1 //! Checking that constant values used in types can be successfully evaluated.
3 //! For concrete constants, this is fairly simple as we can just try and evaluate it.
5 //! When dealing with polymorphic constants, for example `std::mem::size_of::<T>() - 1`,
6 //! this is not as easy.
8 //! In this case we try to build an abstract representation of this constant using
9 //! `thir_abstract_const` which can then be checked for structural equality with other
10 //! generic constants mentioned in the `caller_bounds` of the current environment.
11 use rustc_hir::def::DefKind;
12 use rustc_infer::infer::InferCtxt;
13 use rustc_middle::mir::interpret::ErrorHandled;
15 use rustc_middle::traits::ObligationCause;
16 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
17 use rustc_middle::ty::{self, TyCtxt, TypeVisitable, TypeVisitor};
20 use std::ops::ControlFlow;
22 use crate::traits::ObligationCtxt;
24 /// Check if a given constant can be evaluated.
25 #[instrument(skip(infcx), level = "debug")]
26 pub fn is_const_evaluatable<'tcx>(
27 infcx: &InferCtxt<'tcx>,
29 param_env: ty::ParamEnv<'tcx>,
31 ) -> Result<(), NotConstEvaluatable> {
33 let uv = match ct.kind() {
34 ty::ConstKind::Unevaluated(uv) => uv,
35 // FIXME(generic_const_exprs): this seems wrong but I couldn't find a way to get this to trigger
36 ty::ConstKind::Expr(_) => bug!("unexpected expr in `is_const_evaluatable: {ct:?}"),
37 ty::ConstKind::Param(_)
38 | ty::ConstKind::Bound(_, _)
39 | ty::ConstKind::Placeholder(_)
40 | ty::ConstKind::Value(_)
41 | ty::ConstKind::Error(_) => return Ok(()),
42 ty::ConstKind::Infer(_) => return Err(NotConstEvaluatable::MentionsInfer),
45 if tcx.features().generic_const_exprs {
46 let ct = tcx.expand_abstract_consts(ct);
48 let is_anon_ct = if let ty::ConstKind::Unevaluated(uv) = ct.kind() {
49 tcx.def_kind(uv.def.did) == DefKind::AnonConst
55 if satisfied_from_param_env(tcx, infcx, ct, param_env) {
58 if ct.has_non_region_infer() {
59 return Err(NotConstEvaluatable::MentionsInfer);
60 } else if ct.has_non_region_param() {
61 return Err(NotConstEvaluatable::MentionsParam);
65 let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
67 Err(ErrorHandled::TooGeneric) => Err(NotConstEvaluatable::Error(
71 .delay_span_bug(span, "Missing value for constant, but no error reported?"),
73 Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
77 // FIXME: We should only try to evaluate a given constant here if it is fully concrete
78 // as we don't want to allow things like `[u8; std::mem::size_of::<*mut T>()]`.
80 // We previously did not check this, so we only emit a future compat warning if
81 // const evaluation succeeds and the given constant is still polymorphic for now
82 // and hopefully soon change this to an error.
84 // See #74595 for more details about this.
85 let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
87 // If we're evaluating a generic foreign constant, under a nightly compiler while
88 // the current crate does not enable `feature(generic_const_exprs)`, abort
89 // compilation with a useful error.
91 if tcx.sess.is_nightly_build()
92 && satisfied_from_param_env(
95 tcx.expand_abstract_consts(ct),
101 // Slightly better span than just using `span` alone
102 if span == rustc_span::DUMMY_SP { tcx.def_span(uv.def.did) } else { span },
103 "failed to evaluate generic const expression",
105 .note("the crate this constant originates from uses `#![feature(generic_const_exprs)]`")
106 .span_suggestion_verbose(
107 rustc_span::DUMMY_SP,
108 "consider enabling this feature",
109 "#![feature(generic_const_exprs)]\n",
110 rustc_errors::Applicability::MaybeIncorrect,
115 Err(ErrorHandled::TooGeneric) => {
116 let err = if uv.has_non_region_infer() {
117 NotConstEvaluatable::MentionsInfer
118 } else if uv.has_non_region_param() {
119 NotConstEvaluatable::MentionsParam
121 let guar = infcx.tcx.sess.delay_span_bug(
123 format!("Missing value for constant, but no error reported?"),
125 NotConstEvaluatable::Error(guar)
130 Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
136 #[instrument(skip(infcx, tcx), level = "debug")]
137 fn satisfied_from_param_env<'tcx>(
139 infcx: &InferCtxt<'tcx>,
141 param_env: ty::ParamEnv<'tcx>,
143 // Try to unify with each subtree in the AbstractConst to allow for
144 // `N + 1` being const evaluatable even if theres only a `ConstEvaluatable`
145 // predicate for `(N + 1) * 2`
146 struct Visitor<'a, 'tcx> {
148 param_env: ty::ParamEnv<'tcx>,
150 infcx: &'a InferCtxt<'tcx>,
152 impl<'a, 'tcx> TypeVisitor<'tcx> for Visitor<'a, 'tcx> {
154 fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
155 if let Ok(()) = self.infcx.commit_if_ok(|_| {
156 let ocx = ObligationCtxt::new_in_snapshot(self.infcx);
157 if let Ok(()) = ocx.eq(&ObligationCause::dummy(), self.param_env, c.ty(), self.ct.ty())
158 && let Ok(()) = ocx.eq(&ObligationCause::dummy(), self.param_env, c, self.ct)
159 && ocx.select_all_or_error().is_empty()
167 } else if let ty::ConstKind::Expr(e) = c.kind() {
170 // FIXME(generic_const_exprs): This doesn't recurse into `<T as Trait<U>>::ASSOC`'s substs.
171 // This is currently unobservable as `<T as Trait<{ U + 1 }>>::ASSOC` creates an anon const
172 // with its own `ConstEvaluatable` bound in the param env which we will visit separately.
174 // If we start allowing directly writing `ConstKind::Expr` without an intermediate anon const
175 // this will be incorrect. It might be worth investigating making `predicates_of` elaborate
176 // all of the `ConstEvaluatable` bounds rather than having a visitor here.
177 ControlFlow::CONTINUE
182 for pred in param_env.caller_bounds() {
183 match pred.kind().skip_binder() {
184 ty::PredicateKind::ConstEvaluatable(ce) => {
185 let b_ct = tcx.expand_abstract_consts(ce);
186 let mut v = Visitor { ct, infcx, param_env };
187 let result = b_ct.visit_with(&mut v);
189 if let ControlFlow::Break(()) = result {
190 debug!("is_const_evaluatable: abstract_const ~~> ok");
194 _ => {} // don't care