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_infer::infer::InferCtxt;
12 use rustc_middle::mir::interpret::ErrorHandled;
14 use rustc_middle::traits::ObligationCause;
15 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
16 use rustc_middle::ty::{self, TyCtxt, TypeVisitable, TypeVisitor};
19 use std::ops::ControlFlow;
21 use crate::traits::ObligationCtxt;
23 /// Check if a given constant can be evaluated.
24 #[instrument(skip(infcx), level = "debug")]
25 pub fn is_const_evaluatable<'tcx>(
26 infcx: &InferCtxt<'tcx>,
28 param_env: ty::ParamEnv<'tcx>,
30 ) -> Result<(), NotConstEvaluatable> {
32 let uv = match ct.kind() {
33 ty::ConstKind::Unevaluated(uv) => uv,
34 // FIXME(generic_const_exprs): this seems wrong but I couldn't find a way to get this to trigger
35 ty::ConstKind::Expr(_) => bug!("unexpected expr in `is_const_evaluatable: {ct:?}"),
36 ty::ConstKind::Param(_)
37 | ty::ConstKind::Bound(_, _)
38 | ty::ConstKind::Placeholder(_)
39 | ty::ConstKind::Value(_)
40 | ty::ConstKind::Error(_) => return Ok(()),
41 ty::ConstKind::Infer(_) => return Err(NotConstEvaluatable::MentionsInfer),
44 if tcx.features().generic_const_exprs {
45 if let Some(ct) = tcx.expand_abstract_consts(ct)? {
46 if satisfied_from_param_env(tcx, infcx, ct, param_env)? {
49 if ct.has_non_region_infer() {
50 return Err(NotConstEvaluatable::MentionsInfer);
51 } else if ct.has_non_region_param() {
52 return Err(NotConstEvaluatable::MentionsParam);
55 let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
57 Err(ErrorHandled::TooGeneric) => Err(NotConstEvaluatable::Error(
61 .delay_span_bug(span, "Missing value for constant, but no error reported?"),
63 Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
67 // FIXME: We should only try to evaluate a given constant here if it is fully concrete
68 // as we don't want to allow things like `[u8; std::mem::size_of::<*mut T>()]`.
70 // We previously did not check this, so we only emit a future compat warning if
71 // const evaluation succeeds and the given constant is still polymorphic for now
72 // and hopefully soon change this to an error.
74 // See #74595 for more details about this.
75 let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
77 // If we're evaluating a generic foreign constant, under a nightly compiler while
78 // the current crate does not enable `feature(generic_const_exprs)`, abort
79 // compilation with a useful error.
80 Err(_) if tcx.sess.is_nightly_build()
81 && let Ok(Some(ac)) = tcx.expand_abstract_consts(ct)
82 && let ty::ConstKind::Expr(_) = ac.kind() =>
86 // Slightly better span than just using `span` alone
87 if span == rustc_span::DUMMY_SP { tcx.def_span(uv.def.did) } else { span },
88 "failed to evaluate generic const expression",
90 .note("the crate this constant originates from uses `#![feature(generic_const_exprs)]`")
91 .span_suggestion_verbose(
93 "consider enabling this feature",
94 "#![feature(generic_const_exprs)]\n",
95 rustc_errors::Applicability::MaybeIncorrect,
100 Err(ErrorHandled::TooGeneric) => {
101 let err = if uv.has_non_region_infer() {
102 NotConstEvaluatable::MentionsInfer
103 } else if uv.has_non_region_param() {
104 NotConstEvaluatable::MentionsParam
106 let guar = infcx.tcx.sess.delay_span_bug(span, format!("Missing value for constant, but no error reported?"));
107 NotConstEvaluatable::Error(guar)
112 Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
118 #[instrument(skip(infcx, tcx), level = "debug")]
119 fn satisfied_from_param_env<'tcx>(
121 infcx: &InferCtxt<'tcx>,
123 param_env: ty::ParamEnv<'tcx>,
124 ) -> Result<bool, NotConstEvaluatable> {
125 // Try to unify with each subtree in the AbstractConst to allow for
126 // `N + 1` being const evaluatable even if theres only a `ConstEvaluatable`
127 // predicate for `(N + 1) * 2`
128 struct Visitor<'a, 'tcx> {
130 param_env: ty::ParamEnv<'tcx>,
132 infcx: &'a InferCtxt<'tcx>,
134 impl<'a, 'tcx> TypeVisitor<'tcx> for Visitor<'a, 'tcx> {
136 fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
137 if let Ok(()) = self.infcx.commit_if_ok(|_| {
138 let ocx = ObligationCtxt::new_in_snapshot(self.infcx);
139 if let Ok(()) = ocx.eq(&ObligationCause::dummy(), self.param_env, c.ty(), self.ct.ty())
140 && let Ok(()) = ocx.eq(&ObligationCause::dummy(), self.param_env, c, self.ct)
141 && ocx.select_all_or_error().is_empty()
149 } else if let ty::ConstKind::Expr(e) = c.kind() {
152 ControlFlow::CONTINUE
157 for pred in param_env.caller_bounds() {
158 match pred.kind().skip_binder() {
159 ty::PredicateKind::ConstEvaluatable(ce) => {
160 let b_ct = tcx.expand_abstract_consts(ce)?.unwrap_or(ce);
162 let mut v = Visitor { ct, infcx, param_env };
163 let result = b_ct.visit_with(&mut v);
165 if let ControlFlow::Break(()) = result {
166 debug!("is_const_evaluatable: abstract_const ~~> ok");
170 _ => {} // don't care