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::{self, 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: mir::ConstantKind<'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<'tcx>) -> 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.kind() {
 125                 ty::Adt(adt, _) => self.adt_derive_msg(*adt),
 126                 ty::Dynamic(..) => {
 127                     "trait objects cannot be used in patterns".to_string()
 128                 }
 129                 ty::Opaque(..) => {
 130                     "opaque types cannot be used in patterns".to_string()
 131                 }
 132                 ty::Closure(..) => {
 133                     "closures cannot be used in patterns".to_string()
 134                 }
 135                 ty::Generator(..) | ty::GeneratorWitness(..) => {
 136                     "generators cannot be used in patterns".to_string()
 137                 }
 138                 ty::Float(..) => {
 139                     "floating-point numbers cannot be used in patterns".to_string()
 140                 }
 141                 ty::FnPtr(..) => {
 142                     "function pointers cannot be used in patterns".to_string()
 143                 }
 144                 ty::RawPtr(..) => {
 145                     "raw pointers cannot be used in patterns".to_string()
 146                 }
 147                 _ => {
 148                     bug!("use of a value of `{non_sm_ty}` inside a pattern")
 149                 }
 150             })
 151         })
 152     }
 153
 154     fn type_marked_structural(&self, ty: Ty<'tcx>) -> bool {
 155         ty.is_structural_eq_shallow(self.infcx.tcx)
 156     }
 157
 158     fn to_pat(
 159         &mut self,
 160         cv: mir::ConstantKind<'tcx>,
 161         mir_structural_match_violation: bool,
 162     ) -> Pat<'tcx> {
 163         trace!(self.treat_byte_string_as_slice);
 164         // This method is just a wrapper handling a validity check; the heavy lifting is
 165         // performed by the recursive `recur` method, which is not meant to be
 166         // invoked except by this method.
 167         //
 168         // once indirect_structural_match is a full fledged error, this
 169         // level of indirection can be eliminated
 170
 171         let inlined_const_as_pat = self.recur(cv, mir_structural_match_violation).unwrap();
 172
 173         if self.include_lint_checks && !self.saw_const_match_error.get() {
 174             // If we were able to successfully convert the const to some pat,
 175             // double-check that all types in the const implement `Structural`.
 176
 177             let structural = self.search_for_structural_match_violation(cv.ty());
 178             debug!(
 179                 "search_for_structural_match_violation cv.ty: {:?} returned: {:?}",
 180                 cv.ty(),
 181                 structural
 182             );
 183
 184             // This can occur because const qualification treats all associated constants as
 185             // opaque, whereas `search_for_structural_match_violation` tries to monomorphize them
 186             // before it runs.
 187             //
 188             // FIXME(#73448): Find a way to bring const qualification into parity with
 189             // `search_for_structural_match_violation`.
 190             if structural.is_none() && mir_structural_match_violation {
 191                 warn!("MIR const-checker found novel structural match violation. See #73448.");
 192                 return inlined_const_as_pat;
 193             }
 194
 195             if let Some(msg) = structural {
 196                 if !self.type_may_have_partial_eq_impl(cv.ty()) {
 197                     // span_fatal avoids ICE from resolution of non-existent method (rare case).
 198                     self.tcx().sess.span_fatal(self.span, &msg);
 199                 } else if mir_structural_match_violation && !self.saw_const_match_lint.get() {
 200                     self.tcx().struct_span_lint_hir(
 201                         lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 202                         self.id,
 203                         self.span,
 204                         |lint| {
 205                             lint.build(&msg).emit();
 206                         },
 207                     );
 208                 } else {
 209                     debug!(
 210                         "`search_for_structural_match_violation` found one, but `CustomEq` was \
 211                           not in the qualifs for that `const`"
 212                     );
 213                 }
 214             }
 215         }
 216
 217         inlined_const_as_pat
 218     }
 219
 220     fn type_may_have_partial_eq_impl(&self, ty: Ty<'tcx>) -> bool {
 221         // double-check there even *is* a semantic `PartialEq` to dispatch to.
 222         //
 223         // (If there isn't, then we can safely issue a hard
 224         // error, because that's never worked, due to compiler
 225         // using `PartialEq::eq` in this scenario in the past.)
 226         let partial_eq_trait_id =
 227             self.tcx().require_lang_item(hir::LangItem::PartialEq, Some(self.span));
 228         let obligation: PredicateObligation<'_> = predicate_for_trait_def(
 229             self.tcx(),
 230             self.param_env,
 231             ObligationCause::misc(self.span, self.id),
 232             partial_eq_trait_id,
 233             0,
 234             ty,
 235             &[],
 236         );
 237         // FIXME: should this call a `predicate_must_hold` variant instead?
 238
 239         let has_impl = self.infcx.predicate_may_hold(&obligation);
 240
 241         // Note: To fix rust-lang/rust#65466, we could just remove this type
 242         // walk hack for function pointers, and unconditionally error
 243         // if `PartialEq` is not implemented. However, that breaks stable
 244         // code at the moment, because types like `for <'a> fn(&'a ())` do
 245         // not *yet* implement `PartialEq`. So for now we leave this here.
 246         has_impl
 247             || ty.walk().any(|t| match t.unpack() {
 248                 ty::subst::GenericArgKind::Lifetime(_) => false,
 249                 ty::subst::GenericArgKind::Type(t) => t.is_fn_ptr(),
 250                 ty::subst::GenericArgKind::Const(_) => false,
 251             })
 252     }
 253
 254     fn field_pats(
 255         &self,
 256         vals: impl Iterator<Item = mir::ConstantKind<'tcx>>,
 257     ) -> Result<Vec<FieldPat<'tcx>>, FallbackToConstRef> {
 258         vals.enumerate()
 259             .map(|(idx, val)| {
 260                 let field = Field::new(idx);
 261                 Ok(FieldPat { field, pattern: self.recur(val, false)? })
 262             })
 263             .collect()
 264     }
 265
 266     // Recursive helper for `to_pat`; invoke that (instead of calling this directly).
 267     #[instrument(skip(self), level = "debug")]
 268     fn recur(
 269         &self,
 270         cv: mir::ConstantKind<'tcx>,
 271         mir_structural_match_violation: bool,
 272     ) -> Result<Pat<'tcx>, FallbackToConstRef> {
 273         let id = self.id;
 274         let span = self.span;
 275         let tcx = self.tcx();
 276         let param_env = self.param_env;
 277
 278         let kind = match cv.ty().kind() {
 279             ty::Float(_) => {
 280                 if self.include_lint_checks {
 281                     tcx.struct_span_lint_hir(
 282                         lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
 283                         id,
 284                         span,
 285                         |lint| {
 286                             lint.build("floating-point types cannot be used in patterns").emit();
 287                         },
 288                     );
 289                 }
 290                 PatKind::Constant { value: cv }
 291             }
 292             ty::Adt(adt_def, _) if adt_def.is_union() => {
 293                 // Matching on union fields is unsafe, we can't hide it in constants
 294                 self.saw_const_match_error.set(true);
 295                 let msg = "cannot use unions in constant patterns";
 296                 if self.include_lint_checks {
 297                     tcx.sess.span_err(span, msg);
 298                 } else {
 299                     tcx.sess.delay_span_bug(span, msg);
 300                 }
 301                 PatKind::Wild
 302             }
 303             ty::Adt(..)
 304                 if !self.type_may_have_partial_eq_impl(cv.ty())
 305                     // FIXME(#73448): Find a way to bring const qualification into parity with
 306                     // `search_for_structural_match_violation` and then remove this condition.
 307                     && self.search_for_structural_match_violation(cv.ty()).is_some() =>
 308             {
 309                 // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
 310                 // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
 311                 let msg = self.search_for_structural_match_violation(cv.ty()).unwrap();
 312                 self.saw_const_match_error.set(true);
 313                 if self.include_lint_checks {
 314                     tcx.sess.span_err(self.span, &msg);
 315                 } else {
 316                     tcx.sess.delay_span_bug(self.span, &msg);
 317                 }
 318                 PatKind::Wild
 319             }
 320             // If the type is not structurally comparable, just emit the constant directly,
 321             // causing the pattern match code to treat it opaquely.
 322             // FIXME: This code doesn't emit errors itself, the caller emits the errors.
 323             // So instead of specific errors, you just get blanket errors about the whole
 324             // const type. See
 325             // https://github.com/rust-lang/rust/pull/70743#discussion_r404701963 for
 326             // details.
 327             // Backwards compatibility hack because we can't cause hard errors on these
 328             // types, so we compare them via `PartialEq::eq` at runtime.
 329             ty::Adt(..) if !self.type_marked_structural(cv.ty()) && self.behind_reference.get() => {
 330                 if self.include_lint_checks
 331                     && !self.saw_const_match_error.get()
 332                     && !self.saw_const_match_lint.get()
 333                 {
 334                     self.saw_const_match_lint.set(true);
 335                     tcx.struct_span_lint_hir(
 336                         lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 337                         id,
 338                         span,
 339                         |lint| {
 340                             let msg = format!(
 341                                 "to use a constant of type `{}` in a pattern, \
 342                                  `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 343                                 cv.ty(),
 344                                 cv.ty(),
 345                             );
 346                             lint.build(&msg).emit();
 347                         },
 348                     );
 349                 }
 350                 // Since we are behind a reference, we can just bubble the error up so we get a
 351                 // constant at reference type, making it easy to let the fallback call
 352                 // `PartialEq::eq` on it.
 353                 return Err(fallback_to_const_ref(self));
 354             }
 355             ty::Adt(adt_def, _) if !self.type_marked_structural(cv.ty()) => {
 356                 debug!(
 357                     "adt_def {:?} has !type_marked_structural for cv.ty: {:?}",
 358                     adt_def,
 359                     cv.ty()
 360                 );
 361                 let path = tcx.def_path_str(adt_def.did());
 362                 let msg = format!(
 363                     "to use a constant of type `{}` in a pattern, \
 364                      `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 365                     path, path,
 366                 );
 367                 self.saw_const_match_error.set(true);
 368                 if self.include_lint_checks {
 369                     tcx.sess.span_err(span, &msg);
 370                 } else {
 371                     tcx.sess.delay_span_bug(span, &msg);
 372                 }
 373                 PatKind::Wild
 374             }
 375             ty::Adt(adt_def, substs) if adt_def.is_enum() => {
 376                 let destructured = tcx.destructure_mir_constant(param_env, cv);
 377
 378                 PatKind::Variant {
 379                     adt_def: *adt_def,
 380                     substs,
 381                     variant_index: destructured
 382                         .variant
 383                         .expect("destructed const of adt without variant id"),
 384                     subpatterns: self.field_pats(destructured.fields.iter().copied())?,
 385                 }
 386             }
 387             ty::Tuple(_) | ty::Adt(_, _) => {
 388                 let destructured = tcx.destructure_mir_constant(param_env, cv);
 389                 PatKind::Leaf { subpatterns: self.field_pats(destructured.fields.iter().copied())? }
 390             }
 391             ty::Array(..) => PatKind::Array {
 392                 prefix: tcx
 393                     .destructure_mir_constant(param_env, cv)
 394                     .fields
 395                     .iter()
 396                     .map(|val| self.recur(*val, false))
 397                     .collect::<Result<_, _>>()?,
 398                 slice: None,
 399                 suffix: Vec::new(),
 400             },
 401             ty::Ref(_, pointee_ty, ..) => match *pointee_ty.kind() {
 402                 // These are not allowed and will error elsewhere anyway.
 403                 ty::Dynamic(..) => {
 404                     self.saw_const_match_error.set(true);
 405                     let msg = format!("`{}` cannot be used in patterns", cv.ty());
 406                     if self.include_lint_checks {
 407                         tcx.sess.span_err(span, &msg);
 408                     } else {
 409                         tcx.sess.delay_span_bug(span, &msg);
 410                     }
 411                     PatKind::Wild
 412                 }
 413                 // `&str` is represented as `ConstValue::Slice`, let's keep using this
 414                 // optimization for now.
 415                 ty::Str => PatKind::Constant { value: cv },
 416                 // `b"foo"` produces a `&[u8; 3]`, but you can't use constants of array type when
 417                 // matching against references, you can only use byte string literals.
 418                 // The typechecker has a special case for byte string literals, by treating them
 419                 // as slices. This means we turn `&[T; N]` constants into slice patterns, which
 420                 // has no negative effects on pattern matching, even if we're actually matching on
 421                 // arrays.
 422                 ty::Array(..) if !self.treat_byte_string_as_slice => {
 423                     let old = self.behind_reference.replace(true);
 424                     let array = tcx.deref_mir_constant(self.param_env.and(cv));
 425                     let val = PatKind::Deref {
 426                         subpattern: Pat {
 427                             kind: Box::new(PatKind::Array {
 428                                 prefix: tcx
 429                                     .destructure_mir_constant(param_env, array)
 430                                     .fields
 431                                     .iter()
 432                                     .map(|val| self.recur(*val, false))
 433                                     .collect::<Result<_, _>>()?,
 434                                 slice: None,
 435                                 suffix: vec![],
 436                             }),
 437                             span,
 438                             ty: *pointee_ty,
 439                         },
 440                     };
 441                     self.behind_reference.set(old);
 442                     val
 443                 }
 444                 ty::Array(elem_ty, _) |
 445                 // Cannot merge this with the catch all branch below, because the `const_deref`
 446                 // changes the type from slice to array, we need to keep the original type in the
 447                 // pattern.
 448                 ty::Slice(elem_ty) => {
 449                     let old = self.behind_reference.replace(true);
 450                     let array = tcx.deref_mir_constant(self.param_env.and(cv));
 451                     let val = PatKind::Deref {
 452                         subpattern: Pat {
 453                             kind: Box::new(PatKind::Slice {
 454                                 prefix: tcx
 455                                     .destructure_mir_constant(param_env, array)
 456                                     .fields
 457                                     .iter()
 458                                     .map(|val| self.recur(*val, false))
 459                                     .collect::<Result<_, _>>()?,
 460                                 slice: None,
 461                                 suffix: vec![],
 462                             }),
 463                             span,
 464                             ty: tcx.mk_slice(elem_ty),
 465                         },
 466                     };
 467                     self.behind_reference.set(old);
 468                     val
 469                 }
 470                 // Backwards compatibility hack: support references to non-structural types.
 471                 // We'll lower
 472                 // this pattern to a `PartialEq::eq` comparison and `PartialEq::eq` takes a
 473                 // reference. This makes the rest of the matching logic simpler as it doesn't have
 474                 // to figure out how to get a reference again.
 475                 ty::Adt(adt_def, _) if !self.type_marked_structural(*pointee_ty) => {
 476                     if self.behind_reference.get() {
 477                         if self.include_lint_checks
 478                             && !self.saw_const_match_error.get()
 479                             && !self.saw_const_match_lint.get()
 480                         {
 481                             self.saw_const_match_lint.set(true);
 482                             let msg = self.adt_derive_msg(adt_def);
 483                             self.tcx().struct_span_lint_hir(
 484                                 lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 485                                 self.id,
 486                                 self.span,
 487                                 |lint| {lint.build(&msg).emit();},
 488                             );
 489                         }
 490                         PatKind::Constant { value: cv }
 491                     } else {
 492                         if !self.saw_const_match_error.get() {
 493                             self.saw_const_match_error.set(true);
 494                             let msg = self.adt_derive_msg(adt_def);
 495                             if self.include_lint_checks {
 496                                 tcx.sess.span_err(span, &msg);
 497                             } else {
 498                                 tcx.sess.delay_span_bug(span, &msg);
 499                             }
 500                         }
 501                         PatKind::Wild
 502                     }
 503                 }
 504                 // All other references are converted into deref patterns and then recursively
 505                 // convert the dereferenced constant to a pattern that is the sub-pattern of the
 506                 // deref pattern.
 507                 _ => {
 508                     if !pointee_ty.is_sized(tcx.at(span), param_env) {
 509                         // `tcx.deref_mir_constant()` below will ICE with an unsized type
 510                         // (except slices, which are handled in a separate arm above).
 511                         let msg = format!("cannot use unsized non-slice type `{}` in constant patterns", pointee_ty);
 512                         if self.include_lint_checks {
 513                             tcx.sess.span_err(span, &msg);
 514                         } else {
 515                             tcx.sess.delay_span_bug(span, &msg);
 516                         }
 517                         PatKind::Wild
 518                     } else {
 519                         let old = self.behind_reference.replace(true);
 520                         // In case there are structural-match violations somewhere in this subpattern,
 521                         // we fall back to a const pattern. If we do not do this, we may end up with
 522                         // a !structural-match constant that is not of reference type, which makes it
 523                         // very hard to invoke `PartialEq::eq` on it as a fallback.
 524                         let val = match self.recur(tcx.deref_mir_constant(self.param_env.and(cv)), false) {
 525                             Ok(subpattern) => PatKind::Deref { subpattern },
 526                             Err(_) => PatKind::Constant { value: cv },
 527                         };
 528                         self.behind_reference.set(old);
 529                         val
 530                     }
 531                 }
 532             },
 533             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::FnDef(..) => {
 534                 PatKind::Constant { value: cv }
 535             }
 536             ty::RawPtr(pointee) if pointee.ty.is_sized(tcx.at(span), param_env) => {
 537                 PatKind::Constant { value: cv }
 538             }
 539             // FIXME: these can have very surprising behaviour where optimization levels or other
 540             // compilation choices change the runtime behaviour of the match.
 541             // See https://github.com/rust-lang/rust/issues/70861 for examples.
 542             ty::FnPtr(..) | ty::RawPtr(..) => {
 543                 if self.include_lint_checks
 544                     && !self.saw_const_match_error.get()
 545                     && !self.saw_const_match_lint.get()
 546                 {
 547                     self.saw_const_match_lint.set(true);
 548                     let msg = "function pointers and unsized pointers in patterns behave \
 549                         unpredictably and should not be relied upon. \
 550                         See https://github.com/rust-lang/rust/issues/70861 for details.";
 551                     tcx.struct_span_lint_hir(
 552                         lint::builtin::POINTER_STRUCTURAL_MATCH,
 553                         id,
 554                         span,
 555                         |lint| {
 556                             lint.build(msg).emit();
 557                         },
 558                     );
 559                 }
 560                 PatKind::Constant { value: cv }
 561             }
 562             _ => {
 563                 self.saw_const_match_error.set(true);
 564                 let msg = format!("`{}` cannot be used in patterns", cv.ty());
 565                 if self.include_lint_checks {
 566                     tcx.sess.span_err(span, &msg);
 567                 } else {
 568                     tcx.sess.delay_span_bug(span, &msg);
 569                 }
 570                 PatKind::Wild
 571             }
 572         };
 573
 574         if self.include_lint_checks
 575             && !self.saw_const_match_error.get()
 576             && !self.saw_const_match_lint.get()
 577             && mir_structural_match_violation
 578             // FIXME(#73448): Find a way to bring const qualification into parity with
 579             // `search_for_structural_match_violation` and then remove this condition.
 580             && self.search_for_structural_match_violation(cv.ty()).is_some()
 581         {
 582             self.saw_const_match_lint.set(true);
 583             // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
 584             // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
 585             let msg = self.search_for_structural_match_violation(cv.ty()).unwrap().replace(
 586                 "in a pattern,",
 587                 "in a pattern, the constant's initializer must be trivial or",
 588             );
 589             tcx.struct_span_lint_hir(
 590                 lint::builtin::NONTRIVIAL_STRUCTURAL_MATCH,
 591                 id,
 592                 span,
 593                 |lint| {
 594                     lint.build(&msg).emit();
 595                 },
 596             );
 597         }
 598
 599         Ok(Pat { span, ty: cv.ty(), kind: Box::new(kind) })
 600     }
 601 }