Move const eval machine into its own module

3 files changed, 369 insertions, 344 deletions

diff --git a/src/librustc_mir/const_eval.rs b/src/librustc_mir/const_eval.rs
index 8a3bab2a282..833547d4d05 100644
--- a/src/librustc_mir/const_eval.rs
+++ b/src/librustc_mir/const_eval.rs
@@ -1,21 +1,18 @@
 // Not in interpret to make sure we do not use private implementation details
 
-use std::borrow::{Borrow, Cow};
-use std::collections::hash_map::Entry;
 use std::convert::TryInto;
 use std::error::Error;
 use std::fmt;
 use std::hash::Hash;
 
 use crate::interpret::eval_nullary_intrinsic;
+use crate::interpret::eval_nullary_intrinsic;
 use rustc::hir::def::DefKind;
-use rustc::hir::def_id::DefId;
 use rustc::mir;
 use rustc::mir::interpret::{ConstEvalErr, ErrorHandled, ScalarMaybeUndef};
 use rustc::traits::Reveal;
-use rustc::ty::layout::{self, HasTyCtxt, LayoutOf, VariantIdx};
-use rustc::ty::{self, subst::Subst, Ty, TyCtxt};
-use rustc_data_structures::fx::FxHashMap;
+use rustc::ty::layout::{self, LayoutOf, VariantIdx};
+use rustc::ty::{self, subst::Subst, TyCtxt};
 
 use syntax::{
     source_map::{Span, DUMMY_SP},
@@ -23,17 +20,16 @@ use syntax::{
 };
 
 use crate::interpret::{
-    self, intern_const_alloc_recursive, snapshot, AllocId, Allocation, AssertMessage, ConstValue,
-    GlobalId, ImmTy, Immediate, InterpCx, InterpErrorInfo, InterpResult, MPlaceTy, Machine, Memory,
-    MemoryKind, OpTy, PlaceTy, Pointer, RawConst, RefTracking, Scalar, StackPopCleanup,
+    intern_const_alloc_recursive, Allocation, ConstValue, GlobalId, ImmTy, Immediate, InterpCx,
+    InterpErrorInfo, InterpResult, MPlaceTy, Machine, MemoryKind, OpTy, RawConst, RefTracking,
+    Scalar, StackPopCleanup,
 };
 
-/// Number of steps until the detector even starts doing anything.
-/// Also, a warning is shown to the user when this number is reached.
-const STEPS_UNTIL_DETECTOR_ENABLED: isize = 1_000_000;
-/// The number of steps between loop detector snapshots.
-/// Should be a power of two for performance reasons.
-const DETECTOR_SNAPSHOT_PERIOD: isize = 256;
+mod error;
+mod machine;
+
+pub use error::*;
+pub use machine::*;
 
 /// The `InterpCx` is only meant to be used to do field and index projections into constants for
 /// `simd_shuffle` and const patterns in match arms.
@@ -173,335 +169,6 @@ fn eval_body_using_ecx<'mir, 'tcx>(
     Ok(ret)
 }
 
-#[derive(Clone, Debug)]
-pub enum ConstEvalError {
-    NeedsRfc(String),
-    ConstAccessesStatic,
-}
-
-impl<'tcx> Into<InterpErrorInfo<'tcx>> for ConstEvalError {
-    fn into(self) -> InterpErrorInfo<'tcx> {
-        err_unsup!(Unsupported(self.to_string())).into()
-    }
-}
-
-impl fmt::Display for ConstEvalError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        use self::ConstEvalError::*;
-        match *self {
-            NeedsRfc(ref msg) => {
-                write!(f, "\"{}\" needs an rfc before being allowed inside constants", msg)
-            }
-            ConstAccessesStatic => write!(f, "constant accesses static"),
-        }
-    }
-}
-
-impl Error for ConstEvalError {}
-
-// Extra machine state for CTFE, and the Machine instance
-pub struct CompileTimeInterpreter<'mir, 'tcx> {
-    /// When this value is negative, it indicates the number of interpreter
-    /// steps *until* the loop detector is enabled. When it is positive, it is
-    /// the number of steps after the detector has been enabled modulo the loop
-    /// detector period.
-    pub(super) steps_since_detector_enabled: isize,
-
-    /// Extra state to detect loops.
-    pub(super) loop_detector: snapshot::InfiniteLoopDetector<'mir, 'tcx>,
-}
-
-#[derive(Copy, Clone, Debug)]
-pub struct MemoryExtra {
-    /// Whether this machine may read from statics
-    can_access_statics: bool,
-}
-
-impl<'mir, 'tcx> CompileTimeInterpreter<'mir, 'tcx> {
-    fn new() -> Self {
-        CompileTimeInterpreter {
-            loop_detector: Default::default(),
-            steps_since_detector_enabled: -STEPS_UNTIL_DETECTOR_ENABLED,
-        }
-    }
-}
-
-impl<K: Hash + Eq, V> interpret::AllocMap<K, V> for FxHashMap<K, V> {
-    #[inline(always)]
-    fn contains_key<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> bool
-    where
-        K: Borrow<Q>,
-    {
-        FxHashMap::contains_key(self, k)
-    }
-
-    #[inline(always)]
-    fn insert(&mut self, k: K, v: V) -> Option<V> {
-        FxHashMap::insert(self, k, v)
-    }
-
-    #[inline(always)]
-    fn remove<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> Option<V>
-    where
-        K: Borrow<Q>,
-    {
-        FxHashMap::remove(self, k)
-    }
-
-    #[inline(always)]
-    fn filter_map_collect<T>(&self, mut f: impl FnMut(&K, &V) -> Option<T>) -> Vec<T> {
-        self.iter().filter_map(move |(k, v)| f(k, &*v)).collect()
-    }
-
-    #[inline(always)]
-    fn get_or<E>(&self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&V, E> {
-        match self.get(&k) {
-            Some(v) => Ok(v),
-            None => {
-                vacant()?;
-                bug!("The CTFE machine shouldn't ever need to extend the alloc_map when reading")
-            }
-        }
-    }
-
-    #[inline(always)]
-    fn get_mut_or<E>(&mut self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&mut V, E> {
-        match self.entry(k) {
-            Entry::Occupied(e) => Ok(e.into_mut()),
-            Entry::Vacant(e) => {
-                let v = vacant()?;
-                Ok(e.insert(v))
-            }
-        }
-    }
-}
-
-crate type CompileTimeEvalContext<'mir, 'tcx> =
-    InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>>;
-
-impl interpret::MayLeak for ! {
-    #[inline(always)]
-    fn may_leak(self) -> bool {
-        // `self` is uninhabited
-        self
-    }
-}
-
-impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir, 'tcx> {
-    type MemoryKinds = !;
-    type PointerTag = ();
-    type ExtraFnVal = !;
-
-    type FrameExtra = ();
-    type MemoryExtra = MemoryExtra;
-    type AllocExtra = ();
-
-    type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
-
-    const STATIC_KIND: Option<!> = None; // no copying of statics allowed
-
-    // We do not check for alignment to avoid having to carry an `Align`
-    // in `ConstValue::ByRef`.
-    const CHECK_ALIGN: bool = false;
-
-    #[inline(always)]
-    fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
-        false // for now, we don't enforce validity
-    }
-
-    fn find_mir_or_eval_fn(
-        ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        instance: ty::Instance<'tcx>,
-        args: &[OpTy<'tcx>],
-        ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
-        _unwind: Option<mir::BasicBlock>, // unwinding is not supported in consts
-    ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
-        debug!("find_mir_or_eval_fn: {:?}", instance);
-
-        // Only check non-glue functions
-        if let ty::InstanceDef::Item(def_id) = instance.def {
-            // Execution might have wandered off into other crates, so we cannot do a stability-
-            // sensitive check here.  But we can at least rule out functions that are not const
-            // at all.
-            if ecx.tcx.is_const_fn_raw(def_id) {
-                // If this function is a `const fn` then as an optimization we can query this
-                // evaluation immediately.
-                //
-                // For the moment we only do this for functions which take no arguments
-                // (or all arguments are ZSTs) so that we don't memoize too much.
-                //
-                // Because `#[track_caller]` adds an implicit non-ZST argument, we also cannot
-                // perform this optimization on items tagged with it.
-                let no_implicit_args = !instance.def.requires_caller_location(ecx.tcx());
-                if args.iter().all(|a| a.layout.is_zst()) && no_implicit_args {
-                    let gid = GlobalId { instance, promoted: None };
-                    ecx.eval_const_fn_call(gid, ret)?;
-                    return Ok(None);
-                }
-            } else {
-                // Some functions we support even if they are non-const -- but avoid testing
-                // that for const fn!  We certainly do *not* want to actually call the fn
-                // though, so be sure we return here.
-                return if ecx.hook_panic_fn(instance, args, ret)? {
-                    Ok(None)
-                } else {
-                    throw_unsup_format!("calling non-const function `{}`", instance)
-                };
-            }
-        }
-        // This is a const fn. Call it.
-        Ok(Some(match ecx.load_mir(instance.def, None) {
-            Ok(body) => *body,
-            Err(err) => {
-                if let err_unsup!(NoMirFor(ref path)) = err.kind {
-                    return Err(ConstEvalError::NeedsRfc(format!(
-                        "calling extern function `{}`",
-                        path
-                    ))
-                    .into());
-                }
-                return Err(err);
-            }
-        }))
-    }
-
-    fn call_extra_fn(
-        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        fn_val: !,
-        _args: &[OpTy<'tcx>],
-        _ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
-        _unwind: Option<mir::BasicBlock>,
-    ) -> InterpResult<'tcx> {
-        match fn_val {}
-    }
-
-    fn call_intrinsic(
-        ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        span: Span,
-        instance: ty::Instance<'tcx>,
-        args: &[OpTy<'tcx>],
-        ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
-        _unwind: Option<mir::BasicBlock>,
-    ) -> InterpResult<'tcx> {
-        if ecx.emulate_intrinsic(span, instance, args, ret)? {
-            return Ok(());
-        }
-        // An intrinsic that we do not support
-        let intrinsic_name = ecx.tcx.item_name(instance.def_id());
-        Err(ConstEvalError::NeedsRfc(format!("calling intrinsic `{}`", intrinsic_name)).into())
-    }
-
-    fn assert_panic(
-        ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        _span: Span,
-        msg: &AssertMessage<'tcx>,
-        _unwind: Option<mir::BasicBlock>,
-    ) -> InterpResult<'tcx> {
-        use rustc::mir::interpret::PanicInfo::*;
-        Err(match msg {
-            BoundsCheck { ref len, ref index } => {
-                let len = ecx
-                    .read_immediate(ecx.eval_operand(len, None)?)
-                    .expect("can't eval len")
-                    .to_scalar()?
-                    .to_machine_usize(&*ecx)?;
-                let index = ecx
-                    .read_immediate(ecx.eval_operand(index, None)?)
-                    .expect("can't eval index")
-                    .to_scalar()?
-                    .to_machine_usize(&*ecx)?;
-                err_panic!(BoundsCheck { len, index })
-            }
-            Overflow(op) => err_panic!(Overflow(*op)),
-            OverflowNeg => err_panic!(OverflowNeg),
-            DivisionByZero => err_panic!(DivisionByZero),
-            RemainderByZero => err_panic!(RemainderByZero),
-            ResumedAfterReturn(generator_kind) => err_panic!(ResumedAfterReturn(*generator_kind)),
-            ResumedAfterPanic(generator_kind) => err_panic!(ResumedAfterPanic(*generator_kind)),
-            Panic { .. } => bug!("`Panic` variant cannot occur in MIR"),
-        }
-        .into())
-    }
-
-    fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
-        Err(ConstEvalError::NeedsRfc("pointer-to-integer cast".to_string()).into())
-    }
-
-    fn binary_ptr_op(
-        _ecx: &InterpCx<'mir, 'tcx, Self>,
-        _bin_op: mir::BinOp,
-        _left: ImmTy<'tcx>,
-        _right: ImmTy<'tcx>,
-    ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
-        Err(ConstEvalError::NeedsRfc("pointer arithmetic or comparison".to_string()).into())
-    }
-
-    fn find_foreign_static(
-        _tcx: TyCtxt<'tcx>,
-        _def_id: DefId,
-    ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
-        throw_unsup!(ReadForeignStatic)
-    }
-
-    #[inline(always)]
-    fn init_allocation_extra<'b>(
-        _memory_extra: &MemoryExtra,
-        _id: AllocId,
-        alloc: Cow<'b, Allocation>,
-        _kind: Option<MemoryKind<!>>,
-    ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
-        // We do not use a tag so we can just cheaply forward the allocation
-        (alloc, ())
-    }
-
-    #[inline(always)]
-    fn tag_static_base_pointer(_memory_extra: &MemoryExtra, _id: AllocId) -> Self::PointerTag {
-        ()
-    }
-
-    fn box_alloc(
-        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        _dest: PlaceTy<'tcx>,
-    ) -> InterpResult<'tcx> {
-        Err(ConstEvalError::NeedsRfc("heap allocations via `box` keyword".to_string()).into())
-    }
-
-    fn before_terminator(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
-        {
-            let steps = &mut ecx.machine.steps_since_detector_enabled;
-
-            *steps += 1;
-            if *steps < 0 {
-                return Ok(());
-            }
-
-            *steps %= DETECTOR_SNAPSHOT_PERIOD;
-            if *steps != 0 {
-                return Ok(());
-            }
-        }
-
-        let span = ecx.frame().span;
-        ecx.machine.loop_detector.observe_and_analyze(*ecx.tcx, span, &ecx.memory, &ecx.stack[..])
-    }
-
-    #[inline(always)]
-    fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
-        Ok(())
-    }
-
-    fn before_access_static(
-        memory_extra: &MemoryExtra,
-        _allocation: &Allocation,
-    ) -> InterpResult<'tcx> {
-        if memory_extra.can_access_statics {
-            Ok(())
-        } else {
-            Err(ConstEvalError::ConstAccessesStatic.into())
-        }
-    }
-}
-
 /// Extracts a field of a (variant of a) const.
 // this function uses `unwrap` copiously, because an already validated constant must have valid
 // fields and can thus never fail outside of compiler bugs
diff --git a/src/librustc_mir/const_eval/error.rs b/src/librustc_mir/const_eval/error.rs
new file mode 100644
index 00000000000..8948cc0fc3e
--- /dev/null
+++ b/src/librustc_mir/const_eval/error.rs
@@ -0,0 +1,30 @@
+use std::error::Error;
+use std::fmt;
+
+use crate::interpret::InterpErrorInfo;
+
+#[derive(Clone, Debug)]
+pub enum ConstEvalError {
+    NeedsRfc(String),
+    ConstAccessesStatic,
+}
+
+impl<'tcx> Into<InterpErrorInfo<'tcx>> for ConstEvalError {
+    fn into(self) -> InterpErrorInfo<'tcx> {
+        err_unsup!(Unsupported(self.to_string())).into()
+    }
+}
+
+impl fmt::Display for ConstEvalError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use self::ConstEvalError::*;
+        match *self {
+            NeedsRfc(ref msg) => {
+                write!(f, "\"{}\" needs an rfc before being allowed inside constants", msg)
+            }
+            ConstAccessesStatic => write!(f, "constant accesses static"),
+        }
+    }
+}
+
+impl Error for ConstEvalError {}
diff --git a/src/librustc_mir/const_eval/machine.rs b/src/librustc_mir/const_eval/machine.rs
new file mode 100644
index 00000000000..0e3794ee3ab
--- /dev/null
+++ b/src/librustc_mir/const_eval/machine.rs
@@ -0,0 +1,328 @@
+use rustc::hir::def_id::DefId;
+use rustc::mir;
+use rustc::ty::layout::HasTyCtxt;
+use rustc::ty::{self, Ty, TyCtxt};
+use std::borrow::{Borrow, Cow};
+use std::collections::hash_map::Entry;
+use std::hash::Hash;
+
+use rustc_data_structures::fx::FxHashMap;
+
+use syntax::source_map::Span;
+
+use crate::interpret::{
+    self, snapshot, AllocId, Allocation, AssertMessage, GlobalId, ImmTy, InterpCx, InterpResult,
+    Memory, MemoryKind, OpTy, PlaceTy, Pointer, Scalar,
+};
+
+use super::error::*;
+
+/// Number of steps until the detector even starts doing anything.
+/// Also, a warning is shown to the user when this number is reached.
+const STEPS_UNTIL_DETECTOR_ENABLED: isize = 1_000_000;
+/// The number of steps between loop detector snapshots.
+/// Should be a power of two for performance reasons.
+const DETECTOR_SNAPSHOT_PERIOD: isize = 256;
+
+// Extra machine state for CTFE, and the Machine instance
+pub struct CompileTimeInterpreter<'mir, 'tcx> {
+    /// When this value is negative, it indicates the number of interpreter
+    /// steps *until* the loop detector is enabled. When it is positive, it is
+    /// the number of steps after the detector has been enabled modulo the loop
+    /// detector period.
+    pub(super) steps_since_detector_enabled: isize,
+
+    /// Extra state to detect loops.
+    pub(super) loop_detector: snapshot::InfiniteLoopDetector<'mir, 'tcx>,
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct MemoryExtra {
+    /// Whether this machine may read from statics
+    can_access_statics: bool,
+}
+
+impl<'mir, 'tcx> CompileTimeInterpreter<'mir, 'tcx> {
+    fn new() -> Self {
+        CompileTimeInterpreter {
+            loop_detector: Default::default(),
+            steps_since_detector_enabled: -STEPS_UNTIL_DETECTOR_ENABLED,
+        }
+    }
+}
+
+impl<K: Hash + Eq, V> interpret::AllocMap<K, V> for FxHashMap<K, V> {
+    #[inline(always)]
+    fn contains_key<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> bool
+    where
+        K: Borrow<Q>,
+    {
+        FxHashMap::contains_key(self, k)
+    }
+
+    #[inline(always)]
+    fn insert(&mut self, k: K, v: V) -> Option<V> {
+        FxHashMap::insert(self, k, v)
+    }
+
+    #[inline(always)]
+    fn remove<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> Option<V>
+    where
+        K: Borrow<Q>,
+    {
+        FxHashMap::remove(self, k)
+    }
+
+    #[inline(always)]
+    fn filter_map_collect<T>(&self, mut f: impl FnMut(&K, &V) -> Option<T>) -> Vec<T> {
+        self.iter().filter_map(move |(k, v)| f(k, &*v)).collect()
+    }
+
+    #[inline(always)]
+    fn get_or<E>(&self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&V, E> {
+        match self.get(&k) {
+            Some(v) => Ok(v),
+            None => {
+                vacant()?;
+                bug!("The CTFE machine shouldn't ever need to extend the alloc_map when reading")
+            }
+        }
+    }
+
+    #[inline(always)]
+    fn get_mut_or<E>(&mut self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&mut V, E> {
+        match self.entry(k) {
+            Entry::Occupied(e) => Ok(e.into_mut()),
+            Entry::Vacant(e) => {
+                let v = vacant()?;
+                Ok(e.insert(v))
+            }
+        }
+    }
+}
+
+crate type CompileTimeEvalContext<'mir, 'tcx> =
+    InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>>;
+
+impl interpret::MayLeak for ! {
+    #[inline(always)]
+    fn may_leak(self) -> bool {
+        // `self` is uninhabited
+        self
+    }
+}
+
+impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir, 'tcx> {
+    type MemoryKinds = !;
+    type PointerTag = ();
+    type ExtraFnVal = !;
+
+    type FrameExtra = ();
+    type MemoryExtra = MemoryExtra;
+    type AllocExtra = ();
+
+    type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
+
+    const STATIC_KIND: Option<!> = None; // no copying of statics allowed
+
+    // We do not check for alignment to avoid having to carry an `Align`
+    // in `ConstValue::ByRef`.
+    const CHECK_ALIGN: bool = false;
+
+    #[inline(always)]
+    fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        false // for now, we don't enforce validity
+    }
+
+    fn find_mir_or_eval_fn(
+        ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        instance: ty::Instance<'tcx>,
+        args: &[OpTy<'tcx>],
+        ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
+        _unwind: Option<mir::BasicBlock>, // unwinding is not supported in consts
+    ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
+        debug!("find_mir_or_eval_fn: {:?}", instance);
+
+        // Only check non-glue functions
+        if let ty::InstanceDef::Item(def_id) = instance.def {
+            // Execution might have wandered off into other crates, so we cannot do a stability-
+            // sensitive check here.  But we can at least rule out functions that are not const
+            // at all.
+            if ecx.tcx.is_const_fn_raw(def_id) {
+                // If this function is a `const fn` then as an optimization we can query this
+                // evaluation immediately.
+                //
+                // For the moment we only do this for functions which take no arguments
+                // (or all arguments are ZSTs) so that we don't memoize too much.
+                //
+                // Because `#[track_caller]` adds an implicit non-ZST argument, we also cannot
+                // perform this optimization on items tagged with it.
+                let no_implicit_args = !instance.def.requires_caller_location(ecx.tcx());
+                if args.iter().all(|a| a.layout.is_zst()) && no_implicit_args {
+                    let gid = GlobalId { instance, promoted: None };
+                    ecx.eval_const_fn_call(gid, ret)?;
+                    return Ok(None);
+                }
+            } else {
+                // Some functions we support even if they are non-const -- but avoid testing
+                // that for const fn!  We certainly do *not* want to actually call the fn
+                // though, so be sure we return here.
+                return if ecx.hook_panic_fn(instance, args, ret)? {
+                    Ok(None)
+                } else {
+                    throw_unsup_format!("calling non-const function `{}`", instance)
+                };
+            }
+        }
+        // This is a const fn. Call it.
+        Ok(Some(match ecx.load_mir(instance.def, None) {
+            Ok(body) => *body,
+            Err(err) => {
+                if let err_unsup!(NoMirFor(ref path)) = err.kind {
+                    return Err(ConstEvalError::NeedsRfc(format!(
+                        "calling extern function `{}`",
+                        path
+                    ))
+                    .into());
+                }
+                return Err(err);
+            }
+        }))
+    }
+
+    fn call_extra_fn(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        fn_val: !,
+        _args: &[OpTy<'tcx>],
+        _ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
+        _unwind: Option<mir::BasicBlock>,
+    ) -> InterpResult<'tcx> {
+        match fn_val {}
+    }
+
+    fn call_intrinsic(
+        ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        span: Span,
+        instance: ty::Instance<'tcx>,
+        args: &[OpTy<'tcx>],
+        ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
+        _unwind: Option<mir::BasicBlock>,
+    ) -> InterpResult<'tcx> {
+        if ecx.emulate_intrinsic(span, instance, args, ret)? {
+            return Ok(());
+        }
+        // An intrinsic that we do not support
+        let intrinsic_name = ecx.tcx.item_name(instance.def_id());
+        Err(ConstEvalError::NeedsRfc(format!("calling intrinsic `{}`", intrinsic_name)).into())
+    }
+
+    fn assert_panic(
+        ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _span: Span,
+        msg: &AssertMessage<'tcx>,
+        _unwind: Option<mir::BasicBlock>,
+    ) -> InterpResult<'tcx> {
+        use rustc::mir::interpret::PanicInfo::*;
+        Err(match msg {
+            BoundsCheck { ref len, ref index } => {
+                let len = ecx
+                    .read_immediate(ecx.eval_operand(len, None)?)
+                    .expect("can't eval len")
+                    .to_scalar()?
+                    .to_machine_usize(&*ecx)?;
+                let index = ecx
+                    .read_immediate(ecx.eval_operand(index, None)?)
+                    .expect("can't eval index")
+                    .to_scalar()?
+                    .to_machine_usize(&*ecx)?;
+                err_panic!(BoundsCheck { len, index })
+            }
+            Overflow(op) => err_panic!(Overflow(*op)),
+            OverflowNeg => err_panic!(OverflowNeg),
+            DivisionByZero => err_panic!(DivisionByZero),
+            RemainderByZero => err_panic!(RemainderByZero),
+            ResumedAfterReturn(generator_kind) => err_panic!(ResumedAfterReturn(*generator_kind)),
+            ResumedAfterPanic(generator_kind) => err_panic!(ResumedAfterPanic(*generator_kind)),
+            Panic { .. } => bug!("`Panic` variant cannot occur in MIR"),
+        }
+        .into())
+    }
+
+    fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
+        Err(ConstEvalError::NeedsRfc("pointer-to-integer cast".to_string()).into())
+    }
+
+    fn binary_ptr_op(
+        _ecx: &InterpCx<'mir, 'tcx, Self>,
+        _bin_op: mir::BinOp,
+        _left: ImmTy<'tcx>,
+        _right: ImmTy<'tcx>,
+    ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
+        Err(ConstEvalError::NeedsRfc("pointer arithmetic or comparison".to_string()).into())
+    }
+
+    fn find_foreign_static(
+        _tcx: TyCtxt<'tcx>,
+        _def_id: DefId,
+    ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
+        throw_unsup!(ReadForeignStatic)
+    }
+
+    #[inline(always)]
+    fn init_allocation_extra<'b>(
+        _memory_extra: &MemoryExtra,
+        _id: AllocId,
+        alloc: Cow<'b, Allocation>,
+        _kind: Option<MemoryKind<!>>,
+    ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
+        // We do not use a tag so we can just cheaply forward the allocation
+        (alloc, ())
+    }
+
+    #[inline(always)]
+    fn tag_static_base_pointer(_memory_extra: &MemoryExtra, _id: AllocId) -> Self::PointerTag {
+        ()
+    }
+
+    fn box_alloc(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _dest: PlaceTy<'tcx>,
+    ) -> InterpResult<'tcx> {
+        Err(ConstEvalError::NeedsRfc("heap allocations via `box` keyword".to_string()).into())
+    }
+
+    fn before_terminator(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+        {
+            let steps = &mut ecx.machine.steps_since_detector_enabled;
+
+            *steps += 1;
+            if *steps < 0 {
+                return Ok(());
+            }
+
+            *steps %= DETECTOR_SNAPSHOT_PERIOD;
+            if *steps != 0 {
+                return Ok(());
+            }
+        }
+
+        let span = ecx.frame().span;
+        ecx.machine.loop_detector.observe_and_analyze(*ecx.tcx, span, &ecx.memory, &ecx.stack[..])
+    }
+
+    #[inline(always)]
+    fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+        Ok(())
+    }
+
+    fn before_access_static(
+        memory_extra: &MemoryExtra,
+        _allocation: &Allocation,
+    ) -> InterpResult<'tcx> {
+        if memory_extra.can_access_statics {
+            Ok(())
+        } else {
+            Err(ConstEvalError::ConstAccessesStatic.into())
+        }
+    }
+}