Split up const_eval.rs

src/librustc_mir/const_eval/error.rs

@@ -0,0 +1,42 @@
+use std::error::Error;
+use std::fmt;
+
+use super::InterpCx;
+use crate::interpret::{ConstEvalErr, InterpErrorInfo, Machine};
+#[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 {}
+
+/// Turn an interpreter error into something to report to the user.
+/// As a side-effect, if RUSTC_CTFE_BACKTRACE is set, this prints the backtrace.
+/// Should be called only if the error is actually going to to be reported!
+pub fn error_to_const_error<'mir, 'tcx, M: Machine<'mir, 'tcx>>(
+    ecx: &InterpCx<'mir, 'tcx, M>,
+    mut error: InterpErrorInfo<'tcx>,
+) -> ConstEvalErr<'tcx> {
+    error.print_backtrace();
+    let stacktrace = ecx.generate_stacktrace(None);
+    ConstEvalErr { error: error.kind, stacktrace, span: ecx.tcx.span }
+}

src/librustc_mir/const_eval/eval_queries.rs

@@ -0,0 +1,360 @@
+use super::{error_to_const_error, CompileTimeEvalContext, CompileTimeInterpreter, MemoryExtra};
+use crate::interpret::eval_nullary_intrinsic;
+use crate::interpret::{
+    intern_const_alloc_recursive, Allocation, ConstValue, GlobalId, ImmTy, Immediate, InterpCx,
+    InterpResult, MPlaceTy, MemoryKind, OpTy, RawConst, RefTracking, Scalar, ScalarMaybeUndef,
+    StackPopCleanup,
+};
+use rustc::hir::def::DefKind;
+use rustc::mir;
+use rustc::mir::interpret::{ConstEvalErr, ErrorHandled};
+use rustc::traits::Reveal;
+use rustc::ty::{self, layout, layout::LayoutOf, subst::Subst, TyCtxt};
+use std::convert::TryInto;
+use syntax::source_map::Span;
+
+pub fn note_on_undefined_behavior_error() -> &'static str {
+    "The rules on what exactly is undefined behavior aren't clear, \
+     so this check might be overzealous. Please open an issue on the rustc \
+     repository if you believe it should not be considered undefined behavior."
+}
+
+// Returns a pointer to where the result lives
+fn eval_body_using_ecx<'mir, 'tcx>(
+    ecx: &mut CompileTimeEvalContext<'mir, 'tcx>,
+    cid: GlobalId<'tcx>,
+    body: &'mir mir::Body<'tcx>,
+) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
+    debug!("eval_body_using_ecx: {:?}, {:?}", cid, ecx.param_env);
+    let tcx = ecx.tcx.tcx;
+    let layout = ecx.layout_of(body.return_ty().subst(tcx, cid.instance.substs))?;
+    assert!(!layout.is_unsized());
+    let ret = ecx.allocate(layout, MemoryKind::Stack);
+
+    let name = ty::tls::with(|tcx| tcx.def_path_str(cid.instance.def_id()));
+    let prom = cid.promoted.map_or(String::new(), |p| format!("::promoted[{:?}]", p));
+    trace!("eval_body_using_ecx: pushing stack frame for global: {}{}", name, prom);
+
+    // Assert all args (if any) are zero-sized types; `eval_body_using_ecx` doesn't
+    // make sense if the body is expecting nontrivial arguments.
+    // (The alternative would be to use `eval_fn_call` with an args slice.)
+    for arg in body.args_iter() {
+        let decl = body.local_decls.get(arg).expect("arg missing from local_decls");
+        let layout = ecx.layout_of(decl.ty.subst(tcx, cid.instance.substs))?;
+        assert!(layout.is_zst())
+    }
+
+    ecx.push_stack_frame(
+        cid.instance,
+        body.span,
+        body,
+        Some(ret.into()),
+        StackPopCleanup::None { cleanup: false },
+    )?;
+
+    // The main interpreter loop.
+    ecx.run()?;
+
+    // Intern the result
+    intern_const_alloc_recursive(ecx, tcx.static_mutability(cid.instance.def_id()), ret)?;
+
+    debug!("eval_body_using_ecx done: {:?}", *ret);
+    Ok(ret)
+}
+
+/// 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.
+///
+/// The function containing the `match` that is currently being analyzed may have generic bounds
+/// that inform us about the generic bounds of the constant. E.g., using an associated constant
+/// of a function's generic parameter will require knowledge about the bounds on the generic
+/// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
+pub(super) fn mk_eval_cx<'mir, 'tcx>(
+    tcx: TyCtxt<'tcx>,
+    span: Span,
+    param_env: ty::ParamEnv<'tcx>,
+    can_access_statics: bool,
+) -> CompileTimeEvalContext<'mir, 'tcx> {
+    debug!("mk_eval_cx: {:?}", param_env);
+    InterpCx::new(
+        tcx.at(span),
+        param_env,
+        CompileTimeInterpreter::new(),
+        MemoryExtra { can_access_statics },
+    )
+}
+
+pub(super) fn op_to_const<'tcx>(
+    ecx: &CompileTimeEvalContext<'_, 'tcx>,
+    op: OpTy<'tcx>,
+) -> &'tcx ty::Const<'tcx> {
+    // We do not have value optimizations for everything.
+    // Only scalars and slices, since they are very common.
+    // Note that further down we turn scalars of undefined bits back to `ByRef`. These can result
+    // from scalar unions that are initialized with one of their zero sized variants. We could
+    // instead allow `ConstValue::Scalar` to store `ScalarMaybeUndef`, but that would affect all
+    // the usual cases of extracting e.g. a `usize`, without there being a real use case for the
+    // `Undef` situation.
+    let try_as_immediate = match op.layout.abi {
+        layout::Abi::Scalar(..) => true,
+        layout::Abi::ScalarPair(..) => match op.layout.ty.kind {
+            ty::Ref(_, inner, _) => match inner.kind {
+                ty::Slice(elem) => elem == ecx.tcx.types.u8,
+                ty::Str => true,
+                _ => false,
+            },
+            _ => false,
+        },
+        _ => false,
+    };
+    let immediate = if try_as_immediate {
+        Err(ecx.read_immediate(op).expect("normalization works on validated constants"))
+    } else {
+        // It is guaranteed that any non-slice scalar pair is actually ByRef here.
+        // When we come back from raw const eval, we are always by-ref. The only way our op here is
+        // by-val is if we are in const_field, i.e., if this is (a field of) something that we
+        // "tried to make immediate" before. We wouldn't do that for non-slice scalar pairs or
+        // structs containing such.
+        op.try_as_mplace()
+    };
+    let val = match immediate {
+        Ok(mplace) => {
+            let ptr = mplace.ptr.to_ptr().unwrap();
+            let alloc = ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
+            ConstValue::ByRef { alloc, offset: ptr.offset }
+        }
+        // see comment on `let try_as_immediate` above
+        Err(ImmTy { imm: Immediate::Scalar(x), .. }) => match x {
+            ScalarMaybeUndef::Scalar(s) => ConstValue::Scalar(s),
+            ScalarMaybeUndef::Undef => {
+                // When coming out of "normal CTFE", we'll always have an `Indirect` operand as
+                // argument and we will not need this. The only way we can already have an
+                // `Immediate` is when we are called from `const_field`, and that `Immediate`
+                // comes from a constant so it can happen have `Undef`, because the indirect
+                // memory that was read had undefined bytes.
+                let mplace = op.assert_mem_place();
+                let ptr = mplace.ptr.to_ptr().unwrap();
+                let alloc = ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
+                ConstValue::ByRef { alloc, offset: ptr.offset }
+            }
+        },
+        Err(ImmTy { imm: Immediate::ScalarPair(a, b), .. }) => {
+            let (data, start) = match a.not_undef().unwrap() {
+                Scalar::Ptr(ptr) => {
+                    (ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id), ptr.offset.bytes())
+                }
+                Scalar::Raw { .. } => (
+                    ecx.tcx.intern_const_alloc(Allocation::from_byte_aligned_bytes(b"" as &[u8])),
+                    0,
+                ),
+            };
+            let len = b.to_machine_usize(&ecx.tcx.tcx).unwrap();
+            let start = start.try_into().unwrap();
+            let len: usize = len.try_into().unwrap();
+            ConstValue::Slice { data, start, end: start + len }
+        }
+    };
+    ecx.tcx.mk_const(ty::Const { val: ty::ConstKind::Value(val), ty: op.layout.ty })
+}
+
+fn validate_and_turn_into_const<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    constant: RawConst<'tcx>,
+    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
+) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
+    let cid = key.value;
+    let def_id = cid.instance.def.def_id();
+    let is_static = tcx.is_static(def_id);
+    let ecx = mk_eval_cx(tcx, tcx.def_span(key.value.instance.def_id()), key.param_env, is_static);
+    let val = (|| {
+        let mplace = ecx.raw_const_to_mplace(constant)?;
+        let mut ref_tracking = RefTracking::new(mplace);
+        while let Some((mplace, path)) = ref_tracking.todo.pop() {
+            ecx.validate_operand(mplace.into(), path, Some(&mut ref_tracking))?;
+        }
+        // Now that we validated, turn this into a proper constant.
+        // Statics/promoteds are always `ByRef`, for the rest `op_to_const` decides
+        // whether they become immediates.
+        if is_static || cid.promoted.is_some() {
+            let ptr = mplace.ptr.to_ptr()?;
+            Ok(tcx.mk_const(ty::Const {
+                val: ty::ConstKind::Value(ConstValue::ByRef {
+                    alloc: ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id),
+                    offset: ptr.offset,
+                }),
+                ty: mplace.layout.ty,
+            }))
+        } else {
+            Ok(op_to_const(&ecx, mplace.into()))
+        }
+    })();
+
+    val.map_err(|error| {
+        let err = error_to_const_error(&ecx, error);
+        match err.struct_error(ecx.tcx, "it is undefined behavior to use this value") {
+            Ok(mut diag) => {
+                diag.note(note_on_undefined_behavior_error());
+                diag.emit();
+                ErrorHandled::Reported
+            }
+            Err(err) => err,
+        }
+    })
+}
+
+pub fn const_eval_validated_provider<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
+) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
+    // see comment in const_eval_raw_provider for what we're doing here
+    if key.param_env.reveal == Reveal::All {
+        let mut key = key.clone();
+        key.param_env.reveal = Reveal::UserFacing;
+        match tcx.const_eval_validated(key) {
+            // try again with reveal all as requested
+            Err(ErrorHandled::TooGeneric) => {
+                // Promoteds should never be "too generic" when getting evaluated.
+                // They either don't get evaluated, or we are in a monomorphic context
+                assert!(key.value.promoted.is_none());
+            }
+            // dedupliate calls
+            other => return other,
+        }
+    }
+
+    // We call `const_eval` for zero arg intrinsics, too, in order to cache their value.
+    // Catch such calls and evaluate them instead of trying to load a constant's MIR.
+    if let ty::InstanceDef::Intrinsic(def_id) = key.value.instance.def {
+        let ty = key.value.instance.ty(tcx);
+        let substs = match ty.kind {
+            ty::FnDef(_, substs) => substs,
+            _ => bug!("intrinsic with type {:?}", ty),
+        };
+        return eval_nullary_intrinsic(tcx, key.param_env, def_id, substs).map_err(|error| {
+            let span = tcx.def_span(def_id);
+            let error = ConstEvalErr { error: error.kind, stacktrace: vec![], span };
+            error.report_as_error(tcx.at(span), "could not evaluate nullary intrinsic")
+        });
+    }
+
+    tcx.const_eval_raw(key).and_then(|val| validate_and_turn_into_const(tcx, val, key))
+}
+
+pub fn const_eval_raw_provider<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
+) -> ::rustc::mir::interpret::ConstEvalRawResult<'tcx> {
+    // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
+    // reporting the same error twice here. To resolve this, we check whether we can evaluate the
+    // constant in the more restrictive `Reveal::UserFacing`, which most likely already was
+    // computed. For a large percentage of constants that will already have succeeded. Only
+    // associated constants of generic functions will fail due to not enough monomorphization
+    // information being available.
+
+    // In case we fail in the `UserFacing` variant, we just do the real computation.
+    if key.param_env.reveal == Reveal::All {
+        let mut key = key.clone();
+        key.param_env.reveal = Reveal::UserFacing;
+        match tcx.const_eval_raw(key) {
+            // try again with reveal all as requested
+            Err(ErrorHandled::TooGeneric) => {}
+            // dedupliate calls
+            other => return other,
+        }
+    }
+    if cfg!(debug_assertions) {
+        // Make sure we format the instance even if we do not print it.
+        // This serves as a regression test against an ICE on printing.
+        // The next two lines concatenated contain some discussion:
+        // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
+        // subject/anon_const_instance_printing/near/135980032
+        let instance = key.value.instance.to_string();
+        trace!("const eval: {:?} ({})", key, instance);
+    }
+
+    let cid = key.value;
+    let def_id = cid.instance.def.def_id();
+
+    if def_id.is_local() && tcx.typeck_tables_of(def_id).tainted_by_errors {
+        return Err(ErrorHandled::Reported);
+    }
+
+    let is_static = tcx.is_static(def_id);
+
+    let span = tcx.def_span(cid.instance.def_id());
+    let mut ecx = InterpCx::new(
+        tcx.at(span),
+        key.param_env,
+        CompileTimeInterpreter::new(),
+        MemoryExtra { can_access_statics: is_static },
+    );
+
+    let res = ecx.load_mir(cid.instance.def, cid.promoted);
+    res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, *body))
+        .and_then(|place| {
+            Ok(RawConst { alloc_id: place.ptr.assert_ptr().alloc_id, ty: place.layout.ty })
+        })
+        .map_err(|error| {
+            let err = error_to_const_error(&ecx, error);
+            // errors in statics are always emitted as fatal errors
+            if is_static {
+                // Ensure that if the above error was either `TooGeneric` or `Reported`
+                // an error must be reported.
+                let v = err.report_as_error(ecx.tcx, "could not evaluate static initializer");
+                tcx.sess.delay_span_bug(
+                    err.span,
+                    &format!("static eval failure did not emit an error: {:#?}", v),
+                );
+                v
+            } else if def_id.is_local() {
+                // constant defined in this crate, we can figure out a lint level!
+                match tcx.def_kind(def_id) {
+                    // constants never produce a hard error at the definition site. Anything else is
+                    // a backwards compatibility hazard (and will break old versions of winapi for
+                    // sure)
+                    //
+                    // note that validation may still cause a hard error on this very same constant,
+                    // because any code that existed before validation could not have failed
+                    // validation thus preventing such a hard error from being a backwards
+                    // compatibility hazard
+                    Some(DefKind::Const) | Some(DefKind::AssocConst) => {
+                        let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
+                        err.report_as_lint(
+                            tcx.at(tcx.def_span(def_id)),
+                            "any use of this value will cause an error",
+                            hir_id,
+                            Some(err.span),
+                        )
+                    }
+                    // promoting runtime code is only allowed to error if it references broken
+                    // constants any other kind of error will be reported to the user as a
+                    // deny-by-default lint
+                    _ => {
+                        if let Some(p) = cid.promoted {
+                            let span = tcx.promoted_mir(def_id)[p].span;
+                            if let err_inval!(ReferencedConstant) = err.error {
+                                err.report_as_error(
+                                    tcx.at(span),
+                                    "evaluation of constant expression failed",
+                                )
+                            } else {
+                                err.report_as_lint(
+                                    tcx.at(span),
+                                    "reaching this expression at runtime will panic or abort",
+                                    tcx.hir().as_local_hir_id(def_id).unwrap(),
+                                    Some(err.span),
+                                )
+                            }
+                        // anything else (array lengths, enum initializers, constant patterns) are
+                        // reported as hard errors
+                        } else {
+                            err.report_as_error(ecx.tcx, "evaluation of constant value failed")
+                        }
+                    }
+                }
+            } else {
+                // use of broken constant from other crate
+                err.report_as_error(ecx.tcx, "could not evaluate constant")
+            }
+        })
+}

src/librustc_mir/const_eval/machine.rs

@@ -0,0 +1,364 @@
+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::*;
+
+impl<'mir, 'tcx> InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>> {
+    /// Evaluate a const function where all arguments (if any) are zero-sized types.
+    /// The evaluation is memoized thanks to the query system.
+    ///
+    /// Returns `true` if the call has been evaluated.
+    fn try_eval_const_fn_call(
+        &mut self,
+        instance: ty::Instance<'tcx>,
+        ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
+        args: &[OpTy<'tcx>],
+    ) -> InterpResult<'tcx, bool> {
+        trace!("try_eval_const_fn_call: {:?}", instance);
+        // Because `#[track_caller]` adds an implicit non-ZST argument, we also cannot
+        // perform this optimization on items tagged with it.
+        if instance.def.requires_caller_location(self.tcx()) {
+            return Ok(false);
+        }
+        // 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.
+        if args.iter().any(|a| !a.layout.is_zst()) {
+            return Ok(false);
+        }
+
+        let dest = match ret {
+            Some((dest, _)) => dest,
+            // Don't memoize diverging function calls.
+            None => return Ok(false),
+        };
+
+        let gid = GlobalId { instance, promoted: None };
+
+        let place = self.const_eval_raw(gid)?;
+
+        self.copy_op(place.into(), dest)?;
+
+        self.return_to_block(ret.map(|r| r.1))?;
+        self.dump_place(*dest);
+        return Ok(true);
+    }
+}
+
+/// 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
+    pub(super) can_access_statics: bool,
+}
+
+impl<'mir, 'tcx> CompileTimeInterpreter<'mir, 'tcx> {
+    pub(super) 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 under certain circumstances we
+                // can evaluate call via the query system, thus memoizing all future calls.
+                if ecx.try_eval_const_fn_call(instance, ret, args)? {
+                    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())
+        }
+    }
+}
+
+// Please do not add any code below the above `Machine` trait impl. I (oli-obk) plan more cleanups
+// so we can end up having a file with just that impl, but for now, let's keep the impl discoverable
+// at the bottom of this file.

src/librustc_mir/interpret/terminator.rs

@@ -7,8 +7,7 @@ use rustc_target::spec::abi::Abi;
 use syntax::source_map::Span;
 
 use super::{
-    FnVal, GlobalId, ImmTy, InterpCx, InterpResult, MPlaceTy, Machine, OpTy, PlaceTy,
-    StackPopCleanup,
+    FnVal, ImmTy, InterpCx, InterpResult, MPlaceTy, Machine, OpTy, PlaceTy, StackPopCleanup,
 };
 
 impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
@@ -402,26 +401,6 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    /// Evaluate a const function where all arguments (if any) are zero-sized types.
-    /// The evaluation is memoized thanks to the query system.
-    // FIXME: Consider moving this to `const_eval.rs`.
-    pub(crate) fn eval_const_fn_call(
-        &mut self,
-        gid: GlobalId<'tcx>,
-        ret: Option<(PlaceTy<'tcx, M::PointerTag>, mir::BasicBlock)>,
-    ) -> InterpResult<'tcx> {
-        trace!("eval_const_fn_call: {:?}", gid);
-
-        let place = self.const_eval_raw(gid)?;
-        let dest = ret.ok_or_else(|| err_ub!(Unreachable))?.0;
-
-        self.copy_op(place.into(), dest)?;
-
-        self.return_to_block(ret.map(|r| r.1))?;
-        self.dump_place(*dest);
-        return Ok(());
-    }
-
     fn drop_in_place(
         &mut self,
         place: PlaceTy<'tcx, M::PointerTag>,

src/test/ui/consts/uninhabited-const-issue-61744.rs

@@ -5,15 +5,15 @@ pub const unsafe fn fake_type<T>() -> T {
 }
 
 pub const unsafe fn hint_unreachable() -> ! {
-    fake_type() //~ ERROR cycle detected when const-evaluating `hint_unreachable` [E0391]
+    fake_type() //~ ERROR evaluation of constant value failed
 }
 
 trait Const {
-    const CONSTANT: i32 = unsafe { fake_type() };
+    const CONSTANT: i32 = unsafe { fake_type() }; //~ ERROR any use of this value will cause an err
 }
 
-impl <T> Const for T {}
+impl<T> Const for T {}
 
 pub fn main() -> () {
-    dbg!(i32::CONSTANT);
+    dbg!(i32::CONSTANT); //~ ERROR erroneous constant used
 }

src/test/ui/consts/uninhabited-const-issue-61744.stderr

@@ -1,21 +1,96 @@
-error[E0391]: cycle detected when const-evaluating `hint_unreachable`
+error[E0080]: evaluation of constant value failed
   --> $DIR/uninhabited-const-issue-61744.rs:8:5
    |
+LL |     hint_unreachable()
+   |     ------------------
+   |     |
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+   |     inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
+...
 LL |     fake_type()
    |     ^^^^^^^^^^^
+   |     |
+   |     reached the configured maximum number of stack frames
+   |     inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
+
+error: any use of this value will cause an error
+  --> $DIR/uninhabited-const-issue-61744.rs:12:36
    |
-note: ...which requires const-evaluating `fake_type`...
-  --> $DIR/uninhabited-const-issue-61744.rs:4:5
+LL |     const CONSTANT: i32 = unsafe { fake_type() };
+   |     -------------------------------^^^^^^^^^^^---
+   |                                    |
+   |                                    referenced constant has errors
    |
-LL |     hint_unreachable()
-   |     ^^^^^^^^^^^^^^^^^^
-   = note: ...which again requires const-evaluating `hint_unreachable`, completing the cycle
-note: cycle used when const-evaluating `fake_type`
-  --> $DIR/uninhabited-const-issue-61744.rs:4:5
+   = note: `#[deny(const_err)]` on by default
+
+error[E0080]: erroneous constant used
+  --> $DIR/uninhabited-const-issue-61744.rs:18:10
    |
-LL |     hint_unreachable()
-   |     ^^^^^^^^^^^^^^^^^^
+LL |     dbg!(i32::CONSTANT);
+   |          ^^^^^^^^^^^^^ referenced constant has errors
 
-error: aborting due to previous error
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0391`.
+For more information about this error, try `rustc --explain E0080`.