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::{DefId, LocalDefId};
9use rustc_hir::{AmbigArg, LangItem, 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, Symbol, kw};
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_unbounds<'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 unbounds: 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            unbounds.push(ptr);
96        }
97    });
98    unbounds
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(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(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(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    /// Skip `PointeeSized` bounds.
155    ///
156    /// `PointeeSized` is a "fake bound" insofar as anywhere a `PointeeSized` bound exists, there
157    /// is actually the absence of any bounds. This avoids limitations around non-global where
158    /// clauses being preferred over item bounds (where `PointeeSized` bounds would be
159    /// proven) - which can result in errors when a `PointeeSized` supertrait/bound/predicate is
160    /// added to some items.
161    pub(crate) fn should_skip_sizedness_bound<'hir>(
162        &self,
163        bound: &'hir hir::GenericBound<'tcx>,
164    ) -> bool {
165        bound
166            .trait_ref()
167            .and_then(|tr| tr.trait_def_id())
168            .map(|did| self.tcx().is_lang_item(did, LangItem::PointeeSized))
169            .unwrap_or(false)
170    }
171
172    /// Adds sizedness bounds to a trait, trait alias, parameter, opaque type or associated type.
173    ///
174    /// - On parameters, opaque type and associated types, add default `Sized` bound if no explicit
175    ///   sizedness bounds are present.
176    /// - On traits and trait aliases, add default `MetaSized` supertrait if no explicit sizedness
177    ///   bounds are present.
178    /// - On parameters, opaque type, associated types and trait aliases, add a `MetaSized` bound if
179    ///   a `?Sized` bound is present.
180    pub(crate) fn add_sizedness_bounds(
181        &self,
182        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
183        self_ty: Ty<'tcx>,
184        hir_bounds: &'tcx [hir::GenericBound<'tcx>],
185        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
186        trait_did: Option<LocalDefId>,
187        span: Span,
188    ) {
189        let tcx = self.tcx();
190
191        let meta_sized_did = tcx.require_lang_item(LangItem::MetaSized, span);
192        let pointee_sized_did = tcx.require_lang_item(LangItem::PointeeSized, span);
193
194        // If adding sizedness bounds to a trait, then there are some relevant early exits
195        if let Some(trait_did) = trait_did {
196            let trait_did = trait_did.to_def_id();
197            // Never add a default supertrait to `PointeeSized`.
198            if trait_did == pointee_sized_did {
199                return;
200            }
201            // Don't add default sizedness supertraits to auto traits because it isn't possible to
202            // relax an automatically added supertrait on the defn itself.
203            if tcx.trait_is_auto(trait_did) {
204                return;
205            }
206        } else {
207            // Report invalid unbounds on sizedness-bounded generic parameters.
208            let unbounds = collect_unbounds(hir_bounds, self_ty_where_predicates);
209            self.check_and_report_invalid_unbounds_on_param(unbounds);
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(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.do_not_provide_default_trait_bound(
412                trait_id,
413                hir_bounds,
414                self_ty_where_predicates,
415            )
416        {
417            add_trait_bound(tcx, bounds, self_ty, trait_id, span);
418        }
419    }
420
421    fn do_not_provide_default_trait_bound<'a>(
422        &self,
423        trait_def_id: DefId,
424        hir_bounds: &'a [hir::GenericBound<'tcx>],
425        self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
426    ) -> bool {
427        let collected = collect_bounds(hir_bounds, self_ty_where_predicates, trait_def_id);
428        !collected.any()
429    }
430
431    /// Lower HIR bounds into `bounds` given the self type `param_ty` and the overarching late-bound vars if any.
432    ///
433    /// ### Examples
434    ///
435    /// ```ignore (illustrative)
436    /// fn foo<T>() where for<'a> T: Trait<'a> + Copy {}
437    /// //                ^^^^^^^ ^  ^^^^^^^^^^^^^^^^ `hir_bounds`, in HIR form
438    /// //                |       |
439    /// //                |       `param_ty`, in ty form
440    /// //                `bound_vars`, in ty form
441    ///
442    /// fn bar<T>() where T: for<'a> Trait<'a> + Copy {} // no overarching `bound_vars` here!
443    /// //                ^  ^^^^^^^^^^^^^^^^^^^^^^^^ `hir_bounds`, in HIR form
444    /// //                |
445    /// //                `param_ty`, in ty form
446    /// ```
447    ///
448    /// ### A Note on Binders
449    ///
450    /// There is an implied binder around `param_ty` and `hir_bounds`.
451    /// See `lower_poly_trait_ref` for more details.
452    #[instrument(level = "debug", skip(self, hir_bounds, bounds))]
453    pub(crate) fn lower_bounds<'hir, I: IntoIterator<Item = &'hir hir::GenericBound<'tcx>>>(
454        &self,
455        param_ty: Ty<'tcx>,
456        hir_bounds: I,
457        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
458        bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
459        predicate_filter: PredicateFilter,
460    ) where
461        'tcx: 'hir,
462    {
463        for hir_bound in hir_bounds {
464            if self.should_skip_sizedness_bound(hir_bound) {
465                continue;
466            }
467
468            // In order to avoid cycles, when we're lowering `SelfTraitThatDefines`,
469            // we skip over any traits that don't define the given associated type.
470            if let PredicateFilter::SelfTraitThatDefines(assoc_ident) = predicate_filter {
471                if let Some(trait_ref) = hir_bound.trait_ref()
472                    && let Some(trait_did) = trait_ref.trait_def_id()
473                    && self.tcx().trait_may_define_assoc_item(trait_did, assoc_ident)
474                {
475                    // Okay
476                } else {
477                    continue;
478                }
479            }
480
481            match hir_bound {
482                hir::GenericBound::Trait(poly_trait_ref) => {
483                    let hir::TraitBoundModifiers { constness, polarity } = poly_trait_ref.modifiers;
484                    let _ = self.lower_poly_trait_ref(
485                        &poly_trait_ref.trait_ref,
486                        poly_trait_ref.span,
487                        constness,
488                        polarity,
489                        param_ty,
490                        bounds,
491                        predicate_filter,
492                    );
493                }
494                hir::GenericBound::Outlives(lifetime) => {
495                    // `ConstIfConst` is only interested in `~const` bounds.
496                    if matches!(
497                        predicate_filter,
498                        PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst
499                    ) {
500                        continue;
501                    }
502
503                    let region = self.lower_lifetime(lifetime, RegionInferReason::OutlivesBound);
504                    let bound = ty::Binder::bind_with_vars(
505                        ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(param_ty, region)),
506                        bound_vars,
507                    );
508                    bounds.push((bound.upcast(self.tcx()), lifetime.ident.span));
509                }
510                hir::GenericBound::Use(..) => {
511                    // We don't actually lower `use` into the type layer.
512                }
513            }
514        }
515    }
516
517    /// Lower an associated item constraint from the HIR into `bounds`.
518    ///
519    /// ### A Note on Binders
520    ///
521    /// Given something like `T: for<'a> Iterator<Item = &'a u32>`,
522    /// the `trait_ref` here will be `for<'a> T: Iterator`.
523    /// The `constraint` data however is from *inside* the binder
524    /// (e.g., `&'a u32`) and hence may reference bound regions.
525    #[instrument(level = "debug", skip(self, bounds, duplicates, path_span))]
526    pub(super) fn lower_assoc_item_constraint(
527        &self,
528        hir_ref_id: hir::HirId,
529        trait_ref: ty::PolyTraitRef<'tcx>,
530        constraint: &hir::AssocItemConstraint<'tcx>,
531        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
532        duplicates: &mut FxIndexMap<DefId, Span>,
533        path_span: Span,
534        predicate_filter: PredicateFilter,
535    ) -> Result<(), ErrorGuaranteed> {
536        let tcx = self.tcx();
537
538        let assoc_tag = if constraint.gen_args.parenthesized
539            == hir::GenericArgsParentheses::ReturnTypeNotation
540        {
541            ty::AssocTag::Fn
542        } else if let hir::AssocItemConstraintKind::Equality { term: hir::Term::Const(_) } =
543            constraint.kind
544        {
545            ty::AssocTag::Const
546        } else {
547            ty::AssocTag::Type
548        };
549
550        // Given something like `U: Trait<T = X>`, we want to produce a predicate like
551        // `<U as Trait>::T = X`.
552        // This is somewhat subtle in the event that `T` is defined in a supertrait of `Trait`,
553        // because in that case we need to upcast. I.e., we want to produce
554        // `<B as SuperTrait<i32>>::T == X` for `B: SubTrait<T = X>` where
555        //
556        //     trait SubTrait: SuperTrait<i32> {}
557        //     trait SuperTrait<A> { type T; }
558        let candidate = if self.probe_trait_that_defines_assoc_item(
559            trait_ref.def_id(),
560            assoc_tag,
561            constraint.ident,
562        ) {
563            // Simple case: The assoc item is defined in the current trait.
564            trait_ref
565        } else {
566            // Otherwise, we have to walk through the supertraits to find
567            // one that does define it.
568            self.probe_single_bound_for_assoc_item(
569                || traits::supertraits(tcx, trait_ref),
570                AssocItemQSelf::Trait(trait_ref.def_id()),
571                assoc_tag,
572                constraint.ident,
573                path_span,
574                Some(constraint),
575            )?
576        };
577
578        let assoc_item = self
579            .probe_assoc_item(
580                constraint.ident,
581                assoc_tag,
582                hir_ref_id,
583                constraint.span,
584                candidate.def_id(),
585            )
586            .expect("failed to find associated item");
587
588        duplicates
589            .entry(assoc_item.def_id)
590            .and_modify(|prev_span| {
591                self.dcx().emit_err(errors::ValueOfAssociatedStructAlreadySpecified {
592                    span: constraint.span,
593                    prev_span: *prev_span,
594                    item_name: constraint.ident,
595                    def_path: tcx.def_path_str(assoc_item.container_id(tcx)),
596                });
597            })
598            .or_insert(constraint.span);
599
600        let projection_term = if let ty::AssocTag::Fn = assoc_tag {
601            let bound_vars = tcx.late_bound_vars(constraint.hir_id);
602            ty::Binder::bind_with_vars(
603                self.lower_return_type_notation_ty(candidate, assoc_item.def_id, path_span)?.into(),
604                bound_vars,
605            )
606        } else {
607            // Create the generic arguments for the associated type or constant by joining the
608            // parent arguments (the arguments of the trait) and the own arguments (the ones of
609            // the associated item itself) and construct an alias type using them.
610            let alias_term = candidate.map_bound(|trait_ref| {
611                let item_segment = hir::PathSegment {
612                    ident: constraint.ident,
613                    hir_id: constraint.hir_id,
614                    res: Res::Err,
615                    args: Some(constraint.gen_args),
616                    infer_args: false,
617                };
618
619                let alias_args = self.lower_generic_args_of_assoc_item(
620                    path_span,
621                    assoc_item.def_id,
622                    &item_segment,
623                    trait_ref.args,
624                );
625                debug!(?alias_args);
626
627                ty::AliasTerm::new_from_args(tcx, assoc_item.def_id, alias_args)
628            });
629
630            // Provide the resolved type of the associated constant to `type_of(AnonConst)`.
631            if let Some(const_arg) = constraint.ct() {
632                if let hir::ConstArgKind::Anon(anon_const) = const_arg.kind {
633                    let ty = alias_term
634                        .map_bound(|alias| tcx.type_of(alias.def_id).instantiate(tcx, alias.args));
635                    let ty = check_assoc_const_binding_type(
636                        self,
637                        constraint.ident,
638                        ty,
639                        constraint.hir_id,
640                    );
641                    tcx.feed_anon_const_type(anon_const.def_id, ty::EarlyBinder::bind(ty));
642                }
643            }
644
645            alias_term
646        };
647
648        match constraint.kind {
649            hir::AssocItemConstraintKind::Equality { .. } if let ty::AssocTag::Fn = assoc_tag => {
650                return Err(self.dcx().emit_err(crate::errors::ReturnTypeNotationEqualityBound {
651                    span: constraint.span,
652                }));
653            }
654            // Lower an equality constraint like `Item = u32` as found in HIR bound `T: Iterator<Item = u32>`
655            // to a projection predicate: `<T as Iterator>::Item = u32`.
656            hir::AssocItemConstraintKind::Equality { term } => {
657                let term = match term {
658                    hir::Term::Ty(ty) => self.lower_ty(ty).into(),
659                    hir::Term::Const(ct) => self.lower_const_arg(ct, FeedConstTy::No).into(),
660                };
661
662                // Find any late-bound regions declared in `ty` that are not
663                // declared in the trait-ref or assoc_item. These are not well-formed.
664                //
665                // Example:
666                //
667                //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
668                //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
669                let late_bound_in_projection_ty =
670                    tcx.collect_constrained_late_bound_regions(projection_term);
671                let late_bound_in_term =
672                    tcx.collect_referenced_late_bound_regions(trait_ref.rebind(term));
673                debug!(?late_bound_in_projection_ty);
674                debug!(?late_bound_in_term);
675
676                // FIXME: point at the type params that don't have appropriate lifetimes:
677                // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
678                //                         ----  ----     ^^^^^^^
679                // NOTE(associated_const_equality): This error should be impossible to trigger
680                //                                  with associated const equality constraints.
681                self.validate_late_bound_regions(
682                    late_bound_in_projection_ty,
683                    late_bound_in_term,
684                    |br_name| {
685                        struct_span_code_err!(
686                            self.dcx(),
687                            constraint.span,
688                            E0582,
689                            "binding for associated type `{}` references {}, \
690                             which does not appear in the trait input types",
691                            constraint.ident,
692                            br_name
693                        )
694                    },
695                );
696
697                match predicate_filter {
698                    PredicateFilter::All
699                    | PredicateFilter::SelfOnly
700                    | PredicateFilter::SelfAndAssociatedTypeBounds => {
701                        let bound = projection_term.map_bound(|projection_term| {
702                            ty::ClauseKind::Projection(ty::ProjectionPredicate {
703                                projection_term,
704                                term,
705                            })
706                        });
707                        bounds.push((bound.upcast(tcx), constraint.span));
708                    }
709                    // SelfTraitThatDefines is only interested in trait predicates.
710                    PredicateFilter::SelfTraitThatDefines(_) => {}
711                    // `ConstIfConst` is only interested in `~const` bounds.
712                    PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {}
713                }
714            }
715            // Lower a constraint like `Item: Debug` as found in HIR bound `T: Iterator<Item: Debug>`
716            // to a bound involving a projection: `<T as Iterator>::Item: Debug`.
717            hir::AssocItemConstraintKind::Bound { bounds: hir_bounds } => {
718                match predicate_filter {
719                    PredicateFilter::All
720                    | PredicateFilter::SelfAndAssociatedTypeBounds
721                    | PredicateFilter::ConstIfConst => {
722                        let projection_ty = projection_term
723                            .map_bound(|projection_term| projection_term.expect_ty(self.tcx()));
724                        // Calling `skip_binder` is okay, because `lower_bounds` expects the `param_ty`
725                        // parameter to have a skipped binder.
726                        let param_ty =
727                            Ty::new_alias(tcx, ty::Projection, projection_ty.skip_binder());
728                        self.lower_bounds(
729                            param_ty,
730                            hir_bounds,
731                            bounds,
732                            projection_ty.bound_vars(),
733                            predicate_filter,
734                        );
735                    }
736                    PredicateFilter::SelfOnly
737                    | PredicateFilter::SelfTraitThatDefines(_)
738                    | PredicateFilter::SelfConstIfConst => {}
739                }
740            }
741        }
742        Ok(())
743    }
744
745    /// Lower a type, possibly specially handling the type if it's a return type notation
746    /// which we otherwise deny in other positions.
747    pub fn lower_ty_maybe_return_type_notation(&self, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
748        let hir::TyKind::Path(qpath) = hir_ty.kind else {
749            return self.lower_ty(hir_ty);
750        };
751
752        let tcx = self.tcx();
753        match qpath {
754            hir::QPath::Resolved(opt_self_ty, path)
755                if let [mod_segments @ .., trait_segment, item_segment] = &path.segments[..]
756                    && item_segment.args.is_some_and(|args| {
757                        matches!(
758                            args.parenthesized,
759                            hir::GenericArgsParentheses::ReturnTypeNotation
760                        )
761                    }) =>
762            {
763                // We don't allow generics on the module segments.
764                let _ =
765                    self.prohibit_generic_args(mod_segments.iter(), GenericsArgsErrExtend::None);
766
767                let item_def_id = match path.res {
768                    Res::Def(DefKind::AssocFn, item_def_id) => item_def_id,
769                    Res::Err => {
770                        return Ty::new_error_with_message(
771                            tcx,
772                            hir_ty.span,
773                            "failed to resolve RTN",
774                        );
775                    }
776                    _ => bug!("only expected method resolution for fully qualified RTN"),
777                };
778                let trait_def_id = tcx.parent(item_def_id);
779
780                // Good error for `where Trait::method(..): Send`.
781                let Some(self_ty) = opt_self_ty else {
782                    let guar = self.report_missing_self_ty_for_resolved_path(
783                        trait_def_id,
784                        hir_ty.span,
785                        item_segment,
786                        ty::AssocTag::Type,
787                    );
788                    return Ty::new_error(tcx, guar);
789                };
790                let self_ty = self.lower_ty(self_ty);
791
792                let trait_ref = self.lower_mono_trait_ref(
793                    hir_ty.span,
794                    trait_def_id,
795                    self_ty,
796                    trait_segment,
797                    false,
798                );
799
800                // SUBTLE: As noted at the end of `try_append_return_type_notation_params`
801                // in `resolve_bound_vars`, we stash the explicit bound vars of the where
802                // clause onto the item segment of the RTN type. This allows us to know
803                // how many bound vars are *not* coming from the signature of the function
804                // from lowering RTN itself.
805                //
806                // For example, in `where for<'a> <T as Trait<'a>>::method(..): Other`,
807                // the `late_bound_vars` of the where clause predicate (i.e. this HIR ty's
808                // parent) will include `'a` AND all the early- and late-bound vars of the
809                // method. But when lowering the RTN type, we just want the list of vars
810                // we used to resolve the trait ref. We explicitly stored those back onto
811                // the item segment, since there's no other good place to put them.
812                let candidate =
813                    ty::Binder::bind_with_vars(trait_ref, tcx.late_bound_vars(item_segment.hir_id));
814
815                match self.lower_return_type_notation_ty(candidate, item_def_id, hir_ty.span) {
816                    Ok(ty) => Ty::new_alias(tcx, ty::Projection, ty),
817                    Err(guar) => Ty::new_error(tcx, guar),
818                }
819            }
820            hir::QPath::TypeRelative(hir_self_ty, segment)
821                if segment.args.is_some_and(|args| {
822                    matches!(args.parenthesized, hir::GenericArgsParentheses::ReturnTypeNotation)
823                }) =>
824            {
825                let self_ty = self.lower_ty(hir_self_ty);
826                let (item_def_id, bound) = match self.resolve_type_relative_path(
827                    self_ty,
828                    hir_self_ty,
829                    ty::AssocTag::Fn,
830                    segment,
831                    hir_ty.hir_id,
832                    hir_ty.span,
833                    None,
834                ) {
835                    Ok(result) => result,
836                    Err(guar) => return Ty::new_error(tcx, guar),
837                };
838
839                // Don't let `T::method` resolve to some `for<'a> <T as Tr<'a>>::method`,
840                // which may happen via a higher-ranked where clause or supertrait.
841                // This is the same restrictions as associated types; even though we could
842                // support it, it just makes things a lot more difficult to support in
843                // `resolve_bound_vars`, since we'd need to introduce those as elided
844                // bound vars on the where clause too.
845                if bound.has_bound_vars() {
846                    return Ty::new_error(
847                        tcx,
848                        self.dcx().emit_err(errors::AssociatedItemTraitUninferredGenericParams {
849                            span: hir_ty.span,
850                            inferred_sugg: Some(hir_ty.span.with_hi(segment.ident.span.lo())),
851                            bound: format!("{}::", tcx.anonymize_bound_vars(bound).skip_binder()),
852                            mpart_sugg: None,
853                            what: tcx.def_descr(item_def_id),
854                        }),
855                    );
856                }
857
858                match self.lower_return_type_notation_ty(bound, item_def_id, hir_ty.span) {
859                    Ok(ty) => Ty::new_alias(tcx, ty::Projection, ty),
860                    Err(guar) => Ty::new_error(tcx, guar),
861                }
862            }
863            _ => self.lower_ty(hir_ty),
864        }
865    }
866
867    /// Do the common parts of lowering an RTN type. This involves extending the
868    /// candidate binder to include all of the early- and late-bound vars that are
869    /// defined on the function itself, and constructing a projection to the RPITIT
870    /// return type of that function.
871    fn lower_return_type_notation_ty(
872        &self,
873        candidate: ty::PolyTraitRef<'tcx>,
874        item_def_id: DefId,
875        path_span: Span,
876    ) -> Result<ty::AliasTy<'tcx>, ErrorGuaranteed> {
877        let tcx = self.tcx();
878        let mut emitted_bad_param_err = None;
879        // If we have an method return type bound, then we need to instantiate
880        // the method's early bound params with suitable late-bound params.
881        let mut num_bound_vars = candidate.bound_vars().len();
882        let args = candidate.skip_binder().args.extend_to(tcx, item_def_id, |param, _| {
883            let arg = match param.kind {
884                ty::GenericParamDefKind::Lifetime => ty::Region::new_bound(
885                    tcx,
886                    ty::INNERMOST,
887                    ty::BoundRegion {
888                        var: ty::BoundVar::from_usize(num_bound_vars),
889                        kind: ty::BoundRegionKind::Named(param.def_id, param.name),
890                    },
891                )
892                .into(),
893                ty::GenericParamDefKind::Type { .. } => {
894                    let guar = *emitted_bad_param_err.get_or_insert_with(|| {
895                        self.dcx().emit_err(crate::errors::ReturnTypeNotationIllegalParam::Type {
896                            span: path_span,
897                            param_span: tcx.def_span(param.def_id),
898                        })
899                    });
900                    Ty::new_error(tcx, guar).into()
901                }
902                ty::GenericParamDefKind::Const { .. } => {
903                    let guar = *emitted_bad_param_err.get_or_insert_with(|| {
904                        self.dcx().emit_err(crate::errors::ReturnTypeNotationIllegalParam::Const {
905                            span: path_span,
906                            param_span: tcx.def_span(param.def_id),
907                        })
908                    });
909                    ty::Const::new_error(tcx, guar).into()
910                }
911            };
912            num_bound_vars += 1;
913            arg
914        });
915
916        // Next, we need to check that the return-type notation is being used on
917        // an RPITIT (return-position impl trait in trait) or AFIT (async fn in trait).
918        let output = tcx.fn_sig(item_def_id).skip_binder().output();
919        let output = if let ty::Alias(ty::Projection, alias_ty) = *output.skip_binder().kind()
920            && tcx.is_impl_trait_in_trait(alias_ty.def_id)
921        {
922            alias_ty
923        } else {
924            return Err(self.dcx().emit_err(crate::errors::ReturnTypeNotationOnNonRpitit {
925                span: path_span,
926                ty: tcx.liberate_late_bound_regions(item_def_id, output),
927                fn_span: tcx.hir_span_if_local(item_def_id),
928                note: (),
929            }));
930        };
931
932        // Finally, move the fn return type's bound vars over to account for the early bound
933        // params (and trait ref's late bound params). This logic is very similar to
934        // `rustc_middle::ty::predicate::Clause::instantiate_supertrait`
935        // and it's no coincidence why.
936        let shifted_output = tcx.shift_bound_var_indices(num_bound_vars, output);
937        Ok(ty::EarlyBinder::bind(shifted_output).instantiate(tcx, args))
938    }
939}
940
941/// Detect and reject early-bound & escaping late-bound generic params in the type of assoc const bindings.
942///
943/// FIXME(const_generics): This is a temporary and semi-artificial restriction until the
944/// arrival of *generic const generics*[^1].
945///
946/// It might actually be possible that we can already support early-bound generic params
947/// in such types if we just lifted some more checks in other places, too, for example
948/// inside `HirTyLowerer::lower_anon_const`. However, even if that were the case, we should
949/// probably gate this behind another feature flag.
950///
951/// [^1]: <https://github.com/rust-lang/project-const-generics/issues/28>.
952fn check_assoc_const_binding_type<'tcx>(
953    cx: &dyn HirTyLowerer<'tcx>,
954    assoc_const: Ident,
955    ty: ty::Binder<'tcx, Ty<'tcx>>,
956    hir_id: hir::HirId,
957) -> Ty<'tcx> {
958    // We can't perform the checks for early-bound params during name resolution unlike E0770
959    // because this information depends on *type* resolution.
960    // We can't perform these checks in `resolve_bound_vars` either for the same reason.
961    // Consider the trait ref `for<'a> Trait<'a, C = { &0 }>`. We need to know the fully
962    // resolved type of `Trait::C` in order to know if it references `'a` or not.
963
964    let ty = ty.skip_binder();
965    if !ty.has_param() && !ty.has_escaping_bound_vars() {
966        return ty;
967    }
968
969    let mut collector = GenericParamAndBoundVarCollector {
970        cx,
971        params: Default::default(),
972        vars: Default::default(),
973        depth: ty::INNERMOST,
974    };
975    let mut guar = ty.visit_with(&mut collector).break_value();
976
977    let tcx = cx.tcx();
978    let ty_note = ty
979        .make_suggestable(tcx, false, None)
980        .map(|ty| crate::errors::TyOfAssocConstBindingNote { assoc_const, ty });
981
982    let enclosing_item_owner_id = tcx
983        .hir_parent_owner_iter(hir_id)
984        .find_map(|(owner_id, parent)| parent.generics().map(|_| owner_id))
985        .unwrap();
986    let generics = tcx.generics_of(enclosing_item_owner_id);
987    for index in collector.params {
988        let param = generics.param_at(index as _, tcx);
989        let is_self_param = param.name == kw::SelfUpper;
990        guar.get_or_insert(cx.dcx().emit_err(crate::errors::ParamInTyOfAssocConstBinding {
991            span: assoc_const.span,
992            assoc_const,
993            param_name: param.name,
994            param_def_kind: tcx.def_descr(param.def_id),
995            param_category: if is_self_param {
996                "self"
997            } else if param.kind.is_synthetic() {
998                "synthetic"
999            } else {
1000                "normal"
1001            },
1002            param_defined_here_label:
1003                (!is_self_param).then(|| tcx.def_ident_span(param.def_id).unwrap()),
1004            ty_note,
1005        }));
1006    }
1007    for (var_def_id, var_name) in collector.vars {
1008        guar.get_or_insert(cx.dcx().emit_err(
1009            crate::errors::EscapingBoundVarInTyOfAssocConstBinding {
1010                span: assoc_const.span,
1011                assoc_const,
1012                var_name,
1013                var_def_kind: tcx.def_descr(var_def_id),
1014                var_defined_here_label: tcx.def_ident_span(var_def_id).unwrap(),
1015                ty_note,
1016            },
1017        ));
1018    }
1019
1020    let guar = guar.unwrap_or_else(|| bug!("failed to find gen params or bound vars in ty"));
1021    Ty::new_error(tcx, guar)
1022}
1023
1024struct GenericParamAndBoundVarCollector<'a, 'tcx> {
1025    cx: &'a dyn HirTyLowerer<'tcx>,
1026    params: FxIndexSet<u32>,
1027    vars: FxIndexSet<(DefId, Symbol)>,
1028    depth: ty::DebruijnIndex,
1029}
1030
1031impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'_, 'tcx> {
1032    type Result = ControlFlow<ErrorGuaranteed>;
1033
1034    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
1035        &mut self,
1036        binder: &ty::Binder<'tcx, T>,
1037    ) -> Self::Result {
1038        self.depth.shift_in(1);
1039        let result = binder.super_visit_with(self);
1040        self.depth.shift_out(1);
1041        result
1042    }
1043
1044    fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
1045        match ty.kind() {
1046            ty::Param(param) => {
1047                self.params.insert(param.index);
1048            }
1049            ty::Bound(db, bt) if *db >= self.depth => {
1050                self.vars.insert(match bt.kind {
1051                    ty::BoundTyKind::Param(def_id, name) => (def_id, name),
1052                    ty::BoundTyKind::Anon => {
1053                        let reported = self
1054                            .cx
1055                            .dcx()
1056                            .delayed_bug(format!("unexpected anon bound ty: {:?}", bt.var));
1057                        return ControlFlow::Break(reported);
1058                    }
1059                });
1060            }
1061            _ if ty.has_param() || ty.has_bound_vars() => return ty.super_visit_with(self),
1062            _ => {}
1063        }
1064        ControlFlow::Continue(())
1065    }
1066
1067    fn visit_region(&mut self, re: ty::Region<'tcx>) -> Self::Result {
1068        match re.kind() {
1069            ty::ReEarlyParam(param) => {
1070                self.params.insert(param.index);
1071            }
1072            ty::ReBound(db, br) if db >= self.depth => {
1073                self.vars.insert(match br.kind {
1074                    ty::BoundRegionKind::Named(def_id, name) => (def_id, name),
1075                    ty::BoundRegionKind::Anon | ty::BoundRegionKind::ClosureEnv => {
1076                        let guar = self
1077                            .cx
1078                            .dcx()
1079                            .delayed_bug(format!("unexpected bound region kind: {:?}", br.kind));
1080                        return ControlFlow::Break(guar);
1081                    }
1082                });
1083            }
1084            _ => {}
1085        }
1086        ControlFlow::Continue(())
1087    }
1088
1089    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
1090        match ct.kind() {
1091            ty::ConstKind::Param(param) => {
1092                self.params.insert(param.index);
1093            }
1094            ty::ConstKind::Bound(db, ty::BoundVar { .. }) if db >= self.depth => {
1095                let guar = self.cx.dcx().delayed_bug("unexpected escaping late-bound const var");
1096                return ControlFlow::Break(guar);
1097            }
1098            _ if ct.has_param() || ct.has_bound_vars() => return ct.super_visit_with(self),
1099            _ => {}
1100        }
1101        ControlFlow::Continue(())
1102    }
1103}
rustc_hir_analysis/hir_ty_lowering/bounds.rs

rustc_hir_analysis/hir_ty_lowering/
bounds.rs
