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;
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::{RegionckMode, TyCtxtInferExt};
13 use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
14 use rustc_middle::ty::TypeFoldable;
15 use rustc_middle::ty::{self, Ty, TyCtxt};
16 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
17 use rustc_trait_selection::traits::misc::{can_type_implement_copy, CopyImplementationError};
18 use rustc_trait_selection::traits::predicate_for_trait_def;
19 use rustc_trait_selection::traits::{self, ObligationCause, TraitEngine, TraitEngineExt};
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 nominal types.
51 if let ty::Adt(..) | ty::Error(_) = tcx.type_of(impl_did).kind() {
55 let sp = match tcx.hir().expect_item(impl_did).kind {
56 ItemKind::Impl(ref impl_) => impl_.self_ty.span,
57 _ => bug!("expected Drop impl item"),
60 tcx.sess.emit_err(DropImplOnWrongItem { span: sp });
63 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
64 debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
66 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
68 let self_type = tcx.type_of(impl_did);
69 debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
71 let span = tcx.hir().span(impl_hir_id);
72 let param_env = tcx.param_env(impl_did);
73 assert!(!self_type.has_escaping_bound_vars());
75 debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
77 let cause = traits::ObligationCause::misc(span, impl_hir_id);
78 match can_type_implement_copy(tcx, param_env, self_type, cause) {
80 Err(CopyImplementationError::InfrigingFields(fields)) => {
81 let item = tcx.hir().expect_item(impl_did);
82 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref tr), .. }) = item.kind {
88 let mut err = struct_span_err!(
92 "the trait `Copy` may not be implemented for this type"
94 for (field, ty) in fields {
95 let field_span = tcx.def_span(field.did);
96 err.span_label(field_span, "this field does not implement `Copy`");
97 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
98 // why this field does not implement Copy. This is useful because sometimes
99 // it is not immediately clear why Copy is not implemented for a field, since
100 // all we point at is the field itself.
101 tcx.infer_ctxt().enter(|infcx| {
102 let mut fulfill_cx = traits::FulfillmentContext::new_ignoring_regions();
103 fulfill_cx.register_bound(
107 tcx.lang_items().copy_trait().unwrap(),
108 traits::ObligationCause::dummy_with_span(field_span),
110 for error in fulfill_cx.select_all_or_error(&infcx) {
111 let error_predicate = error.obligation.predicate;
112 // Only note if it's not the root obligation, otherwise it's trivial and
113 // should be self-explanatory (i.e. a field literally doesn't implement Copy).
115 // FIXME: This error could be more descriptive, especially if the error_predicate
116 // contains a foreign type or if it's a deeply nested type...
117 if error_predicate != error.root_obligation.predicate {
119 error.obligation.cause.span,
121 "the `Copy` impl for `{}` requires that `{}`",
131 Err(CopyImplementationError::NotAnAdt) => {
132 let item = tcx.hir().expect_item(impl_did);
134 if let ItemKind::Impl(ref impl_) = item.kind { impl_.self_ty.span } else { span };
136 tcx.sess.emit_err(CopyImplOnNonAdt { span });
138 Err(CopyImplementationError::HasDestructor) => {
139 tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
144 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
145 debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
147 // Just compute this for the side-effects, in particular reporting
148 // errors; other parts of the code may demand it for the info of
150 let span = tcx.def_span(impl_did);
151 tcx.at(span).coerce_unsized_info(impl_did);
154 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
155 debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
157 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
158 let span = tcx.hir().span(impl_hir_id);
160 let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
162 let source = tcx.type_of(impl_did);
163 assert!(!source.has_escaping_bound_vars());
165 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
166 assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
168 trait_ref.substs.type_at(1)
171 debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
173 let param_env = tcx.param_env(impl_did);
175 let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
177 tcx.infer_ctxt().enter(|infcx| {
178 let cause = ObligationCause::misc(span, impl_hir_id);
180 use rustc_type_ir::sty::TyKind::*;
181 match (source.kind(), target.kind()) {
182 (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
183 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
184 (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
185 (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
186 if def_a.is_struct() && def_b.is_struct() =>
189 let source_path = tcx.def_path_str(def_a.did());
190 let target_path = tcx.def_path_str(def_b.did());
193 "the trait `DispatchFromDyn` may only be implemented \
194 for a coercion between structures with the same \
195 definition; expected `{}`, found `{}`",
196 source_path, target_path,
203 if def_a.repr().c() || def_a.repr().packed() {
205 "structs implementing `DispatchFromDyn` may not have \
206 `#[repr(packed)]` or `#[repr(C)]`",
211 let fields = &def_a.non_enum_variant().fields;
213 let coerced_fields = fields
216 let ty_a = field.ty(tcx, substs_a);
217 let ty_b = field.ty(tcx, substs_b);
219 if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
220 if layout.is_zst() && layout.align.abi.bytes() == 1 {
221 // ignore ZST fields with alignment of 1 byte
226 if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
227 if ok.obligations.is_empty() {
229 "the trait `DispatchFromDyn` may only be implemented \
230 for structs containing the field being coerced, \
231 ZST fields with 1 byte alignment, and nothing else",
234 "extra field `{}` of type `{}` is not allowed",
245 .collect::<Vec<_>>();
247 if coerced_fields.is_empty() {
249 "the trait `DispatchFromDyn` may only be implemented \
250 for a coercion between structures with a single field \
251 being coerced, none found",
254 } else if coerced_fields.len() > 1 {
256 "implementing the `DispatchFromDyn` trait requires multiple coercions",
259 "the trait `DispatchFromDyn` may only be implemented \
260 for a coercion between structures with a single field \
264 "currently, {} fields need coercions: {}",
265 coerced_fields.len(),
270 "`{}` (`{}` to `{}`)",
272 field.ty(tcx, substs_a),
273 field.ty(tcx, substs_b),
281 let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
283 for field in coerced_fields {
284 let predicate = predicate_for_trait_def(
288 dispatch_from_dyn_trait,
290 field.ty(tcx, substs_a),
291 &[field.ty(tcx, substs_b).into()],
294 fulfill_cx.register_predicate_obligation(&infcx, predicate);
297 // Check that all transitive obligations are satisfied.
298 let errors = fulfill_cx.select_all_or_error(&infcx);
299 if !errors.is_empty() {
300 infcx.report_fulfillment_errors(&errors, None, false);
303 // Finally, resolve all regions.
304 let outlives_env = OutlivesEnvironment::new(param_env);
305 infcx.resolve_regions_and_report_errors(
306 impl_did.to_def_id(),
308 RegionckMode::default(),
314 "the trait `DispatchFromDyn` may only be implemented \
315 for a coercion between structures",
323 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
324 debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
326 // this provider should only get invoked for local def-ids
327 let impl_did = impl_did.expect_local();
328 let span = tcx.def_span(impl_did);
330 let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
332 let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
333 tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
336 let source = tcx.type_of(impl_did);
337 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
338 assert_eq!(trait_ref.def_id, coerce_unsized_trait);
339 let target = trait_ref.substs.type_at(1);
340 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
342 let param_env = tcx.param_env(impl_did);
343 assert!(!source.has_escaping_bound_vars());
345 let err_info = CoerceUnsizedInfo { custom_kind: None };
347 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
349 tcx.infer_ctxt().enter(|infcx| {
350 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
351 let cause = ObligationCause::misc(span, impl_hir_id);
352 let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
353 mt_b: ty::TypeAndMut<'tcx>,
354 mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
355 if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
357 .report_mismatched_types(
361 ty::error::TypeError::Mutability,
365 (mt_a.ty, mt_b.ty, unsize_trait, None)
367 let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
368 (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
369 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
370 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
371 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
372 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
375 (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
376 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
377 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
380 (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
381 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
384 (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
385 if def_a.is_struct() && def_b.is_struct() =>
388 let source_path = tcx.def_path_str(def_a.did());
389 let target_path = tcx.def_path_str(def_b.did());
394 "the trait `CoerceUnsized` may only be implemented \
395 for a coercion between structures with the same \
396 definition; expected `{}`, found `{}`",
404 // Here we are considering a case of converting
405 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
406 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
408 // struct Foo<T, U> {
413 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
414 // to `Foo<T, [i32]>`. That impl would look like:
416 // impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
418 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
419 // when this coercion occurs, we would be changing the
420 // field `ptr` from a thin pointer of type `*mut [i32;
421 // 3]` to a fat pointer of type `*mut [i32]` (with
422 // extra data `3`). **The purpose of this check is to
423 // make sure that we know how to do this conversion.**
425 // To check if this impl is legal, we would walk down
426 // the fields of `Foo` and consider their types with
427 // both substitutes. We are looking to find that
428 // exactly one (non-phantom) field has changed its
429 // type, which we will expect to be the pointer that
430 // is becoming fat (we could probably generalize this
431 // to multiple thin pointers of the same type becoming
432 // fat, but we don't). In this case:
434 // - `extra` has type `T` before and type `T` after
435 // - `ptr` has type `*mut U` before and type `*mut V` after
437 // Since just one field changed, we would then check
438 // that `*mut U: CoerceUnsized<*mut V>` is implemented
439 // (in other words, that we know how to do this
440 // conversion). This will work out because `U:
441 // Unsize<V>`, and we have a builtin rule that `*mut
442 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
443 let fields = &def_a.non_enum_variant().fields;
444 let diff_fields = fields
447 .filter_map(|(i, f)| {
448 let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
450 if tcx.type_of(f.did).is_phantom_data() {
451 // Ignore PhantomData fields
455 // Ignore fields that aren't changed; it may
456 // be that we could get away with subtyping or
457 // something more accepting, but we use
458 // equality because we want to be able to
459 // perform this check without computing
460 // variance where possible. (This is because
461 // we may have to evaluate constraint
462 // expressions in the course of execution.)
464 if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
465 if ok.obligations.is_empty() {
470 // Collect up all fields that were significantly changed
471 // i.e., those that contain T in coerce_unsized T -> U
474 .collect::<Vec<_>>();
476 if diff_fields.is_empty() {
481 "the trait `CoerceUnsized` may only be implemented \
482 for a coercion between structures with one field \
483 being coerced, none found"
487 } else if diff_fields.len() > 1 {
488 let item = tcx.hir().expect_item(impl_did);
489 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) =
494 tcx.def_span(impl_did)
501 "implementing the trait \
502 `CoerceUnsized` requires multiple \
506 "`CoerceUnsized` may only be implemented for \
507 a coercion between structures with one field being coerced",
510 "currently, {} fields need coercions: {}",
515 format!("`{}` (`{}` to `{}`)", fields[i].name, a, b)
520 .span_label(span, "requires multiple coercions")
525 let (i, a, b) = diff_fields[0];
526 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
527 (a, b, coerce_unsized_trait, Some(kind))
535 "the trait `CoerceUnsized` may only be implemented \
536 for a coercion between structures"
543 let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
545 // Register an obligation for `A: Trait<B>`.
546 let cause = traits::ObligationCause::misc(span, impl_hir_id);
547 let predicate = predicate_for_trait_def(
556 fulfill_cx.register_predicate_obligation(&infcx, predicate);
558 // Check that all transitive obligations are satisfied.
559 let errors = fulfill_cx.select_all_or_error(&infcx);
560 if !errors.is_empty() {
561 infcx.report_fulfillment_errors(&errors, None, false);
564 // Finally, resolve all regions.
565 let outlives_env = OutlivesEnvironment::new(param_env);
566 infcx.resolve_regions_and_report_errors(
567 impl_did.to_def_id(),
569 RegionckMode::default(),
572 CoerceUnsizedInfo { custom_kind: kind }