impl<'_x, $($typaram),*> SpecializedDecoder<$crate::mir::Place<'_x>>
	for $DecoderName<$($typaram),*> {
	fn specialized_decode(
	&mut self
	) -> Result<$crate::mir::Place<'_x>, Self::Error> {
	unsafe { transmute(decode_place(self)) }
	}
	}

	impl<'_x, $($typaram),*> SpecializedDecoder<ty::Region<'_x>>
	for $DecoderName<$($typaram),*> {
	fn specialized_decode(&mut self) -> Result<ty::Region<'_x>, Self::Error> {
	unsafe { transmute(decode_region(self)) }
	}
	}

	impl<'_x, '_y, '_z, $($typaram),*> SpecializedDecoder<&'_x ty::List<&'_y ty::TyS<'_z>>>
	for $DecoderName<$($typaram),*>
	where &'_x ty::List<&'_y ty::TyS<'_z>>: UseSpecializedDecodable {
	fn specialized_decode(&mut self)
	-> Result<&'_x ty::List<&'_y ty::TyS<'_z>>, Self::Error> {
	unsafe { transmute(decode_ty_slice(self)) }
	}
	}

	impl<'_x, $($typaram),*> SpecializedDecoder<&'_x ty::AdtDef>
	for $DecoderName<$($typaram),*> {
	fn specialized_decode(&mut self) -> Result<&'_x ty::AdtDef, Self::Error> {
	unsafe { transmute(decode_adt_def(self)) }
	}
	}

	impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x ty::List<ty::ExistentialPredicate<'_y>>>
	for $DecoderName<$($typaram),*>
	where &'_x ty::List<ty::ExistentialPredicate<'_y>>: UseSpecializedDecodable {
	fn specialized_decode(&mut self)
	-> Result<&'_x ty::List<ty::ExistentialPredicate<'_y>>, Self::Error> {
	unsafe { transmute(decode_existential_predicate_slice(self)) }
	}
	}

	impl<'_x, $($typaram),*> SpecializedDecoder<CanonicalVarInfos<'_x>>
	for $DecoderName<$($typaram),*> {
	fn specialized_decode(&mut self)
	-> Result<CanonicalVarInfos<'_x>, Self::Error> {
	unsafe { transmute(decode_canonical_var_infos(self)) }
	}
	}

	impl<'_x, '_y, $($typaram),*> SpecializedDecoder<&'_x $crate::ty::Const<'_y>>
	for $DecoderName<$($typaram),*>
	where &'_x $crate::ty::Const<'_y>: UseSpecializedDecodable {
	fn specialized_decode(&mut self) -> Result<&'_x ty::Const<'_y>, Self::Error> {
	unsafe { transmute(decode_const(self)) }
	}
	}

	impl<'_x, $($typaram),*> SpecializedDecoder<&'_x $crate::mir::interpret::Allocation>
	for $DecoderName<$($typaram),*> {
	fn specialized_decode(
	&mut self
	) -> Result<&'_x $crate::mir::interpret::Allocation, Self::Error> {
	unsafe { transmute(decode_allocation(self)) }
	}
	}
	}
	};
	}