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