compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

   1 //! Checking that constant values used in types can be successfully evaluated.
   2 //!
   3 //! For concrete constants, this is fairly simple as we can just try and evaluate it.
   4 //!
   5 //! When dealing with polymorphic constants, for example `std::mem::size_of::<T>() - 1`,
   6 //! this is not as easy.
   7 //!
   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_errors::ErrorGuaranteed;
  12 use rustc_hir::def::DefKind;
  13 use rustc_infer::infer::InferCtxt;
  14 use rustc_middle::mir::interpret::ErrorHandled;
  15 use rustc_middle::ty::abstract_const::{
  16     walk_abstract_const, AbstractConst, FailureKind, Node, NotConstEvaluatable,
  17 };
  18 use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
  19 use rustc_session::lint;
  20 use rustc_span::Span;
  21
  22 use std::iter;
  23 use std::ops::ControlFlow;
  24
  25 pub struct ConstUnifyCtxt<'tcx> {
  26     pub tcx: TyCtxt<'tcx>,
  27     pub param_env: ty::ParamEnv<'tcx>,
  28 }
  29
  30 impl<'tcx> ConstUnifyCtxt<'tcx> {
  31     // Substitutes generics repeatedly to allow AbstractConsts to unify where a
  32     // ConstKind::Unevaluated could be turned into an AbstractConst that would unify e.g.
  33     // Param(N) should unify with Param(T), substs: [Unevaluated("T2", [Unevaluated("T3", [Param(N)])])]
  34     #[inline]
  35     #[instrument(skip(self), level = "debug")]
  36     fn try_replace_substs_in_root(
  37         &self,
  38         mut abstr_const: AbstractConst<'tcx>,
  39     ) -> Option<AbstractConst<'tcx>> {
  40         while let Node::Leaf(ct) = abstr_const.root(self.tcx) {
  41             match AbstractConst::from_const(self.tcx, ct) {
  42                 Ok(Some(act)) => abstr_const = act,
  43                 Ok(None) => break,
  44                 Err(_) => return None,
  45             }
  46         }
  47
  48         Some(abstr_const)
  49     }
  50
  51     /// Tries to unify two abstract constants using structural equality.
  52     #[instrument(skip(self), level = "debug")]
  53     pub fn try_unify(&self, a: AbstractConst<'tcx>, b: AbstractConst<'tcx>) -> bool {
  54         let a = if let Some(a) = self.try_replace_substs_in_root(a) {
  55             a
  56         } else {
  57             return true;
  58         };
  59
  60         let b = if let Some(b) = self.try_replace_substs_in_root(b) {
  61             b
  62         } else {
  63             return true;
  64         };
  65
  66         let a_root = a.root(self.tcx);
  67         let b_root = b.root(self.tcx);
  68         debug!(?a_root, ?b_root);
  69
  70         match (a_root, b_root) {
  71             (Node::Leaf(a_ct), Node::Leaf(b_ct)) => {
  72                 let a_ct = a_ct.eval(self.tcx, self.param_env);
  73                 debug!("a_ct evaluated: {:?}", a_ct);
  74                 let b_ct = b_ct.eval(self.tcx, self.param_env);
  75                 debug!("b_ct evaluated: {:?}", b_ct);
  76
  77                 if a_ct.ty() != b_ct.ty() {
  78                     return false;
  79                 }
  80
  81                 match (a_ct.kind(), b_ct.kind()) {
  82                     // We can just unify errors with everything to reduce the amount of
  83                     // emitted errors here.
  84                     (ty::ConstKind::Error(_), _) | (_, ty::ConstKind::Error(_)) => true,
  85                     (ty::ConstKind::Param(a_param), ty::ConstKind::Param(b_param)) => {
  86                         a_param == b_param
  87                     }
  88                     (ty::ConstKind::Value(a_val), ty::ConstKind::Value(b_val)) => a_val == b_val,
  89                     // If we have `fn a<const N: usize>() -> [u8; N + 1]` and `fn b<const M: usize>() -> [u8; 1 + M]`
  90                     // we do not want to use `assert_eq!(a(), b())` to infer that `N` and `M` have to be `1`. This
  91                     // means that we only allow inference variables if they are equal.
  92                     (ty::ConstKind::Infer(a_val), ty::ConstKind::Infer(b_val)) => a_val == b_val,
  93                     // We expand generic anonymous constants at the start of this function, so this
  94                     // branch should only be taking when dealing with associated constants, at
  95                     // which point directly comparing them seems like the desired behavior.
  96                     //
  97                     // FIXME(generic_const_exprs): This isn't actually the case.
  98                     // We also take this branch for concrete anonymous constants and
  99                     // expand generic anonymous constants with concrete substs.
 100                     (ty::ConstKind::Unevaluated(a_uv), ty::ConstKind::Unevaluated(b_uv)) => {
 101                         a_uv == b_uv
 102                     }
 103                     // FIXME(generic_const_exprs): We may want to either actually try
 104                     // to evaluate `a_ct` and `b_ct` if they are are fully concrete or something like
 105                     // this, for now we just return false here.
 106                     _ => false,
 107                 }
 108             }
 109             (Node::Binop(a_op, al, ar), Node::Binop(b_op, bl, br)) if a_op == b_op => {
 110                 self.try_unify(a.subtree(al), b.subtree(bl))
 111                     && self.try_unify(a.subtree(ar), b.subtree(br))
 112             }
 113             (Node::UnaryOp(a_op, av), Node::UnaryOp(b_op, bv)) if a_op == b_op => {
 114                 self.try_unify(a.subtree(av), b.subtree(bv))
 115             }
 116             (Node::FunctionCall(a_f, a_args), Node::FunctionCall(b_f, b_args))
 117                 if a_args.len() == b_args.len() =>
 118             {
 119                 self.try_unify(a.subtree(a_f), b.subtree(b_f))
 120                     && iter::zip(a_args, b_args)
 121                         .all(|(&an, &bn)| self.try_unify(a.subtree(an), b.subtree(bn)))
 122             }
 123             (Node::Cast(a_kind, a_operand, a_ty), Node::Cast(b_kind, b_operand, b_ty))
 124                 if (a_ty == b_ty) && (a_kind == b_kind) =>
 125             {
 126                 self.try_unify(a.subtree(a_operand), b.subtree(b_operand))
 127             }
 128             // use this over `_ => false` to make adding variants to `Node` less error prone
 129             (Node::Cast(..), _)
 130             | (Node::FunctionCall(..), _)
 131             | (Node::UnaryOp(..), _)
 132             | (Node::Binop(..), _)
 133             | (Node::Leaf(..), _) => false,
 134         }
 135     }
 136 }
 137
 138 #[instrument(skip(tcx), level = "debug")]
 139 pub fn try_unify_abstract_consts<'tcx>(
 140     tcx: TyCtxt<'tcx>,
 141     (a, b): (ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>),
 142     param_env: ty::ParamEnv<'tcx>,
 143 ) -> bool {
 144     (|| {
 145         if let Some(a) = AbstractConst::new(tcx, a)? {
 146             if let Some(b) = AbstractConst::new(tcx, b)? {
 147                 let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
 148                 return Ok(const_unify_ctxt.try_unify(a, b));
 149             }
 150         }
 151
 152         Ok(false)
 153     })()
 154     .unwrap_or_else(|_: ErrorGuaranteed| true)
 155     // FIXME(generic_const_exprs): We should instead have this
 156     // method return the resulting `ty::Const` and return `ConstKind::Error`
 157     // on `ErrorGuaranteed`.
 158 }
 159
 160 /// Check if a given constant can be evaluated.
 161 #[instrument(skip(infcx), level = "debug")]
 162 pub fn is_const_evaluatable<'cx, 'tcx>(
 163     infcx: &InferCtxt<'cx, 'tcx>,
 164     uv: ty::Unevaluated<'tcx, ()>,
 165     param_env: ty::ParamEnv<'tcx>,
 166     span: Span,
 167 ) -> Result<(), NotConstEvaluatable> {
 168     let tcx = infcx.tcx;
 169
 170     if tcx.features().generic_const_exprs {
 171         if let Some(ct) = AbstractConst::new(tcx, uv)? {
 172             if satisfied_from_param_env(tcx, ct, param_env)? {
 173                 return Ok(());
 174             }
 175             match ct.unify_failure_kind(tcx) {
 176                 FailureKind::MentionsInfer => {
 177                     return Err(NotConstEvaluatable::MentionsInfer);
 178                 }
 179                 FailureKind::MentionsParam => {
 180                     return Err(NotConstEvaluatable::MentionsParam);
 181                 }
 182                 // returned below
 183                 FailureKind::Concrete => {}
 184             }
 185         }
 186         let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
 187         match concrete {
 188             Err(ErrorHandled::TooGeneric) => {
 189                 Err(NotConstEvaluatable::Error(infcx.tcx.sess.delay_span_bug(
 190                     span,
 191                     format!("Missing value for constant, but no error reported?"),
 192                 )))
 193             }
 194             Err(ErrorHandled::Linted) => {
 195                 let reported = infcx
 196                     .tcx
 197                     .sess
 198                     .delay_span_bug(span, "constant in type had error reported as lint");
 199                 Err(NotConstEvaluatable::Error(reported))
 200             }
 201             Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
 202             Ok(_) => Ok(()),
 203         }
 204     } else {
 205         // FIXME: We should only try to evaluate a given constant here if it is fully concrete
 206         // as we don't want to allow things like `[u8; std::mem::size_of::<*mut T>()]`.
 207         //
 208         // We previously did not check this, so we only emit a future compat warning if
 209         // const evaluation succeeds and the given constant is still polymorphic for now
 210         // and hopefully soon change this to an error.
 211         //
 212         // See #74595 for more details about this.
 213         let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
 214
 215         match concrete {
 216           // If we're evaluating a foreign constant, under a nightly compiler without generic
 217           // const exprs, AND it would've passed if that expression had been evaluated with
 218           // generic const exprs, then suggest using generic const exprs.
 219           Err(_) if tcx.sess.is_nightly_build()
 220             && let Ok(Some(ct)) = AbstractConst::new(tcx, uv)
 221             && satisfied_from_param_env(tcx, ct, param_env) == Ok(true) => {
 222               tcx.sess
 223                   .struct_span_fatal(
 224                       // Slightly better span than just using `span` alone
 225                       if span == rustc_span::DUMMY_SP { tcx.def_span(uv.def.did) } else { span },
 226                       "failed to evaluate generic const expression",
 227                   )
 228                   .note("the crate this constant originates from uses `#![feature(generic_const_exprs)]`")
 229                   .span_suggestion_verbose(
 230                       rustc_span::DUMMY_SP,
 231                       "consider enabling this feature",
 232                       "#![feature(generic_const_exprs)]\n",
 233                       rustc_errors::Applicability::MaybeIncorrect,
 234                   )
 235                   .emit()
 236             }
 237
 238             Err(ErrorHandled::TooGeneric) => Err(if uv.has_infer_types_or_consts() {
 239                 NotConstEvaluatable::MentionsInfer
 240                 } else if uv.has_param_types_or_consts() {
 241                 NotConstEvaluatable::MentionsParam
 242             } else {
 243                 let guar = infcx.tcx.sess.delay_span_bug(span, format!("Missing value for constant, but no error reported?"));
 244                 NotConstEvaluatable::Error(guar)
 245             }),
 246             Err(ErrorHandled::Linted) => {
 247                 let reported =
 248                     infcx.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 249                 Err(NotConstEvaluatable::Error(reported))
 250             }
 251             Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
 252             Ok(_) => {
 253               if uv.substs.has_param_types_or_consts() {
 254                   assert!(matches!(infcx.tcx.def_kind(uv.def.did), DefKind::AnonConst));
 255                   let mir_body = infcx.tcx.mir_for_ctfe_opt_const_arg(uv.def);
 256
 257                   if mir_body.is_polymorphic {
 258                       let Some(local_def_id) = uv.def.did.as_local() else { return Ok(()) };
 259                       tcx.struct_span_lint_hir(
 260                           lint::builtin::CONST_EVALUATABLE_UNCHECKED,
 261                           tcx.hir().local_def_id_to_hir_id(local_def_id),
 262                           span,
 263                           |err| {
 264                               err.build("cannot use constants which depend on generic parameters in types").emit();
 265                         })
 266                   }
 267               }
 268
 269               Ok(())
 270             },
 271         }
 272     }
 273 }
 274
 275 #[instrument(skip(tcx), level = "debug")]
 276 fn satisfied_from_param_env<'tcx>(
 277     tcx: TyCtxt<'tcx>,
 278     ct: AbstractConst<'tcx>,
 279     param_env: ty::ParamEnv<'tcx>,
 280 ) -> Result<bool, NotConstEvaluatable> {
 281     for pred in param_env.caller_bounds() {
 282         match pred.kind().skip_binder() {
 283             ty::PredicateKind::ConstEvaluatable(uv) => {
 284                 if let Some(b_ct) = AbstractConst::new(tcx, uv)? {
 285                     let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
 286
 287                     // Try to unify with each subtree in the AbstractConst to allow for
 288                     // `N + 1` being const evaluatable even if theres only a `ConstEvaluatable`
 289                     // predicate for `(N + 1) * 2`
 290                     let result = walk_abstract_const(tcx, b_ct, |b_ct| {
 291                         match const_unify_ctxt.try_unify(ct, b_ct) {
 292                             true => ControlFlow::BREAK,
 293                             false => ControlFlow::CONTINUE,
 294                         }
 295                     });
 296
 297                     if let ControlFlow::Break(()) = result {
 298                         debug!("is_const_evaluatable: abstract_const ~~> ok");
 299                         return Ok(true);
 300                     }
 301                 }
 302             }
 303             _ => {} // don't care
 304         }
 305     }
 306
 307     Ok(false)
 308 }