Add hooks for Miri panic unwinding

[rust.git] / src / librustc_mir / const_eval.rs

// Not in interpret to make sure we do not use private implementation details

use std::fmt;
use std::error::Error;
use std::borrow::{Borrow, Cow};
use std::hash::Hash;
use std::collections::hash_map::Entry;
use std::convert::TryInto;

use rustc::hir::def::DefKind;
use rustc::hir::def_id::DefId;
use rustc::middle::lang_items::PanicLocationLangItem;
use rustc::mir::interpret::{ConstEvalErr, ErrorHandled, ScalarMaybeUndef};
use rustc::mir;
use rustc::ty::{self, Ty, TyCtxt, subst::Subst};
use rustc::ty::layout::{self, LayoutOf, VariantIdx};
use rustc::traits::Reveal;
use rustc_data_structures::fx::FxHashMap;
use crate::interpret::eval_nullary_intrinsic;

use syntax::{source_map::{Span, DUMMY_SP}, symbol::Symbol};

use crate::interpret::{self,
    PlaceTy, MPlaceTy, OpTy, ImmTy, Immediate, Scalar, Pointer,
    RawConst, ConstValue, Machine,
    InterpResult, InterpErrorInfo, GlobalId, InterpCx, StackPopCleanup,
    Allocation, AllocId, MemoryKind, Memory, StackPopInfo,
    snapshot, RefTracking, intern_const_alloc_recursive,
};

/// 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;

/// 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.
fn mk_eval_cx<'mir, 'tcx>(
    tcx: TyCtxt<'tcx>,
    span: Span,
    param_env: ty::ParamEnv<'tcx>,
) -> CompileTimeEvalContext<'mir, 'tcx> {
    debug!("mk_eval_cx: {:?}", param_env);
    InterpCx::new(tcx.at(span), param_env, CompileTimeInterpreter::new(), Default::default())
}

fn op_to_const<'tcx>(
    ecx: &CompileTimeEvalContext<'_, 'tcx>,
    op: OpTy<'tcx>,
) -> &'tcx ty::Const<'tcx> {
    // We do not have value optmizations 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: op.layout.ty })
}

// 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!(body.arg_count == 0);
    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)
}

#[derive(Clone, Debug)]
pub enum ConstEvalError {
    NeedsRfc(String),
}

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
                )
            }
        }
    }
}

impl Error for ConstEvalError {
    fn description(&self) -> &str {
        use self::ConstEvalError::*;
        match *self {
            NeedsRfc(_) => "this feature needs an rfc before being allowed inside constants",
        }
    }

    fn cause(&self) -> Option<&dyn Error> {
        None
    }
}

// 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>,
}

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 = ();
    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_fn(
        ecx: &mut InterpCx<'mir, 'tcx, Self>,
        instance: ty::Instance<'tcx>,
        args: &[OpTy<'tcx>],
        dest: Option<PlaceTy<'tcx>>,
        ret: Option<mir::BasicBlock>,
        _unwind: Option<mir::BasicBlock> // unwinding is not supported in consts
    ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
        debug!("eval_fn_call: {:?}", 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) {
                // 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_fn(instance, args, dest)? {
                    ecx.goto_block(ret)?; // fully evaluated and done
                    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>],
        _dest: Option<PlaceTy<'tcx>>,
        _ret: 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>],
        dest: Option<PlaceTy<'tcx>>,
    ) -> InterpResult<'tcx> {
        if ecx.emulate_intrinsic(span, instance, args, dest)? {
            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 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 tag_allocation<'b>(
        _memory_extra: &(),
        _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: &(),
        _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(())
    }

    /// Called immediately before a stack frame gets popped.
    #[inline(always)]
    fn stack_pop(
        _ecx: &mut InterpCx<'mir, 'tcx, Self>, _extra: ()) -> InterpResult<'tcx, StackPopInfo> {
        // Const-eval mode does not support unwinding from panics
        Ok(StackPopInfo::Normal)
    }
}

/// 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
pub fn const_field<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    variant: Option<VariantIdx>,
    field: mir::Field,
    value: &'tcx ty::Const<'tcx>,
) -> &'tcx ty::Const<'tcx> {
    trace!("const_field: {:?}, {:?}", field, value);
    let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
    // get the operand again
    let op = ecx.eval_const_to_op(value, None).unwrap();
    // downcast
    let down = match variant {
        None => op,
        Some(variant) => ecx.operand_downcast(op, variant).unwrap(),
    };
    // then project
    let field = ecx.operand_field(down, field.index() as u64).unwrap();
    // and finally move back to the const world, always normalizing because
    // this is not called for statics.
    op_to_const(&ecx, field)
}

pub fn const_caller_location<'tcx>(
    tcx: TyCtxt<'tcx>,
    (file, line, col): (Symbol, u32, u32),
) -> &'tcx ty::Const<'tcx> {
    trace!("const_caller_location: {}:{}:{}", file, line, col);
    let mut ecx = mk_eval_cx(tcx, DUMMY_SP, ty::ParamEnv::reveal_all());

    let loc_ty = tcx.mk_imm_ref(
        tcx.lifetimes.re_static,
        tcx.type_of(tcx.require_lang_item(PanicLocationLangItem, None))
            .subst(tcx, tcx.mk_substs([tcx.lifetimes.re_static.into()].iter())),
    );
    let loc_place = ecx.alloc_caller_location(file, line, col).unwrap();
    intern_const_alloc_recursive(&mut ecx, None, loc_place).unwrap();
    let loc_const = ty::Const {
        ty: loc_ty,
        val: ConstValue::Scalar(loc_place.ptr.into()),
    };

    tcx.mk_const(loc_const)
}

// this function uses `unwrap` copiously, because an already validated constant must have valid
// fields and can thus never fail outside of compiler bugs
pub fn const_variant_index<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    val: &'tcx ty::Const<'tcx>,
) -> VariantIdx {
    trace!("const_variant_index: {:?}", val);
    let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
    let op = ecx.eval_const_to_op(val, None).unwrap();
    ecx.read_discriminant(op).unwrap().1
}

/// 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 }
}

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."
}

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 ecx = mk_eval_cx(tcx, tcx.def_span(key.value.instance.def_id()), key.param_env);
    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.
        let def_id = cid.instance.def.def_id();
        if tcx.is_static(def_id) || cid.promoted.is_some() {
            let ptr = mplace.ptr.to_ptr()?;
            Ok(tcx.mk_const(ty::Const {
                val: 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_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(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 span = tcx.def_span(cid.instance.def_id());
    let mut ecx = InterpCx::new(
        tcx.at(span),
        key.param_env,
        CompileTimeInterpreter::new(),
        Default::default()
    );

    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 tcx.is_static(def_id) {
            // 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")
        }
    })
}