compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

   1 use rustc_hir as hir;
   2 use rustc_index::vec::Idx;
   3 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
   4 use rustc_middle::mir::Field;
   5 use rustc_middle::thir::{FieldPat, Pat, PatKind};
   6 use rustc_middle::ty::print::with_no_trimmed_paths;
   7 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
   8 use rustc_session::lint;
   9 use rustc_span::Span;
  10 use rustc_trait_selection::traits::predicate_for_trait_def;
  11 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  12 use rustc_trait_selection::traits::{self, ObligationCause, PredicateObligation};
  13
  14 use std::cell::Cell;
  15
  16 use super::PatCtxt;
  17
  18 impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
  19     /// Converts an evaluated constant to a pattern (if possible).
  20     /// This means aggregate values (like structs and enums) are converted
  21     /// to a pattern that matches the value (as if you'd compared via structural equality).
  22     #[instrument(level = "debug", skip(self))]
  23     pub(super) fn const_to_pat(
  24         &self,
  25         cv: &'tcx ty::Const<'tcx>,
  26         id: hir::HirId,
  27         span: Span,
  28         mir_structural_match_violation: bool,
  29     ) -> Pat<'tcx> {
  30         let pat = self.tcx.infer_ctxt().enter(|infcx| {
  31             let mut convert = ConstToPat::new(self, id, span, infcx);
  32             convert.to_pat(cv, mir_structural_match_violation)
  33         });
  34
  35         debug!(?pat);
  36         pat
  37     }
  38 }
  39
  40 struct ConstToPat<'a, 'tcx> {
  41     id: hir::HirId,
  42     span: Span,
  43     param_env: ty::ParamEnv<'tcx>,
  44
  45     // This tracks if we emitted some hard error for a given const value, so that
  46     // we will not subsequently issue an irrelevant lint for the same const
  47     // value.
  48     saw_const_match_error: Cell<bool>,
  49
  50     // This tracks if we emitted some diagnostic for a given const value, so that
  51     // we will not subsequently issue an irrelevant lint for the same const
  52     // value.
  53     saw_const_match_lint: Cell<bool>,
  54
  55     // For backcompat we need to keep allowing non-structurally-eq types behind references.
  56     // See also all the `cant-hide-behind` tests.
  57     behind_reference: Cell<bool>,
  58
  59     // inference context used for checking `T: Structural` bounds.
  60     infcx: InferCtxt<'a, 'tcx>,
  61
  62     include_lint_checks: bool,
  63
  64     treat_byte_string_as_slice: bool,
  65 }
  66
  67 mod fallback_to_const_ref {
  68     #[derive(Debug)]
  69     /// This error type signals that we encountered a non-struct-eq situation behind a reference.
  70     /// We bubble this up in order to get back to the reference destructuring and make that emit
  71     /// a const pattern instead of a deref pattern. This allows us to simply call `PartialEq::eq`
  72     /// on such patterns (since that function takes a reference) and not have to jump through any
  73     /// hoops to get a reference to the value.
  74     pub(super) struct FallbackToConstRef(());
  75
  76     pub(super) fn fallback_to_const_ref<'a, 'tcx>(
  77         c2p: &super::ConstToPat<'a, 'tcx>,
  78     ) -> FallbackToConstRef {
  79         assert!(c2p.behind_reference.get());
  80         FallbackToConstRef(())
  81     }
  82 }
  83 use fallback_to_const_ref::{fallback_to_const_ref, FallbackToConstRef};
  84
  85 impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
  86     fn new(
  87         pat_ctxt: &PatCtxt<'_, 'tcx>,
  88         id: hir::HirId,
  89         span: Span,
  90         infcx: InferCtxt<'a, 'tcx>,
  91     ) -> Self {
  92         trace!(?pat_ctxt.typeck_results.hir_owner);
  93         ConstToPat {
  94             id,
  95             span,
  96             infcx,
  97             param_env: pat_ctxt.param_env,
  98             include_lint_checks: pat_ctxt.include_lint_checks,
  99             saw_const_match_error: Cell::new(false),
 100             saw_const_match_lint: Cell::new(false),
 101             behind_reference: Cell::new(false),
 102             treat_byte_string_as_slice: pat_ctxt
 103                 .typeck_results
 104                 .treat_byte_string_as_slice
 105                 .contains(&id.local_id),
 106         }
 107     }
 108
 109     fn tcx(&self) -> TyCtxt<'tcx> {
 110         self.infcx.tcx
 111     }
 112
 113     fn adt_derive_msg(&self, adt_def: &AdtDef) -> String {
 114         let path = self.tcx().def_path_str(adt_def.did);
 115         format!(
 116             "to use a constant of type `{}` in a pattern, \
 117             `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 118             path, path,
 119         )
 120     }
 121
 122     fn search_for_structural_match_violation(&self, ty: Ty<'tcx>) -> Option<String> {
 123         traits::search_for_structural_match_violation(self.span, self.tcx(), ty).map(|non_sm_ty| {
 124             with_no_trimmed_paths(|| match non_sm_ty {
 125                 traits::NonStructuralMatchTy::Adt(adt) => self.adt_derive_msg(adt),
 126                 traits::NonStructuralMatchTy::Dynamic => {
 127                     "trait objects cannot be used in patterns".to_string()
 128                 }
 129                 traits::NonStructuralMatchTy::Opaque => {
 130                     "opaque types cannot be used in patterns".to_string()
 131                 }
 132                 traits::NonStructuralMatchTy::Closure => {
 133                     "closures cannot be used in patterns".to_string()
 134                 }
 135                 traits::NonStructuralMatchTy::Generator => {
 136                     "generators cannot be used in patterns".to_string()
 137                 }
 138                 traits::NonStructuralMatchTy::Param => {
 139                     bug!("use of a constant whose type is a parameter inside a pattern")
 140                 }
 141                 traits::NonStructuralMatchTy::Projection => {
 142                     bug!("use of a constant whose type is a projection inside a pattern")
 143                 }
 144                 traits::NonStructuralMatchTy::Foreign => {
 145                     bug!("use of a value of a foreign type inside a pattern")
 146                 }
 147             })
 148         })
 149     }
 150
 151     fn type_marked_structural(&self, ty: Ty<'tcx>) -> bool {
 152         ty.is_structural_eq_shallow(self.infcx.tcx)
 153     }
 154
 155     fn to_pat(
 156         &mut self,
 157         cv: &'tcx ty::Const<'tcx>,
 158         mir_structural_match_violation: bool,
 159     ) -> Pat<'tcx> {
 160         trace!(self.treat_byte_string_as_slice);
 161         // This method is just a wrapper handling a validity check; the heavy lifting is
 162         // performed by the recursive `recur` method, which is not meant to be
 163         // invoked except by this method.
 164         //
 165         // once indirect_structural_match is a full fledged error, this
 166         // level of indirection can be eliminated
 167
 168         let inlined_const_as_pat = self.recur(cv, mir_structural_match_violation).unwrap();
 169
 170         if self.include_lint_checks && !self.saw_const_match_error.get() {
 171             // If we were able to successfully convert the const to some pat,
 172             // double-check that all types in the const implement `Structural`.
 173
 174             let structural = self.search_for_structural_match_violation(cv.ty);
 175             debug!(
 176                 "search_for_structural_match_violation cv.ty: {:?} returned: {:?}",
 177                 cv.ty, structural
 178             );
 179
 180             // This can occur because const qualification treats all associated constants as
 181             // opaque, whereas `search_for_structural_match_violation` tries to monomorphize them
 182             // before it runs.
 183             //
 184             // FIXME(#73448): Find a way to bring const qualification into parity with
 185             // `search_for_structural_match_violation`.
 186             if structural.is_none() && mir_structural_match_violation {
 187                 warn!("MIR const-checker found novel structural match violation. See #73448.");
 188                 return inlined_const_as_pat;
 189             }
 190
 191             if let Some(msg) = structural {
 192                 if !self.type_may_have_partial_eq_impl(cv.ty) {
 193                     // span_fatal avoids ICE from resolution of non-existent method (rare case).
 194                     self.tcx().sess.span_fatal(self.span, &msg);
 195                 } else if mir_structural_match_violation && !self.saw_const_match_lint.get() {
 196                     self.tcx().struct_span_lint_hir(
 197                         lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 198                         self.id,
 199                         self.span,
 200                         |lint| lint.build(&msg).emit(),
 201                     );
 202                 } else {
 203                     debug!(
 204                         "`search_for_structural_match_violation` found one, but `CustomEq` was \
 205                           not in the qualifs for that `const`"
 206                     );
 207                 }
 208             }
 209         }
 210
 211         inlined_const_as_pat
 212     }
 213
 214     fn type_may_have_partial_eq_impl(&self, ty: Ty<'tcx>) -> bool {
 215         // double-check there even *is* a semantic `PartialEq` to dispatch to.
 216         //
 217         // (If there isn't, then we can safely issue a hard
 218         // error, because that's never worked, due to compiler
 219         // using `PartialEq::eq` in this scenario in the past.)
 220         let partial_eq_trait_id =
 221             self.tcx().require_lang_item(hir::LangItem::PartialEq, Some(self.span));
 222         let obligation: PredicateObligation<'_> = predicate_for_trait_def(
 223             self.tcx(),
 224             self.param_env,
 225             ObligationCause::misc(self.span, self.id),
 226             partial_eq_trait_id,
 227             0,
 228             ty,
 229             &[],
 230         );
 231         // FIXME: should this call a `predicate_must_hold` variant instead?
 232
 233         let has_impl = self.infcx.predicate_may_hold(&obligation);
 234
 235         // Note: To fix rust-lang/rust#65466, we could just remove this type
 236         // walk hack for function pointers, and unconditionally error
 237         // if `PartialEq` is not implemented. However, that breaks stable
 238         // code at the moment, because types like `for <'a> fn(&'a ())` do
 239         // not *yet* implement `PartialEq`. So for now we leave this here.
 240         has_impl
 241             || ty.walk().any(|t| match t.unpack() {
 242                 ty::subst::GenericArgKind::Lifetime(_) => false,
 243                 ty::subst::GenericArgKind::Type(t) => t.is_fn_ptr(),
 244                 ty::subst::GenericArgKind::Const(_) => false,
 245             })
 246     }
 247
 248     fn field_pats(
 249         &self,
 250         vals: impl Iterator<Item = &'tcx ty::Const<'tcx>>,
 251     ) -> Result<Vec<FieldPat<'tcx>>, FallbackToConstRef> {
 252         vals.enumerate()
 253             .map(|(idx, val)| {
 254                 let field = Field::new(idx);
 255                 Ok(FieldPat { field, pattern: self.recur(val, false)? })
 256             })
 257             .collect()
 258     }
 259
 260     // Recursive helper for `to_pat`; invoke that (instead of calling this directly).
 261     fn recur(
 262         &self,
 263         cv: &'tcx ty::Const<'tcx>,
 264         mir_structural_match_violation: bool,
 265     ) -> Result<Pat<'tcx>, FallbackToConstRef> {
 266         let id = self.id;
 267         let span = self.span;
 268         let tcx = self.tcx();
 269         let param_env = self.param_env;
 270
 271         let kind = match cv.ty.kind() {
 272             ty::Float(_) => {
 273                 if self.include_lint_checks {
 274                     tcx.struct_span_lint_hir(
 275                         lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
 276                         id,
 277                         span,
 278                         |lint| lint.build("floating-point types cannot be used in patterns").emit(),
 279                     );
 280                 }
 281                 PatKind::Constant { value: cv }
 282             }
 283             ty::Adt(adt_def, _) if adt_def.is_union() => {
 284                 // Matching on union fields is unsafe, we can't hide it in constants
 285                 self.saw_const_match_error.set(true);
 286                 let msg = "cannot use unions in constant patterns";
 287                 if self.include_lint_checks {
 288                     tcx.sess.span_err(span, msg);
 289                 } else {
 290                     tcx.sess.delay_span_bug(span, msg)
 291                 }
 292                 PatKind::Wild
 293             }
 294             ty::Adt(..)
 295                 if !self.type_may_have_partial_eq_impl(cv.ty)
 296                     // FIXME(#73448): Find a way to bring const qualification into parity with
 297                     // `search_for_structural_match_violation` and then remove this condition.
 298                     && self.search_for_structural_match_violation(cv.ty).is_some() =>
 299             {
 300                 // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
 301                 // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
 302                 let msg = self.search_for_structural_match_violation(cv.ty).unwrap();
 303                 self.saw_const_match_error.set(true);
 304                 if self.include_lint_checks {
 305                     tcx.sess.span_err(self.span, &msg);
 306                 } else {
 307                     tcx.sess.delay_span_bug(self.span, &msg)
 308                 }
 309                 PatKind::Wild
 310             }
 311             // If the type is not structurally comparable, just emit the constant directly,
 312             // causing the pattern match code to treat it opaquely.
 313             // FIXME: This code doesn't emit errors itself, the caller emits the errors.
 314             // So instead of specific errors, you just get blanket errors about the whole
 315             // const type. See
 316             // https://github.com/rust-lang/rust/pull/70743#discussion_r404701963 for
 317             // details.
 318             // Backwards compatibility hack because we can't cause hard errors on these
 319             // types, so we compare them via `PartialEq::eq` at runtime.
 320             ty::Adt(..) if !self.type_marked_structural(cv.ty) && self.behind_reference.get() => {
 321                 if self.include_lint_checks
 322                     && !self.saw_const_match_error.get()
 323                     && !self.saw_const_match_lint.get()
 324                 {
 325                     self.saw_const_match_lint.set(true);
 326                     tcx.struct_span_lint_hir(
 327                         lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 328                         id,
 329                         span,
 330                         |lint| {
 331                             let msg = format!(
 332                                 "to use a constant of type `{}` in a pattern, \
 333                                  `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 334                                 cv.ty, cv.ty,
 335                             );
 336                             lint.build(&msg).emit()
 337                         },
 338                     );
 339                 }
 340                 // Since we are behind a reference, we can just bubble the error up so we get a
 341                 // constant at reference type, making it easy to let the fallback call
 342                 // `PartialEq::eq` on it.
 343                 return Err(fallback_to_const_ref(self));
 344             }
 345             ty::Adt(adt_def, _) if !self.type_marked_structural(cv.ty) => {
 346                 debug!("adt_def {:?} has !type_marked_structural for cv.ty: {:?}", adt_def, cv.ty);
 347                 let path = tcx.def_path_str(adt_def.did);
 348                 let msg = format!(
 349                     "to use a constant of type `{}` in a pattern, \
 350                      `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 351                     path, path,
 352                 );
 353                 self.saw_const_match_error.set(true);
 354                 if self.include_lint_checks {
 355                     tcx.sess.span_err(span, &msg);
 356                 } else {
 357                     tcx.sess.delay_span_bug(span, &msg)
 358                 }
 359                 PatKind::Wild
 360             }
 361             ty::Adt(adt_def, substs) if adt_def.is_enum() => {
 362                 let destructured = tcx.destructure_const(param_env.and(cv));
 363                 PatKind::Variant {
 364                     adt_def,
 365                     substs,
 366                     variant_index: destructured
 367                         .variant
 368                         .expect("destructed const of adt without variant id"),
 369                     subpatterns: self.field_pats(destructured.fields.iter().copied())?,
 370                 }
 371             }
 372             ty::Tuple(_) | ty::Adt(_, _) => {
 373                 let destructured = tcx.destructure_const(param_env.and(cv));
 374                 PatKind::Leaf { subpatterns: self.field_pats(destructured.fields.iter().copied())? }
 375             }
 376             ty::Array(..) => PatKind::Array {
 377                 prefix: tcx
 378                     .destructure_const(param_env.and(cv))
 379                     .fields
 380                     .iter()
 381                     .map(|val| self.recur(val, false))
 382                     .collect::<Result<_, _>>()?,
 383                 slice: None,
 384                 suffix: Vec::new(),
 385             },
 386             ty::Ref(_, pointee_ty, ..) => match *pointee_ty.kind() {
 387                 // These are not allowed and will error elsewhere anyway.
 388                 ty::Dynamic(..) => {
 389                     self.saw_const_match_error.set(true);
 390                     let msg = format!("`{}` cannot be used in patterns", cv.ty);
 391                     if self.include_lint_checks {
 392                         tcx.sess.span_err(span, &msg);
 393                     } else {
 394                         tcx.sess.delay_span_bug(span, &msg)
 395                     }
 396                     PatKind::Wild
 397                 }
 398                 // `&str` is represented as `ConstValue::Slice`, let's keep using this
 399                 // optimization for now.
 400                 ty::Str => PatKind::Constant { value: cv },
 401                 // `b"foo"` produces a `&[u8; 3]`, but you can't use constants of array type when
 402                 // matching against references, you can only use byte string literals.
 403                 // The typechecker has a special case for byte string literals, by treating them
 404                 // as slices. This means we turn `&[T; N]` constants into slice patterns, which
 405                 // has no negative effects on pattern matching, even if we're actually matching on
 406                 // arrays.
 407                 ty::Array(..) if !self.treat_byte_string_as_slice => {
 408                     let old = self.behind_reference.replace(true);
 409                     let array = tcx.deref_const(self.param_env.and(cv));
 410                     let val = PatKind::Deref {
 411                         subpattern: Pat {
 412                             kind: Box::new(PatKind::Array {
 413                                 prefix: tcx
 414                                     .destructure_const(param_env.and(array))
 415                                     .fields
 416                                     .iter()
 417                                     .map(|val| self.recur(val, false))
 418                                     .collect::<Result<_, _>>()?,
 419                                 slice: None,
 420                                 suffix: vec![],
 421                             }),
 422                             span,
 423                             ty: pointee_ty,
 424                         },
 425                     };
 426                     self.behind_reference.set(old);
 427                     val
 428                 }
 429                 ty::Array(elem_ty, _) |
 430                 // Cannot merge this with the catch all branch below, because the `const_deref`
 431                 // changes the type from slice to array, we need to keep the original type in the
 432                 // pattern.
 433                 ty::Slice(elem_ty) => {
 434                     let old = self.behind_reference.replace(true);
 435                     let array = tcx.deref_const(self.param_env.and(cv));
 436                     let val = PatKind::Deref {
 437                         subpattern: Pat {
 438                             kind: Box::new(PatKind::Slice {
 439                                 prefix: tcx
 440                                     .destructure_const(param_env.and(array))
 441                                     .fields
 442                                     .iter()
 443                                     .map(|val| self.recur(val, false))
 444                                     .collect::<Result<_, _>>()?,
 445                                 slice: None,
 446                                 suffix: vec![],
 447                             }),
 448                             span,
 449                             ty: tcx.mk_slice(elem_ty),
 450                         },
 451                     };
 452                     self.behind_reference.set(old);
 453                     val
 454                 }
 455                 // Backwards compatibility hack: support references to non-structural types.
 456                 // We'll lower
 457                 // this pattern to a `PartialEq::eq` comparison and `PartialEq::eq` takes a
 458                 // reference. This makes the rest of the matching logic simpler as it doesn't have
 459                 // to figure out how to get a reference again.
 460                 ty::Adt(adt_def, _) if !self.type_marked_structural(pointee_ty) => {
 461                     if self.behind_reference.get() {
 462                         if self.include_lint_checks
 463                             && !self.saw_const_match_error.get()
 464                             && !self.saw_const_match_lint.get()
 465                         {
 466                             self.saw_const_match_lint.set(true);
 467                             let msg = self.adt_derive_msg(adt_def);
 468                             self.tcx().struct_span_lint_hir(
 469                                 lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 470                                 self.id,
 471                                 self.span,
 472                                 |lint| lint.build(&msg).emit(),
 473                             );
 474                         }
 475                         PatKind::Constant { value: cv }
 476                     } else {
 477                         if !self.saw_const_match_error.get() {
 478                             self.saw_const_match_error.set(true);
 479                             let msg = self.adt_derive_msg(adt_def);
 480                             if self.include_lint_checks {
 481                                 tcx.sess.span_err(span, &msg);
 482                             } else {
 483                                 tcx.sess.delay_span_bug(span, &msg)
 484                             }
 485                         }
 486                         PatKind::Wild
 487                     }
 488                 }
 489                 // All other references are converted into deref patterns and then recursively
 490                 // convert the dereferenced constant to a pattern that is the sub-pattern of the
 491                 // deref pattern.
 492                 _ => {
 493                     if !pointee_ty.is_sized(tcx.at(span), param_env) {
 494                         // `tcx.deref_const()` below will ICE with an unsized type
 495                         // (except slices, which are handled in a separate arm above).
 496                         let msg = format!("cannot use unsized non-slice type `{}` in constant patterns", pointee_ty);
 497                         if self.include_lint_checks {
 498                             tcx.sess.span_err(span, &msg);
 499                         } else {
 500                             tcx.sess.delay_span_bug(span, &msg);
 501                         }
 502                         PatKind::Wild
 503                     } else {
 504                         let old = self.behind_reference.replace(true);
 505                         // In case there are structural-match violations somewhere in this subpattern,
 506                         // we fall back to a const pattern. If we do not do this, we may end up with
 507                         // a !structural-match constant that is not of reference type, which makes it
 508                         // very hard to invoke `PartialEq::eq` on it as a fallback.
 509                         let val = match self.recur(tcx.deref_const(self.param_env.and(cv)), false) {
 510                             Ok(subpattern) => PatKind::Deref { subpattern },
 511                             Err(_) => PatKind::Constant { value: cv },
 512                         };
 513                         self.behind_reference.set(old);
 514                         val
 515                     }
 516                 }
 517             },
 518             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::FnDef(..) => {
 519                 PatKind::Constant { value: cv }
 520             }
 521             ty::RawPtr(pointee) if pointee.ty.is_sized(tcx.at(span), param_env) => {
 522                 PatKind::Constant { value: cv }
 523             }
 524             // FIXME: these can have very suprising behaviour where optimization levels or other
 525             // compilation choices change the runtime behaviour of the match.
 526             // See https://github.com/rust-lang/rust/issues/70861 for examples.
 527             ty::FnPtr(..) | ty::RawPtr(..) => {
 528                 if self.include_lint_checks
 529                     && !self.saw_const_match_error.get()
 530                     && !self.saw_const_match_lint.get()
 531                 {
 532                     self.saw_const_match_lint.set(true);
 533                     let msg = "function pointers and unsized pointers in patterns behave \
 534                         unpredictably and should not be relied upon. \
 535                         See https://github.com/rust-lang/rust/issues/70861 for details.";
 536                     tcx.struct_span_lint_hir(
 537                         lint::builtin::POINTER_STRUCTURAL_MATCH,
 538                         id,
 539                         span,
 540                         |lint| lint.build(&msg).emit(),
 541                     );
 542                 }
 543                 PatKind::Constant { value: cv }
 544             }
 545             _ => {
 546                 self.saw_const_match_error.set(true);
 547                 let msg = format!("`{}` cannot be used in patterns", cv.ty);
 548                 if self.include_lint_checks {
 549                     tcx.sess.span_err(span, &msg);
 550                 } else {
 551                     tcx.sess.delay_span_bug(span, &msg)
 552                 }
 553                 PatKind::Wild
 554             }
 555         };
 556
 557         if self.include_lint_checks
 558             && !self.saw_const_match_error.get()
 559             && !self.saw_const_match_lint.get()
 560             && mir_structural_match_violation
 561             // FIXME(#73448): Find a way to bring const qualification into parity with
 562             // `search_for_structural_match_violation` and then remove this condition.
 563             && self.search_for_structural_match_violation(cv.ty).is_some()
 564         {
 565             self.saw_const_match_lint.set(true);
 566             // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
 567             // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
 568             let msg = self.search_for_structural_match_violation(cv.ty).unwrap().replace(
 569                 "in a pattern,",
 570                 "in a pattern, the constant's initializer must be trivial or",
 571             );
 572             tcx.struct_span_lint_hir(
 573                 lint::builtin::NONTRIVIAL_STRUCTURAL_MATCH,
 574                 id,
 575                 span,
 576                 |lint| lint.build(&msg).emit(),
 577             );
 578         }
 579
 580         Ok(Pat { span, ty: cv.ty, kind: Box::new(kind) })
 581     }
 582 }