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::outlives::env::OutlivesEnvironment;
11 use rustc_infer::infer::TyCtxtInferExt;
12 use rustc_infer::infer::{self, RegionResolutionError};
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::{
17 type_allowed_to_implement_copy, CopyImplementationError, InfringingFieldsReason,
19 use rustc_trait_selection::traits::predicate_for_trait_def;
20 use rustc_trait_selection::traits::{self, ObligationCause};
21 use std::collections::BTreeMap;
23 pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
24 let lang_items = tcx.lang_items();
25 Checker { tcx, trait_def_id }
26 .check(lang_items.drop_trait(), visit_implementation_of_drop)
27 .check(lang_items.copy_trait(), visit_implementation_of_copy)
28 .check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
29 .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn);
32 struct Checker<'tcx> {
37 impl<'tcx> Checker<'tcx> {
38 fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
40 F: FnMut(TyCtxt<'tcx>, LocalDefId),
42 if Some(self.trait_def_id) == trait_def_id {
43 for &impl_def_id in self.tcx.hir().trait_impls(self.trait_def_id) {
44 f(self.tcx, impl_def_id);
51 fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
52 // Destructors only work on local ADT types.
53 match tcx.type_of(impl_did).kind() {
54 ty::Adt(def, _) if def.did().is_local() => return,
55 ty::Error(_) => return,
59 let impl_ = tcx.hir().expect_item(impl_did).expect_impl();
61 tcx.sess.emit_err(DropImplOnWrongItem { span: impl_.self_ty.span });
64 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
65 debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
67 let self_type = tcx.type_of(impl_did);
68 debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
70 let param_env = tcx.param_env(impl_did);
71 assert!(!self_type.has_escaping_bound_vars());
73 debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
75 let span = match tcx.hir().expect_item(impl_did).kind {
76 ItemKind::Impl(hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. }) => return,
77 ItemKind::Impl(impl_) => impl_.self_ty.span,
78 _ => bug!("expected Copy impl item"),
81 let cause = traits::ObligationCause::misc(span, impl_did);
82 match type_allowed_to_implement_copy(tcx, param_env, self_type, cause) {
84 Err(CopyImplementationError::InfrigingFields(fields)) => {
85 let mut err = struct_span_err!(
89 "the trait `Copy` may not be implemented for this type"
92 // We'll try to suggest constraining type parameters to fulfill the requirements of
93 // their `Copy` implementation.
94 let mut errors: BTreeMap<_, Vec<_>> = Default::default();
95 let mut bounds = vec![];
97 for (field, ty, reason) in fields {
98 let field_span = tcx.def_span(field.did);
99 err.span_label(field_span, "this field does not implement `Copy`");
102 InfringingFieldsReason::Fulfill(fulfillment_errors) => {
103 for error in fulfillment_errors {
104 let error_predicate = error.obligation.predicate;
105 // Only note if it's not the root obligation, otherwise it's trivial and
106 // should be self-explanatory (i.e. a field literally doesn't implement Copy).
108 // FIXME: This error could be more descriptive, especially if the error_predicate
109 // contains a foreign type or if it's a deeply nested type...
110 if error_predicate != error.root_obligation.predicate {
112 .entry((ty.to_string(), error_predicate.to_string()))
114 .push(error.obligation.cause.span);
116 if let ty::PredicateKind::Clause(ty::Clause::Trait(
119 polarity: ty::ImplPolarity::Positive,
122 )) = error_predicate.kind().skip_binder()
124 let ty = trait_ref.self_ty();
125 if let ty::Param(_) = ty.kind() {
128 trait_ref.print_only_trait_path().to_string(),
129 Some(trait_ref.def_id),
135 InfringingFieldsReason::Regions(region_errors) => {
136 for error in region_errors {
137 let ty = ty.to_string();
139 RegionResolutionError::ConcreteFailure(origin, a, b) => {
140 let predicate = format!("{b}: {a}");
142 .entry((ty.clone(), predicate.clone()))
144 .push(origin.span());
145 if let ty::RegionKind::ReEarlyBound(ebr) = *b && ebr.has_name() {
146 bounds.push((b.to_string(), a.to_string(), None));
149 RegionResolutionError::GenericBoundFailure(origin, a, b) => {
150 let predicate = format!("{a}: {b}");
152 .entry((ty.clone(), predicate.clone()))
154 .push(origin.span());
155 if let infer::region_constraints::GenericKind::Param(_) = a {
156 bounds.push((a.to_string(), b.to_string(), None));
165 for ((ty, error_predicate), spans) in errors {
166 let span: MultiSpan = spans.into();
169 &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
172 suggest_constraining_type_params(
174 tcx.hir().get_generics(impl_did).expect("impls always have generics"),
176 bounds.iter().map(|(param, constraint, def_id)| {
177 (param.as_str(), constraint.as_str(), *def_id)
183 Err(CopyImplementationError::NotAnAdt) => {
184 tcx.sess.emit_err(CopyImplOnNonAdt { span });
186 Err(CopyImplementationError::HasDestructor) => {
187 tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
192 fn visit_implementation_of_coerce_unsized(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
193 debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
195 // Just compute this for the side-effects, in particular reporting
196 // errors; other parts of the code may demand it for the info of
198 let span = tcx.def_span(impl_did);
199 tcx.at(span).coerce_unsized_info(impl_did);
202 fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
203 debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
205 let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
206 let span = tcx.hir().span(impl_hir_id);
208 let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
210 let source = tcx.type_of(impl_did);
211 assert!(!source.has_escaping_bound_vars());
213 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().subst_identity();
214 assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
216 trait_ref.substs.type_at(1)
219 debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
221 let param_env = tcx.param_env(impl_did);
223 let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
225 let infcx = tcx.infer_ctxt().build();
226 let cause = ObligationCause::misc(span, impl_did);
228 use rustc_type_ir::sty::TyKind::*;
229 match (source.kind(), target.kind()) {
230 (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
231 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
232 (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
233 (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
234 if def_a.is_struct() && def_b.is_struct() =>
237 let source_path = tcx.def_path_str(def_a.did());
238 let target_path = tcx.def_path_str(def_b.did());
241 "the trait `DispatchFromDyn` may only be implemented \
242 for a coercion between structures with the same \
243 definition; expected `{}`, found `{}`",
244 source_path, target_path,
251 if def_a.repr().c() || def_a.repr().packed() {
253 "structs implementing `DispatchFromDyn` may not have \
254 `#[repr(packed)]` or `#[repr(C)]`",
259 let fields = &def_a.non_enum_variant().fields;
261 let coerced_fields = fields
264 let ty_a = field.ty(tcx, substs_a);
265 let ty_b = field.ty(tcx, substs_b);
267 if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
268 if layout.is_zst() && layout.align.abi.bytes() == 1 {
269 // ignore ZST fields with alignment of 1 byte
274 if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
275 if ok.obligations.is_empty() {
277 "the trait `DispatchFromDyn` may only be implemented \
278 for structs containing the field being coerced, \
279 ZST fields with 1 byte alignment, and nothing else",
282 "extra field `{}` of type `{}` is not allowed",
293 .collect::<Vec<_>>();
295 if coerced_fields.is_empty() {
297 "the trait `DispatchFromDyn` may only be implemented \
298 for a coercion between structures with a single field \
299 being coerced, none found",
302 } else if coerced_fields.len() > 1 {
303 create_err("implementing the `DispatchFromDyn` trait requires multiple coercions")
305 "the trait `DispatchFromDyn` may only be implemented \
306 for a coercion between structures with a single field \
310 "currently, {} fields need coercions: {}",
311 coerced_fields.len(),
316 "`{}` (`{}` to `{}`)",
318 field.ty(tcx, substs_a),
319 field.ty(tcx, substs_b),
327 let errors = traits::fully_solve_obligations(
329 coerced_fields.into_iter().map(|field| {
330 predicate_for_trait_def(
334 dispatch_from_dyn_trait,
336 [field.ty(tcx, substs_a), field.ty(tcx, substs_b)],
340 if !errors.is_empty() {
341 infcx.err_ctxt().report_fulfillment_errors(&errors, None);
344 // Finally, resolve all regions.
345 let outlives_env = OutlivesEnvironment::new(param_env);
348 .check_region_obligations_and_report_errors(impl_did, &outlives_env);
353 "the trait `DispatchFromDyn` may only be implemented \
354 for a coercion between structures",
361 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
362 debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
364 // this provider should only get invoked for local def-ids
365 let impl_did = impl_did.expect_local();
366 let span = tcx.def_span(impl_did);
368 let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
370 let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
371 tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
374 let source = tcx.type_of(impl_did);
375 let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().subst_identity();
376 assert_eq!(trait_ref.def_id, coerce_unsized_trait);
377 let target = trait_ref.substs.type_at(1);
378 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
380 let param_env = tcx.param_env(impl_did);
381 assert!(!source.has_escaping_bound_vars());
383 let err_info = CoerceUnsizedInfo { custom_kind: None };
385 debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
387 let infcx = tcx.infer_ctxt().build();
388 let cause = ObligationCause::misc(span, impl_did);
389 let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
390 mt_b: ty::TypeAndMut<'tcx>,
391 mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
392 if mt_a.mutbl < mt_b.mutbl {
395 .report_mismatched_types(
399 ty::error::TypeError::Mutability,
403 (mt_a.ty, mt_b.ty, unsize_trait, None)
405 let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
406 (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
407 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
408 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
409 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
410 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
413 (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
414 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
415 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
418 (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty)),
420 (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
421 if def_a.is_struct() && def_b.is_struct() =>
424 let source_path = tcx.def_path_str(def_a.did());
425 let target_path = tcx.def_path_str(def_b.did());
430 "the trait `CoerceUnsized` may only be implemented \
431 for a coercion between structures with the same \
432 definition; expected `{}`, found `{}`",
440 // Here we are considering a case of converting
441 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
442 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
444 // struct Foo<T, U> {
449 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
450 // to `Foo<T, [i32]>`. That impl would look like:
452 // impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
454 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
455 // when this coercion occurs, we would be changing the
456 // field `ptr` from a thin pointer of type `*mut [i32;
457 // 3]` to a fat pointer of type `*mut [i32]` (with
458 // extra data `3`). **The purpose of this check is to
459 // make sure that we know how to do this conversion.**
461 // To check if this impl is legal, we would walk down
462 // the fields of `Foo` and consider their types with
463 // both substitutes. We are looking to find that
464 // exactly one (non-phantom) field has changed its
465 // type, which we will expect to be the pointer that
466 // is becoming fat (we could probably generalize this
467 // to multiple thin pointers of the same type becoming
468 // fat, but we don't). In this case:
470 // - `extra` has type `T` before and type `T` after
471 // - `ptr` has type `*mut U` before and type `*mut V` after
473 // Since just one field changed, we would then check
474 // that `*mut U: CoerceUnsized<*mut V>` is implemented
475 // (in other words, that we know how to do this
476 // conversion). This will work out because `U:
477 // Unsize<V>`, and we have a builtin rule that `*mut
478 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
479 let fields = &def_a.non_enum_variant().fields;
480 let diff_fields = fields
483 .filter_map(|(i, f)| {
484 let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
486 if tcx.type_of(f.did).is_phantom_data() {
487 // Ignore PhantomData fields
491 // Ignore fields that aren't changed; it may
492 // be that we could get away with subtyping or
493 // something more accepting, but we use
494 // equality because we want to be able to
495 // perform this check without computing
496 // variance where possible. (This is because
497 // we may have to evaluate constraint
498 // expressions in the course of execution.)
500 if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
501 if ok.obligations.is_empty() {
506 // Collect up all fields that were significantly changed
507 // i.e., those that contain T in coerce_unsized T -> U
510 .collect::<Vec<_>>();
512 if diff_fields.is_empty() {
517 "the trait `CoerceUnsized` may only be implemented \
518 for a coercion between structures with one field \
519 being coerced, none found"
523 } else if diff_fields.len() > 1 {
524 let item = tcx.hir().expect_item(impl_did);
525 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(t), .. }) = &item.kind {
528 tcx.def_span(impl_did)
535 "implementing the trait \
536 `CoerceUnsized` requires multiple \
540 "`CoerceUnsized` may only be implemented for \
541 a coercion between structures with one field being coerced",
544 "currently, {} fields need coercions: {}",
548 .map(|&(i, a, b)| { format!("`{}` (`{}` to `{}`)", fields[i].name, a, b) })
552 .span_label(span, "requires multiple coercions")
557 let (i, a, b) = diff_fields[0];
558 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
559 (a, b, coerce_unsized_trait, Some(kind))
567 "the trait `CoerceUnsized` may only be implemented \
568 for a coercion between structures"
575 // Register an obligation for `A: Trait<B>`.
576 let cause = traits::ObligationCause::misc(span, impl_did);
578 predicate_for_trait_def(tcx, param_env, cause, trait_def_id, 0, [source, target]);
579 let errors = traits::fully_solve_obligation(&infcx, predicate);
580 if !errors.is_empty() {
581 infcx.err_ctxt().report_fulfillment_errors(&errors, None);
584 // Finally, resolve all regions.
585 let outlives_env = OutlivesEnvironment::new(param_env);
586 let _ = infcx.err_ctxt().check_region_obligations_and_report_errors(impl_did, &outlives_env);
588 CoerceUnsizedInfo { custom_kind: kind }