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;
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};
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(
25 cv: mir::ConstantKind<'tcx>,
28 mir_structural_match_violation: bool,
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)
37 struct ConstToPat<'a, 'tcx> {
40 param_env: ty::ParamEnv<'tcx>,
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
45 saw_const_match_error: Cell<bool>,
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
50 saw_const_match_lint: Cell<bool>,
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>,
56 // inference context used for checking `T: Structural` bounds.
57 infcx: InferCtxt<'a, 'tcx>,
59 include_lint_checks: bool,
61 treat_byte_string_as_slice: bool,
64 mod fallback_to_const_ref {
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(());
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(())
80 use fallback_to_const_ref::{fallback_to_const_ref, FallbackToConstRef};
82 impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
84 pat_ctxt: &PatCtxt<'_, 'tcx>,
87 infcx: InferCtxt<'a, 'tcx>,
89 trace!(?pat_ctxt.typeck_results.hir_owner);
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
101 .treat_byte_string_as_slice
102 .contains(&id.local_id),
106 fn tcx(&self) -> TyCtxt<'tcx> {
110 fn adt_derive_msg(&self, adt_def: AdtDef<'tcx>) -> String {
111 let path = self.tcx().def_path_str(adt_def.did());
113 "to use a constant of type `{}` in a pattern, \
114 `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
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),
124 "trait objects cannot be used in patterns".to_string()
127 "opaque types cannot be used in patterns".to_string()
130 "closures cannot be used in patterns".to_string()
132 ty::Generator(..) | ty::GeneratorWitness(..) => {
133 "generators cannot be used in patterns".to_string()
136 "floating-point numbers cannot be used in patterns".to_string()
139 "function pointers cannot be used in patterns".to_string()
142 "raw pointers cannot be used in patterns".to_string()
145 bug!("use of a value of `{non_sm_ty}` inside a pattern")
151 fn type_marked_structural(&self, ty: Ty<'tcx>) -> bool {
152 ty.is_structural_eq_shallow(self.infcx.tcx)
157 cv: mir::ConstantKind<'tcx>,
158 mir_structural_match_violation: bool,
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.
165 // once indirect_structural_match is a full fledged error, this
166 // level of indirection can be eliminated
168 let inlined_const_as_pat =
169 self.recur(cv, mir_structural_match_violation).unwrap_or_else(|_| Pat {
172 kind: Box::new(PatKind::Constant { value: cv }),
175 if self.include_lint_checks && !self.saw_const_match_error.get() {
176 // If we were able to successfully convert the const to some pat,
177 // double-check that all types in the const implement `Structural`.
179 let structural = self.search_for_structural_match_violation(cv.ty());
181 "search_for_structural_match_violation cv.ty: {:?} returned: {:?}",
186 // This can occur because const qualification treats all associated constants as
187 // opaque, whereas `search_for_structural_match_violation` tries to monomorphize them
190 // FIXME(#73448): Find a way to bring const qualification into parity with
191 // `search_for_structural_match_violation`.
192 if structural.is_none() && mir_structural_match_violation {
193 warn!("MIR const-checker found novel structural match violation. See #73448.");
194 return inlined_const_as_pat;
197 if let Some(msg) = structural {
198 if !self.type_may_have_partial_eq_impl(cv.ty()) {
199 // span_fatal avoids ICE from resolution of non-existent method (rare case).
200 self.tcx().sess.span_fatal(self.span, &msg);
201 } else if mir_structural_match_violation && !self.saw_const_match_lint.get() {
202 self.tcx().struct_span_lint_hir(
203 lint::builtin::INDIRECT_STRUCTURAL_MATCH,
207 lint.build(&msg).emit();
212 "`search_for_structural_match_violation` found one, but `CustomEq` was \
213 not in the qualifs for that `const`"
222 fn type_may_have_partial_eq_impl(&self, ty: Ty<'tcx>) -> bool {
223 // double-check there even *is* a semantic `PartialEq` to dispatch to.
225 // (If there isn't, then we can safely issue a hard
226 // error, because that's never worked, due to compiler
227 // using `PartialEq::eq` in this scenario in the past.)
228 let partial_eq_trait_id =
229 self.tcx().require_lang_item(hir::LangItem::PartialEq, Some(self.span));
230 let obligation: PredicateObligation<'_> = predicate_for_trait_def(
233 ObligationCause::misc(self.span, self.id),
239 // FIXME: should this call a `predicate_must_hold` variant instead?
241 let has_impl = self.infcx.predicate_may_hold(&obligation);
243 // Note: To fix rust-lang/rust#65466, we could just remove this type
244 // walk hack for function pointers, and unconditionally error
245 // if `PartialEq` is not implemented. However, that breaks stable
246 // code at the moment, because types like `for <'a> fn(&'a ())` do
247 // not *yet* implement `PartialEq`. So for now we leave this here.
249 || ty.walk().any(|t| match t.unpack() {
250 ty::subst::GenericArgKind::Lifetime(_) => false,
251 ty::subst::GenericArgKind::Type(t) => t.is_fn_ptr(),
252 ty::subst::GenericArgKind::Const(_) => false,
258 vals: impl Iterator<Item = mir::ConstantKind<'tcx>>,
259 ) -> Result<Vec<FieldPat<'tcx>>, FallbackToConstRef> {
262 let field = Field::new(idx);
263 Ok(FieldPat { field, pattern: self.recur(val, false)? })
268 // Recursive helper for `to_pat`; invoke that (instead of calling this directly).
269 #[instrument(skip(self), level = "debug")]
272 cv: mir::ConstantKind<'tcx>,
273 mir_structural_match_violation: bool,
274 ) -> Result<Pat<'tcx>, FallbackToConstRef> {
276 let span = self.span;
277 let tcx = self.tcx();
278 let param_env = self.param_env;
280 let kind = match cv.ty().kind() {
282 if self.include_lint_checks {
283 tcx.struct_span_lint_hir(
284 lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
288 lint.build("floating-point types cannot be used in patterns").emit();
292 PatKind::Constant { value: cv }
294 ty::Adt(adt_def, _) if adt_def.is_union() => {
295 // Matching on union fields is unsafe, we can't hide it in constants
296 self.saw_const_match_error.set(true);
297 let msg = "cannot use unions in constant patterns";
298 if self.include_lint_checks {
299 tcx.sess.span_err(span, msg);
301 tcx.sess.delay_span_bug(span, msg);
306 if !self.type_may_have_partial_eq_impl(cv.ty())
307 // FIXME(#73448): Find a way to bring const qualification into parity with
308 // `search_for_structural_match_violation` and then remove this condition.
309 && self.search_for_structural_match_violation(cv.ty()).is_some() =>
311 // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
312 // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
313 let msg = self.search_for_structural_match_violation(cv.ty()).unwrap();
314 self.saw_const_match_error.set(true);
315 if self.include_lint_checks {
316 tcx.sess.span_err(self.span, &msg);
318 tcx.sess.delay_span_bug(self.span, &msg);
322 // If the type is not structurally comparable, just emit the constant directly,
323 // causing the pattern match code to treat it opaquely.
324 // FIXME: This code doesn't emit errors itself, the caller emits the errors.
325 // So instead of specific errors, you just get blanket errors about the whole
327 // https://github.com/rust-lang/rust/pull/70743#discussion_r404701963 for
329 // Backwards compatibility hack because we can't cause hard errors on these
330 // types, so we compare them via `PartialEq::eq` at runtime.
331 ty::Adt(..) if !self.type_marked_structural(cv.ty()) && self.behind_reference.get() => {
332 if self.include_lint_checks
333 && !self.saw_const_match_error.get()
334 && !self.saw_const_match_lint.get()
336 self.saw_const_match_lint.set(true);
337 tcx.struct_span_lint_hir(
338 lint::builtin::INDIRECT_STRUCTURAL_MATCH,
343 "to use a constant of type `{}` in a pattern, \
344 `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
348 lint.build(&msg).emit();
352 // Since we are behind a reference, we can just bubble the error up so we get a
353 // constant at reference type, making it easy to let the fallback call
354 // `PartialEq::eq` on it.
355 return Err(fallback_to_const_ref(self));
357 ty::Adt(adt_def, _) if !self.type_marked_structural(cv.ty()) => {
359 "adt_def {:?} has !type_marked_structural for cv.ty: {:?}",
363 let path = tcx.def_path_str(adt_def.did());
365 "to use a constant of type `{}` in a pattern, \
366 `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
369 self.saw_const_match_error.set(true);
370 if self.include_lint_checks {
371 tcx.sess.span_err(span, &msg);
373 tcx.sess.delay_span_bug(span, &msg);
377 ty::Adt(adt_def, substs) if adt_def.is_enum() => {
378 let destructured = tcx.destructure_mir_constant(param_env, cv);
383 variant_index: destructured
385 .expect("destructed const of adt without variant id"),
386 subpatterns: self.field_pats(destructured.fields.iter().copied())?,
389 ty::Tuple(_) | ty::Adt(_, _) => {
390 let destructured = tcx.destructure_mir_constant(param_env, cv);
391 PatKind::Leaf { subpatterns: self.field_pats(destructured.fields.iter().copied())? }
393 ty::Array(..) => PatKind::Array {
395 .destructure_mir_constant(param_env, cv)
398 .map(|val| self.recur(*val, false))
399 .collect::<Result<_, _>>()?,
403 ty::Ref(_, pointee_ty, ..) => match *pointee_ty.kind() {
404 // These are not allowed and will error elsewhere anyway.
406 self.saw_const_match_error.set(true);
407 let msg = format!("`{}` cannot be used in patterns", cv.ty());
408 if self.include_lint_checks {
409 tcx.sess.span_err(span, &msg);
411 tcx.sess.delay_span_bug(span, &msg);
415 // `&str` is represented as `ConstValue::Slice`, let's keep using this
416 // optimization for now.
417 ty::Str => PatKind::Constant { value: cv },
418 // `b"foo"` produces a `&[u8; 3]`, but you can't use constants of array type when
419 // matching against references, you can only use byte string literals.
420 // The typechecker has a special case for byte string literals, by treating them
421 // as slices. This means we turn `&[T; N]` constants into slice patterns, which
422 // has no negative effects on pattern matching, even if we're actually matching on
424 ty::Array(..) if !self.treat_byte_string_as_slice => {
425 let old = self.behind_reference.replace(true);
426 let array = tcx.deref_mir_constant(self.param_env.and(cv));
427 let val = PatKind::Deref {
429 kind: Box::new(PatKind::Array {
431 .destructure_mir_constant(param_env, array)
434 .map(|val| self.recur(*val, false))
435 .collect::<Result<_, _>>()?,
443 self.behind_reference.set(old);
446 ty::Array(elem_ty, _) |
447 // Cannot merge this with the catch all branch below, because the `const_deref`
448 // changes the type from slice to array, we need to keep the original type in the
450 ty::Slice(elem_ty) => {
451 let old = self.behind_reference.replace(true);
452 let array = tcx.deref_mir_constant(self.param_env.and(cv));
453 let val = PatKind::Deref {
455 kind: Box::new(PatKind::Slice {
457 .destructure_mir_constant(param_env, array)
460 .map(|val| self.recur(*val, false))
461 .collect::<Result<_, _>>()?,
466 ty: tcx.mk_slice(elem_ty),
469 self.behind_reference.set(old);
472 // Backwards compatibility hack: support references to non-structural types.
474 // this pattern to a `PartialEq::eq` comparison and `PartialEq::eq` takes a
475 // reference. This makes the rest of the matching logic simpler as it doesn't have
476 // to figure out how to get a reference again.
477 ty::Adt(adt_def, _) if !self.type_marked_structural(*pointee_ty) => {
478 if self.behind_reference.get() {
479 if self.include_lint_checks
480 && !self.saw_const_match_error.get()
481 && !self.saw_const_match_lint.get()
483 self.saw_const_match_lint.set(true);
484 let msg = self.adt_derive_msg(adt_def);
485 self.tcx().struct_span_lint_hir(
486 lint::builtin::INDIRECT_STRUCTURAL_MATCH,
489 |lint| {lint.build(&msg).emit();},
492 PatKind::Constant { value: cv }
494 if !self.saw_const_match_error.get() {
495 self.saw_const_match_error.set(true);
496 let msg = self.adt_derive_msg(adt_def);
497 if self.include_lint_checks {
498 tcx.sess.span_err(span, &msg);
500 tcx.sess.delay_span_bug(span, &msg);
506 // All other references are converted into deref patterns and then recursively
507 // convert the dereferenced constant to a pattern that is the sub-pattern of the
510 if !pointee_ty.is_sized(tcx.at(span), param_env) {
511 // `tcx.deref_mir_constant()` below will ICE with an unsized type
512 // (except slices, which are handled in a separate arm above).
513 let msg = format!("cannot use unsized non-slice type `{}` in constant patterns", pointee_ty);
514 if self.include_lint_checks {
515 tcx.sess.span_err(span, &msg);
517 tcx.sess.delay_span_bug(span, &msg);
521 let old = self.behind_reference.replace(true);
522 // In case there are structural-match violations somewhere in this subpattern,
523 // we fall back to a const pattern. If we do not do this, we may end up with
524 // a !structural-match constant that is not of reference type, which makes it
525 // very hard to invoke `PartialEq::eq` on it as a fallback.
526 let val = match self.recur(tcx.deref_mir_constant(self.param_env.and(cv)), false) {
527 Ok(subpattern) => PatKind::Deref { subpattern },
528 Err(_) => PatKind::Constant { value: cv },
530 self.behind_reference.set(old);
535 ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::FnDef(..) => {
536 PatKind::Constant { value: cv }
538 ty::RawPtr(pointee) if pointee.ty.is_sized(tcx.at(span), param_env) => {
539 PatKind::Constant { value: cv }
541 // FIXME: these can have very surprising behaviour where optimization levels or other
542 // compilation choices change the runtime behaviour of the match.
543 // See https://github.com/rust-lang/rust/issues/70861 for examples.
544 ty::FnPtr(..) | ty::RawPtr(..) => {
545 if self.include_lint_checks
546 && !self.saw_const_match_error.get()
547 && !self.saw_const_match_lint.get()
549 self.saw_const_match_lint.set(true);
550 let msg = "function pointers and unsized pointers in patterns behave \
551 unpredictably and should not be relied upon. \
552 See https://github.com/rust-lang/rust/issues/70861 for details.";
553 tcx.struct_span_lint_hir(
554 lint::builtin::POINTER_STRUCTURAL_MATCH,
558 lint.build(msg).emit();
562 PatKind::Constant { value: cv }
565 self.saw_const_match_error.set(true);
566 let msg = format!("`{}` cannot be used in patterns", cv.ty());
567 if self.include_lint_checks {
568 tcx.sess.span_err(span, &msg);
570 tcx.sess.delay_span_bug(span, &msg);
576 if self.include_lint_checks
577 && !self.saw_const_match_error.get()
578 && !self.saw_const_match_lint.get()
579 && mir_structural_match_violation
580 // FIXME(#73448): Find a way to bring const qualification into parity with
581 // `search_for_structural_match_violation` and then remove this condition.
582 && self.search_for_structural_match_violation(cv.ty()).is_some()
584 self.saw_const_match_lint.set(true);
585 // Obtain the actual type that isn't annotated. If we just looked at `cv.ty` we
586 // could get `Option<NonStructEq>`, even though `Option` is annotated with derive.
587 let msg = self.search_for_structural_match_violation(cv.ty()).unwrap().replace(
589 "in a pattern, the constant's initializer must be trivial or",
591 tcx.struct_span_lint_hir(
592 lint::builtin::NONTRIVIAL_STRUCTURAL_MATCH,
596 lint.build(&msg).emit();
601 Ok(Pat { span, ty: cv.ty(), kind: Box::new(kind) })