rustc_ty_utils/
ty.rs

1use rustc_data_structures::fx::FxHashSet;
2use rustc_hir as hir;
3use rustc_hir::def::DefKind;
4use rustc_index::bit_set::DenseBitSet;
5use rustc_infer::infer::TyCtxtInferExt;
6use rustc_middle::bug;
7use rustc_middle::query::Providers;
8use rustc_middle::ty::{
9    self, SizedTraitKind, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor, Upcast,
10    fold_regions,
11};
12use rustc_span::DUMMY_SP;
13use rustc_span::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
14use rustc_trait_selection::traits;
15use tracing::instrument;
16
17/// If `ty` implements the given `sizedness` trait, returns `None`. Otherwise, returns the type
18/// that must implement the given `sizedness` for `ty` to implement it.
19#[instrument(level = "debug", skip(tcx), ret)]
20fn sizedness_constraint_for_ty<'tcx>(
21    tcx: TyCtxt<'tcx>,
22    sizedness: SizedTraitKind,
23    ty: Ty<'tcx>,
24) -> Option<Ty<'tcx>> {
25    match ty.kind() {
26        // Always `Sized` or `MetaSized`
27        ty::Bool
28        | ty::Char
29        | ty::Int(..)
30        | ty::Uint(..)
31        | ty::Float(..)
32        | ty::RawPtr(..)
33        | ty::Ref(..)
34        | ty::FnDef(..)
35        | ty::FnPtr(..)
36        | ty::Array(..)
37        | ty::Closure(..)
38        | ty::CoroutineClosure(..)
39        | ty::Coroutine(..)
40        | ty::CoroutineWitness(..)
41        | ty::Never => None,
42
43        ty::Str | ty::Slice(..) | ty::Dynamic(_, _, ty::Dyn) => match sizedness {
44            // Never `Sized`
45            SizedTraitKind::Sized => Some(ty),
46            // Always `MetaSized`
47            SizedTraitKind::MetaSized => None,
48        },
49
50        // Maybe `Sized` or `MetaSized`
51        ty::Param(..) | ty::Alias(..) | ty::Error(_) => Some(ty),
52
53        // We cannot instantiate the binder, so just return the *original* type back,
54        // but only if the inner type has a sized constraint. Thus we skip the binder,
55        // but don't actually use the result from `sized_constraint_for_ty`.
56        ty::UnsafeBinder(inner_ty) => {
57            sizedness_constraint_for_ty(tcx, sizedness, inner_ty.skip_binder()).map(|_| ty)
58        }
59
60        // Never `MetaSized` or `Sized`
61        ty::Foreign(..) => Some(ty),
62
63        // Recursive cases
64        ty::Pat(ty, _) => sizedness_constraint_for_ty(tcx, sizedness, *ty),
65
66        ty::Tuple(tys) => {
67            tys.last().and_then(|&ty| sizedness_constraint_for_ty(tcx, sizedness, ty))
68        }
69
70        ty::Adt(adt, args) => adt.sizedness_constraint(tcx, sizedness).and_then(|intermediate| {
71            let ty = intermediate.instantiate(tcx, args);
72            sizedness_constraint_for_ty(tcx, sizedness, ty)
73        }),
74
75        ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => {
76            bug!("unexpected type `{ty:?}` in `sizedness_constraint_for_ty`")
77        }
78    }
79}
80
81fn defaultness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Defaultness {
82    match tcx.hir_node_by_def_id(def_id) {
83        hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.defaultness,
84        hir::Node::ImplItem(hir::ImplItem { defaultness, .. })
85        | hir::Node::TraitItem(hir::TraitItem { defaultness, .. }) => *defaultness,
86        node => {
87            bug!("`defaultness` called on {:?}", node);
88        }
89    }
90}
91
92/// Returns the type of the last field of a struct ("the constraint") which must implement the
93/// `sizedness` trait for the whole ADT to be considered to implement that `sizedness` trait.
94/// `def_id` is assumed to be the `AdtDef` of a struct and will panic otherwise.
95///
96/// For `Sized`, there are only a few options for the types in the constraint:
97///     - an meta-sized type (str, slices, trait objects, etc)
98///     - an pointee-sized type (extern types)
99///     - a type parameter or projection whose sizedness can't be known
100///
101/// For `MetaSized`, there are only a few options for the types in the constraint:
102///     - an pointee-sized type (extern types)
103///     - a type parameter or projection whose sizedness can't be known
104#[instrument(level = "debug", skip(tcx), ret)]
105fn adt_sizedness_constraint<'tcx>(
106    tcx: TyCtxt<'tcx>,
107    (def_id, sizedness): (DefId, SizedTraitKind),
108) -> Option<ty::EarlyBinder<'tcx, Ty<'tcx>>> {
109    if let Some(def_id) = def_id.as_local() {
110        if let ty::Representability::Infinite(_) = tcx.representability(def_id) {
111            return None;
112        }
113    }
114    let def = tcx.adt_def(def_id);
115
116    if !def.is_struct() {
117        bug!("`adt_sizedness_constraint` called on non-struct type: {def:?}");
118    }
119
120    let tail_def = def.non_enum_variant().tail_opt()?;
121    let tail_ty = tcx.type_of(tail_def.did).instantiate_identity();
122
123    let constraint_ty = sizedness_constraint_for_ty(tcx, sizedness, tail_ty)?;
124
125    // perf hack: if there is a `constraint_ty: {Meta,}Sized` bound, then we know
126    // that the type is sized and do not need to check it on the impl.
127    let sizedness_trait_def_id = sizedness.require_lang_item(tcx);
128    let predicates = tcx.predicates_of(def.did()).predicates;
129    if predicates.iter().any(|(p, _)| {
130        p.as_trait_clause().is_some_and(|trait_pred| {
131            trait_pred.def_id() == sizedness_trait_def_id
132                && trait_pred.self_ty().skip_binder() == constraint_ty
133        })
134    }) {
135        return None;
136    }
137
138    Some(ty::EarlyBinder::bind(constraint_ty))
139}
140
141/// See `ParamEnv` struct definition for details.
142fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
143    // Compute the bounds on Self and the type parameters.
144    let ty::InstantiatedPredicates { mut predicates, .. } =
145        tcx.predicates_of(def_id).instantiate_identity(tcx);
146
147    // Finally, we have to normalize the bounds in the environment, in
148    // case they contain any associated type projections. This process
149    // can yield errors if the put in illegal associated types, like
150    // `<i32 as Foo>::Bar` where `i32` does not implement `Foo`. We
151    // report these errors right here; this doesn't actually feel
152    // right to me, because constructing the environment feels like a
153    // kind of an "idempotent" action, but I'm not sure where would be
154    // a better place. In practice, we construct environments for
155    // every fn once during type checking, and we'll abort if there
156    // are any errors at that point, so outside of type inference you can be
157    // sure that this will succeed without errors anyway.
158
159    if tcx.def_kind(def_id) == DefKind::AssocFn
160        && let assoc_item = tcx.associated_item(def_id)
161        && assoc_item.container == ty::AssocItemContainer::Trait
162        && assoc_item.defaultness(tcx).has_value()
163    {
164        let sig = tcx.fn_sig(def_id).instantiate_identity();
165        // We accounted for the binder of the fn sig, so skip the binder.
166        sig.skip_binder().visit_with(&mut ImplTraitInTraitFinder {
167            tcx,
168            fn_def_id: def_id,
169            bound_vars: sig.bound_vars(),
170            predicates: &mut predicates,
171            seen: FxHashSet::default(),
172            depth: ty::INNERMOST,
173        });
174    }
175
176    // We extend the param-env of our item with the const conditions of the item,
177    // since we're allowed to assume `[const]` bounds hold within the item itself.
178    if tcx.is_conditionally_const(def_id) {
179        predicates.extend(
180            tcx.const_conditions(def_id).instantiate_identity(tcx).into_iter().map(
181                |(trait_ref, _)| trait_ref.to_host_effect_clause(tcx, ty::BoundConstness::Maybe),
182            ),
183        );
184    }
185
186    let local_did = def_id.as_local();
187
188    let unnormalized_env = ty::ParamEnv::new(tcx.mk_clauses(&predicates));
189
190    let body_id = local_did.unwrap_or(CRATE_DEF_ID);
191    let cause = traits::ObligationCause::misc(tcx.def_span(def_id), body_id);
192    traits::normalize_param_env_or_error(tcx, unnormalized_env, cause)
193}
194
195/// Walk through a function type, gathering all RPITITs and installing a
196/// `NormalizesTo(Projection(RPITIT) -> Opaque(RPITIT))` predicate into the
197/// predicates list. This allows us to observe that an RPITIT projects to
198/// its corresponding opaque within the body of a default-body trait method.
199struct ImplTraitInTraitFinder<'a, 'tcx> {
200    tcx: TyCtxt<'tcx>,
201    predicates: &'a mut Vec<ty::Clause<'tcx>>,
202    fn_def_id: DefId,
203    bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
204    seen: FxHashSet<DefId>,
205    depth: ty::DebruijnIndex,
206}
207
208impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ImplTraitInTraitFinder<'_, 'tcx> {
209    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(&mut self, binder: &ty::Binder<'tcx, T>) {
210        self.depth.shift_in(1);
211        binder.super_visit_with(self);
212        self.depth.shift_out(1);
213    }
214
215    fn visit_ty(&mut self, ty: Ty<'tcx>) {
216        if let ty::Alias(ty::Projection, unshifted_alias_ty) = *ty.kind()
217            && let Some(
218                ty::ImplTraitInTraitData::Trait { fn_def_id, .. }
219                | ty::ImplTraitInTraitData::Impl { fn_def_id, .. },
220            ) = self.tcx.opt_rpitit_info(unshifted_alias_ty.def_id)
221            && fn_def_id == self.fn_def_id
222            && self.seen.insert(unshifted_alias_ty.def_id)
223        {
224            // We have entered some binders as we've walked into the
225            // bounds of the RPITIT. Shift these binders back out when
226            // constructing the top-level projection predicate.
227            let shifted_alias_ty = fold_regions(self.tcx, unshifted_alias_ty, |re, depth| {
228                if let ty::ReBound(index, bv) = re.kind() {
229                    if depth != ty::INNERMOST {
230                        return ty::Region::new_error_with_message(
231                            self.tcx,
232                            DUMMY_SP,
233                            "we shouldn't walk non-predicate binders with `impl Trait`...",
234                        );
235                    }
236                    ty::Region::new_bound(self.tcx, index.shifted_out_to_binder(self.depth), bv)
237                } else {
238                    re
239                }
240            });
241
242            // If we're lowering to associated item, install the opaque type which is just
243            // the `type_of` of the trait's associated item. If we're using the old lowering
244            // strategy, then just reinterpret the associated type like an opaque :^)
245            let default_ty = self
246                .tcx
247                .type_of(shifted_alias_ty.def_id)
248                .instantiate(self.tcx, shifted_alias_ty.args);
249
250            self.predicates.push(
251                ty::Binder::bind_with_vars(
252                    ty::ProjectionPredicate {
253                        projection_term: shifted_alias_ty.into(),
254                        term: default_ty.into(),
255                    },
256                    self.bound_vars,
257                )
258                .upcast(self.tcx),
259            );
260
261            // We walk the *un-shifted* alias ty, because we're tracking the de bruijn
262            // binder depth, and if we were to walk `shifted_alias_ty` instead, we'd
263            // have to reset `self.depth` back to `ty::INNERMOST` or something. It's
264            // easier to just do this.
265            for bound in self
266                .tcx
267                .item_bounds(unshifted_alias_ty.def_id)
268                .iter_instantiated(self.tcx, unshifted_alias_ty.args)
269            {
270                bound.visit_with(self);
271            }
272        }
273
274        ty.super_visit_with(self)
275    }
276}
277
278fn typing_env_normalized_for_post_analysis(tcx: TyCtxt<'_>, def_id: DefId) -> ty::TypingEnv<'_> {
279    ty::TypingEnv::non_body_analysis(tcx, def_id).with_post_analysis_normalized(tcx)
280}
281
282/// Check if a function is async.
283fn asyncness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Asyncness {
284    let node = tcx.hir_node_by_def_id(def_id);
285    node.fn_sig().map_or(ty::Asyncness::No, |sig| match sig.header.asyncness {
286        hir::IsAsync::Async(_) => ty::Asyncness::Yes,
287        hir::IsAsync::NotAsync => ty::Asyncness::No,
288    })
289}
290
291fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> DenseBitSet<u32> {
292    let def = tcx.adt_def(def_id);
293    let num_params = tcx.generics_of(def_id).count();
294
295    let maybe_unsizing_param_idx = |arg: ty::GenericArg<'tcx>| match arg.kind() {
296        ty::GenericArgKind::Type(ty) => match ty.kind() {
297            ty::Param(p) => Some(p.index),
298            _ => None,
299        },
300
301        // We can't unsize a lifetime
302        ty::GenericArgKind::Lifetime(_) => None,
303
304        ty::GenericArgKind::Const(ct) => match ct.kind() {
305            ty::ConstKind::Param(p) => Some(p.index),
306            _ => None,
307        },
308    };
309
310    // The last field of the structure has to exist and contain type/const parameters.
311    let Some((tail_field, prefix_fields)) = def.non_enum_variant().fields.raw.split_last() else {
312        return DenseBitSet::new_empty(num_params);
313    };
314
315    let mut unsizing_params = DenseBitSet::new_empty(num_params);
316    for arg in tcx.type_of(tail_field.did).instantiate_identity().walk() {
317        if let Some(i) = maybe_unsizing_param_idx(arg) {
318            unsizing_params.insert(i);
319        }
320    }
321
322    // Ensure none of the other fields mention the parameters used
323    // in unsizing.
324    for field in prefix_fields {
325        for arg in tcx.type_of(field.did).instantiate_identity().walk() {
326            if let Some(i) = maybe_unsizing_param_idx(arg) {
327                unsizing_params.remove(i);
328            }
329        }
330    }
331
332    unsizing_params
333}
334
335fn impl_self_is_guaranteed_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_def_id: DefId) -> bool {
336    debug_assert_eq!(tcx.def_kind(impl_def_id), DefKind::Impl { of_trait: true });
337
338    let infcx = tcx.infer_ctxt().ignoring_regions().build(ty::TypingMode::non_body_analysis());
339
340    let ocx = traits::ObligationCtxt::new(&infcx);
341    let cause = traits::ObligationCause::dummy();
342    let param_env = tcx.param_env(impl_def_id);
343
344    let tail = tcx.struct_tail_raw(
345        tcx.type_of(impl_def_id).instantiate_identity(),
346        |ty| {
347            ocx.structurally_normalize_ty(&cause, param_env, ty).unwrap_or_else(|_| {
348                Ty::new_error_with_message(
349                    tcx,
350                    tcx.def_span(impl_def_id),
351                    "struct tail should be computable",
352                )
353            })
354        },
355        || (),
356    );
357
358    match tail.kind() {
359        ty::Dynamic(_, _, ty::Dyn) | ty::Slice(_) | ty::Str => true,
360        ty::Bool
361        | ty::Char
362        | ty::Int(_)
363        | ty::Uint(_)
364        | ty::Float(_)
365        | ty::Adt(_, _)
366        | ty::Foreign(_)
367        | ty::Array(_, _)
368        | ty::Pat(_, _)
369        | ty::RawPtr(_, _)
370        | ty::Ref(_, _, _)
371        | ty::FnDef(_, _)
372        | ty::FnPtr(_, _)
373        | ty::UnsafeBinder(_)
374        | ty::Closure(_, _)
375        | ty::CoroutineClosure(_, _)
376        | ty::Coroutine(_, _)
377        | ty::CoroutineWitness(_, _)
378        | ty::Never
379        | ty::Tuple(_)
380        | ty::Alias(_, _)
381        | ty::Param(_)
382        | ty::Bound(_, _)
383        | ty::Placeholder(_)
384        | ty::Infer(_)
385        | ty::Error(_) => false,
386    }
387}
388
389pub(crate) fn provide(providers: &mut Providers) {
390    *providers = Providers {
391        asyncness,
392        adt_sizedness_constraint,
393        param_env,
394        typing_env_normalized_for_post_analysis,
395        defaultness,
396        unsizing_params_for_adt,
397        impl_self_is_guaranteed_unsized,
398        ..*providers
399    };
400}
rustc_ty_utils/ty.rs

rustc_ty_utils/
ty.rs
