1 //! Check properties that are required by built-in traits and set
2 //! up data structures required by type-checking/codegen.
4 use crate::errors::{CopyImplOnNonAdt, CopyImplOnTypeWithDtor, DropImplOnWrongItem};
5 use rustc_errors::{struct_span_err, MultiSpan};
7 use rustc_hir::def_id::{DefId, LocalDefId};
8 use rustc_hir::lang_items::LangItem;
9 use rustc_hir::ItemKind;
10 use rustc_infer::infer;
11 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
12 use rustc_infer::infer::TyCtxtInferExt;
13 use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
14 use rustc_middle::ty::{self, suggest_constraining_type_params, Ty, TyCtxt, TypeVisitable};
15 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
16 use rustc_trait_selection::traits::misc::{can_type_implement_copy, CopyImplementationError};
17 use rustc_trait_selection::traits::predicate_for_trait_def;
18 use rustc_trait_selection::traits::{self, ObligationCause};
19 use std::collections::BTreeMap;
21 pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
22 let lang_items = tcx.lang_items();
23 Checker { tcx, trait_def_id }
24 .check(lang_items.drop_trait(), visit_implementation_of_drop)
25 .check(lang_items.copy_trait(), visit_implementation_of_copy)
26 .check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
27 .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn);
30 struct Checker<'tcx> {
35 impl<'tcx> Checker<'tcx> {
36 fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
38 F: FnMut(TyCtxt<'tcx>, LocalDefId),
40 if Some(self.trait_def_id) == trait_def_id {
41 for &impl_def_id in self.tcx.hir().trait_impls(self.trait_def_id) {
42 f(self.tcx, impl_def_id);
49 fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
50 // Destructors only work on local ADT types.
51 match tcx.type_of(impl_did).kind() {
52 ty::Adt(def, _) if def.did().is_local() => return,
53 ty::Error(_) => return,
57 let sp = match tcx.hir().expect_item(impl_did).kind {
58 ItemKind::Impl(ref impl_) => impl_.self_ty.span,
59 _ => bug!("expected Drop impl item"),
62 tcx.sess.emit_err(DropImplOnWrongItem { span: sp });
65 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
66 debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
68 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
70 let self_type = tcx.type_of(impl_did);
71 debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
73 let span = tcx.hir().span(impl_hir_id);
74 let param_env = tcx.param_env(impl_did);
75 assert!(!self_type.has_escaping_bound_vars());
77 debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
79 let cause = traits::ObligationCause::misc(span, impl_hir_id);
80 match can_type_implement_copy(tcx, param_env, self_type, cause) {
82 Err(CopyImplementationError::InfrigingFields(fields)) => {
83 let item = tcx.hir().expect_item(impl_did);
84 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref tr), .. }) = item.kind {
90 let mut err = struct_span_err!(
94 "the trait `Copy` may not be implemented for this type"
97 // We'll try to suggest constraining type parameters to fulfill the requirements of
98 // their `Copy` implementation.
99 let mut errors: BTreeMap<_, Vec<_>> = Default::default();
100 let mut bounds = vec![];
102 for (field, ty) in fields {
103 let field_span = tcx.def_span(field.did);
104 let field_ty_span = match tcx.hir().get_if_local(field.did) {
105 Some(hir::Node::Field(field_def)) => field_def.ty.span,
108 err.span_label(field_span, "this field does not implement `Copy`");
109 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
110 // why this field does not implement Copy. This is useful because sometimes
111 // it is not immediately clear why Copy is not implemented for a field, since
112 // all we point at is the field itself.
113 tcx.infer_ctxt().ignoring_regions().enter(|infcx| {
114 for error in traits::fully_solve_bound(
116 traits::ObligationCause::dummy_with_span(field_ty_span),
119 tcx.lang_items().copy_trait().unwrap(),
121 let error_predicate = error.obligation.predicate;
122 // Only note if it's not the root obligation, otherwise it's trivial and
123 // should be self-explanatory (i.e. a field literally doesn't implement Copy).
125 // FIXME: This error could be more descriptive, especially if the error_predicate
126 // contains a foreign type or if it's a deeply nested type...
127 if error_predicate != error.root_obligation.predicate {
129 .entry((ty.to_string(), error_predicate.to_string()))
131 .push(error.obligation.cause.span);
133 if let ty::PredicateKind::Trait(ty::TraitPredicate {
135 polarity: ty::ImplPolarity::Positive,
137 }) = error_predicate.kind().skip_binder()
139 let ty = trait_ref.self_ty();
140 if let ty::Param(_) = ty.kind() {
143 trait_ref.print_only_trait_path().to_string(),
144 Some(trait_ref.def_id),
151 for ((ty, error_predicate), spans) in errors {
152 let span: MultiSpan = spans.into();
155 &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
158 suggest_constraining_type_params(
160 tcx.hir().get_generics(impl_did).expect("impls always have generics"),
162 bounds.iter().map(|(param, constraint, def_id)| {
163 (param.as_str(), constraint.as_str(), *def_id)
168 Err(CopyImplementationError::NotAnAdt) => {
169 let item = tcx.hir().expect_item(impl_did);
171 if let ItemKind::Impl(ref impl_) = item.kind { impl_.self_ty.span } else { span };
173 tcx.sess.emit_err(CopyImplOnNonAdt { span });
175 Err(CopyImplementationError::HasDestructor) => {
176 tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
181 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
182 debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
184 // Just compute this for the side-effects, in particular reporting
185 // errors; other parts of the code may demand it for the info of
187 let span = tcx.def_span(impl_did);
188 tcx.at(span).coerce_unsized_info(impl_did);
191 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
192 debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
194 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
195 let span = tcx.hir().span(impl_hir_id);
197 let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
199 let source = tcx.type_of(impl_did);
200 assert!(!source.has_escaping_bound_vars());
202 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
203 assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
205 trait_ref.substs.type_at(1)
208 debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
210 let param_env = tcx.param_env(impl_did);
212 let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
214 tcx.infer_ctxt().enter(|infcx| {
215 let cause = ObligationCause::misc(span, impl_hir_id);
217 use rustc_type_ir::sty::TyKind::*;
218 match (source.kind(), target.kind()) {
219 (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
220 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
221 (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
222 (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
223 if def_a.is_struct() && def_b.is_struct() =>
226 let source_path = tcx.def_path_str(def_a.did());
227 let target_path = tcx.def_path_str(def_b.did());
230 "the trait `DispatchFromDyn` may only be implemented \
231 for a coercion between structures with the same \
232 definition; expected `{}`, found `{}`",
233 source_path, target_path,
240 if def_a.repr().c() || def_a.repr().packed() {
242 "structs implementing `DispatchFromDyn` may not have \
243 `#[repr(packed)]` or `#[repr(C)]`",
248 let fields = &def_a.non_enum_variant().fields;
250 let coerced_fields = fields
253 let ty_a = field.ty(tcx, substs_a);
254 let ty_b = field.ty(tcx, substs_b);
256 if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
257 if layout.is_zst() && layout.align.abi.bytes() == 1 {
258 // ignore ZST fields with alignment of 1 byte
263 if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
264 if ok.obligations.is_empty() {
266 "the trait `DispatchFromDyn` may only be implemented \
267 for structs containing the field being coerced, \
268 ZST fields with 1 byte alignment, and nothing else",
271 "extra field `{}` of type `{}` is not allowed",
282 .collect::<Vec<_>>();
284 if coerced_fields.is_empty() {
286 "the trait `DispatchFromDyn` may only be implemented \
287 for a coercion between structures with a single field \
288 being coerced, none found",
291 } else if coerced_fields.len() > 1 {
293 "implementing the `DispatchFromDyn` trait requires multiple coercions",
296 "the trait `DispatchFromDyn` may only be implemented \
297 for a coercion between structures with a single field \
301 "currently, {} fields need coercions: {}",
302 coerced_fields.len(),
307 "`{}` (`{}` to `{}`)",
309 field.ty(tcx, substs_a),
310 field.ty(tcx, substs_b),
318 let errors = traits::fully_solve_obligations(
320 coerced_fields.into_iter().map(|field| {
321 predicate_for_trait_def(
325 dispatch_from_dyn_trait,
327 field.ty(tcx, substs_a),
328 &[field.ty(tcx, substs_b).into()],
332 if !errors.is_empty() {
333 infcx.report_fulfillment_errors(&errors, None, false);
336 // Finally, resolve all regions.
337 let outlives_env = OutlivesEnvironment::new(param_env);
338 infcx.check_region_obligations_and_report_errors(impl_did, &outlives_env);
343 "the trait `DispatchFromDyn` may only be implemented \
344 for a coercion between structures",
352 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
353 debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
355 // this provider should only get invoked for local def-ids
356 let impl_did = impl_did.expect_local();
357 let span = tcx.def_span(impl_did);
359 let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
361 let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
362 tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
365 let source = tcx.type_of(impl_did);
366 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
367 assert_eq!(trait_ref.def_id, coerce_unsized_trait);
368 let target = trait_ref.substs.type_at(1);
369 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
371 let param_env = tcx.param_env(impl_did);
372 assert!(!source.has_escaping_bound_vars());
374 let err_info = CoerceUnsizedInfo { custom_kind: None };
376 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
378 tcx.infer_ctxt().enter(|infcx| {
379 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
380 let cause = ObligationCause::misc(span, impl_hir_id);
381 let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
382 mt_b: ty::TypeAndMut<'tcx>,
383 mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
384 if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
386 .report_mismatched_types(
390 ty::error::TypeError::Mutability,
394 (mt_a.ty, mt_b.ty, unsize_trait, None)
396 let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
397 (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
398 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
399 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
400 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
401 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
404 (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
405 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
406 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
409 (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
410 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
413 (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
414 if def_a.is_struct() && def_b.is_struct() =>
417 let source_path = tcx.def_path_str(def_a.did());
418 let target_path = tcx.def_path_str(def_b.did());
423 "the trait `CoerceUnsized` may only be implemented \
424 for a coercion between structures with the same \
425 definition; expected `{}`, found `{}`",
433 // Here we are considering a case of converting
434 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
435 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
437 // struct Foo<T, U> {
442 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
443 // to `Foo<T, [i32]>`. That impl would look like:
445 // impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
447 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
448 // when this coercion occurs, we would be changing the
449 // field `ptr` from a thin pointer of type `*mut [i32;
450 // 3]` to a fat pointer of type `*mut [i32]` (with
451 // extra data `3`). **The purpose of this check is to
452 // make sure that we know how to do this conversion.**
454 // To check if this impl is legal, we would walk down
455 // the fields of `Foo` and consider their types with
456 // both substitutes. We are looking to find that
457 // exactly one (non-phantom) field has changed its
458 // type, which we will expect to be the pointer that
459 // is becoming fat (we could probably generalize this
460 // to multiple thin pointers of the same type becoming
461 // fat, but we don't). In this case:
463 // - `extra` has type `T` before and type `T` after
464 // - `ptr` has type `*mut U` before and type `*mut V` after
466 // Since just one field changed, we would then check
467 // that `*mut U: CoerceUnsized<*mut V>` is implemented
468 // (in other words, that we know how to do this
469 // conversion). This will work out because `U:
470 // Unsize<V>`, and we have a builtin rule that `*mut
471 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
472 let fields = &def_a.non_enum_variant().fields;
473 let diff_fields = fields
476 .filter_map(|(i, f)| {
477 let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
479 if tcx.type_of(f.did).is_phantom_data() {
480 // Ignore PhantomData fields
484 // Ignore fields that aren't changed; it may
485 // be that we could get away with subtyping or
486 // something more accepting, but we use
487 // equality because we want to be able to
488 // perform this check without computing
489 // variance where possible. (This is because
490 // we may have to evaluate constraint
491 // expressions in the course of execution.)
493 if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
494 if ok.obligations.is_empty() {
499 // Collect up all fields that were significantly changed
500 // i.e., those that contain T in coerce_unsized T -> U
503 .collect::<Vec<_>>();
505 if diff_fields.is_empty() {
510 "the trait `CoerceUnsized` may only be implemented \
511 for a coercion between structures with one field \
512 being coerced, none found"
516 } else if diff_fields.len() > 1 {
517 let item = tcx.hir().expect_item(impl_did);
518 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) =
523 tcx.def_span(impl_did)
530 "implementing the trait \
531 `CoerceUnsized` requires multiple \
535 "`CoerceUnsized` may only be implemented for \
536 a coercion between structures with one field being coerced",
539 "currently, {} fields need coercions: {}",
544 format!("`{}` (`{}` to `{}`)", fields[i].name, a, b)
549 .span_label(span, "requires multiple coercions")
554 let (i, a, b) = diff_fields[0];
555 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
556 (a, b, coerce_unsized_trait, Some(kind))
564 "the trait `CoerceUnsized` may only be implemented \
565 for a coercion between structures"
572 // Register an obligation for `A: Trait<B>`.
573 let cause = traits::ObligationCause::misc(span, impl_hir_id);
574 let predicate = predicate_for_trait_def(
583 let errors = traits::fully_solve_obligation(&infcx, predicate);
584 if !errors.is_empty() {
585 infcx.report_fulfillment_errors(&errors, None, false);
588 // Finally, resolve all regions.
589 let outlives_env = OutlivesEnvironment::new(param_env);
590 infcx.check_region_obligations_and_report_errors(impl_did, &outlives_env);
592 CoerceUnsizedInfo { custom_kind: kind }