rustc_hir_analysis/hir_ty_lowering/
bounds.rs

1use std::ops::ControlFlow;
2
3use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
4use rustc_errors::codes::*;
5use rustc_errors::struct_span_code_err;
6use rustc_hir as hir;
7use rustc_hir::def::{DefKind, Res};
8use rustc_hir::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
9use rustc_hir::{AmbigArg, PolyTraitRef};
10use rustc_middle::bug;
11use rustc_middle::ty::{
12    self as ty, IsSuggestable, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
13    TypeVisitor, Upcast,
14};
15use rustc_span::{ErrorGuaranteed, Ident, Span, kw, sym};
16use rustc_trait_selection::traits;
17use smallvec::SmallVec;
18use tracing::{debug, instrument};
19
20use super::errors::GenericsArgsErrExtend;
21use crate::errors;
22use crate::hir_ty_lowering::{
23    AssocItemQSelf, FeedConstTy, HirTyLowerer, PredicateFilter, RegionInferReason,
24};
25
26#[derive(Debug, Default)]
27struct CollectedBound {
28    /// `Trait`
29    positive: bool,
30    /// `?Trait`
31    maybe: bool,
32    /// `!Trait`
33    negative: bool,
34}
35
36impl CollectedBound {
37    /// Returns `true` if any of `Trait`, `?Trait` or `!Trait` were encountered.
38    fn any(&self) -> bool {
39        self.positive || self.maybe || self.negative
40    }
41}
42
43#[derive(Debug)]
44struct CollectedSizednessBounds {
45    // Collected `Sized` bounds
46    sized: CollectedBound,
47    // Collected `MetaSized` bounds
48    meta_sized: CollectedBound,
49    // Collected `PointeeSized` bounds
50    pointee_sized: CollectedBound,
51}
52
53impl CollectedSizednessBounds {
54    /// Returns `true` if any of `Trait`, `?Trait` or `!Trait` were encountered for `Sized`,
55    /// `MetaSized` or `PointeeSized`.
56    fn any(&self) -> bool {
57        self.sized.any() || self.meta_sized.any() || self.pointee_sized.any()
58    }
59}
60
61fn search_bounds_for<'tcx>(
62    hir_bounds: &'tcx [hir::GenericBound<'tcx>],
63    self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
64    mut f: impl FnMut(&'tcx PolyTraitRef<'tcx>),
65) {
66    let mut search_bounds = |hir_bounds: &'tcx [hir::GenericBound<'tcx>]| {
67        for hir_bound in hir_bounds {
68            let hir::GenericBound::Trait(ptr) = hir_bound else {
69                continue;
70            };
71
72            f(ptr)
73        }
74    };
75
76    search_bounds(hir_bounds);
77    if let Some((self_ty, where_clause)) = self_ty_where_predicates {
78        for clause in where_clause {
79            if let hir::WherePredicateKind::BoundPredicate(pred) = clause.kind
80                && pred.is_param_bound(self_ty.to_def_id())
81            {
82                search_bounds(pred.bounds);
83            }
84        }
85    }
86}
87
88fn collect_relaxed_bounds<'tcx>(
89    hir_bounds: &'tcx [hir::GenericBound<'tcx>],
90    self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
91) -> SmallVec<[&'tcx PolyTraitRef<'tcx>; 1]> {
92    let mut relaxed_bounds: SmallVec<[_; 1]> = SmallVec::new();
93    search_bounds_for(hir_bounds, self_ty_where_predicates, |ptr| {
94        if matches!(ptr.modifiers.polarity, hir::BoundPolarity::Maybe(_)) {
95            relaxed_bounds.push(ptr);
96        }
97    });
98    relaxed_bounds
99}
100
101fn collect_bounds<'a, 'tcx>(
102    hir_bounds: &'a [hir::GenericBound<'tcx>],
103    self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
104    target_did: DefId,
105) -> CollectedBound {
106    let mut collect_into = CollectedBound::default();
107    search_bounds_for(hir_bounds, self_ty_where_predicates, |ptr| {
108        if !matches!(ptr.trait_ref.path.res, Res::Def(DefKind::Trait, did) if did == target_did) {
109            return;
110        }
111
112        match ptr.modifiers.polarity {
113            hir::BoundPolarity::Maybe(_) => collect_into.maybe = true,
114            hir::BoundPolarity::Negative(_) => collect_into.negative = true,
115            hir::BoundPolarity::Positive => collect_into.positive = true,
116        }
117    });
118    collect_into
119}
120
121fn collect_sizedness_bounds<'tcx>(
122    tcx: TyCtxt<'tcx>,
123    hir_bounds: &'tcx [hir::GenericBound<'tcx>],
124    self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
125    span: Span,
126) -> CollectedSizednessBounds {
127    let sized_did = tcx.require_lang_item(hir::LangItem::Sized, span);
128    let sized = collect_bounds(hir_bounds, self_ty_where_predicates, sized_did);
129
130    let meta_sized_did = tcx.require_lang_item(hir::LangItem::MetaSized, span);
131    let meta_sized = collect_bounds(hir_bounds, self_ty_where_predicates, meta_sized_did);
132
133    let pointee_sized_did = tcx.require_lang_item(hir::LangItem::PointeeSized, span);
134    let pointee_sized = collect_bounds(hir_bounds, self_ty_where_predicates, pointee_sized_did);
135
136    CollectedSizednessBounds { sized, meta_sized, pointee_sized }
137}
138
139/// Add a trait bound for `did`.
140fn add_trait_bound<'tcx>(
141    tcx: TyCtxt<'tcx>,
142    bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
143    self_ty: Ty<'tcx>,
144    did: DefId,
145    span: Span,
146) {
147    let trait_ref = ty::TraitRef::new(tcx, did, [self_ty]);
148    // Preferable to put sizedness obligations first, since we report better errors for `Sized`
149    // ambiguity.
150    bounds.insert(0, (trait_ref.upcast(tcx), span));
151}
152
153impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
154    /// Adds sizedness bounds to a trait, trait alias, parameter, opaque type or associated type.
155    ///
156    /// - On parameters, opaque type and associated types, add default `Sized` bound if no explicit
157    ///   sizedness bounds are present.
158    /// - On traits and trait aliases, add default `MetaSized` supertrait if no explicit sizedness
159    ///   bounds are present.
160    /// - On parameters, opaque type, associated types and trait aliases, add a `MetaSized` bound if
161    ///   a `?Sized` bound is present.
162    pub(crate) fn add_sizedness_bounds(
163        &self,
164        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
165        self_ty: Ty<'tcx>,
166        hir_bounds: &'tcx [hir::GenericBound<'tcx>],
167        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
168        trait_did: Option<LocalDefId>,
169        span: Span,
170    ) {
171        let tcx = self.tcx();
172
173        // Skip adding any default bounds if `#![rustc_no_implicit_bounds]`
174        if tcx.has_attr(CRATE_DEF_ID, sym::rustc_no_implicit_bounds) {
175            return;
176        }
177
178        let meta_sized_did = tcx.require_lang_item(hir::LangItem::MetaSized, span);
179        let pointee_sized_did = tcx.require_lang_item(hir::LangItem::PointeeSized, span);
180
181        // If adding sizedness bounds to a trait, then there are some relevant early exits
182        if let Some(trait_did) = trait_did {
183            let trait_did = trait_did.to_def_id();
184            // Never add a default supertrait to `PointeeSized`.
185            if trait_did == pointee_sized_did {
186                return;
187            }
188            // Don't add default sizedness supertraits to auto traits because it isn't possible to
189            // relax an automatically added supertrait on the defn itself.
190            if tcx.trait_is_auto(trait_did) {
191                return;
192            }
193        } else {
194            // Report invalid relaxed bounds.
195            // FIXME: Since we only call this validation function here in this function, we only
196            //        fully validate relaxed bounds in contexts where we perform
197            //        "sized elaboration". In most cases that doesn't matter because we *usually*
198            //        reject such relaxed bounds outright during AST lowering.
199            //        However, this can easily get out of sync! Ideally, we would perform this step
200            //        where we are guaranteed to catch *all* bounds like in
201            //        `Self::lower_poly_trait_ref`. List of concrete issues:
202            //        FIXME(more_maybe_bounds): We don't call this for e.g., trait object tys or
203            //                                  supertrait bounds!
204            //        FIXME(trait_alias, #143122): We don't call it for the RHS. Arguably however,
205            //                                       AST lowering should reject them outright.
206            //        FIXME(associated_type_bounds): We don't call this for them. However, AST
207            //                                       lowering should reject them outright (#135229).
208            let bounds = collect_relaxed_bounds(hir_bounds, self_ty_where_predicates);
209            self.check_and_report_invalid_relaxed_bounds(bounds);
210        }
211
212        let collected = collect_sizedness_bounds(tcx, hir_bounds, self_ty_where_predicates, span);
213        if (collected.sized.maybe || collected.sized.negative)
214            && !collected.sized.positive
215            && !collected.meta_sized.any()
216            && !collected.pointee_sized.any()
217        {
218            // `?Sized` is equivalent to `MetaSized` (but only add the bound if there aren't any
219            // other explicit ones) - this can happen for trait aliases as well as bounds.
220            add_trait_bound(tcx, bounds, self_ty, meta_sized_did, span);
221        } else if !collected.any() {
222            if trait_did.is_some() {
223                // If there are no explicit sizedness bounds on a trait then add a default
224                // `MetaSized` supertrait.
225                add_trait_bound(tcx, bounds, self_ty, meta_sized_did, span);
226            } else {
227                // If there are no explicit sizedness bounds on a parameter then add a default
228                // `Sized` bound.
229                let sized_did = tcx.require_lang_item(hir::LangItem::Sized, span);
230                add_trait_bound(tcx, bounds, self_ty, sized_did, span);
231            }
232        }
233    }
234
235    /// Checks whether `Self: DefaultAutoTrait` bounds should be added on trait super bounds
236    /// or associated items.
237    ///
238    /// To keep backward compatibility with existing code, `experimental_default_bounds` bounds
239    /// should be added everywhere, including super bounds. However this causes a huge performance
240    /// costs. For optimization purposes instead of adding default supertraits, bounds
241    /// are added to the associated items:
242    ///
243    /// ```ignore(illustrative)
244    /// // Default bounds are generated in the following way:
245    /// trait Trait {
246    ///     fn foo(&self) where Self: Leak {}
247    /// }
248    ///
249    /// // instead of this:
250    /// trait Trait: Leak {
251    ///     fn foo(&self) {}
252    /// }
253    /// ```
254    /// It is not always possible to do this because of backward compatibility:
255    ///
256    /// ```ignore(illustrative)
257    /// pub trait Trait<Rhs = Self> {}
258    /// pub trait Trait1 : Trait {}
259    /// //~^ ERROR: `Rhs` requires `DefaultAutoTrait`, but `Self` is not `DefaultAutoTrait`
260    /// ```
261    ///
262    /// or:
263    ///
264    /// ```ignore(illustrative)
265    /// trait Trait {
266    ///     type Type where Self: Sized;
267    /// }
268    /// trait Trait2<T> : Trait<Type = T> {}
269    /// //~^ ERROR: `DefaultAutoTrait` required for `Trait2`, by implicit  `Self: DefaultAutoTrait` in `Trait::Type`
270    /// ```
271    ///
272    /// Therefore, `experimental_default_bounds` are still being added to supertraits if
273    /// the `SelfTyParam` or `AssocItemConstraint` were found in a trait header.
274    fn requires_default_supertraits(
275        &self,
276        hir_bounds: &'tcx [hir::GenericBound<'tcx>],
277        hir_generics: &'tcx hir::Generics<'tcx>,
278    ) -> bool {
279        struct TraitInfoCollector;
280
281        impl<'tcx> hir::intravisit::Visitor<'tcx> for TraitInfoCollector {
282            type Result = ControlFlow<()>;
283
284            fn visit_assoc_item_constraint(
285                &mut self,
286                _constraint: &'tcx hir::AssocItemConstraint<'tcx>,
287            ) -> Self::Result {
288                ControlFlow::Break(())
289            }
290
291            fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx, AmbigArg>) -> Self::Result {
292                if matches!(
293                    &t.kind,
294                    hir::TyKind::Path(hir::QPath::Resolved(
295                        _,
296                        hir::Path { res: hir::def::Res::SelfTyParam { .. }, .. },
297                    ))
298                ) {
299                    return ControlFlow::Break(());
300                }
301                hir::intravisit::walk_ty(self, t)
302            }
303        }
304
305        let mut found = false;
306        for bound in hir_bounds {
307            found |= hir::intravisit::walk_param_bound(&mut TraitInfoCollector, bound).is_break();
308        }
309        found |= hir::intravisit::walk_generics(&mut TraitInfoCollector, hir_generics).is_break();
310        found
311    }
312
313    /// Implicitly add `Self: DefaultAutoTrait` clauses on trait associated items if
314    /// they are not added as super trait bounds to the trait itself. See
315    /// `requires_default_supertraits` for more information.
316    pub(crate) fn add_default_trait_item_bounds(
317        &self,
318        trait_item: &hir::TraitItem<'tcx>,
319        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
320    ) {
321        let tcx = self.tcx();
322        if !tcx.sess.opts.unstable_opts.experimental_default_bounds {
323            return;
324        }
325
326        let parent = tcx.local_parent(trait_item.hir_id().owner.def_id);
327        let hir::Node::Item(parent_trait) = tcx.hir_node_by_def_id(parent) else {
328            unreachable!();
329        };
330
331        let (trait_generics, trait_bounds) = match parent_trait.kind {
332            hir::ItemKind::Trait(_, _, _, _, generics, supertraits, _) => (generics, supertraits),
333            hir::ItemKind::TraitAlias(_, generics, supertraits) => (generics, supertraits),
334            _ => unreachable!(),
335        };
336
337        if !self.requires_default_supertraits(trait_bounds, trait_generics) {
338            let self_ty_where_predicates = (parent, trait_item.generics.predicates);
339            self.add_default_traits(
340                bounds,
341                tcx.types.self_param,
342                &[],
343                Some(self_ty_where_predicates),
344                trait_item.span,
345            );
346        }
347    }
348
349    /// Lazily sets `experimental_default_bounds` to true on trait super bounds.
350    /// See `requires_default_supertraits` for more information.
351    pub(crate) fn add_default_super_traits(
352        &self,
353        trait_def_id: LocalDefId,
354        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
355        hir_bounds: &'tcx [hir::GenericBound<'tcx>],
356        hir_generics: &'tcx hir::Generics<'tcx>,
357        span: Span,
358    ) {
359        if !self.tcx().sess.opts.unstable_opts.experimental_default_bounds {
360            return;
361        }
362
363        assert!(matches!(self.tcx().def_kind(trait_def_id), DefKind::Trait | DefKind::TraitAlias));
364        if self.requires_default_supertraits(hir_bounds, hir_generics) {
365            let self_ty_where_predicates = (trait_def_id, hir_generics.predicates);
366            self.add_default_traits(
367                bounds,
368                self.tcx().types.self_param,
369                hir_bounds,
370                Some(self_ty_where_predicates),
371                span,
372            );
373        }
374    }
375
376    pub(crate) fn add_default_traits(
377        &self,
378        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
379        self_ty: Ty<'tcx>,
380        hir_bounds: &[hir::GenericBound<'tcx>],
381        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
382        span: Span,
383    ) {
384        self.tcx().default_traits().iter().for_each(|default_trait| {
385            self.add_default_trait(
386                *default_trait,
387                bounds,
388                self_ty,
389                hir_bounds,
390                self_ty_where_predicates,
391                span,
392            );
393        });
394    }
395
396    /// Add a `experimental_default_bounds` bound to the `bounds` if appropriate.
397    ///
398    /// Doesn't add the bound if the HIR bounds contain any of `Trait`, `?Trait` or `!Trait`.
399    pub(crate) fn add_default_trait(
400        &self,
401        trait_: hir::LangItem,
402        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
403        self_ty: Ty<'tcx>,
404        hir_bounds: &[hir::GenericBound<'tcx>],
405        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
406        span: Span,
407    ) {
408        let tcx = self.tcx();
409        let trait_id = tcx.lang_items().get(trait_);
410        if let Some(trait_id) = trait_id
411            && self.should_add_default_traits(trait_id, hir_bounds, self_ty_where_predicates)
412        {
413            add_trait_bound(tcx, bounds, self_ty, trait_id, span);
414        }
415    }
416
417    /// Returns `true` if default trait bound should be added.
418    fn should_add_default_traits<'a>(
419        &self,
420        trait_def_id: DefId,
421        hir_bounds: &'a [hir::GenericBound<'tcx>],
422        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
423    ) -> bool {
424        let collected = collect_bounds(hir_bounds, self_ty_where_predicates, trait_def_id);
425        !self.tcx().has_attr(CRATE_DEF_ID, sym::rustc_no_implicit_bounds) && !collected.any()
426    }
427
428    /// Lower HIR bounds into `bounds` given the self type `param_ty` and the overarching late-bound vars if any.
429    ///
430    /// ### Examples
431    ///
432    /// ```ignore (illustrative)
433    /// fn foo<T>() where for<'a> T: Trait<'a> + Copy {}
434    /// //                ^^^^^^^ ^  ^^^^^^^^^^^^^^^^ `hir_bounds`, in HIR form
435    /// //                |       |
436    /// //                |       `param_ty`, in ty form
437    /// //                `bound_vars`, in ty form
438    ///
439    /// fn bar<T>() where T: for<'a> Trait<'a> + Copy {} // no overarching `bound_vars` here!
440    /// //                ^  ^^^^^^^^^^^^^^^^^^^^^^^^ `hir_bounds`, in HIR form
441    /// //                |
442    /// //                `param_ty`, in ty form
443    /// ```
444    ///
445    /// ### A Note on Binders
446    ///
447    /// There is an implied binder around `param_ty` and `hir_bounds`.
448    /// See `lower_poly_trait_ref` for more details.
449    #[instrument(level = "debug", skip(self, hir_bounds, bounds))]
450    pub(crate) fn lower_bounds<'hir, I: IntoIterator<Item = &'hir hir::GenericBound<'tcx>>>(
451        &self,
452        param_ty: Ty<'tcx>,
453        hir_bounds: I,
454        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
455        bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
456        predicate_filter: PredicateFilter,
457    ) where
458        'tcx: 'hir,
459    {
460        for hir_bound in hir_bounds {
461            // In order to avoid cycles, when we're lowering `SelfTraitThatDefines`,
462            // we skip over any traits that don't define the given associated type.
463            if let PredicateFilter::SelfTraitThatDefines(assoc_ident) = predicate_filter {
464                if let Some(trait_ref) = hir_bound.trait_ref()
465                    && let Some(trait_did) = trait_ref.trait_def_id()
466                    && self.tcx().trait_may_define_assoc_item(trait_did, assoc_ident)
467                {
468                    // Okay
469                } else {
470                    continue;
471                }
472            }
473
474            match hir_bound {
475                hir::GenericBound::Trait(poly_trait_ref) => {
476                    let _ = self.lower_poly_trait_ref(
477                        poly_trait_ref,
478                        param_ty,
479                        bounds,
480                        predicate_filter,
481                    );
482                }
483                hir::GenericBound::Outlives(lifetime) => {
484                    // `ConstIfConst` is only interested in `[const]` bounds.
485                    if matches!(
486                        predicate_filter,
487                        PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst
488                    ) {
489                        continue;
490                    }
491
492                    let region = self.lower_lifetime(lifetime, RegionInferReason::OutlivesBound);
493                    let bound = ty::Binder::bind_with_vars(
494                        ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(param_ty, region)),
495                        bound_vars,
496                    );
497                    bounds.push((bound.upcast(self.tcx()), lifetime.ident.span));
498                }
499                hir::GenericBound::Use(..) => {
500                    // We don't actually lower `use` into the type layer.
501                }
502            }
503        }
504    }
505
506    /// Lower an associated item constraint from the HIR into `bounds`.
507    ///
508    /// ### A Note on Binders
509    ///
510    /// Given something like `T: for<'a> Iterator<Item = &'a u32>`,
511    /// the `trait_ref` here will be `for<'a> T: Iterator`.
512    /// The `constraint` data however is from *inside* the binder
513    /// (e.g., `&'a u32`) and hence may reference bound regions.
514    #[instrument(level = "debug", skip(self, bounds, duplicates, path_span))]
515    pub(super) fn lower_assoc_item_constraint(
516        &self,
517        hir_ref_id: hir::HirId,
518        trait_ref: ty::PolyTraitRef<'tcx>,
519        constraint: &hir::AssocItemConstraint<'tcx>,
520        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
521        duplicates: &mut FxIndexMap<DefId, Span>,
522        path_span: Span,
523        predicate_filter: PredicateFilter,
524    ) -> Result<(), ErrorGuaranteed> {
525        let tcx = self.tcx();
526
527        let assoc_tag = if constraint.gen_args.parenthesized
528            == hir::GenericArgsParentheses::ReturnTypeNotation
529        {
530            ty::AssocTag::Fn
531        } else if let hir::AssocItemConstraintKind::Equality { term: hir::Term::Const(_) } =
532            constraint.kind
533        {
534            ty::AssocTag::Const
535        } else {
536            ty::AssocTag::Type
537        };
538
539        // Given something like `U: Trait<T = X>`, we want to produce a predicate like
540        // `<U as Trait>::T = X`.
541        // This is somewhat subtle in the event that `T` is defined in a supertrait of `Trait`,
542        // because in that case we need to upcast. I.e., we want to produce
543        // `<B as SuperTrait<i32>>::T == X` for `B: SubTrait<T = X>` where
544        //
545        //     trait SubTrait: SuperTrait<i32> {}
546        //     trait SuperTrait<A> { type T; }
547        let candidate = if self.probe_trait_that_defines_assoc_item(
548            trait_ref.def_id(),
549            assoc_tag,
550            constraint.ident,
551        ) {
552            // Simple case: The assoc item is defined in the current trait.
553            trait_ref
554        } else {
555            // Otherwise, we have to walk through the supertraits to find
556            // one that does define it.
557            self.probe_single_bound_for_assoc_item(
558                || traits::supertraits(tcx, trait_ref),
559                AssocItemQSelf::Trait(trait_ref.def_id()),
560                assoc_tag,
561                constraint.ident,
562                path_span,
563                Some(constraint),
564            )?
565        };
566
567        let assoc_item = self
568            .probe_assoc_item(
569                constraint.ident,
570                assoc_tag,
571                hir_ref_id,
572                constraint.span,
573                candidate.def_id(),
574            )
575            .expect("failed to find associated item");
576
577        duplicates
578            .entry(assoc_item.def_id)
579            .and_modify(|prev_span| {
580                self.dcx().emit_err(errors::ValueOfAssociatedStructAlreadySpecified {
581                    span: constraint.span,
582                    prev_span: *prev_span,
583                    item_name: constraint.ident,
584                    def_path: tcx.def_path_str(assoc_item.container_id(tcx)),
585                });
586            })
587            .or_insert(constraint.span);
588
589        let projection_term = if let ty::AssocTag::Fn = assoc_tag {
590            let bound_vars = tcx.late_bound_vars(constraint.hir_id);
591            ty::Binder::bind_with_vars(
592                self.lower_return_type_notation_ty(candidate, assoc_item.def_id, path_span)?.into(),
593                bound_vars,
594            )
595        } else {
596            // Create the generic arguments for the associated type or constant by joining the
597            // parent arguments (the arguments of the trait) and the own arguments (the ones of
598            // the associated item itself) and construct an alias type using them.
599            let alias_term = candidate.map_bound(|trait_ref| {
600                let item_segment = hir::PathSegment {
601                    ident: constraint.ident,
602                    hir_id: constraint.hir_id,
603                    res: Res::Err,
604                    args: Some(constraint.gen_args),
605                    infer_args: false,
606                };
607
608                let alias_args = self.lower_generic_args_of_assoc_item(
609                    path_span,
610                    assoc_item.def_id,
611                    &item_segment,
612                    trait_ref.args,
613                );
614                debug!(?alias_args);
615
616                ty::AliasTerm::new_from_args(tcx, assoc_item.def_id, alias_args)
617            });
618
619            // Provide the resolved type of the associated constant to `type_of(AnonConst)`.
620            if let Some(const_arg) = constraint.ct() {
621                if let hir::ConstArgKind::Anon(anon_const) = const_arg.kind {
622                    let ty = alias_term
623                        .map_bound(|alias| tcx.type_of(alias.def_id).instantiate(tcx, alias.args));
624                    let ty = check_assoc_const_binding_type(
625                        self,
626                        constraint.ident,
627                        ty,
628                        constraint.hir_id,
629                    );
630                    tcx.feed_anon_const_type(anon_const.def_id, ty::EarlyBinder::bind(ty));
631                }
632            }
633
634            alias_term
635        };
636
637        match constraint.kind {
638            hir::AssocItemConstraintKind::Equality { .. } if let ty::AssocTag::Fn = assoc_tag => {
639                return Err(self.dcx().emit_err(crate::errors::ReturnTypeNotationEqualityBound {
640                    span: constraint.span,
641                }));
642            }
643            // Lower an equality constraint like `Item = u32` as found in HIR bound `T: Iterator<Item = u32>`
644            // to a projection predicate: `<T as Iterator>::Item = u32`.
645            hir::AssocItemConstraintKind::Equality { term } => {
646                let term = match term {
647                    hir::Term::Ty(ty) => self.lower_ty(ty).into(),
648                    hir::Term::Const(ct) => self.lower_const_arg(ct, FeedConstTy::No).into(),
649                };
650
651                // Find any late-bound regions declared in `ty` that are not
652                // declared in the trait-ref or assoc_item. These are not well-formed.
653                //
654                // Example:
655                //
656                //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
657                //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
658                let late_bound_in_projection_ty =
659                    tcx.collect_constrained_late_bound_regions(projection_term);
660                let late_bound_in_term =
661                    tcx.collect_referenced_late_bound_regions(trait_ref.rebind(term));
662                debug!(?late_bound_in_projection_ty);
663                debug!(?late_bound_in_term);
664
665                // FIXME: point at the type params that don't have appropriate lifetimes:
666                // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
667                //                         ----  ----     ^^^^^^^
668                // NOTE(associated_const_equality): This error should be impossible to trigger
669                //                                  with associated const equality constraints.
670                self.validate_late_bound_regions(
671                    late_bound_in_projection_ty,
672                    late_bound_in_term,
673                    |br_name| {
674                        struct_span_code_err!(
675                            self.dcx(),
676                            constraint.span,
677                            E0582,
678                            "binding for associated type `{}` references {}, \
679                             which does not appear in the trait input types",
680                            constraint.ident,
681                            br_name
682                        )
683                    },
684                );
685
686                match predicate_filter {
687                    PredicateFilter::All
688                    | PredicateFilter::SelfOnly
689                    | PredicateFilter::SelfAndAssociatedTypeBounds => {
690                        let bound = projection_term.map_bound(|projection_term| {
691                            ty::ClauseKind::Projection(ty::ProjectionPredicate {
692                                projection_term,
693                                term,
694                            })
695                        });
696                        bounds.push((bound.upcast(tcx), constraint.span));
697                    }
698                    // SelfTraitThatDefines is only interested in trait predicates.
699                    PredicateFilter::SelfTraitThatDefines(_) => {}
700                    // `ConstIfConst` is only interested in `[const]` bounds.
701                    PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {}
702                }
703            }
704            // Lower a constraint like `Item: Debug` as found in HIR bound `T: Iterator<Item: Debug>`
705            // to a bound involving a projection: `<T as Iterator>::Item: Debug`.
706            hir::AssocItemConstraintKind::Bound { bounds: hir_bounds } => {
707                match predicate_filter {
708                    PredicateFilter::All
709                    | PredicateFilter::SelfAndAssociatedTypeBounds
710                    | PredicateFilter::ConstIfConst => {
711                        let projection_ty = projection_term
712                            .map_bound(|projection_term| projection_term.expect_ty(self.tcx()));
713                        // Calling `skip_binder` is okay, because `lower_bounds` expects the `param_ty`
714                        // parameter to have a skipped binder.
715                        let param_ty =
716                            Ty::new_alias(tcx, ty::Projection, projection_ty.skip_binder());
717                        self.lower_bounds(
718                            param_ty,
719                            hir_bounds,
720                            bounds,
721                            projection_ty.bound_vars(),
722                            predicate_filter,
723                        );
724                    }
725                    PredicateFilter::SelfOnly
726                    | PredicateFilter::SelfTraitThatDefines(_)
727                    | PredicateFilter::SelfConstIfConst => {}
728                }
729            }
730        }
731        Ok(())
732    }
733
734    /// Lower a type, possibly specially handling the type if it's a return type notation
735    /// which we otherwise deny in other positions.
736    pub fn lower_ty_maybe_return_type_notation(&self, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
737        let hir::TyKind::Path(qpath) = hir_ty.kind else {
738            return self.lower_ty(hir_ty);
739        };
740
741        let tcx = self.tcx();
742        match qpath {
743            hir::QPath::Resolved(opt_self_ty, path)
744                if let [mod_segments @ .., trait_segment, item_segment] = &path.segments[..]
745                    && item_segment.args.is_some_and(|args| {
746                        matches!(
747                            args.parenthesized,
748                            hir::GenericArgsParentheses::ReturnTypeNotation
749                        )
750                    }) =>
751            {
752                // We don't allow generics on the module segments.
753                let _ =
754                    self.prohibit_generic_args(mod_segments.iter(), GenericsArgsErrExtend::None);
755
756                let item_def_id = match path.res {
757                    Res::Def(DefKind::AssocFn, item_def_id) => item_def_id,
758                    Res::Err => {
759                        return Ty::new_error_with_message(
760                            tcx,
761                            hir_ty.span,
762                            "failed to resolve RTN",
763                        );
764                    }
765                    _ => bug!("only expected method resolution for fully qualified RTN"),
766                };
767                let trait_def_id = tcx.parent(item_def_id);
768
769                // Good error for `where Trait::method(..): Send`.
770                let Some(self_ty) = opt_self_ty else {
771                    let guar = self.report_missing_self_ty_for_resolved_path(
772                        trait_def_id,
773                        hir_ty.span,
774                        item_segment,
775                        ty::AssocTag::Type,
776                    );
777                    return Ty::new_error(tcx, guar);
778                };
779                let self_ty = self.lower_ty(self_ty);
780
781                let trait_ref = self.lower_mono_trait_ref(
782                    hir_ty.span,
783                    trait_def_id,
784                    self_ty,
785                    trait_segment,
786                    false,
787                );
788
789                // SUBTLE: As noted at the end of `try_append_return_type_notation_params`
790                // in `resolve_bound_vars`, we stash the explicit bound vars of the where
791                // clause onto the item segment of the RTN type. This allows us to know
792                // how many bound vars are *not* coming from the signature of the function
793                // from lowering RTN itself.
794                //
795                // For example, in `where for<'a> <T as Trait<'a>>::method(..): Other`,
796                // the `late_bound_vars` of the where clause predicate (i.e. this HIR ty's
797                // parent) will include `'a` AND all the early- and late-bound vars of the
798                // method. But when lowering the RTN type, we just want the list of vars
799                // we used to resolve the trait ref. We explicitly stored those back onto
800                // the item segment, since there's no other good place to put them.
801                let candidate =
802                    ty::Binder::bind_with_vars(trait_ref, tcx.late_bound_vars(item_segment.hir_id));
803
804                match self.lower_return_type_notation_ty(candidate, item_def_id, hir_ty.span) {
805                    Ok(ty) => Ty::new_alias(tcx, ty::Projection, ty),
806                    Err(guar) => Ty::new_error(tcx, guar),
807                }
808            }
809            hir::QPath::TypeRelative(hir_self_ty, segment)
810                if segment.args.is_some_and(|args| {
811                    matches!(args.parenthesized, hir::GenericArgsParentheses::ReturnTypeNotation)
812                }) =>
813            {
814                let self_ty = self.lower_ty(hir_self_ty);
815                let (item_def_id, bound) = match self.resolve_type_relative_path(
816                    self_ty,
817                    hir_self_ty,
818                    ty::AssocTag::Fn,
819                    segment,
820                    hir_ty.hir_id,
821                    hir_ty.span,
822                    None,
823                ) {
824                    Ok(result) => result,
825                    Err(guar) => return Ty::new_error(tcx, guar),
826                };
827
828                // Don't let `T::method` resolve to some `for<'a> <T as Tr<'a>>::method`,
829                // which may happen via a higher-ranked where clause or supertrait.
830                // This is the same restrictions as associated types; even though we could
831                // support it, it just makes things a lot more difficult to support in
832                // `resolve_bound_vars`, since we'd need to introduce those as elided
833                // bound vars on the where clause too.
834                if bound.has_bound_vars() {
835                    return Ty::new_error(
836                        tcx,
837                        self.dcx().emit_err(errors::AssociatedItemTraitUninferredGenericParams {
838                            span: hir_ty.span,
839                            inferred_sugg: Some(hir_ty.span.with_hi(segment.ident.span.lo())),
840                            bound: format!("{}::", tcx.anonymize_bound_vars(bound).skip_binder()),
841                            mpart_sugg: None,
842                            what: tcx.def_descr(item_def_id),
843                        }),
844                    );
845                }
846
847                match self.lower_return_type_notation_ty(bound, item_def_id, hir_ty.span) {
848                    Ok(ty) => Ty::new_alias(tcx, ty::Projection, ty),
849                    Err(guar) => Ty::new_error(tcx, guar),
850                }
851            }
852            _ => self.lower_ty(hir_ty),
853        }
854    }
855
856    /// Do the common parts of lowering an RTN type. This involves extending the
857    /// candidate binder to include all of the early- and late-bound vars that are
858    /// defined on the function itself, and constructing a projection to the RPITIT
859    /// return type of that function.
860    fn lower_return_type_notation_ty(
861        &self,
862        candidate: ty::PolyTraitRef<'tcx>,
863        item_def_id: DefId,
864        path_span: Span,
865    ) -> Result<ty::AliasTy<'tcx>, ErrorGuaranteed> {
866        let tcx = self.tcx();
867        let mut emitted_bad_param_err = None;
868        // If we have an method return type bound, then we need to instantiate
869        // the method's early bound params with suitable late-bound params.
870        let mut num_bound_vars = candidate.bound_vars().len();
871        let args = candidate.skip_binder().args.extend_to(tcx, item_def_id, |param, _| {
872            let arg = match param.kind {
873                ty::GenericParamDefKind::Lifetime => ty::Region::new_bound(
874                    tcx,
875                    ty::INNERMOST,
876                    ty::BoundRegion {
877                        var: ty::BoundVar::from_usize(num_bound_vars),
878                        kind: ty::BoundRegionKind::Named(param.def_id),
879                    },
880                )
881                .into(),
882                ty::GenericParamDefKind::Type { .. } => {
883                    let guar = *emitted_bad_param_err.get_or_insert_with(|| {
884                        self.dcx().emit_err(crate::errors::ReturnTypeNotationIllegalParam::Type {
885                            span: path_span,
886                            param_span: tcx.def_span(param.def_id),
887                        })
888                    });
889                    Ty::new_error(tcx, guar).into()
890                }
891                ty::GenericParamDefKind::Const { .. } => {
892                    let guar = *emitted_bad_param_err.get_or_insert_with(|| {
893                        self.dcx().emit_err(crate::errors::ReturnTypeNotationIllegalParam::Const {
894                            span: path_span,
895                            param_span: tcx.def_span(param.def_id),
896                        })
897                    });
898                    ty::Const::new_error(tcx, guar).into()
899                }
900            };
901            num_bound_vars += 1;
902            arg
903        });
904
905        // Next, we need to check that the return-type notation is being used on
906        // an RPITIT (return-position impl trait in trait) or AFIT (async fn in trait).
907        let output = tcx.fn_sig(item_def_id).skip_binder().output();
908        let output = if let ty::Alias(ty::Projection, alias_ty) = *output.skip_binder().kind()
909            && tcx.is_impl_trait_in_trait(alias_ty.def_id)
910        {
911            alias_ty
912        } else {
913            return Err(self.dcx().emit_err(crate::errors::ReturnTypeNotationOnNonRpitit {
914                span: path_span,
915                ty: tcx.liberate_late_bound_regions(item_def_id, output),
916                fn_span: tcx.hir_span_if_local(item_def_id),
917                note: (),
918            }));
919        };
920
921        // Finally, move the fn return type's bound vars over to account for the early bound
922        // params (and trait ref's late bound params). This logic is very similar to
923        // `rustc_middle::ty::predicate::Clause::instantiate_supertrait`
924        // and it's no coincidence why.
925        let shifted_output = tcx.shift_bound_var_indices(num_bound_vars, output);
926        Ok(ty::EarlyBinder::bind(shifted_output).instantiate(tcx, args))
927    }
928}
929
930/// Detect and reject early-bound & escaping late-bound generic params in the type of assoc const bindings.
931///
932/// FIXME(const_generics): This is a temporary and semi-artificial restriction until the
933/// arrival of *generic const generics*[^1].
934///
935/// It might actually be possible that we can already support early-bound generic params
936/// in such types if we just lifted some more checks in other places, too, for example
937/// inside `HirTyLowerer::lower_anon_const`. However, even if that were the case, we should
938/// probably gate this behind another feature flag.
939///
940/// [^1]: <https://github.com/rust-lang/project-const-generics/issues/28>.
941fn check_assoc_const_binding_type<'tcx>(
942    cx: &dyn HirTyLowerer<'tcx>,
943    assoc_const: Ident,
944    ty: ty::Binder<'tcx, Ty<'tcx>>,
945    hir_id: hir::HirId,
946) -> Ty<'tcx> {
947    // We can't perform the checks for early-bound params during name resolution unlike E0770
948    // because this information depends on *type* resolution.
949    // We can't perform these checks in `resolve_bound_vars` either for the same reason.
950    // Consider the trait ref `for<'a> Trait<'a, C = { &0 }>`. We need to know the fully
951    // resolved type of `Trait::C` in order to know if it references `'a` or not.
952
953    let ty = ty.skip_binder();
954    if !ty.has_param() && !ty.has_escaping_bound_vars() {
955        return ty;
956    }
957
958    let mut collector = GenericParamAndBoundVarCollector {
959        cx,
960        params: Default::default(),
961        vars: Default::default(),
962        depth: ty::INNERMOST,
963    };
964    let mut guar = ty.visit_with(&mut collector).break_value();
965
966    let tcx = cx.tcx();
967    let ty_note = ty
968        .make_suggestable(tcx, false, None)
969        .map(|ty| crate::errors::TyOfAssocConstBindingNote { assoc_const, ty });
970
971    let enclosing_item_owner_id = tcx
972        .hir_parent_owner_iter(hir_id)
973        .find_map(|(owner_id, parent)| parent.generics().map(|_| owner_id))
974        .unwrap();
975    let generics = tcx.generics_of(enclosing_item_owner_id);
976    for index in collector.params {
977        let param = generics.param_at(index as _, tcx);
978        let is_self_param = param.name == kw::SelfUpper;
979        guar.get_or_insert(cx.dcx().emit_err(crate::errors::ParamInTyOfAssocConstBinding {
980            span: assoc_const.span,
981            assoc_const,
982            param_name: param.name,
983            param_def_kind: tcx.def_descr(param.def_id),
984            param_category: if is_self_param {
985                "self"
986            } else if param.kind.is_synthetic() {
987                "synthetic"
988            } else {
989                "normal"
990            },
991            param_defined_here_label:
992                (!is_self_param).then(|| tcx.def_ident_span(param.def_id).unwrap()),
993            ty_note,
994        }));
995    }
996    for var_def_id in collector.vars {
997        guar.get_or_insert(cx.dcx().emit_err(
998            crate::errors::EscapingBoundVarInTyOfAssocConstBinding {
999                span: assoc_const.span,
1000                assoc_const,
1001                var_name: cx.tcx().item_name(var_def_id),
1002                var_def_kind: tcx.def_descr(var_def_id),
1003                var_defined_here_label: tcx.def_ident_span(var_def_id).unwrap(),
1004                ty_note,
1005            },
1006        ));
1007    }
1008
1009    let guar = guar.unwrap_or_else(|| bug!("failed to find gen params or bound vars in ty"));
1010    Ty::new_error(tcx, guar)
1011}
1012
1013struct GenericParamAndBoundVarCollector<'a, 'tcx> {
1014    cx: &'a dyn HirTyLowerer<'tcx>,
1015    params: FxIndexSet<u32>,
1016    vars: FxIndexSet<DefId>,
1017    depth: ty::DebruijnIndex,
1018}
1019
1020impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'_, 'tcx> {
1021    type Result = ControlFlow<ErrorGuaranteed>;
1022
1023    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
1024        &mut self,
1025        binder: &ty::Binder<'tcx, T>,
1026    ) -> Self::Result {
1027        self.depth.shift_in(1);
1028        let result = binder.super_visit_with(self);
1029        self.depth.shift_out(1);
1030        result
1031    }
1032
1033    fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
1034        match ty.kind() {
1035            ty::Param(param) => {
1036                self.params.insert(param.index);
1037            }
1038            ty::Bound(db, bt) if *db >= self.depth => {
1039                self.vars.insert(match bt.kind {
1040                    ty::BoundTyKind::Param(def_id) => def_id,
1041                    ty::BoundTyKind::Anon => {
1042                        let reported = self
1043                            .cx
1044                            .dcx()
1045                            .delayed_bug(format!("unexpected anon bound ty: {:?}", bt.var));
1046                        return ControlFlow::Break(reported);
1047                    }
1048                });
1049            }
1050            _ if ty.has_param() || ty.has_bound_vars() => return ty.super_visit_with(self),
1051            _ => {}
1052        }
1053        ControlFlow::Continue(())
1054    }
1055
1056    fn visit_region(&mut self, re: ty::Region<'tcx>) -> Self::Result {
1057        match re.kind() {
1058            ty::ReEarlyParam(param) => {
1059                self.params.insert(param.index);
1060            }
1061            ty::ReBound(db, br) if db >= self.depth => {
1062                self.vars.insert(match br.kind {
1063                    ty::BoundRegionKind::Named(def_id) => def_id,
1064                    ty::BoundRegionKind::Anon | ty::BoundRegionKind::ClosureEnv => {
1065                        let guar = self
1066                            .cx
1067                            .dcx()
1068                            .delayed_bug(format!("unexpected bound region kind: {:?}", br.kind));
1069                        return ControlFlow::Break(guar);
1070                    }
1071                    ty::BoundRegionKind::NamedAnon(_) => bug!("only used for pretty printing"),
1072                });
1073            }
1074            _ => {}
1075        }
1076        ControlFlow::Continue(())
1077    }
1078
1079    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
1080        match ct.kind() {
1081            ty::ConstKind::Param(param) => {
1082                self.params.insert(param.index);
1083            }
1084            ty::ConstKind::Bound(db, ty::BoundVar { .. }) if db >= self.depth => {
1085                let guar = self.cx.dcx().delayed_bug("unexpected escaping late-bound const var");
1086                return ControlFlow::Break(guar);
1087            }
1088            _ if ct.has_param() || ct.has_bound_vars() => return ct.super_visit_with(self),
1089            _ => {}
1090        }
1091        ControlFlow::Continue(())
1092    }
1093}
rustc_hir_analysis/hir_ty_lowering/bounds.rs

rustc_hir_analysis/hir_ty_lowering/
bounds.rs
