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::TypeErrCtxtExt;
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 param_env = tcx.param_env(impl_did);
74 assert!(!self_type.has_escaping_bound_vars());
76 debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
78 let span = match tcx.hir().expect_item(impl_did).kind {
79 ItemKind::Impl(hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. }) => return,
80 ItemKind::Impl(impl_) => impl_.self_ty.span,
81 _ => bug!("expected Copy impl item"),
84 let cause = traits::ObligationCause::misc(span, impl_hir_id);
85 match can_type_implement_copy(tcx, param_env, self_type, cause) {
87 Err(CopyImplementationError::InfrigingFields(fields)) => {
88 let mut err = struct_span_err!(
92 "the trait `Copy` may not be implemented for this type"
95 // We'll try to suggest constraining type parameters to fulfill the requirements of
96 // their `Copy` implementation.
97 let mut errors: BTreeMap<_, Vec<_>> = Default::default();
98 let mut bounds = vec![];
100 for (field, ty) in fields {
101 let field_span = tcx.def_span(field.did);
102 let field_ty_span = match tcx.hir().get_if_local(field.did) {
103 Some(hir::Node::Field(field_def)) => field_def.ty.span,
106 err.span_label(field_span, "this field does not implement `Copy`");
107 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
108 // why this field does not implement Copy. This is useful because sometimes
109 // it is not immediately clear why Copy is not implemented for a field, since
110 // all we point at is the field itself.
111 let infcx = tcx.infer_ctxt().ignoring_regions().build();
112 for error in traits::fully_solve_bound(
114 traits::ObligationCause::dummy_with_span(field_ty_span),
117 tcx.require_lang_item(LangItem::Copy, Some(span)),
119 let error_predicate = error.obligation.predicate;
120 // Only note if it's not the root obligation, otherwise it's trivial and
121 // should be self-explanatory (i.e. a field literally doesn't implement Copy).
123 // FIXME: This error could be more descriptive, especially if the error_predicate
124 // contains a foreign type or if it's a deeply nested type...
125 if error_predicate != error.root_obligation.predicate {
127 .entry((ty.to_string(), error_predicate.to_string()))
129 .push(error.obligation.cause.span);
131 if let ty::PredicateKind::Clause(ty::Clause::Trait(ty::TraitPredicate {
133 polarity: ty::ImplPolarity::Positive,
135 })) = error_predicate.kind().skip_binder()
137 let ty = trait_ref.self_ty();
138 if let ty::Param(_) = ty.kind() {
141 trait_ref.print_only_trait_path().to_string(),
142 Some(trait_ref.def_id),
148 for ((ty, error_predicate), spans) in errors {
149 let span: MultiSpan = spans.into();
152 &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
155 suggest_constraining_type_params(
157 tcx.hir().get_generics(impl_did).expect("impls always have generics"),
159 bounds.iter().map(|(param, constraint, def_id)| {
160 (param.as_str(), constraint.as_str(), *def_id)
165 Err(CopyImplementationError::NotAnAdt) => {
166 tcx.sess.emit_err(CopyImplOnNonAdt { span });
168 Err(CopyImplementationError::HasDestructor) => {
169 tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
174 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
175 debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
177 // Just compute this for the side-effects, in particular reporting
178 // errors; other parts of the code may demand it for the info of
180 let span = tcx.def_span(impl_did);
181 tcx.at(span).coerce_unsized_info(impl_did);
184 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
185 debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
187 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
188 let span = tcx.hir().span(impl_hir_id);
190 let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
192 let source = tcx.type_of(impl_did);
193 assert!(!source.has_escaping_bound_vars());
195 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
196 assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
198 trait_ref.substs.type_at(1)
201 debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
203 let param_env = tcx.param_env(impl_did);
205 let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
207 let infcx = tcx.infer_ctxt().build();
208 let cause = ObligationCause::misc(span, impl_hir_id);
210 use rustc_type_ir::sty::TyKind::*;
211 match (source.kind(), target.kind()) {
212 (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
213 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
214 (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
215 (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
216 if def_a.is_struct() && def_b.is_struct() =>
219 let source_path = tcx.def_path_str(def_a.did());
220 let target_path = tcx.def_path_str(def_b.did());
223 "the trait `DispatchFromDyn` may only be implemented \
224 for a coercion between structures with the same \
225 definition; expected `{}`, found `{}`",
226 source_path, target_path,
233 if def_a.repr().c() || def_a.repr().packed() {
235 "structs implementing `DispatchFromDyn` may not have \
236 `#[repr(packed)]` or `#[repr(C)]`",
241 let fields = &def_a.non_enum_variant().fields;
243 let coerced_fields = fields
246 let ty_a = field.ty(tcx, substs_a);
247 let ty_b = field.ty(tcx, substs_b);
249 if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
250 if layout.is_zst() && layout.align.abi.bytes() == 1 {
251 // ignore ZST fields with alignment of 1 byte
256 if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
257 if ok.obligations.is_empty() {
259 "the trait `DispatchFromDyn` may only be implemented \
260 for structs containing the field being coerced, \
261 ZST fields with 1 byte alignment, and nothing else",
264 "extra field `{}` of type `{}` is not allowed",
275 .collect::<Vec<_>>();
277 if coerced_fields.is_empty() {
279 "the trait `DispatchFromDyn` may only be implemented \
280 for a coercion between structures with a single field \
281 being coerced, none found",
284 } else if coerced_fields.len() > 1 {
285 create_err("implementing the `DispatchFromDyn` trait requires multiple coercions")
287 "the trait `DispatchFromDyn` may only be implemented \
288 for a coercion between structures with a single field \
292 "currently, {} fields need coercions: {}",
293 coerced_fields.len(),
298 "`{}` (`{}` to `{}`)",
300 field.ty(tcx, substs_a),
301 field.ty(tcx, substs_b),
309 let errors = traits::fully_solve_obligations(
311 coerced_fields.into_iter().map(|field| {
312 predicate_for_trait_def(
316 dispatch_from_dyn_trait,
318 [field.ty(tcx, substs_a), field.ty(tcx, substs_b)],
322 if !errors.is_empty() {
323 infcx.err_ctxt().report_fulfillment_errors(&errors, None);
326 // Finally, resolve all regions.
327 let outlives_env = OutlivesEnvironment::new(param_env);
328 infcx.check_region_obligations_and_report_errors(impl_did, &outlives_env);
333 "the trait `DispatchFromDyn` may only be implemented \
334 for a coercion between structures",
341 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
342 debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
344 // this provider should only get invoked for local def-ids
345 let impl_did = impl_did.expect_local();
346 let span = tcx.def_span(impl_did);
348 let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
350 let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
351 tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
354 let source = tcx.type_of(impl_did);
355 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
356 assert_eq!(trait_ref.def_id, coerce_unsized_trait);
357 let target = trait_ref.substs.type_at(1);
358 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
360 let param_env = tcx.param_env(impl_did);
361 assert!(!source.has_escaping_bound_vars());
363 let err_info = CoerceUnsizedInfo { custom_kind: None };
365 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
367 let infcx = tcx.infer_ctxt().build();
368 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
369 let cause = ObligationCause::misc(span, impl_hir_id);
370 let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
371 mt_b: ty::TypeAndMut<'tcx>,
372 mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
373 if mt_a.mutbl < mt_b.mutbl {
376 .report_mismatched_types(
380 ty::error::TypeError::Mutability,
384 (mt_a.ty, mt_b.ty, unsize_trait, None)
386 let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
387 (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
388 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
389 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
390 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
391 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
394 (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
395 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
396 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
399 (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty)),
401 (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
402 if def_a.is_struct() && def_b.is_struct() =>
405 let source_path = tcx.def_path_str(def_a.did());
406 let target_path = tcx.def_path_str(def_b.did());
411 "the trait `CoerceUnsized` may only be implemented \
412 for a coercion between structures with the same \
413 definition; expected `{}`, found `{}`",
421 // Here we are considering a case of converting
422 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
423 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
425 // struct Foo<T, U> {
430 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
431 // to `Foo<T, [i32]>`. That impl would look like:
433 // impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
435 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
436 // when this coercion occurs, we would be changing the
437 // field `ptr` from a thin pointer of type `*mut [i32;
438 // 3]` to a fat pointer of type `*mut [i32]` (with
439 // extra data `3`). **The purpose of this check is to
440 // make sure that we know how to do this conversion.**
442 // To check if this impl is legal, we would walk down
443 // the fields of `Foo` and consider their types with
444 // both substitutes. We are looking to find that
445 // exactly one (non-phantom) field has changed its
446 // type, which we will expect to be the pointer that
447 // is becoming fat (we could probably generalize this
448 // to multiple thin pointers of the same type becoming
449 // fat, but we don't). In this case:
451 // - `extra` has type `T` before and type `T` after
452 // - `ptr` has type `*mut U` before and type `*mut V` after
454 // Since just one field changed, we would then check
455 // that `*mut U: CoerceUnsized<*mut V>` is implemented
456 // (in other words, that we know how to do this
457 // conversion). This will work out because `U:
458 // Unsize<V>`, and we have a builtin rule that `*mut
459 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
460 let fields = &def_a.non_enum_variant().fields;
461 let diff_fields = fields
464 .filter_map(|(i, f)| {
465 let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
467 if tcx.type_of(f.did).is_phantom_data() {
468 // Ignore PhantomData fields
472 // Ignore fields that aren't changed; it may
473 // be that we could get away with subtyping or
474 // something more accepting, but we use
475 // equality because we want to be able to
476 // perform this check without computing
477 // variance where possible. (This is because
478 // we may have to evaluate constraint
479 // expressions in the course of execution.)
481 if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
482 if ok.obligations.is_empty() {
487 // Collect up all fields that were significantly changed
488 // i.e., those that contain T in coerce_unsized T -> U
491 .collect::<Vec<_>>();
493 if diff_fields.is_empty() {
498 "the trait `CoerceUnsized` may only be implemented \
499 for a coercion between structures with one field \
500 being coerced, none found"
504 } else if diff_fields.len() > 1 {
505 let item = tcx.hir().expect_item(impl_did);
507 if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) = item.kind {
510 tcx.def_span(impl_did)
517 "implementing the trait \
518 `CoerceUnsized` requires multiple \
522 "`CoerceUnsized` may only be implemented for \
523 a coercion between structures with one field being coerced",
526 "currently, {} fields need coercions: {}",
530 .map(|&(i, a, b)| { format!("`{}` (`{}` to `{}`)", fields[i].name, a, b) })
534 .span_label(span, "requires multiple coercions")
539 let (i, a, b) = diff_fields[0];
540 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
541 (a, b, coerce_unsized_trait, Some(kind))
549 "the trait `CoerceUnsized` may only be implemented \
550 for a coercion between structures"
557 // Register an obligation for `A: Trait<B>`.
558 let cause = traits::ObligationCause::misc(span, impl_hir_id);
560 predicate_for_trait_def(tcx, param_env, cause, trait_def_id, 0, [source, target]);
561 let errors = traits::fully_solve_obligation(&infcx, predicate);
562 if !errors.is_empty() {
563 infcx.err_ctxt().report_fulfillment_errors(&errors, None);
566 // Finally, resolve all regions.
567 let outlives_env = OutlivesEnvironment::new(param_env);
568 infcx.check_region_obligations_and_report_errors(impl_did, &outlives_env);
570 CoerceUnsizedInfo { custom_kind: kind }