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), ret)]
  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     ) -> Box<Pat<'tcx>> {
  30         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 }
  36
  37 struct ConstToPat<'a, 'tcx> {
  38     id: hir::HirId,
  39     span: Span,
  40     param_env: ty::ParamEnv<'tcx>,
  41
  42     // This tracks if we emitted some hard error for a given const value, so that
  43     // we will not subsequently issue an irrelevant lint for the same const
  44     // value.
  45     saw_const_match_error: Cell<bool>,
  46
  47     // This tracks if we emitted some diagnostic for a given const value, so that
  48     // we will not subsequently issue an irrelevant lint for the same const
  49     // value.
  50     saw_const_match_lint: Cell<bool>,
  51
  52     // For backcompat we need to keep allowing non-structurally-eq types behind references.
  53     // See also all the `cant-hide-behind` tests.
  54     behind_reference: Cell<bool>,
  55
  56     // inference context used for checking `T: Structural` bounds.
  57     infcx: InferCtxt<'a, 'tcx>,
  58
  59     include_lint_checks: bool,
  60
  61     treat_byte_string_as_slice: bool,
  62 }
  63
  64 mod fallback_to_const_ref {
  65     #[derive(Debug)]
  66     /// This error type signals that we encountered a non-struct-eq situation behind a reference.
  67     /// We bubble this up in order to get back to the reference destructuring and make that emit
  68     /// a const pattern instead of a deref pattern. This allows us to simply call `PartialEq::eq`
  69     /// on such patterns (since that function takes a reference) and not have to jump through any
  70     /// hoops to get a reference to the value.
  71     pub(super) struct FallbackToConstRef(());
  72
  73     pub(super) fn fallback_to_const_ref<'a, 'tcx>(
  74         c2p: &super::ConstToPat<'a, 'tcx>,
  75     ) -> FallbackToConstRef {
  76         assert!(c2p.behind_reference.get());
  77         FallbackToConstRef(())
  78     }
  79 }
  80 use fallback_to_const_ref::{fallback_to_const_ref, FallbackToConstRef};
  81
  82 impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
  83     fn new(
  84         pat_ctxt: &PatCtxt<'_, 'tcx>,
  85         id: hir::HirId,
  86         span: Span,
  87         infcx: InferCtxt<'a, 'tcx>,
  88     ) -> Self {
  89         trace!(?pat_ctxt.typeck_results.hir_owner);
  90         ConstToPat {
  91             id,
  92             span,
  93             infcx,
  94             param_env: pat_ctxt.param_env,
  95             include_lint_checks: pat_ctxt.include_lint_checks,
  96             saw_const_match_error: Cell::new(false),
  97             saw_const_match_lint: Cell::new(false),
  98             behind_reference: Cell::new(false),
  99             treat_byte_string_as_slice: pat_ctxt
 100                 .typeck_results
 101                 .treat_byte_string_as_slice
 102                 .contains(&id.local_id),
 103         }
 104     }
 105
 106     fn tcx(&self) -> TyCtxt<'tcx> {
 107         self.infcx.tcx
 108     }
 109
 110     fn adt_derive_msg(&self, adt_def: AdtDef<'tcx>) -> String {
 111         let path = self.tcx().def_path_str(adt_def.did());
 112         format!(
 113             "to use a constant of type `{}` in a pattern, \
 114             `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 115             path, path,
 116         )
 117     }
 118
 119     fn search_for_structural_match_violation(&self, ty: Ty<'tcx>) -> Option<String> {
 120         traits::search_for_structural_match_violation(self.span, self.tcx(), ty).map(|non_sm_ty| {
 121             with_no_trimmed_paths!(match non_sm_ty.kind() {
 122                 ty::Adt(adt, _) => self.adt_derive_msg(*adt),
 123                 ty::Dynamic(..) => {
 124                     "trait objects cannot be used in patterns".to_string()
 125                 }
 126                 ty::Opaque(..) => {
 127                     "opaque types cannot be used in patterns".to_string()
 128                 }
 129                 ty::Closure(..) => {
 130                     "closures cannot be used in patterns".to_string()
 131                 }
 132                 ty::Generator(..) | ty::GeneratorWitness(..) => {
 133                     "generators cannot be used in patterns".to_string()
 134                 }
 135                 ty::Float(..) => {
 136                     "floating-point numbers cannot be used in patterns".to_string()
 137                 }
 138                 ty::FnPtr(..) => {
 139                     "function pointers cannot be used in patterns".to_string()
 140                 }
 141                 ty::RawPtr(..) => {
 142                     "raw pointers cannot be used in patterns".to_string()
 143                 }
 144                 _ => {
 145                     bug!("use of a value of `{non_sm_ty}` 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: mir::ConstantKind<'tcx>,
 158         mir_structural_match_violation: bool,
 159     ) -> Box<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 =
 169             self.recur(cv, mir_structural_match_violation).unwrap_or_else(|_| {
 170                 Box::new(Pat {
 171                     span: self.span,
 172                     ty: cv.ty(),
 173                     kind: PatKind::Constant { value: cv },
 174                 })
 175             });
 176
 177         if self.include_lint_checks && !self.saw_const_match_error.get() {
 178             // If we were able to successfully convert the const to some pat,
 179             // double-check that all types in the const implement `Structural`.
 180
 181             let structural = self.search_for_structural_match_violation(cv.ty());
 182             debug!(
 183                 "search_for_structural_match_violation cv.ty: {:?} returned: {:?}",
 184                 cv.ty(),
 185                 structural
 186             );
 187
 188             // This can occur because const qualification treats all associated constants as
 189             // opaque, whereas `search_for_structural_match_violation` tries to monomorphize them
 190             // before it runs.
 191             //
 192             // FIXME(#73448): Find a way to bring const qualification into parity with
 193             // `search_for_structural_match_violation`.
 194             if structural.is_none() && mir_structural_match_violation {
 195                 warn!("MIR const-checker found novel structural match violation. See #73448.");
 196                 return inlined_const_as_pat;
 197             }
 198
 199             if let Some(msg) = structural {
 200                 if !self.type_may_have_partial_eq_impl(cv.ty()) {
 201                     // span_fatal avoids ICE from resolution of non-existent method (rare case).
 202                     self.tcx().sess.span_fatal(self.span, &msg);
 203                 } else if mir_structural_match_violation && !self.saw_const_match_lint.get() {
 204                     self.tcx().struct_span_lint_hir(
 205                         lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 206                         self.id,
 207                         self.span,
 208                         |lint| {
 209                             lint.build(&msg).emit();
 210                         },
 211                     );
 212                 } else {
 213                     debug!(
 214                         "`search_for_structural_match_violation` found one, but `CustomEq` was \
 215                           not in the qualifs for that `const`"
 216                     );
 217                 }
 218             }
 219         }
 220
 221         inlined_const_as_pat
 222     }
 223
 224     fn type_may_have_partial_eq_impl(&self, ty: Ty<'tcx>) -> bool {
 225         // double-check there even *is* a semantic `PartialEq` to dispatch to.
 226         //
 227         // (If there isn't, then we can safely issue a hard
 228         // error, because that's never worked, due to compiler
 229         // using `PartialEq::eq` in this scenario in the past.)
 230         let partial_eq_trait_id =
 231             self.tcx().require_lang_item(hir::LangItem::PartialEq, Some(self.span));
 232         let obligation: PredicateObligation<'_> = predicate_for_trait_def(
 233             self.tcx(),
 234             self.param_env,
 235             ObligationCause::misc(self.span, self.id),
 236             partial_eq_trait_id,
 237             0,
 238             ty,
 239             &[],
 240         );
 241         // FIXME: should this call a `predicate_must_hold` variant instead?
 242
 243         let has_impl = self.infcx.predicate_may_hold(&obligation);
 244
 245         // Note: To fix rust-lang/rust#65466, we could just remove this type
 246         // walk hack for function pointers, and unconditionally error
 247         // if `PartialEq` is not implemented. However, that breaks stable
 248         // code at the moment, because types like `for <'a> fn(&'a ())` do
 249         // not *yet* implement `PartialEq`. So for now we leave this here.
 250         has_impl
 251             || ty.walk().any(|t| match t.unpack() {
 252                 ty::subst::GenericArgKind::Lifetime(_) => false,
 253                 ty::subst::GenericArgKind::Type(t) => t.is_fn_ptr(),
 254                 ty::subst::GenericArgKind::Const(_) => false,
 255             })
 256     }
 257
 258     fn field_pats(
 259         &self,
 260         vals: impl Iterator<Item = mir::ConstantKind<'tcx>>,
 261     ) -> Result<Vec<FieldPat<'tcx>>, FallbackToConstRef> {
 262         vals.enumerate()
 263             .map(|(idx, val)| {
 264                 let field = Field::new(idx);
 265                 Ok(FieldPat { field, pattern: self.recur(val, false)? })
 266             })
 267             .collect()
 268     }
 269
 270     // Recursive helper for `to_pat`; invoke that (instead of calling this directly).
 271     #[instrument(skip(self), level = "debug")]
 272     fn recur(
 273         &self,
 274         cv: mir::ConstantKind<'tcx>,
 275         mir_structural_match_violation: bool,
 276     ) -> Result<Box<Pat<'tcx>>, FallbackToConstRef> {
 277         let id = self.id;
 278         let span = self.span;
 279         let tcx = self.tcx();
 280         let param_env = self.param_env;
 281
 282         let kind = match cv.ty().kind() {
 283             ty::Float(_) => {
 284                 if self.include_lint_checks {
 285                     tcx.struct_span_lint_hir(
 286                         lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
 287                         id,
 288                         span,
 289                         |lint| {
 290                             lint.build("floating-point types cannot be used in patterns").emit();
 291                         },
 292                     );
 293                 }
 294                 PatKind::Constant { value: cv }
 295             }
 296             ty::Adt(adt_def, _) if adt_def.is_union() => {
 297                 // Matching on union fields is unsafe, we can't hide it in constants
 298                 self.saw_const_match_error.set(true);
 299                 let msg = "cannot use unions in constant patterns";
 300                 if self.include_lint_checks {
 301                     tcx.sess.span_err(span, msg);
 302                 } else {
 303                     tcx.sess.delay_span_bug(span, msg);
 304                 }
 305                 PatKind::Wild
 306             }
 307             ty::Adt(..)
 308                 if !self.type_may_have_partial_eq_impl(cv.ty())
 309                     // FIXME(#73448): Find a way to bring const qualification into parity with
 310                     // `search_for_structural_match_violation` and then remove this condition.
 311                     && self.search_for_structural_match_violation(cv.ty()).is_some() =>
 312             {
 313                 // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
 314                 // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
 315                 let msg = self.search_for_structural_match_violation(cv.ty()).unwrap();
 316                 self.saw_const_match_error.set(true);
 317                 if self.include_lint_checks {
 318                     tcx.sess.span_err(self.span, &msg);
 319                 } else {
 320                     tcx.sess.delay_span_bug(self.span, &msg);
 321                 }
 322                 PatKind::Wild
 323             }
 324             // If the type is not structurally comparable, just emit the constant directly,
 325             // causing the pattern match code to treat it opaquely.
 326             // FIXME: This code doesn't emit errors itself, the caller emits the errors.
 327             // So instead of specific errors, you just get blanket errors about the whole
 328             // const type. See
 329             // https://github.com/rust-lang/rust/pull/70743#discussion_r404701963 for
 330             // details.
 331             // Backwards compatibility hack because we can't cause hard errors on these
 332             // types, so we compare them via `PartialEq::eq` at runtime.
 333             ty::Adt(..) if !self.type_marked_structural(cv.ty()) && self.behind_reference.get() => {
 334                 if self.include_lint_checks
 335                     && !self.saw_const_match_error.get()
 336                     && !self.saw_const_match_lint.get()
 337                 {
 338                     self.saw_const_match_lint.set(true);
 339                     tcx.struct_span_lint_hir(
 340                         lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 341                         id,
 342                         span,
 343                         |lint| {
 344                             let msg = format!(
 345                                 "to use a constant of type `{}` in a pattern, \
 346                                  `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 347                                 cv.ty(),
 348                                 cv.ty(),
 349                             );
 350                             lint.build(&msg).emit();
 351                         },
 352                     );
 353                 }
 354                 // Since we are behind a reference, we can just bubble the error up so we get a
 355                 // constant at reference type, making it easy to let the fallback call
 356                 // `PartialEq::eq` on it.
 357                 return Err(fallback_to_const_ref(self));
 358             }
 359             ty::Adt(adt_def, _) if !self.type_marked_structural(cv.ty()) => {
 360                 debug!(
 361                     "adt_def {:?} has !type_marked_structural for cv.ty: {:?}",
 362                     adt_def,
 363                     cv.ty()
 364                 );
 365                 let path = tcx.def_path_str(adt_def.did());
 366                 let msg = format!(
 367                     "to use a constant of type `{}` in a pattern, \
 368                      `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 369                     path, path,
 370                 );
 371                 self.saw_const_match_error.set(true);
 372                 if self.include_lint_checks {
 373                     tcx.sess.span_err(span, &msg);
 374                 } else {
 375                     tcx.sess.delay_span_bug(span, &msg);
 376                 }
 377                 PatKind::Wild
 378             }
 379             ty::Adt(adt_def, substs) if adt_def.is_enum() => {
 380                 let destructured = tcx.destructure_mir_constant(param_env, cv);
 381
 382                 PatKind::Variant {
 383                     adt_def: *adt_def,
 384                     substs,
 385                     variant_index: destructured
 386                         .variant
 387                         .expect("destructed const of adt without variant id"),
 388                     subpatterns: self.field_pats(destructured.fields.iter().copied())?,
 389                 }
 390             }
 391             ty::Tuple(_) | ty::Adt(_, _) => {
 392                 let destructured = tcx.destructure_mir_constant(param_env, cv);
 393                 PatKind::Leaf { subpatterns: self.field_pats(destructured.fields.iter().copied())? }
 394             }
 395             ty::Array(..) => PatKind::Array {
 396                 prefix: tcx
 397                     .destructure_mir_constant(param_env, cv)
 398                     .fields
 399                     .iter()
 400                     .map(|val| self.recur(*val, false))
 401                     .collect::<Result<_, _>>()?,
 402                 slice: None,
 403                 suffix: Box::new([]),
 404             },
 405             ty::Ref(_, pointee_ty, ..) => match *pointee_ty.kind() {
 406                 // These are not allowed and will error elsewhere anyway.
 407                 ty::Dynamic(..) => {
 408                     self.saw_const_match_error.set(true);
 409                     let msg = format!("`{}` cannot be used in patterns", cv.ty());
 410                     if self.include_lint_checks {
 411                         tcx.sess.span_err(span, &msg);
 412                     } else {
 413                         tcx.sess.delay_span_bug(span, &msg);
 414                     }
 415                     PatKind::Wild
 416                 }
 417                 // `&str` is represented as `ConstValue::Slice`, let's keep using this
 418                 // optimization for now.
 419                 ty::Str => PatKind::Constant { value: cv },
 420                 // `b"foo"` produces a `&[u8; 3]`, but you can't use constants of array type when
 421                 // matching against references, you can only use byte string literals.
 422                 // The typechecker has a special case for byte string literals, by treating them
 423                 // as slices. This means we turn `&[T; N]` constants into slice patterns, which
 424                 // has no negative effects on pattern matching, even if we're actually matching on
 425                 // arrays.
 426                 ty::Array(..) if !self.treat_byte_string_as_slice => {
 427                     let old = self.behind_reference.replace(true);
 428                     let array = tcx.deref_mir_constant(self.param_env.and(cv));
 429                     let val = PatKind::Deref {
 430                         subpattern: Box::new(Pat {
 431                             kind: PatKind::Array {
 432                                 prefix: tcx
 433                                     .destructure_mir_constant(param_env, array)
 434                                     .fields
 435                                     .iter()
 436                                     .map(|val| self.recur(*val, false))
 437                                     .collect::<Result<_, _>>()?,
 438                                 slice: None,
 439                                 suffix: Box::new([]),
 440                             },
 441                             span,
 442                             ty: *pointee_ty,
 443                         }),
 444                     };
 445                     self.behind_reference.set(old);
 446                     val
 447                 }
 448                 ty::Array(elem_ty, _) |
 449                 // Cannot merge this with the catch all branch below, because the `const_deref`
 450                 // changes the type from slice to array, we need to keep the original type in the
 451                 // pattern.
 452                 ty::Slice(elem_ty) => {
 453                     let old = self.behind_reference.replace(true);
 454                     let array = tcx.deref_mir_constant(self.param_env.and(cv));
 455                     let val = PatKind::Deref {
 456                         subpattern: Box::new(Pat {
 457                             kind: PatKind::Slice {
 458                                 prefix: tcx
 459                                     .destructure_mir_constant(param_env, array)
 460                                     .fields
 461                                     .iter()
 462                                     .map(|val| self.recur(*val, false))
 463                                     .collect::<Result<_, _>>()?,
 464                                 slice: None,
 465                                 suffix: Box::new([]),
 466                             },
 467                             span,
 468                             ty: tcx.mk_slice(elem_ty),
 469                         }),
 470                     };
 471                     self.behind_reference.set(old);
 472                     val
 473                 }
 474                 // Backwards compatibility hack: support references to non-structural types.
 475                 // We'll lower
 476                 // this pattern to a `PartialEq::eq` comparison and `PartialEq::eq` takes a
 477                 // reference. This makes the rest of the matching logic simpler as it doesn't have
 478                 // to figure out how to get a reference again.
 479                 ty::Adt(adt_def, _) if !self.type_marked_structural(*pointee_ty) => {
 480                     if self.behind_reference.get() {
 481                         if self.include_lint_checks
 482                             && !self.saw_const_match_error.get()
 483                             && !self.saw_const_match_lint.get()
 484                         {
 485                             self.saw_const_match_lint.set(true);
 486                             let msg = self.adt_derive_msg(adt_def);
 487                             self.tcx().struct_span_lint_hir(
 488                                 lint::builtin::INDIRECT_STRUCTURAL_MATCH,
 489                                 self.id,
 490                                 self.span,
 491                                 |lint| {lint.build(&msg).emit();},
 492                             );
 493                         }
 494                         PatKind::Constant { value: cv }
 495                     } else {
 496                         if !self.saw_const_match_error.get() {
 497                             self.saw_const_match_error.set(true);
 498                             let msg = self.adt_derive_msg(adt_def);
 499                             if self.include_lint_checks {
 500                                 tcx.sess.span_err(span, &msg);
 501                             } else {
 502                                 tcx.sess.delay_span_bug(span, &msg);
 503                             }
 504                         }
 505                         PatKind::Wild
 506                     }
 507                 }
 508                 // All other references are converted into deref patterns and then recursively
 509                 // convert the dereferenced constant to a pattern that is the sub-pattern of the
 510                 // deref pattern.
 511                 _ => {
 512                     if !pointee_ty.is_sized(tcx.at(span), param_env) {
 513                         // `tcx.deref_mir_constant()` below will ICE with an unsized type
 514                         // (except slices, which are handled in a separate arm above).
 515                         let msg = format!("cannot use unsized non-slice type `{}` in constant patterns", pointee_ty);
 516                         if self.include_lint_checks {
 517                             tcx.sess.span_err(span, &msg);
 518                         } else {
 519                             tcx.sess.delay_span_bug(span, &msg);
 520                         }
 521                         PatKind::Wild
 522                     } else {
 523                         let old = self.behind_reference.replace(true);
 524                         // In case there are structural-match violations somewhere in this subpattern,
 525                         // we fall back to a const pattern. If we do not do this, we may end up with
 526                         // a !structural-match constant that is not of reference type, which makes it
 527                         // very hard to invoke `PartialEq::eq` on it as a fallback.
 528                         let val = match self.recur(tcx.deref_mir_constant(self.param_env.and(cv)), false) {
 529                             Ok(subpattern) => PatKind::Deref { subpattern },
 530                             Err(_) => PatKind::Constant { value: cv },
 531                         };
 532                         self.behind_reference.set(old);
 533                         val
 534                     }
 535                 }
 536             },
 537             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::FnDef(..) => {
 538                 PatKind::Constant { value: cv }
 539             }
 540             ty::RawPtr(pointee) if pointee.ty.is_sized(tcx.at(span), param_env) => {
 541                 PatKind::Constant { value: cv }
 542             }
 543             // FIXME: these can have very surprising behaviour where optimization levels or other
 544             // compilation choices change the runtime behaviour of the match.
 545             // See https://github.com/rust-lang/rust/issues/70861 for examples.
 546             ty::FnPtr(..) | ty::RawPtr(..) => {
 547                 if self.include_lint_checks
 548                     && !self.saw_const_match_error.get()
 549                     && !self.saw_const_match_lint.get()
 550                 {
 551                     self.saw_const_match_lint.set(true);
 552                     let msg = "function pointers and unsized pointers in patterns behave \
 553                         unpredictably and should not be relied upon. \
 554                         See https://github.com/rust-lang/rust/issues/70861 for details.";
 555                     tcx.struct_span_lint_hir(
 556                         lint::builtin::POINTER_STRUCTURAL_MATCH,
 557                         id,
 558                         span,
 559                         |lint| {
 560                             lint.build(msg).emit();
 561                         },
 562                     );
 563                 }
 564                 PatKind::Constant { value: cv }
 565             }
 566             _ => {
 567                 self.saw_const_match_error.set(true);
 568                 let msg = format!("`{}` cannot be used in patterns", cv.ty());
 569                 if self.include_lint_checks {
 570                     tcx.sess.span_err(span, &msg);
 571                 } else {
 572                     tcx.sess.delay_span_bug(span, &msg);
 573                 }
 574                 PatKind::Wild
 575             }
 576         };
 577
 578         if self.include_lint_checks
 579             && !self.saw_const_match_error.get()
 580             && !self.saw_const_match_lint.get()
 581             && mir_structural_match_violation
 582             // FIXME(#73448): Find a way to bring const qualification into parity with
 583             // `search_for_structural_match_violation` and then remove this condition.
 584             && self.search_for_structural_match_violation(cv.ty()).is_some()
 585         {
 586             self.saw_const_match_lint.set(true);
 587             // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
 588             // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
 589             let msg = self.search_for_structural_match_violation(cv.ty()).unwrap().replace(
 590                 "in a pattern,",
 591                 "in a pattern, the constant's initializer must be trivial or",
 592             );
 593             tcx.struct_span_lint_hir(
 594                 lint::builtin::NONTRIVIAL_STRUCTURAL_MATCH,
 595                 id,
 596                 span,
 597                 |lint| {
 598                     lint.build(&msg).emit();
 599                 },
 600             );
 601         }
 602
 603         Ok(Box::new(Pat { span, ty: cv.ty(), kind }))
 604     }
 605 }