compiler/rustc_const_eval/src/interpret/machine.rs - toolchain/rustc - Git at Google

 //! This module contains everything needed to instantiate an interpreter.
 //! This separation exists to ensure that no fancy miri features like
 //! interpreting common C functions leak into CTFE.

 use std::borrow::{Borrow, Cow};
 use std::fmt::Debug;
 use std::hash::Hash;

 use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
 use rustc_middle::mir;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::DefId;
 use rustc_target::abi::Size;
 use rustc_target::spec::abi::Abi as CallAbi;

 use super::{
     AllocId, AllocRange, Allocation, ConstAllocation, Frame, ImmTy, InterpCx, InterpResult,
     MemoryKind, OpTy, Operand, PlaceTy, Pointer, Provenance, Scalar, StackPopUnwind,
 };

 /// Data returned by Machine::stack_pop,
 /// to provide further control over the popping of the stack frame
 #[derive(Eq, PartialEq, Debug, Copy, Clone)]
 pub enum StackPopJump {
     /// Indicates that no special handling should be
     /// done - we'll either return normally or unwind
     /// based on the terminator for the function
     /// we're leaving.
     Normal,

     /// Indicates that we should *not* jump to the return/unwind address, as the callback already
     /// took care of everything.
     NoJump,
 }

 /// Whether this kind of memory is allowed to leak
 pub trait MayLeak: Copy {
     fn may_leak(self) -> bool;
 }

 /// The functionality needed by memory to manage its allocations
 pub trait AllocMap<K: Hash + Eq, V> {
     /// Tests if the map contains the given key.
     /// Deliberately takes `&mut` because that is sufficient, and some implementations
     /// can be more efficient then (using `RefCell::get_mut`).
     fn contains_key<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> bool
     where
         K: Borrow<Q>;

     /// Inserts a new entry into the map.
     fn insert(&mut self, k: K, v: V) -> Option<V>;

     /// Removes an entry from the map.
     fn remove<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> Option<V>
     where
         K: Borrow<Q>;

     /// Returns data based on the keys and values in the map.
     fn filter_map_collect<T>(&self, f: impl FnMut(&K, &V) -> Option<T>) -> Vec<T>;

     /// Returns a reference to entry `k`. If no such entry exists, call
     /// `vacant` and either forward its error, or add its result to the map
     /// and return a reference to *that*.
     fn get_or<E>(&self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&V, E>;

     /// Returns a mutable reference to entry `k`. If no such entry exists, call
     /// `vacant` and either forward its error, or add its result to the map
     /// and return a reference to *that*.
     fn get_mut_or<E>(&mut self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&mut V, E>;

     /// Read-only lookup.
     fn get(&self, k: K) -> Option<&V> {
         self.get_or(k, || Err(())).ok()
     }

     /// Mutable lookup.
     fn get_mut(&mut self, k: K) -> Option<&mut V> {
         self.get_mut_or(k, || Err(())).ok()
     }
 }

 /// Methods of this trait signifies a point where CTFE evaluation would fail
 /// and some use case dependent behaviour can instead be applied.
 pub trait Machine<'mir, 'tcx>: Sized {
     /// Additional memory kinds a machine wishes to distinguish from the builtin ones
     type MemoryKind: Debug + std::fmt::Display + MayLeak + Eq + 'static;

     /// Pointers are "tagged" with provenance information; typically the `AllocId` they belong to.
     type Provenance: Provenance + Eq + Hash + 'static;

     /// When getting the AllocId of a pointer, some extra data is also obtained from the provenance
     /// that is passed to memory access hooks so they can do things with it.
     type ProvenanceExtra: Copy + 'static;

     /// Machines can define extra (non-instance) things that represent values of function pointers.
     /// For example, Miri uses this to return a function pointer from `dlsym`
     /// that can later be called to execute the right thing.
     type ExtraFnVal: Debug + Copy;

     /// Extra data stored in every call frame.
     type FrameExtra;

     /// Extra data stored in every allocation.
     type AllocExtra: Debug + Clone + 'static;

     /// Memory's allocation map
     type MemoryMap: AllocMap<
             AllocId,
             (MemoryKind<Self::MemoryKind>, Allocation<Self::Provenance, Self::AllocExtra>),
         > + Default
         + Clone;

     /// The memory kind to use for copied global memory (held in `tcx`) --
     /// or None if such memory should not be mutated and thus any such attempt will cause
     /// a `ModifiedStatic` error to be raised.
     /// Statics are copied under two circumstances: When they are mutated, and when
     /// `adjust_allocation` (see below) returns an owned allocation
     /// that is added to the memory so that the work is not done twice.
     const GLOBAL_KIND: Option<Self::MemoryKind>;

     /// Should the machine panic on allocation failures?
     const PANIC_ON_ALLOC_FAIL: bool;

     /// Whether memory accesses should be alignment-checked.
     fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

     /// Whether, when checking alignment, we should look at the actual address and thus support
     /// custom alignment logic based on whatever the integer address happens to be.
     ///
     /// If this returns true, Provenance::OFFSET_IS_ADDR must be true.
     fn use_addr_for_alignment_check(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

     /// Whether to enforce the validity invariant
     fn enforce_validity(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

     /// Whether function calls should be [ABI](CallAbi)-checked.
     fn enforce_abi(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
         true
     }

     /// Whether CheckedBinOp MIR statements should actually check for overflow.
     fn checked_binop_checks_overflow(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

     /// Entry point for obtaining the MIR of anything that should get evaluated.
     /// So not just functions and shims, but also const/static initializers, anonymous
     /// constants, ...
     fn load_mir(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         instance: ty::InstanceDef<'tcx>,
     ) -> InterpResult<'tcx, &'tcx mir::Body<'tcx>> {
         Ok(ecx.tcx.instance_mir(instance))
     }

     /// Entry point to all function calls.
     ///
     /// Returns either the mir to use for the call, or `None` if execution should
     /// just proceed (which usually means this hook did all the work that the
     /// called function should usually have done). In the latter case, it is
     /// this hook's responsibility to advance the instruction pointer!
     /// (This is to support functions like `__rust_maybe_catch_panic` that neither find a MIR
     /// nor just jump to `ret`, but instead push their own stack frame.)
     /// Passing `dest`and `ret` in the same `Option` proved very annoying when only one of them
     /// was used.
     fn find_mir_or_eval_fn(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         instance: ty::Instance<'tcx>,
         abi: CallAbi,
         args: &[OpTy<'tcx, Self::Provenance>],
         destination: &PlaceTy<'tcx, Self::Provenance>,
         target: Option<mir::BasicBlock>,
         unwind: StackPopUnwind,
     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>>;

     /// Execute `fn_val`.  It is the hook's responsibility to advance the instruction
     /// pointer as appropriate.
     fn call_extra_fn(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         fn_val: Self::ExtraFnVal,
         abi: CallAbi,
         args: &[OpTy<'tcx, Self::Provenance>],
         destination: &PlaceTy<'tcx, Self::Provenance>,
         target: Option<mir::BasicBlock>,
         unwind: StackPopUnwind,
     ) -> InterpResult<'tcx>;

     /// Directly process an intrinsic without pushing a stack frame. It is the hook's
     /// responsibility to advance the instruction pointer as appropriate.
     fn call_intrinsic(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx, Self::Provenance>],
         destination: &PlaceTy<'tcx, Self::Provenance>,
         target: Option<mir::BasicBlock>,
         unwind: StackPopUnwind,
     ) -> InterpResult<'tcx>;

     /// Called to evaluate `Assert` MIR terminators that trigger a panic.
     fn assert_panic(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         msg: &mir::AssertMessage<'tcx>,
         unwind: Option<mir::BasicBlock>,
     ) -> InterpResult<'tcx>;

     /// Called to evaluate `Abort` MIR terminator.
     fn abort(_ecx: &mut InterpCx<'mir, 'tcx, Self>, _msg: String) -> InterpResult<'tcx, !> {
         throw_unsup_format!("aborting execution is not supported")
     }

     /// Called for all binary operations where the LHS has pointer type.
     ///
     /// Returns a (value, overflowed) pair if the operation succeeded
     fn binary_ptr_op(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         bin_op: mir::BinOp,
         left: &ImmTy<'tcx, Self::Provenance>,
         right: &ImmTy<'tcx, Self::Provenance>,
     ) -> InterpResult<'tcx, (Scalar<Self::Provenance>, bool, Ty<'tcx>)>;

     /// Called to write the specified `local` from the `frame`.
     /// Since writing a ZST is not actually accessing memory or locals, this is never invoked
     /// for ZST reads.
     ///
     /// Due to borrow checker trouble, we indicate the `frame` as an index rather than an `&mut
     /// Frame`.
     #[inline]
     fn access_local_mut<'a>(
         ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
         frame: usize,
         local: mir::Local,
     ) -> InterpResult<'tcx, &'a mut Operand<Self::Provenance>>
     where
         'tcx: 'mir,
     {
         ecx.stack_mut()[frame].locals[local].access_mut()
     }

     /// Called before a basic block terminator is executed.
     /// You can use this to detect endlessly running programs.
     #[inline]
     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
         Ok(())
     }

     /// Called before a global allocation is accessed.
     /// `def_id` is `Some` if this is the "lazy" allocation of a static.
     #[inline]
     fn before_access_global(
         _tcx: TyCtxt<'tcx>,
         _machine: &Self,
         _alloc_id: AllocId,
         _allocation: ConstAllocation<'tcx>,
         _static_def_id: Option<DefId>,
         _is_write: bool,
     ) -> InterpResult<'tcx> {
         Ok(())
     }

     /// Return the `AllocId` for the given thread-local static in the current thread.
     fn thread_local_static_base_pointer(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         def_id: DefId,
     ) -> InterpResult<'tcx, Pointer<Self::Provenance>> {
         throw_unsup!(ThreadLocalStatic(def_id))
     }

     /// Return the root pointer for the given `extern static`.
     fn extern_static_base_pointer(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         def_id: DefId,
     ) -> InterpResult<'tcx, Pointer<Self::Provenance>>;

     /// Return a "base" pointer for the given allocation: the one that is used for direct
     /// accesses to this static/const/fn allocation, or the one returned from the heap allocator.
     ///
     /// Not called on `extern` or thread-local statics (those use the methods above).
     fn adjust_alloc_base_pointer(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         ptr: Pointer,
     ) -> Pointer<Self::Provenance>;

     /// "Int-to-pointer cast"
     fn ptr_from_addr_cast(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         addr: u64,
     ) -> InterpResult<'tcx, Pointer<Option<Self::Provenance>>>;

     /// Marks a pointer as exposed, allowing it's provenance
     /// to be recovered. "Pointer-to-int cast"
     fn expose_ptr(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         ptr: Pointer<Self::Provenance>,
     ) -> InterpResult<'tcx>;

     /// Convert a pointer with provenance into an allocation-offset pair
     /// and extra provenance info.
     ///
     /// The returned `AllocId` must be the same as `ptr.provenance.get_alloc_id()`.
     ///
     /// When this fails, that means the pointer does not point to a live allocation.
     fn ptr_get_alloc(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         ptr: Pointer<Self::Provenance>,
     ) -> Option<(AllocId, Size, Self::ProvenanceExtra)>;

     /// Called to adjust allocations to the Provenance and AllocExtra of this machine.
     ///
     /// The way we construct allocations is to always first construct it without extra and then add
     /// the extra. This keeps uniform code paths for handling both allocations created by CTFE for
     /// globals, and allocations created by Miri during evaluation.
     ///
     /// `kind` is the kind of the allocation being adjusted; it can be `None` when
     /// it's a global and `GLOBAL_KIND` is `None`.
     ///
     /// This should avoid copying if no work has to be done! If this returns an owned
     /// allocation (because a copy had to be done to adjust things), machine memory will
     /// cache the result. (This relies on `AllocMap::get_or` being able to add the
     /// owned allocation to the map even when the map is shared.)
     ///
     /// This must only fail if `alloc` contains provenance.
     fn adjust_allocation<'b>(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         id: AllocId,
         alloc: Cow<'b, Allocation>,
         kind: Option<MemoryKind<Self::MemoryKind>>,
     ) -> InterpResult<'tcx, Cow<'b, Allocation<Self::Provenance, Self::AllocExtra>>>;

     fn eval_inline_asm(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _template: &'tcx [InlineAsmTemplatePiece],
         _operands: &[mir::InlineAsmOperand<'tcx>],
         _options: InlineAsmOptions,
     ) -> InterpResult<'tcx> {
         throw_unsup_format!("inline assembly is not supported")
     }

     /// Hook for performing extra checks on a memory read access.
     ///
     /// Takes read-only access to the allocation so we can keep all the memory read
     /// operations take `&self`. Use a `RefCell` in `AllocExtra` if you
     /// need to mutate.
     #[inline(always)]
     fn before_memory_read(
         _tcx: TyCtxt<'tcx>,
         _machine: &Self,
         _alloc_extra: &Self::AllocExtra,
         _prov: (AllocId, Self::ProvenanceExtra),
         _range: AllocRange,
     ) -> InterpResult<'tcx> {
         Ok(())
     }

     /// Hook for performing extra checks on a memory write access.
     #[inline(always)]
     fn before_memory_write(
         _tcx: TyCtxt<'tcx>,
         _machine: &mut Self,
         _alloc_extra: &mut Self::AllocExtra,
         _prov: (AllocId, Self::ProvenanceExtra),
         _range: AllocRange,
     ) -> InterpResult<'tcx> {
         Ok(())
     }

     /// Hook for performing extra operations on a memory deallocation.
     #[inline(always)]
     fn before_memory_deallocation(
         _tcx: TyCtxt<'tcx>,
         _machine: &mut Self,
         _alloc_extra: &mut Self::AllocExtra,
         _prov: (AllocId, Self::ProvenanceExtra),
         _range: AllocRange,
     ) -> InterpResult<'tcx> {
         Ok(())
     }

     /// Executes a retagging operation.
     #[inline]
     fn retag(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _kind: mir::RetagKind,
         _place: &PlaceTy<'tcx, Self::Provenance>,
     ) -> InterpResult<'tcx> {
         Ok(())
     }

     /// Called immediately before a new stack frame gets pushed.
     fn init_frame_extra(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         frame: Frame<'mir, 'tcx, Self::Provenance>,
     ) -> InterpResult<'tcx, Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>>;

     /// Borrow the current thread's stack.
     fn stack<'a>(
         ecx: &'a InterpCx<'mir, 'tcx, Self>,
     ) -> &'a [Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>];

     /// Mutably borrow the current thread's stack.
     fn stack_mut<'a>(
         ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
     ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>>;

     /// Called immediately after a stack frame got pushed and its locals got initialized.
     fn after_stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
         Ok(())
     }

     /// Called immediately after a stack frame got popped, but before jumping back to the caller.
     /// The `locals` have already been destroyed!
     fn after_stack_pop(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _frame: Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
         unwinding: bool,
     ) -> InterpResult<'tcx, StackPopJump> {
         // By default, we do not support unwinding from panics
         assert!(!unwinding);
         Ok(StackPopJump::Normal)
     }
 }

 // A lot of the flexibility above is just needed for `Miri`, but all "compile-time" machines
 // (CTFE and ConstProp) use the same instance.  Here, we share that code.
 pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     type Provenance = AllocId;
     type ProvenanceExtra = ();

     type ExtraFnVal = !;

     type MemoryMap =
         rustc_data_structures::fx::FxIndexMap<AllocId, (MemoryKind<Self::MemoryKind>, Allocation)>;
     const GLOBAL_KIND: Option<Self::MemoryKind> = None; // no copying of globals from `tcx` to machine memory

     type AllocExtra = ();
     type FrameExtra = ();

     #[inline(always)]
     fn use_addr_for_alignment_check(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
         // We do not support `use_addr`.
         false
     }

     #[inline(always)]
     fn checked_binop_checks_overflow(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
         true
     }

     #[inline(always)]
     fn call_extra_fn(
         _ecx: &mut InterpCx<$mir, $tcx, Self>,
         fn_val: !,
         _abi: CallAbi,
         _args: &[OpTy<$tcx>],
         _destination: &PlaceTy<$tcx, Self::Provenance>,
         _target: Option<mir::BasicBlock>,
         _unwind: StackPopUnwind,
     ) -> InterpResult<$tcx> {
         match fn_val {}
     }

     #[inline(always)]
     fn adjust_allocation<'b>(
         _ecx: &InterpCx<$mir, $tcx, Self>,
         _id: AllocId,
         alloc: Cow<'b, Allocation>,
         _kind: Option<MemoryKind<Self::MemoryKind>>,
     ) -> InterpResult<$tcx, Cow<'b, Allocation<Self::Provenance>>> {
         Ok(alloc)
     }

     fn extern_static_base_pointer(
         ecx: &InterpCx<$mir, $tcx, Self>,
         def_id: DefId,
     ) -> InterpResult<$tcx, Pointer> {
         // Use the `AllocId` associated with the `DefId`. Any actual *access* will fail.
         Ok(Pointer::new(ecx.tcx.create_static_alloc(def_id), Size::ZERO))
     }

     #[inline(always)]
     fn adjust_alloc_base_pointer(
         _ecx: &InterpCx<$mir, $tcx, Self>,
         ptr: Pointer<AllocId>,
     ) -> Pointer<AllocId> {
         ptr
     }

     #[inline(always)]
     fn ptr_from_addr_cast(
         _ecx: &InterpCx<$mir, $tcx, Self>,
         addr: u64,
     ) -> InterpResult<$tcx, Pointer<Option<AllocId>>> {
         // Allow these casts, but make the pointer not dereferenceable.
         // (I.e., they behave like transmutation.)
         // This is correct because no pointers can ever be exposed in compile-time evaluation.
         Ok(Pointer::from_addr(addr))
     }

     #[inline(always)]
     fn ptr_get_alloc(
         _ecx: &InterpCx<$mir, $tcx, Self>,
         ptr: Pointer<AllocId>,
     ) -> Option<(AllocId, Size, Self::ProvenanceExtra)> {
         // We know `offset` is relative to the allocation, so we can use `into_parts`.
         let (alloc_id, offset) = ptr.into_parts();
         Some((alloc_id, offset, ()))
     }
 }
	//! This module contains everything needed to instantiate an interpreter.
	//! This separation exists to ensure that no fancy miri features like
	//! interpreting common C functions leak into CTFE.

	use std::borrow::{Borrow, Cow};
	use std::fmt::Debug;
	use std::hash::Hash;

	use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
	use rustc_middle::mir;
	use rustc_middle::ty::{self, Ty, TyCtxt};
	use rustc_span::def_id::DefId;
	use rustc_target::abi::Size;
	use rustc_target::spec::abi::Abi as CallAbi;

	use super::{
	AllocId, AllocRange, Allocation, ConstAllocation, Frame, ImmTy, InterpCx, InterpResult,
	MemoryKind, OpTy, Operand, PlaceTy, Pointer, Provenance, Scalar, StackPopUnwind,
	};

	/// Data returned by Machine::stack_pop,
	/// to provide further control over the popping of the stack frame
	#[derive(Eq, PartialEq, Debug, Copy, Clone)]
	pub enum StackPopJump {
	/// Indicates that no special handling should be
	/// done - we'll either return normally or unwind
	/// based on the terminator for the function
	/// we're leaving.
	Normal,

	/// Indicates that we should not jump to the return/unwind address, as the callback already
	/// took care of everything.
	NoJump,
	}

	/// Whether this kind of memory is allowed to leak
	pub trait MayLeak: Copy {
	fn may_leak(self) -> bool;
	}

	/// The functionality needed by memory to manage its allocations
	pub trait AllocMap<K: Hash + Eq, V> {
	/// Tests if the map contains the given key.
	/// Deliberately takes `&mut` because that is sufficient, and some implementations
	/// can be more efficient then (using `RefCell::get_mut`).
	fn contains_key<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> bool
	where
	K: Borrow<Q>;

	/// Inserts a new entry into the map.
	fn insert(&mut self, k: K, v: V) -> Option<V>;

	/// Removes an entry from the map.
	fn remove<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> Option<V>
	where
	K: Borrow<Q>;

	/// Returns data based on the keys and values in the map.
	fn filter_map_collect<T>(&self, f: impl FnMut(&K, &V) -> Option<T>) -> Vec<T>;

	/// Returns a reference to entry `k`. If no such entry exists, call
	/// `vacant` and either forward its error, or add its result to the map
	/// and return a reference to that.
	fn get_or<E>(&self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&V, E>;

	/// Returns a mutable reference to entry `k`. If no such entry exists, call
	/// `vacant` and either forward its error, or add its result to the map
	/// and return a reference to that.
	fn get_mut_or<E>(&mut self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&mut V, E>;

	/// Read-only lookup.
	fn get(&self, k: K) -> Option<&V> {
	self.get_or(k, \|\| Err(())).ok()
	}

	/// Mutable lookup.
	fn get_mut(&mut self, k: K) -> Option<&mut V> {
	self.get_mut_or(k, \|\| Err(())).ok()
	}
	}

	/// Methods of this trait signifies a point where CTFE evaluation would fail
	/// and some use case dependent behaviour can instead be applied.
	pub trait Machine<'mir, 'tcx>: Sized {
	/// Additional memory kinds a machine wishes to distinguish from the builtin ones
	type MemoryKind: Debug + std::fmt::Display + MayLeak + Eq + 'static;

	/// Pointers are "tagged" with provenance information; typically the `AllocId` they belong to.
	type Provenance: Provenance + Eq + Hash + 'static;

	/// When getting the AllocId of a pointer, some extra data is also obtained from the provenance
	/// that is passed to memory access hooks so they can do things with it.
	type ProvenanceExtra: Copy + 'static;

	/// Machines can define extra (non-instance) things that represent values of function pointers.
	/// For example, Miri uses this to return a function pointer from `dlsym`
	/// that can later be called to execute the right thing.
	type ExtraFnVal: Debug + Copy;

	/// Extra data stored in every call frame.
	type FrameExtra;

	/// Extra data stored in every allocation.
	type AllocExtra: Debug + Clone + 'static;

	/// Memory's allocation map
	type MemoryMap: AllocMap<
	AllocId,
	(MemoryKind<Self::MemoryKind>, Allocation<Self::Provenance, Self::AllocExtra>),
	> + Default
	+ Clone;

	/// The memory kind to use for copied global memory (held in `tcx`) --
	/// or None if such memory should not be mutated and thus any such attempt will cause
	/// a `ModifiedStatic` error to be raised.
	/// Statics are copied under two circumstances: When they are mutated, and when
	/// `adjust_allocation` (see below) returns an owned allocation
	/// that is added to the memory so that the work is not done twice.
	const GLOBAL_KIND: Option<Self::MemoryKind>;

	/// Should the machine panic on allocation failures?
	const PANIC_ON_ALLOC_FAIL: bool;

	/// Whether memory accesses should be alignment-checked.
	fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

	/// Whether, when checking alignment, we should look at the actual address and thus support
	/// custom alignment logic based on whatever the integer address happens to be.
	///
	/// If this returns true, Provenance::OFFSET_IS_ADDR must be true.
	fn use_addr_for_alignment_check(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

	/// Whether to enforce the validity invariant
	fn enforce_validity(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

	/// Whether function calls should be [ABI](CallAbi)-checked.
	fn enforce_abi(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
	true
	}

	/// Whether CheckedBinOp MIR statements should actually check for overflow.
	fn checked_binop_checks_overflow(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;

	/// Entry point for obtaining the MIR of anything that should get evaluated.
	/// So not just functions and shims, but also const/static initializers, anonymous
	/// constants, ...
	fn load_mir(
	ecx: &InterpCx<'mir, 'tcx, Self>,
	instance: ty::InstanceDef<'tcx>,
	) -> InterpResult<'tcx, &'tcx mir::Body<'tcx>> {
	Ok(ecx.tcx.instance_mir(instance))
	}

	/// Entry point to all function calls.
	///
	/// Returns either the mir to use for the call, or `None` if execution should
	/// just proceed (which usually means this hook did all the work that the
	/// called function should usually have done). In the latter case, it is
	/// this hook's responsibility to advance the instruction pointer!
	/// (This is to support functions like `__rust_maybe_catch_panic` that neither find a MIR
	/// nor just jump to `ret`, but instead push their own stack frame.)
	/// Passing `dest`and `ret` in the same `Option` proved very annoying when only one of them
	/// was used.
	fn find_mir_or_eval_fn(
	ecx: &mut InterpCx<'mir, 'tcx, Self>,
	instance: ty::Instance<'tcx>,
	abi: CallAbi,
	args: &[OpTy<'tcx, Self::Provenance>],
	destination: &PlaceTy<'tcx, Self::Provenance>,
	target: Option<mir::BasicBlock>,
	unwind: StackPopUnwind,
	) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>>;

	/// Execute `fn_val`. It is the hook's responsibility to advance the instruction
	/// pointer as appropriate.
	fn call_extra_fn(
	ecx: &mut InterpCx<'mir, 'tcx, Self>,
	fn_val: Self::ExtraFnVal,
	abi: CallAbi,
	args: &[OpTy<'tcx, Self::Provenance>],
	destination: &PlaceTy<'tcx, Self::Provenance>,
	target: Option<mir::BasicBlock>,
	unwind: StackPopUnwind,
	) -> InterpResult<'tcx>;

	/// Directly process an intrinsic without pushing a stack frame. It is the hook's
	/// responsibility to advance the instruction pointer as appropriate.
	fn call_intrinsic(
	ecx: &mut InterpCx<'mir, 'tcx, Self>,
	instance: ty::Instance<'tcx>,
	args: &[OpTy<'tcx, Self::Provenance>],
	destination: &PlaceTy<'tcx, Self::Provenance>,
	target: Option<mir::BasicBlock>,
	unwind: StackPopUnwind,
	) -> InterpResult<'tcx>;

	/// Called to evaluate `Assert` MIR terminators that trigger a panic.
	fn assert_panic(
	ecx: &mut InterpCx<'mir, 'tcx, Self>,
	msg: &mir::AssertMessage<'tcx>,
	unwind: Option<mir::BasicBlock>,
	) -> InterpResult<'tcx>;

	/// Called to evaluate `Abort` MIR terminator.
	fn abort(_ecx: &mut InterpCx<'mir, 'tcx, Self>, _msg: String) -> InterpResult<'tcx, !> {
	throw_unsup_format!("aborting execution is not supported")
	}

	/// Called for all binary operations where the LHS has pointer type.
	///
	/// Returns a (value, overflowed) pair if the operation succeeded
	fn binary_ptr_op(
	ecx: &InterpCx<'mir, 'tcx, Self>,
	bin_op: mir::BinOp,
	left: &ImmTy<'tcx, Self::Provenance>,
	right: &ImmTy<'tcx, Self::Provenance>,
	) -> InterpResult<'tcx, (Scalar<Self::Provenance>, bool, Ty<'tcx>)>;

	/// Called to write the specified `local` from the `frame`.
	/// Since writing a ZST is not actually accessing memory or locals, this is never invoked
	/// for ZST reads.
	///
	/// Due to borrow checker trouble, we indicate the `frame` as an index rather than an `&mut
	/// Frame`.
	#[inline]
	fn access_local_mut<'a>(
	ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
	frame: usize,
	local: mir::Local,
	) -> InterpResult<'tcx, &'a mut Operand<Self::Provenance>>
	where
	'tcx: 'mir,
	{
	ecx.stack_mut()[frame].locals[local].access_mut()
	}

	/// Called before a basic block terminator is executed.
	/// You can use this to detect endlessly running programs.
	#[inline]
	fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
	Ok(())
	}

	/// Called before a global allocation is accessed.
	/// `def_id` is `Some` if this is the "lazy" allocation of a static.
	#[inline]
	fn before_access_global(
	_tcx: TyCtxt<'tcx>,
	_machine: &Self,
	_alloc_id: AllocId,
	_allocation: ConstAllocation<'tcx>,
	_static_def_id: Option<DefId>,
	_is_write: bool,
	) -> InterpResult<'tcx> {
	Ok(())
	}

	/// Return the `AllocId` for the given thread-local static in the current thread.
	fn thread_local_static_base_pointer(
	_ecx: &mut InterpCx<'mir, 'tcx, Self>,
	def_id: DefId,
	) -> InterpResult<'tcx, Pointer<Self::Provenance>> {
	throw_unsup!(ThreadLocalStatic(def_id))
	}

	/// Return the root pointer for the given `extern static`.
	fn extern_static_base_pointer(
	ecx: &InterpCx<'mir, 'tcx, Self>,
	def_id: DefId,
	) -> InterpResult<'tcx, Pointer<Self::Provenance>>;

	/// Return a "base" pointer for the given allocation: the one that is used for direct
	/// accesses to this static/const/fn allocation, or the one returned from the heap allocator.
	///
	/// Not called on `extern` or thread-local statics (those use the methods above).
	fn adjust_alloc_base_pointer(
	ecx: &InterpCx<'mir, 'tcx, Self>,
	ptr: Pointer,
	) -> Pointer<Self::Provenance>;

	/// "Int-to-pointer cast"
	fn ptr_from_addr_cast(
	ecx: &InterpCx<'mir, 'tcx, Self>,
	addr: u64,
	) -> InterpResult<'tcx, Pointer<Option<Self::Provenance>>>;

	/// Marks a pointer as exposed, allowing it's provenance
	/// to be recovered. "Pointer-to-int cast"
	fn expose_ptr(
	ecx: &mut InterpCx<'mir, 'tcx, Self>,
	ptr: Pointer<Self::Provenance>,
	) -> InterpResult<'tcx>;

	/// Convert a pointer with provenance into an allocation-offset pair
	/// and extra provenance info.
	///
	/// The returned `AllocId` must be the same as `ptr.provenance.get_alloc_id()`.
	///
	/// When this fails, that means the pointer does not point to a live allocation.
	fn ptr_get_alloc(
	ecx: &InterpCx<'mir, 'tcx, Self>,
	ptr: Pointer<Self::Provenance>,
	) -> Option<(AllocId, Size, Self::ProvenanceExtra)>;

	/// Called to adjust allocations to the Provenance and AllocExtra of this machine.
	///
	/// The way we construct allocations is to always first construct it without extra and then add
	/// the extra. This keeps uniform code paths for handling both allocations created by CTFE for
	/// globals, and allocations created by Miri during evaluation.
	///
	/// `kind` is the kind of the allocation being adjusted; it can be `None` when
	/// it's a global and `GLOBAL_KIND` is `None`.
	///
	/// This should avoid copying if no work has to be done! If this returns an owned
	/// allocation (because a copy had to be done to adjust things), machine memory will
	/// cache the result. (This relies on `AllocMap::get_or` being able to add the
	/// owned allocation to the map even when the map is shared.)
	///
	/// This must only fail if `alloc` contains provenance.
	fn adjust_allocation<'b>(
	ecx: &InterpCx<'mir, 'tcx, Self>,
	id: AllocId,
	alloc: Cow<'b, Allocation>,
	kind: Option<MemoryKind<Self::MemoryKind>>,
	) -> InterpResult<'tcx, Cow<'b, Allocation<Self::Provenance, Self::AllocExtra>>>;

	fn eval_inline_asm(
	_ecx: &mut InterpCx<'mir, 'tcx, Self>,
	_template: &'tcx [InlineAsmTemplatePiece],
	_operands: &[mir::InlineAsmOperand<'tcx>],
	_options: InlineAsmOptions,
	) -> InterpResult<'tcx> {
	throw_unsup_format!("inline assembly is not supported")
	}

	/// Hook for performing extra checks on a memory read access.
	///
	/// Takes read-only access to the allocation so we can keep all the memory read
	/// operations take `&self`. Use a `RefCell` in `AllocExtra` if you
	/// need to mutate.
	#[inline(always)]
	fn before_memory_read(
	_tcx: TyCtxt<'tcx>,
	_machine: &Self,
	_alloc_extra: &Self::AllocExtra,
	_prov: (AllocId, Self::ProvenanceExtra),
	_range: AllocRange,
	) -> InterpResult<'tcx> {
	Ok(())
	}

	/// Hook for performing extra checks on a memory write access.
	#[inline(always)]
	fn before_memory_write(
	_tcx: TyCtxt<'tcx>,
	_machine: &mut Self,
	_alloc_extra: &mut Self::AllocExtra,
	_prov: (AllocId, Self::ProvenanceExtra),
	_range: AllocRange,
	) -> InterpResult<'tcx> {
	Ok(())
	}

	/// Hook for performing extra operations on a memory deallocation.
	#[inline(always)]
	fn before_memory_deallocation(
	_tcx: TyCtxt<'tcx>,
	_machine: &mut Self,
	_alloc_extra: &mut Self::AllocExtra,
	_prov: (AllocId, Self::ProvenanceExtra),
	_range: AllocRange,
	) -> InterpResult<'tcx> {
	Ok(())
	}

	/// Executes a retagging operation.
	#[inline]
	fn retag(
	_ecx: &mut InterpCx<'mir, 'tcx, Self>,
	_kind: mir::RetagKind,
	_place: &PlaceTy<'tcx, Self::Provenance>,
	) -> InterpResult<'tcx> {
	Ok(())
	}

	/// Called immediately before a new stack frame gets pushed.
	fn init_frame_extra(
	ecx: &mut InterpCx<'mir, 'tcx, Self>,
	frame: Frame<'mir, 'tcx, Self::Provenance>,
	) -> InterpResult<'tcx, Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>>;

	/// Borrow the current thread's stack.
	fn stack<'a>(
	ecx: &'a InterpCx<'mir, 'tcx, Self>,
	) -> &'a [Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>];

	/// Mutably borrow the current thread's stack.
	fn stack_mut<'a>(
	ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
	) -> &'a mut Vec<Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>>;

	/// Called immediately after a stack frame got pushed and its locals got initialized.
	fn after_stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
	Ok(())
	}

	/// Called immediately after a stack frame got popped, but before jumping back to the caller.
	/// The `locals` have already been destroyed!
	fn after_stack_pop(
	_ecx: &mut InterpCx<'mir, 'tcx, Self>,
	_frame: Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
	unwinding: bool,
	) -> InterpResult<'tcx, StackPopJump> {
	// By default, we do not support unwinding from panics
	assert!(!unwinding);
	Ok(StackPopJump::Normal)
	}
	}

	// A lot of the flexibility above is just needed for `Miri`, but all "compile-time" machines
	// (CTFE and ConstProp) use the same instance. Here, we share that code.
	pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
	type Provenance = AllocId;
	type ProvenanceExtra = ();

	type ExtraFnVal = !;

	type MemoryMap =
	rustc_data_structures::fx::FxIndexMap<AllocId, (MemoryKind<Self::MemoryKind>, Allocation)>;
	const GLOBAL_KIND: Option<Self::MemoryKind> = None; // no copying of globals from `tcx` to machine memory

	type AllocExtra = ();
	type FrameExtra = ();

	#[inline(always)]
	fn use_addr_for_alignment_check(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
	// We do not support `use_addr`.
	false
	}

	#[inline(always)]
	fn checked_binop_checks_overflow(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
	true
	}

	#[inline(always)]
	fn call_extra_fn(
	_ecx: &mut InterpCx<$mir, $tcx, Self>,
	fn_val: !,
	_abi: CallAbi,
	_args: &[OpTy<$tcx>],
	_destination: &PlaceTy<$tcx, Self::Provenance>,
	_target: Option<mir::BasicBlock>,
	_unwind: StackPopUnwind,
	) -> InterpResult<$tcx> {
	match fn_val {}
	}

	#[inline(always)]
	fn adjust_allocation<'b>(
	_ecx: &InterpCx<$mir, $tcx, Self>,
	_id: AllocId,
	alloc: Cow<'b, Allocation>,
	_kind: Option<MemoryKind<Self::MemoryKind>>,
	) -> InterpResult<$tcx, Cow<'b, Allocation<Self::Provenance>>> {
	Ok(alloc)
	}

	fn extern_static_base_pointer(
	ecx: &InterpCx<$mir, $tcx, Self>,
	def_id: DefId,
	) -> InterpResult<$tcx, Pointer> {
	// Use the `AllocId` associated with the `DefId`. Any actual access will fail.
	Ok(Pointer::new(ecx.tcx.create_static_alloc(def_id), Size::ZERO))
	}

	#[inline(always)]
	fn adjust_alloc_base_pointer(
	_ecx: &InterpCx<$mir, $tcx, Self>,
	ptr: Pointer<AllocId>,
	) -> Pointer<AllocId> {
	ptr
	}

	#[inline(always)]
	fn ptr_from_addr_cast(
	_ecx: &InterpCx<$mir, $tcx, Self>,
	addr: u64,
	) -> InterpResult<$tcx, Pointer<Option<AllocId>>> {
	// Allow these casts, but make the pointer not dereferenceable.
	// (I.e., they behave like transmutation.)
	// This is correct because no pointers can ever be exposed in compile-time evaluation.
	Ok(Pointer::from_addr(addr))
	}

	#[inline(always)]
	fn ptr_get_alloc(
	_ecx: &InterpCx<$mir, $tcx, Self>,
	ptr: Pointer<AllocId>,
	) -> Option<(AllocId, Size, Self::ProvenanceExtra)> {
	// We know `offset` is relative to the allocation, so we can use `into_parts`.
	let (alloc_id, offset) = ptr.into_parts();
	Some((alloc_id, offset, ()))
	}
	}