1 // Not in interpret to make sure we do not use private implementation details
5 use std::borrow::{Borrow, Cow};
7 use std::collections::hash_map::Entry;
8 use std::convert::TryInto;
10 use rustc::hir::def::DefKind;
11 use rustc::hir::def_id::DefId;
12 use rustc::middle::lang_items::PanicLocationLangItem;
13 use rustc::mir::interpret::{ConstEvalErr, ErrorHandled, ScalarMaybeUndef};
15 use rustc::ty::{self, Ty, TyCtxt, subst::Subst};
16 use rustc::ty::layout::{self, LayoutOf, VariantIdx};
17 use rustc::traits::Reveal;
18 use rustc_data_structures::fx::FxHashMap;
19 use crate::interpret::eval_nullary_intrinsic;
21 use syntax::{source_map::{Span, DUMMY_SP}, symbol::Symbol};
23 use crate::interpret::{self,
24 PlaceTy, MPlaceTy, OpTy, ImmTy, Immediate, Scalar, Pointer,
25 RawConst, ConstValue, Machine,
26 InterpResult, InterpErrorInfo, GlobalId, InterpCx, StackPopCleanup,
27 Allocation, AllocId, MemoryKind, Memory,
28 snapshot, RefTracking, intern_const_alloc_recursive,
31 /// Number of steps until the detector even starts doing anything.
32 /// Also, a warning is shown to the user when this number is reached.
33 const STEPS_UNTIL_DETECTOR_ENABLED: isize = 1_000_000;
34 /// The number of steps between loop detector snapshots.
35 /// Should be a power of two for performance reasons.
36 const DETECTOR_SNAPSHOT_PERIOD: isize = 256;
38 /// The `InterpCx` is only meant to be used to do field and index projections into constants for
39 /// `simd_shuffle` and const patterns in match arms.
41 /// The function containing the `match` that is currently being analyzed may have generic bounds
42 /// that inform us about the generic bounds of the constant. E.g., using an associated constant
43 /// of a function's generic parameter will require knowledge about the bounds on the generic
44 /// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
45 fn mk_eval_cx<'mir, 'tcx>(
48 param_env: ty::ParamEnv<'tcx>,
49 ) -> CompileTimeEvalContext<'mir, 'tcx> {
50 debug!("mk_eval_cx: {:?}", param_env);
51 InterpCx::new(tcx.at(span), param_env, CompileTimeInterpreter::new(), Default::default())
55 ecx: &CompileTimeEvalContext<'_, 'tcx>,
57 ) -> &'tcx ty::Const<'tcx> {
58 // We do not have value optimizations for everything.
59 // Only scalars and slices, since they are very common.
60 // Note that further down we turn scalars of undefined bits back to `ByRef`. These can result
61 // from scalar unions that are initialized with one of their zero sized variants. We could
62 // instead allow `ConstValue::Scalar` to store `ScalarMaybeUndef`, but that would affect all
63 // the usual cases of extracting e.g. a `usize`, without there being a real use case for the
65 let try_as_immediate = match op.layout.abi {
66 layout::Abi::Scalar(..) => true,
67 layout::Abi::ScalarPair(..) => match op.layout.ty.kind {
68 ty::Ref(_, inner, _) => match inner.kind {
69 ty::Slice(elem) => elem == ecx.tcx.types.u8,
77 let immediate = if try_as_immediate {
78 Err(ecx.read_immediate(op).expect("normalization works on validated constants"))
80 // It is guaranteed that any non-slice scalar pair is actually ByRef here.
81 // When we come back from raw const eval, we are always by-ref. The only way our op here is
82 // by-val is if we are in const_field, i.e., if this is (a field of) something that we
83 // "tried to make immediate" before. We wouldn't do that for non-slice scalar pairs or
84 // structs containing such.
87 let val = match immediate {
89 let ptr = mplace.ptr.to_ptr().unwrap();
90 let alloc = ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
91 ConstValue::ByRef { alloc, offset: ptr.offset }
93 // see comment on `let try_as_immediate` above
94 Err(ImmTy { imm: Immediate::Scalar(x), .. }) => match x {
95 ScalarMaybeUndef::Scalar(s) => ConstValue::Scalar(s),
96 ScalarMaybeUndef::Undef => {
97 // When coming out of "normal CTFE", we'll always have an `Indirect` operand as
98 // argument and we will not need this. The only way we can already have an
99 // `Immediate` is when we are called from `const_field`, and that `Immediate`
100 // comes from a constant so it can happen have `Undef`, because the indirect
101 // memory that was read had undefined bytes.
102 let mplace = op.assert_mem_place();
103 let ptr = mplace.ptr.to_ptr().unwrap();
104 let alloc = ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
105 ConstValue::ByRef { alloc, offset: ptr.offset }
108 Err(ImmTy { imm: Immediate::ScalarPair(a, b), .. }) => {
109 let (data, start) = match a.not_undef().unwrap() {
110 Scalar::Ptr(ptr) => (
111 ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id),
114 Scalar::Raw { .. } => (
115 ecx.tcx.intern_const_alloc(Allocation::from_byte_aligned_bytes(
121 let len = b.to_machine_usize(&ecx.tcx.tcx).unwrap();
122 let start = start.try_into().unwrap();
123 let len: usize = len.try_into().unwrap();
131 ecx.tcx.mk_const(ty::Const { val: ty::ConstKind::Value(val), ty: op.layout.ty })
134 // Returns a pointer to where the result lives
135 fn eval_body_using_ecx<'mir, 'tcx>(
136 ecx: &mut CompileTimeEvalContext<'mir, 'tcx>,
138 body: &'mir mir::Body<'tcx>,
139 ) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
140 debug!("eval_body_using_ecx: {:?}, {:?}", cid, ecx.param_env);
141 let tcx = ecx.tcx.tcx;
142 let layout = ecx.layout_of(body.return_ty().subst(tcx, cid.instance.substs))?;
143 assert!(!layout.is_unsized());
144 let ret = ecx.allocate(layout, MemoryKind::Stack);
146 let name = ty::tls::with(|tcx| tcx.def_path_str(cid.instance.def_id()));
147 let prom = cid.promoted.map_or(String::new(), |p| format!("::promoted[{:?}]", p));
148 trace!("eval_body_using_ecx: pushing stack frame for global: {}{}", name, prom);
150 // Assert all args (if any) are zero-sized types; `eval_body_using_ecx` doesn't
151 // make sense if the body is expecting nontrivial arguments.
152 // (The alternative would be to use `eval_fn_call` with an args slice.)
153 for arg in body.args_iter() {
154 let decl = body.local_decls.get(arg).expect("arg missing from local_decls");
155 let layout = ecx.layout_of(decl.ty.subst(tcx, cid.instance.substs))?;
156 assert!(layout.is_zst())
159 ecx.push_stack_frame(
164 StackPopCleanup::None { cleanup: false },
167 // The main interpreter loop.
171 intern_const_alloc_recursive(ecx, tcx.static_mutability(cid.instance.def_id()), ret)?;
173 debug!("eval_body_using_ecx done: {:?}", *ret);
177 #[derive(Clone, Debug)]
178 pub enum ConstEvalError {
182 impl<'tcx> Into<InterpErrorInfo<'tcx>> for ConstEvalError {
183 fn into(self) -> InterpErrorInfo<'tcx> {
184 err_unsup!(Unsupported(self.to_string())).into()
188 impl fmt::Display for ConstEvalError {
189 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
190 use self::ConstEvalError::*;
192 NeedsRfc(ref msg) => {
195 "\"{}\" needs an rfc before being allowed inside constants",
203 impl Error for ConstEvalError {
204 fn description(&self) -> &str {
205 use self::ConstEvalError::*;
207 NeedsRfc(_) => "this feature needs an rfc before being allowed inside constants",
211 fn cause(&self) -> Option<&dyn Error> {
216 // Extra machine state for CTFE, and the Machine instance
217 pub struct CompileTimeInterpreter<'mir, 'tcx> {
218 /// When this value is negative, it indicates the number of interpreter
219 /// steps *until* the loop detector is enabled. When it is positive, it is
220 /// the number of steps after the detector has been enabled modulo the loop
222 pub(super) steps_since_detector_enabled: isize,
224 /// Extra state to detect loops.
225 pub(super) loop_detector: snapshot::InfiniteLoopDetector<'mir, 'tcx>,
228 impl<'mir, 'tcx> CompileTimeInterpreter<'mir, 'tcx> {
230 CompileTimeInterpreter {
231 loop_detector: Default::default(),
232 steps_since_detector_enabled: -STEPS_UNTIL_DETECTOR_ENABLED,
237 impl<K: Hash + Eq, V> interpret::AllocMap<K, V> for FxHashMap<K, V> {
239 fn contains_key<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> bool
242 FxHashMap::contains_key(self, k)
246 fn insert(&mut self, k: K, v: V) -> Option<V>
248 FxHashMap::insert(self, k, v)
252 fn remove<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> Option<V>
255 FxHashMap::remove(self, k)
259 fn filter_map_collect<T>(&self, mut f: impl FnMut(&K, &V) -> Option<T>) -> Vec<T> {
261 .filter_map(move |(k, v)| f(k, &*v))
269 vacant: impl FnOnce() -> Result<V, E>
276 bug!("The CTFE machine shouldn't ever need to extend the alloc_map when reading")
285 vacant: impl FnOnce() -> Result<V, E>
286 ) -> Result<&mut V, E>
288 match self.entry(k) {
289 Entry::Occupied(e) => Ok(e.into_mut()),
290 Entry::Vacant(e) => {
298 crate type CompileTimeEvalContext<'mir, 'tcx> =
299 InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>>;
301 impl interpret::MayLeak for ! {
303 fn may_leak(self) -> bool {
304 // `self` is uninhabited
309 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir, 'tcx> {
310 type MemoryKinds = !;
311 type PointerTag = ();
314 type FrameExtra = ();
315 type MemoryExtra = ();
316 type AllocExtra = ();
318 type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
320 const STATIC_KIND: Option<!> = None; // no copying of statics allowed
322 // We do not check for alignment to avoid having to carry an `Align`
323 // in `ConstValue::ByRef`.
324 const CHECK_ALIGN: bool = false;
327 fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
328 false // for now, we don't enforce validity
331 fn find_mir_or_eval_fn(
332 ecx: &mut InterpCx<'mir, 'tcx, Self>,
333 instance: ty::Instance<'tcx>,
335 ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
336 _unwind: Option<mir::BasicBlock> // unwinding is not supported in consts
337 ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
338 debug!("find_mir_or_eval_fn: {:?}", instance);
340 // If this function is a `const fn` then as an optimization we can query this
341 // evaluation immediately.
343 // For the moment we only do this for functions which take no arguments
344 // (or all arguments are ZSTs) so that we don't memoize too much.
345 if ecx.tcx.is_const_fn_raw(instance.def.def_id()) &&
346 args.iter().all(|a| a.layout.is_zst())
348 let gid = GlobalId { instance, promoted: None };
349 ecx.eval_const_fn_call(gid, ret)?;
353 // Only check non-glue functions
354 if let ty::InstanceDef::Item(def_id) = instance.def {
355 // Execution might have wandered off into other crates, so we cannot do a stability-
356 // sensitive check here. But we can at least rule out functions that are not const
358 if !ecx.tcx.is_const_fn_raw(def_id) {
359 // Some functions we support even if they are non-const -- but avoid testing
360 // that for const fn! We certainly do *not* want to actually call the fn
361 // though, so be sure we return here.
362 return if ecx.hook_panic_fn(instance, args, ret)? {
365 throw_unsup_format!("calling non-const function `{}`", instance)
369 // This is a const fn. Call it.
370 Ok(Some(match ecx.load_mir(instance.def, None) {
373 if let err_unsup!(NoMirFor(ref path)) = err.kind {
375 ConstEvalError::NeedsRfc(format!("calling extern function `{}`", path))
385 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
387 _args: &[OpTy<'tcx>],
388 _ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
389 _unwind: Option<mir::BasicBlock>
390 ) -> InterpResult<'tcx> {
395 ecx: &mut InterpCx<'mir, 'tcx, Self>,
397 instance: ty::Instance<'tcx>,
399 ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
400 _unwind: Option<mir::BasicBlock>
401 ) -> InterpResult<'tcx> {
402 if ecx.emulate_intrinsic(span, instance, args, ret)? {
405 // An intrinsic that we do not support
406 let intrinsic_name = ecx.tcx.item_name(instance.def_id());
408 ConstEvalError::NeedsRfc(format!("calling intrinsic `{}`", intrinsic_name)).into()
413 _mem: &Memory<'mir, 'tcx, Self>,
415 ) -> InterpResult<'tcx, u64> {
417 ConstEvalError::NeedsRfc("pointer-to-integer cast".to_string()).into(),
422 _ecx: &InterpCx<'mir, 'tcx, Self>,
426 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
428 ConstEvalError::NeedsRfc("pointer arithmetic or comparison".to_string()).into(),
432 fn find_foreign_static(
435 ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
436 throw_unsup!(ReadForeignStatic)
440 fn tag_allocation<'b>(
443 alloc: Cow<'b, Allocation>,
444 _kind: Option<MemoryKind<!>>,
445 ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
446 // We do not use a tag so we can just cheaply forward the allocation
451 fn tag_static_base_pointer(
454 ) -> Self::PointerTag {
459 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
460 _dest: PlaceTy<'tcx>,
461 ) -> InterpResult<'tcx> {
463 ConstEvalError::NeedsRfc("heap allocations via `box` keyword".to_string()).into(),
467 fn before_terminator(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
469 let steps = &mut ecx.machine.steps_since_detector_enabled;
476 *steps %= DETECTOR_SNAPSHOT_PERIOD;
482 let span = ecx.frame().span;
483 ecx.machine.loop_detector.observe_and_analyze(
492 fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
497 /// Extracts a field of a (variant of a) const.
498 // this function uses `unwrap` copiously, because an already validated constant must have valid
499 // fields and can thus never fail outside of compiler bugs
500 pub fn const_field<'tcx>(
502 param_env: ty::ParamEnv<'tcx>,
503 variant: Option<VariantIdx>,
505 value: &'tcx ty::Const<'tcx>,
506 ) -> &'tcx ty::Const<'tcx> {
507 trace!("const_field: {:?}, {:?}", field, value);
508 let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
509 // get the operand again
510 let op = ecx.eval_const_to_op(value, None).unwrap();
512 let down = match variant {
514 Some(variant) => ecx.operand_downcast(op, variant).unwrap(),
517 let field = ecx.operand_field(down, field.index() as u64).unwrap();
518 // and finally move back to the const world, always normalizing because
519 // this is not called for statics.
520 op_to_const(&ecx, field)
523 pub fn const_caller_location<'tcx>(
525 (file, line, col): (Symbol, u32, u32),
526 ) -> &'tcx ty::Const<'tcx> {
527 trace!("const_caller_location: {}:{}:{}", file, line, col);
528 let mut ecx = mk_eval_cx(tcx, DUMMY_SP, ty::ParamEnv::reveal_all());
530 let loc_ty = tcx.mk_imm_ref(
531 tcx.lifetimes.re_static,
532 tcx.type_of(tcx.require_lang_item(PanicLocationLangItem, None))
533 .subst(tcx, tcx.mk_substs([tcx.lifetimes.re_static.into()].iter())),
535 let loc_place = ecx.alloc_caller_location(file, line, col).unwrap();
536 intern_const_alloc_recursive(&mut ecx, None, loc_place).unwrap();
537 let loc_const = ty::Const {
539 val: ty::ConstKind::Value(ConstValue::Scalar(loc_place.ptr.into())),
542 tcx.mk_const(loc_const)
545 // this function uses `unwrap` copiously, because an already validated constant must have valid
546 // fields and can thus never fail outside of compiler bugs
547 pub fn const_variant_index<'tcx>(
549 param_env: ty::ParamEnv<'tcx>,
550 val: &'tcx ty::Const<'tcx>,
552 trace!("const_variant_index: {:?}", val);
553 let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
554 let op = ecx.eval_const_to_op(val, None).unwrap();
555 ecx.read_discriminant(op).unwrap().1
558 /// Turn an interpreter error into something to report to the user.
559 /// As a side-effect, if RUSTC_CTFE_BACKTRACE is set, this prints the backtrace.
560 /// Should be called only if the error is actually going to to be reported!
561 pub fn error_to_const_error<'mir, 'tcx, M: Machine<'mir, 'tcx>>(
562 ecx: &InterpCx<'mir, 'tcx, M>,
563 mut error: InterpErrorInfo<'tcx>,
564 ) -> ConstEvalErr<'tcx> {
565 error.print_backtrace();
566 let stacktrace = ecx.generate_stacktrace(None);
567 ConstEvalErr { error: error.kind, stacktrace, span: ecx.tcx.span }
570 pub fn note_on_undefined_behavior_error() -> &'static str {
571 "The rules on what exactly is undefined behavior aren't clear, \
572 so this check might be overzealous. Please open an issue on the rustc \
573 repository if you believe it should not be considered undefined behavior."
576 fn validate_and_turn_into_const<'tcx>(
578 constant: RawConst<'tcx>,
579 key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
580 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
582 let ecx = mk_eval_cx(tcx, tcx.def_span(key.value.instance.def_id()), key.param_env);
584 let mplace = ecx.raw_const_to_mplace(constant)?;
585 let mut ref_tracking = RefTracking::new(mplace);
586 while let Some((mplace, path)) = ref_tracking.todo.pop() {
587 ecx.validate_operand(
590 Some(&mut ref_tracking),
593 // Now that we validated, turn this into a proper constant.
594 // Statics/promoteds are always `ByRef`, for the rest `op_to_const` decides
595 // whether they become immediates.
596 let def_id = cid.instance.def.def_id();
597 if tcx.is_static(def_id) || cid.promoted.is_some() {
598 let ptr = mplace.ptr.to_ptr()?;
599 Ok(tcx.mk_const(ty::Const {
600 val: ty::ConstKind::Value(ConstValue::ByRef {
601 alloc: ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id),
604 ty: mplace.layout.ty,
607 Ok(op_to_const(&ecx, mplace.into()))
611 val.map_err(|error| {
612 let err = error_to_const_error(&ecx, error);
613 match err.struct_error(ecx.tcx, "it is undefined behavior to use this value") {
615 diag.note(note_on_undefined_behavior_error());
617 ErrorHandled::Reported
624 pub fn const_eval_provider<'tcx>(
626 key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
627 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
628 // see comment in const_eval_raw_provider for what we're doing here
629 if key.param_env.reveal == Reveal::All {
630 let mut key = key.clone();
631 key.param_env.reveal = Reveal::UserFacing;
632 match tcx.const_eval(key) {
633 // try again with reveal all as requested
634 Err(ErrorHandled::TooGeneric) => {
635 // Promoteds should never be "too generic" when getting evaluated.
636 // They either don't get evaluated, or we are in a monomorphic context
637 assert!(key.value.promoted.is_none());
640 other => return other,
644 // We call `const_eval` for zero arg intrinsics, too, in order to cache their value.
645 // Catch such calls and evaluate them instead of trying to load a constant's MIR.
646 if let ty::InstanceDef::Intrinsic(def_id) = key.value.instance.def {
647 let ty = key.value.instance.ty(tcx);
648 let substs = match ty.kind {
649 ty::FnDef(_, substs) => substs,
650 _ => bug!("intrinsic with type {:?}", ty),
652 return eval_nullary_intrinsic(tcx, key.param_env, def_id, substs)
654 let span = tcx.def_span(def_id);
655 let error = ConstEvalErr { error: error.kind, stacktrace: vec![], span };
656 error.report_as_error(tcx.at(span), "could not evaluate nullary intrinsic")
660 tcx.const_eval_raw(key).and_then(|val| {
661 validate_and_turn_into_const(tcx, val, key)
665 pub fn const_eval_raw_provider<'tcx>(
667 key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
668 ) -> ::rustc::mir::interpret::ConstEvalRawResult<'tcx> {
669 // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
670 // reporting the same error twice here. To resolve this, we check whether we can evaluate the
671 // constant in the more restrictive `Reveal::UserFacing`, which most likely already was
672 // computed. For a large percentage of constants that will already have succeeded. Only
673 // associated constants of generic functions will fail due to not enough monomorphization
674 // information being available.
676 // In case we fail in the `UserFacing` variant, we just do the real computation.
677 if key.param_env.reveal == Reveal::All {
678 let mut key = key.clone();
679 key.param_env.reveal = Reveal::UserFacing;
680 match tcx.const_eval_raw(key) {
681 // try again with reveal all as requested
682 Err(ErrorHandled::TooGeneric) => {},
684 other => return other,
687 if cfg!(debug_assertions) {
688 // Make sure we format the instance even if we do not print it.
689 // This serves as a regression test against an ICE on printing.
690 // The next two lines concatenated contain some discussion:
691 // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
692 // subject/anon_const_instance_printing/near/135980032
693 let instance = key.value.instance.to_string();
694 trace!("const eval: {:?} ({})", key, instance);
698 let def_id = cid.instance.def.def_id();
700 if def_id.is_local() && tcx.typeck_tables_of(def_id).tainted_by_errors {
701 return Err(ErrorHandled::Reported);
704 let span = tcx.def_span(cid.instance.def_id());
705 let mut ecx = InterpCx::new(
708 CompileTimeInterpreter::new(),
712 let res = ecx.load_mir(cid.instance.def, cid.promoted);
714 |body| eval_body_using_ecx(&mut ecx, cid, body)
717 alloc_id: place.ptr.assert_ptr().alloc_id,
721 let err = error_to_const_error(&ecx, error);
722 // errors in statics are always emitted as fatal errors
723 if tcx.is_static(def_id) {
724 // Ensure that if the above error was either `TooGeneric` or `Reported`
725 // an error must be reported.
726 let v = err.report_as_error(ecx.tcx, "could not evaluate static initializer");
727 tcx.sess.delay_span_bug(
729 &format!("static eval failure did not emit an error: {:#?}", v)
732 } else if def_id.is_local() {
733 // constant defined in this crate, we can figure out a lint level!
734 match tcx.def_kind(def_id) {
735 // constants never produce a hard error at the definition site. Anything else is
736 // a backwards compatibility hazard (and will break old versions of winapi for sure)
738 // note that validation may still cause a hard error on this very same constant,
739 // because any code that existed before validation could not have failed validation
740 // thus preventing such a hard error from being a backwards compatibility hazard
741 Some(DefKind::Const) | Some(DefKind::AssocConst) => {
742 let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
744 tcx.at(tcx.def_span(def_id)),
745 "any use of this value will cause an error",
750 // promoting runtime code is only allowed to error if it references broken constants
751 // any other kind of error will be reported to the user as a deny-by-default lint
752 _ => if let Some(p) = cid.promoted {
753 let span = tcx.promoted_mir(def_id)[p].span;
754 if let err_inval!(ReferencedConstant) = err.error {
757 "evaluation of constant expression failed",
762 "reaching this expression at runtime will panic or abort",
763 tcx.hir().as_local_hir_id(def_id).unwrap(),
767 // anything else (array lengths, enum initializers, constant patterns) are reported
772 "evaluation of constant value failed",
777 // use of broken constant from other crate
778 err.report_as_error(ecx.tcx, "could not evaluate constant")