1 // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 // Not in interpret to make sure we do not use private implementation details
14 use std::error::Error;
15 use std::borrow::{Borrow, Cow};
17 use std::collections::hash_map::Entry;
19 use rustc::hir::{self, def_id::DefId};
20 use rustc::hir::def::Def;
21 use rustc::mir::interpret::{ConstEvalErr, ErrorHandled};
23 use rustc::ty::{self, TyCtxt, Instance, query::TyCtxtAt};
24 use rustc::ty::layout::{self, LayoutOf, TyLayout, VariantIdx};
25 use rustc::ty::subst::Subst;
26 use rustc::traits::Reveal;
27 use rustc_data_structures::indexed_vec::IndexVec;
28 use rustc_data_structures::fx::FxHashMap;
29 use rustc::util::common::ErrorReported;
31 use syntax::ast::Mutability;
32 use syntax::source_map::{Span, DUMMY_SP};
34 use crate::interpret::{self,
35 PlaceTy, MPlaceTy, MemPlace, OpTy, Operand, Immediate, Scalar, RawConst, ConstValue, Pointer,
36 EvalResult, EvalError, EvalErrorKind, GlobalId, EvalContext, StackPopCleanup,
37 Allocation, AllocId, MemoryKind,
38 snapshot, RefTracking,
41 /// Number of steps until the detector even starts doing anything.
42 /// Also, a warning is shown to the user when this number is reached.
43 const STEPS_UNTIL_DETECTOR_ENABLED: isize = 1_000_000;
44 /// The number of steps between loop detector snapshots.
45 /// Should be a power of two for performance reasons.
46 const DETECTOR_SNAPSHOT_PERIOD: isize = 256;
48 pub fn mk_borrowck_eval_cx<'a, 'mir, 'tcx>(
49 tcx: TyCtxt<'a, 'tcx, 'tcx>,
50 instance: Instance<'tcx>,
51 mir: &'mir mir::Mir<'tcx>,
53 ) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'mir, 'tcx>> {
54 debug!("mk_borrowck_eval_cx: {:?}", instance);
55 let param_env = tcx.param_env(instance.def_id());
56 let mut ecx = EvalContext::new(tcx.at(span), param_env, CompileTimeInterpreter::new());
57 // insert a stack frame so any queries have the correct substs
58 // cannot use `push_stack_frame`; if we do `const_prop` explodes
59 ecx.stack.push(interpret::Frame {
60 block: mir::START_BLOCK,
61 locals: IndexVec::new(),
66 return_to_block: StackPopCleanup::Goto(None), // never pop
73 pub fn mk_eval_cx<'a, 'tcx>(
74 tcx: TyCtxt<'a, 'tcx, 'tcx>,
75 instance: Instance<'tcx>,
76 param_env: ty::ParamEnv<'tcx>,
77 ) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'tcx, 'tcx>> {
78 debug!("mk_eval_cx: {:?}, {:?}", instance, param_env);
79 let span = tcx.def_span(instance.def_id());
80 let mut ecx = EvalContext::new(tcx.at(span), param_env, CompileTimeInterpreter::new());
81 let mir = ecx.load_mir(instance.def)?;
82 // insert a stack frame so any queries have the correct substs
88 StackPopCleanup::Goto(None), // never pop
93 pub(crate) fn eval_promoted<'a, 'mir, 'tcx>(
94 tcx: TyCtxt<'a, 'tcx, 'tcx>,
96 mir: &'mir mir::Mir<'tcx>,
97 param_env: ty::ParamEnv<'tcx>,
98 ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
99 let mut ecx = mk_borrowck_eval_cx(tcx, cid.instance, mir, DUMMY_SP).unwrap();
100 eval_body_using_ecx(&mut ecx, cid, Some(mir), param_env)
103 // FIXME: These two conversion functions are bad hacks. We should just always use allocations.
104 pub fn op_to_const<'tcx>(
105 ecx: &CompileTimeEvalContext<'_, '_, 'tcx>,
108 ) -> EvalResult<'tcx, &'tcx ty::Const<'tcx>> {
109 // We do not normalize just any data. Only scalar layout and fat pointers.
110 let normalize = may_normalize
111 && match op.layout.abi {
112 layout::Abi::Scalar(..) => true,
113 layout::Abi::ScalarPair(..) => {
114 // Must be a fat pointer
115 op.layout.ty.builtin_deref(true).is_some()
119 let normalized_op = if normalize {
120 ecx.try_read_immediate(op)?
123 Operand::Indirect(mplace) => Err(mplace),
124 Operand::Immediate(val) => Ok(val)
127 let val = match normalized_op {
128 Err(MemPlace { ptr, align, meta }) => {
129 // extract alloc-offset pair
130 assert!(meta.is_none());
131 let ptr = ptr.to_ptr()?;
132 let alloc = ecx.memory.get(ptr.alloc_id)?;
133 assert!(alloc.align >= align);
134 assert!(alloc.bytes.len() as u64 - ptr.offset.bytes() >= op.layout.size.bytes());
135 let mut alloc = alloc.clone();
137 // FIXME shouldn't it be the case that `mark_static_initialized` has already
138 // interned this? I thought that is the entire point of that `FinishStatic` stuff?
139 let alloc = ecx.tcx.intern_const_alloc(alloc);
140 ConstValue::ByRef(ptr.alloc_id, alloc, ptr.offset)
142 Ok(Immediate::Scalar(x)) =>
143 ConstValue::Scalar(x.not_undef()?),
144 Ok(Immediate::ScalarPair(a, b)) =>
145 ConstValue::ScalarPair(a.not_undef()?, b.not_undef()?),
147 Ok(ty::Const::from_const_value(ecx.tcx.tcx, val, op.layout.ty))
149 pub fn const_to_op<'tcx>(
150 ecx: &CompileTimeEvalContext<'_, '_, 'tcx>,
151 cnst: &ty::Const<'tcx>,
152 ) -> EvalResult<'tcx, OpTy<'tcx>> {
153 let op = ecx.const_value_to_op(cnst.val)?;
154 Ok(OpTy { op, layout: ecx.layout_of(cnst.ty)? })
157 fn eval_body_and_ecx<'a, 'mir, 'tcx>(
158 tcx: TyCtxt<'a, 'tcx, 'tcx>,
160 mir: Option<&'mir mir::Mir<'tcx>>,
161 param_env: ty::ParamEnv<'tcx>,
162 ) -> (EvalResult<'tcx, MPlaceTy<'tcx>>, CompileTimeEvalContext<'a, 'mir, 'tcx>) {
163 // we start out with the best span we have
164 // and try improving it down the road when more information is available
165 let span = tcx.def_span(cid.instance.def_id());
166 let span = mir.map(|mir| mir.span).unwrap_or(span);
167 let mut ecx = EvalContext::new(tcx.at(span), param_env, CompileTimeInterpreter::new());
168 let r = eval_body_using_ecx(&mut ecx, cid, mir, param_env);
172 // Returns a pointer to where the result lives
173 fn eval_body_using_ecx<'mir, 'tcx>(
174 ecx: &mut CompileTimeEvalContext<'_, 'mir, 'tcx>,
176 mir: Option<&'mir mir::Mir<'tcx>>,
177 param_env: ty::ParamEnv<'tcx>,
178 ) -> EvalResult<'tcx, MPlaceTy<'tcx>> {
179 debug!("eval_body_using_ecx: {:?}, {:?}", cid, param_env);
180 let tcx = ecx.tcx.tcx;
181 let mut mir = match mir {
183 None => ecx.load_mir(cid.instance.def)?,
185 if let Some(index) = cid.promoted {
186 mir = &mir.promoted[index];
188 let layout = ecx.layout_of(mir.return_ty().subst(tcx, cid.instance.substs))?;
189 assert!(!layout.is_unsized());
190 let ret = ecx.allocate(layout, MemoryKind::Stack)?;
192 let name = ty::tls::with(|tcx| tcx.item_path_str(cid.instance.def_id()));
193 let prom = cid.promoted.map_or(String::new(), |p| format!("::promoted[{:?}]", p));
194 trace!("eval_body_using_ecx: pushing stack frame for global: {}{}", name, prom);
195 assert!(mir.arg_count == 0);
196 ecx.push_stack_frame(
201 StackPopCleanup::None { cleanup: false },
204 // The main interpreter loop.
208 let internally_mutable = !layout.ty.is_freeze(tcx, param_env, mir.span);
209 let is_static = tcx.is_static(cid.instance.def_id());
210 let mutability = if is_static == Some(hir::Mutability::MutMutable) || internally_mutable {
213 Mutability::Immutable
215 ecx.memory.intern_static(ret.ptr.to_ptr()?.alloc_id, mutability)?;
217 debug!("eval_body_using_ecx done: {:?}", *ret);
221 impl<'tcx> Into<EvalError<'tcx>> for ConstEvalError {
222 fn into(self) -> EvalError<'tcx> {
223 EvalErrorKind::MachineError(self.to_string()).into()
227 #[derive(Clone, Debug)]
228 enum ConstEvalError {
232 impl fmt::Display for ConstEvalError {
233 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
234 use self::ConstEvalError::*;
236 NeedsRfc(ref msg) => {
239 "\"{}\" needs an rfc before being allowed inside constants",
247 impl Error for ConstEvalError {
248 fn description(&self) -> &str {
249 use self::ConstEvalError::*;
251 NeedsRfc(_) => "this feature needs an rfc before being allowed inside constants",
255 fn cause(&self) -> Option<&dyn Error> {
260 // Extra machine state for CTFE, and the Machine instance
261 pub struct CompileTimeInterpreter<'a, 'mir, 'tcx: 'a+'mir> {
262 /// When this value is negative, it indicates the number of interpreter
263 /// steps *until* the loop detector is enabled. When it is positive, it is
264 /// the number of steps after the detector has been enabled modulo the loop
266 pub(super) steps_since_detector_enabled: isize,
268 /// Extra state to detect loops.
269 pub(super) loop_detector: snapshot::InfiniteLoopDetector<'a, 'mir, 'tcx>,
272 impl<'a, 'mir, 'tcx> CompileTimeInterpreter<'a, 'mir, 'tcx> {
274 CompileTimeInterpreter {
275 loop_detector: Default::default(),
276 steps_since_detector_enabled: -STEPS_UNTIL_DETECTOR_ENABLED,
281 impl<K: Hash + Eq, V> interpret::AllocMap<K, V> for FxHashMap<K, V> {
283 fn contains_key<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> bool
286 FxHashMap::contains_key(self, k)
290 fn insert(&mut self, k: K, v: V) -> Option<V>
292 FxHashMap::insert(self, k, v)
296 fn remove<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> Option<V>
299 FxHashMap::remove(self, k)
303 fn filter_map_collect<T>(&self, mut f: impl FnMut(&K, &V) -> Option<T>) -> Vec<T> {
305 .filter_map(move |(k, v)| f(k, &*v))
313 vacant: impl FnOnce() -> Result<V, E>
320 bug!("The CTFE machine shouldn't ever need to extend the alloc_map when reading")
329 vacant: impl FnOnce() -> Result<V, E>
330 ) -> Result<&mut V, E>
332 match self.entry(k) {
333 Entry::Occupied(e) => Ok(e.into_mut()),
334 Entry::Vacant(e) => {
342 type CompileTimeEvalContext<'a, 'mir, 'tcx> =
343 EvalContext<'a, 'mir, 'tcx, CompileTimeInterpreter<'a, 'mir, 'tcx>>;
345 impl interpret::MayLeak for ! {
347 fn may_leak(self) -> bool {
348 // `self` is uninhabited
353 impl<'a, 'mir, 'tcx> interpret::Machine<'a, 'mir, 'tcx>
354 for CompileTimeInterpreter<'a, 'mir, 'tcx>
356 type MemoryKinds = !;
357 type PointerTag = ();
359 type FrameExtra = ();
360 type MemoryExtra = ();
361 type AllocExtra = ();
363 type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
365 const STATIC_KIND: Option<!> = None; // no copying of statics allowed
368 fn enforce_validity(_ecx: &EvalContext<'a, 'mir, 'tcx, Self>) -> bool {
369 false // for now, we don't enforce validity
373 ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>,
374 instance: ty::Instance<'tcx>,
376 dest: Option<PlaceTy<'tcx>>,
377 ret: Option<mir::BasicBlock>,
378 ) -> EvalResult<'tcx, Option<&'mir mir::Mir<'tcx>>> {
379 debug!("eval_fn_call: {:?}", instance);
380 // Execution might have wandered off into other crates, so we cannot to a stability-
381 // sensitive check here. But we can at least rule out functions that are not const
383 if !ecx.tcx.is_const_fn_raw(instance.def_id()) {
384 // Some functions we support even if they are non-const -- but avoid testing
385 // that for const fn! We certainly do *not* want to actually call the fn
386 // though, so be sure we return here.
387 return if ecx.hook_fn(instance, args, dest)? {
388 ecx.goto_block(ret)?; // fully evaluated and done
391 err!(MachineError(format!("calling non-const function `{}`", instance)))
394 // This is a const fn. Call it.
395 Ok(Some(match ecx.load_mir(instance.def) {
398 if let EvalErrorKind::NoMirFor(ref path) = err.kind {
400 ConstEvalError::NeedsRfc(format!("calling extern function `{}`", path))
410 ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>,
411 instance: ty::Instance<'tcx>,
414 ) -> EvalResult<'tcx> {
415 if ecx.emulate_intrinsic(instance, args, dest)? {
418 // An intrinsic that we do not support
419 let intrinsic_name = &ecx.tcx.item_name(instance.def_id()).as_str()[..];
421 ConstEvalError::NeedsRfc(format!("calling intrinsic `{}`", intrinsic_name)).into()
426 _ecx: &EvalContext<'a, 'mir, 'tcx, Self>,
429 _left_layout: TyLayout<'tcx>,
431 _right_layout: TyLayout<'tcx>,
432 ) -> EvalResult<'tcx, (Scalar, bool)> {
434 ConstEvalError::NeedsRfc("pointer arithmetic or comparison".to_string()).into(),
438 fn find_foreign_static(
440 _tcx: TyCtxtAt<'a, 'tcx, 'tcx>,
442 ) -> EvalResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
443 err!(ReadForeignStatic)
447 fn adjust_static_allocation<'b>(
448 alloc: &'b Allocation,
450 ) -> Cow<'b, Allocation<Self::PointerTag>> {
451 // We do not use a tag so we can just cheaply forward the reference
456 _ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>,
457 _dest: PlaceTy<'tcx>,
458 ) -> EvalResult<'tcx> {
460 ConstEvalError::NeedsRfc("heap allocations via `box` keyword".to_string()).into(),
464 fn before_terminator(ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>) -> EvalResult<'tcx> {
466 let steps = &mut ecx.machine.steps_since_detector_enabled;
473 *steps %= DETECTOR_SNAPSHOT_PERIOD;
479 let span = ecx.frame().span;
480 ecx.machine.loop_detector.observe_and_analyze(
489 fn tag_new_allocation(
490 _ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>,
492 _kind: MemoryKind<Self::MemoryKinds>,
493 ) -> EvalResult<'tcx, Pointer> {
499 _ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>,
500 ) -> EvalResult<'tcx> {
504 /// Called immediately before a stack frame gets popped
507 _ecx: &mut EvalContext<'a, 'mir, 'tcx, Self>,
509 ) -> EvalResult<'tcx> {
514 /// Project to a field of a (variant of a) const
515 pub fn const_field<'a, 'tcx>(
516 tcx: TyCtxt<'a, 'tcx, 'tcx>,
517 param_env: ty::ParamEnv<'tcx>,
518 instance: ty::Instance<'tcx>,
519 variant: Option<VariantIdx>,
521 value: &'tcx ty::Const<'tcx>,
522 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
523 trace!("const_field: {:?}, {:?}, {:?}", instance, field, value);
524 let ecx = mk_eval_cx(tcx, instance, param_env).unwrap();
526 // get the operand again
527 let op = const_to_op(&ecx, value)?;
529 let down = match variant {
531 Some(variant) => ecx.operand_downcast(op, variant)?
534 let field = ecx.operand_field(down, field.index() as u64)?;
535 // and finally move back to the const world, always normalizing because
536 // this is not called for statics.
537 op_to_const(&ecx, field, true)
539 result.map_err(|error| {
540 let err = error_to_const_error(&ecx, error);
541 err.report_as_error(ecx.tcx, "could not access field of constant");
542 ErrorHandled::Reported
546 pub fn const_variant_index<'a, 'tcx>(
547 tcx: TyCtxt<'a, 'tcx, 'tcx>,
548 param_env: ty::ParamEnv<'tcx>,
549 instance: ty::Instance<'tcx>,
550 val: &'tcx ty::Const<'tcx>,
551 ) -> EvalResult<'tcx, VariantIdx> {
552 trace!("const_variant_index: {:?}, {:?}", instance, val);
553 let ecx = mk_eval_cx(tcx, instance, param_env).unwrap();
554 let op = const_to_op(&ecx, val)?;
555 Ok(ecx.read_discriminant(op)?.1)
558 pub fn error_to_const_error<'a, 'mir, 'tcx>(
559 ecx: &EvalContext<'a, 'mir, 'tcx, CompileTimeInterpreter<'a, 'mir, 'tcx>>,
560 mut error: EvalError<'tcx>
561 ) -> ConstEvalErr<'tcx> {
562 error.print_backtrace();
563 let stacktrace = ecx.generate_stacktrace(None);
564 ConstEvalErr { error: error.kind, stacktrace, span: ecx.tcx.span }
567 fn validate_and_turn_into_const<'a, 'tcx>(
568 tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
569 constant: RawConst<'tcx>,
570 key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
571 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
573 let ecx = mk_eval_cx(tcx, cid.instance, key.param_env).unwrap();
575 let op = ecx.raw_const_to_mplace(constant)?.into();
576 // FIXME: Once the visitor infrastructure landed, change validation to
577 // work directly on `MPlaceTy`.
578 let mut ref_tracking = RefTracking::new(op);
579 while let Some((op, path)) = ref_tracking.todo.pop() {
580 ecx.validate_operand(
583 Some(&mut ref_tracking),
584 /* const_mode */ true,
587 // Now that we validated, turn this into a proper constant
588 let def_id = cid.instance.def.def_id();
589 let normalize = tcx.is_static(def_id).is_none() && cid.promoted.is_none();
590 op_to_const(&ecx, op, normalize)
593 val.map_err(|error| {
594 let err = error_to_const_error(&ecx, error);
595 match err.struct_error(ecx.tcx, "it is undefined behavior to use this value") {
597 diag.note("The rules on what exactly is undefined behavior aren't clear, \
598 so this check might be overzealous. Please open an issue on the rust compiler \
599 repository if you believe it should not be considered undefined behavior",
602 ErrorHandled::Reported
609 pub fn const_eval_provider<'a, 'tcx>(
610 tcx: TyCtxt<'a, 'tcx, 'tcx>,
611 key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
612 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
613 // see comment in const_eval_provider for what we're doing here
614 if key.param_env.reveal == Reveal::All {
615 let mut key = key.clone();
616 key.param_env.reveal = Reveal::UserFacing;
617 match tcx.const_eval(key) {
618 // try again with reveal all as requested
619 Err(ErrorHandled::TooGeneric) => {
620 // Promoteds should never be "too generic" when getting evaluated.
621 // They either don't get evaluated, or we are in a monomorphic context
622 assert!(key.value.promoted.is_none());
625 other => return other,
628 tcx.const_eval_raw(key).and_then(|val| {
629 validate_and_turn_into_const(tcx, val, key)
633 pub fn const_eval_raw_provider<'a, 'tcx>(
634 tcx: TyCtxt<'a, 'tcx, 'tcx>,
635 key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
636 ) -> ::rustc::mir::interpret::ConstEvalRawResult<'tcx> {
637 // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
638 // reporting the same error twice here. To resolve this, we check whether we can evaluate the
639 // constant in the more restrictive `Reveal::UserFacing`, which most likely already was
640 // computed. For a large percentage of constants that will already have succeeded. Only
641 // associated constants of generic functions will fail due to not enough monomorphization
642 // information being available.
644 // In case we fail in the `UserFacing` variant, we just do the real computation.
645 if key.param_env.reveal == Reveal::All {
646 let mut key = key.clone();
647 key.param_env.reveal = Reveal::UserFacing;
648 match tcx.const_eval_raw(key) {
649 // try again with reveal all as requested
650 Err(ErrorHandled::TooGeneric) => {},
652 other => return other,
655 // the first trace is for replicating an ice
656 // There's no tracking issue, but the next two lines concatenated link to the discussion on
657 // zulip. It's not really possible to test this, because it doesn't show up in diagnostics
659 // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
660 // subject/anon_const_instance_printing/near/135980032
661 trace!("const eval: {}", key.value.instance);
662 trace!("const eval: {:?}", key);
665 let def_id = cid.instance.def.def_id();
667 if let Some(id) = tcx.hir().as_local_node_id(def_id) {
668 let tables = tcx.typeck_tables_of(def_id);
670 // Do match-check before building MIR
671 if let Err(ErrorReported) = tcx.check_match(def_id) {
672 return Err(ErrorHandled::Reported)
675 if let hir::BodyOwnerKind::Const = tcx.hir().body_owner_kind(id) {
676 tcx.mir_const_qualif(def_id);
679 // Do not continue into miri if typeck errors occurred; it will fail horribly
680 if tables.tainted_by_errors {
681 return Err(ErrorHandled::Reported)
685 let (res, ecx) = eval_body_and_ecx(tcx, cid, None, key.param_env);
686 res.and_then(|place| {
688 alloc_id: place.to_ptr().expect("we allocated this ptr!").alloc_id,
692 let err = error_to_const_error(&ecx, error);
693 // errors in statics are always emitted as fatal errors
694 if tcx.is_static(def_id).is_some() {
695 let reported_err = err.report_as_error(ecx.tcx,
696 "could not evaluate static initializer");
697 // Ensure that if the above error was either `TooGeneric` or `Reported`
698 // an error must be reported.
699 if tcx.sess.err_count() == 0 {
700 tcx.sess.delay_span_bug(err.span,
701 &format!("static eval failure did not emit an error: {:#?}",
705 } else if def_id.is_local() {
706 // constant defined in this crate, we can figure out a lint level!
707 match tcx.describe_def(def_id) {
708 // constants never produce a hard error at the definition site. Anything else is
709 // a backwards compatibility hazard (and will break old versions of winapi for sure)
711 // note that validation may still cause a hard error on this very same constant,
712 // because any code that existed before validation could not have failed validation
713 // thus preventing such a hard error from being a backwards compatibility hazard
714 Some(Def::Const(_)) | Some(Def::AssociatedConst(_)) => {
715 let node_id = tcx.hir().as_local_node_id(def_id).unwrap();
717 tcx.at(tcx.def_span(def_id)),
718 "any use of this value will cause an error",
722 // promoting runtime code is only allowed to error if it references broken constants
723 // any other kind of error will be reported to the user as a deny-by-default lint
724 _ => if let Some(p) = cid.promoted {
725 let span = tcx.optimized_mir(def_id).promoted[p].span;
726 if let EvalErrorKind::ReferencedConstant = err.error {
729 "evaluation of constant expression failed",
734 "reaching this expression at runtime will panic or abort",
735 tcx.hir().as_local_node_id(def_id).unwrap(),
738 // anything else (array lengths, enum initializers, constant patterns) are reported
743 "evaluation of constant value failed",
748 // use of broken constant from other crate
749 err.report_as_error(ecx.tcx, "could not evaluate constant")