rustc_lint/
impl_trait_overcaptures.rs

1use std::assert_matches::debug_assert_matches;
2use std::cell::LazyCell;
3
4use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
5use rustc_data_structures::unord::UnordSet;
6use rustc_errors::{LintDiagnostic, Subdiagnostic};
7use rustc_hir as hir;
8use rustc_hir::def::DefKind;
9use rustc_hir::def_id::{DefId, LocalDefId};
10use rustc_infer::infer::TyCtxtInferExt;
11use rustc_infer::infer::outlives::env::OutlivesEnvironment;
12use rustc_macros::LintDiagnostic;
13use rustc_middle::middle::resolve_bound_vars::ResolvedArg;
14use rustc_middle::ty::relate::{
15    Relate, RelateResult, TypeRelation, structurally_relate_consts, structurally_relate_tys,
16};
17use rustc_middle::ty::{
18    self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
19};
20use rustc_middle::{bug, span_bug};
21use rustc_session::lint::FutureIncompatibilityReason;
22use rustc_session::{declare_lint, declare_lint_pass};
23use rustc_span::edition::Edition;
24use rustc_span::{Span, Symbol};
25use rustc_trait_selection::errors::{
26    AddPreciseCapturingForOvercapture, impl_trait_overcapture_suggestion,
27};
28use rustc_trait_selection::regions::OutlivesEnvironmentBuildExt;
29use rustc_trait_selection::traits::ObligationCtxt;
30
31use crate::{LateContext, LateLintPass, fluent_generated as fluent};
32
33declare_lint! {
34    /// The `impl_trait_overcaptures` lint warns against cases where lifetime
35    /// capture behavior will differ in edition 2024.
36    ///
37    /// In the 2024 edition, `impl Trait`s will capture all lifetimes in scope,
38    /// rather than just the lifetimes that are mentioned in the bounds of the type.
39    /// Often these sets are equal, but if not, it means that the `impl Trait` may
40    /// cause erroneous borrow-checker errors.
41    ///
42    /// ### Example
43    ///
44    /// ```rust,compile_fail,edition2021
45    /// # #![deny(impl_trait_overcaptures)]
46    /// # use std::fmt::Display;
47    /// let mut x = vec![];
48    /// x.push(1);
49    ///
50    /// fn test(x: &Vec<i32>) -> impl Display {
51    ///     x[0]
52    /// }
53    ///
54    /// let element = test(&x);
55    /// x.push(2);
56    /// println!("{element}");
57    /// ```
58    ///
59    /// {{produces}}
60    ///
61    /// ### Explanation
62    ///
63    /// In edition < 2024, the returned `impl Display` doesn't capture the
64    /// lifetime from the `&Vec<i32>`, so the vector can be mutably borrowed
65    /// while the `impl Display` is live.
66    ///
67    /// To fix this, we can explicitly state that the `impl Display` doesn't
68    /// capture any lifetimes, using `impl Display + use<>`.
69    pub IMPL_TRAIT_OVERCAPTURES,
70    Allow,
71    "`impl Trait` will capture more lifetimes than possibly intended in edition 2024",
72    @future_incompatible = FutureIncompatibleInfo {
73        reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
74        reference: "<https://doc.rust-lang.org/nightly/edition-guide/rust-2024/rpit-lifetime-capture.html>",
75    };
76}
77
78declare_lint! {
79    /// The `impl_trait_redundant_captures` lint warns against cases where use of the
80    /// precise capturing `use<...>` syntax is not needed.
81    ///
82    /// In the 2024 edition, `impl Trait`s will capture all lifetimes in scope.
83    /// If precise-capturing `use<...>` syntax is used, and the set of parameters
84    /// that are captures are *equal* to the set of parameters in scope, then
85    /// the syntax is redundant, and can be removed.
86    ///
87    /// ### Example
88    ///
89    /// ```rust,edition2024,compile_fail
90    /// # #![deny(impl_trait_redundant_captures)]
91    /// fn test<'a>(x: &'a i32) -> impl Sized + use<'a> { x }
92    /// ```
93    ///
94    /// {{produces}}
95    ///
96    /// ### Explanation
97    ///
98    /// To fix this, remove the `use<'a>`, since the lifetime is already captured
99    /// since it is in scope.
100    pub IMPL_TRAIT_REDUNDANT_CAPTURES,
101    Allow,
102    "redundant precise-capturing `use<...>` syntax on an `impl Trait`",
103}
104
105declare_lint_pass!(
106    /// Lint for opaque types that will begin capturing in-scope but unmentioned lifetimes
107    /// in edition 2024.
108    ImplTraitOvercaptures => [IMPL_TRAIT_OVERCAPTURES, IMPL_TRAIT_REDUNDANT_CAPTURES]
109);
110
111impl<'tcx> LateLintPass<'tcx> for ImplTraitOvercaptures {
112    fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx hir::Item<'tcx>) {
113        match &it.kind {
114            hir::ItemKind::Fn { .. } => check_fn(cx.tcx, it.owner_id.def_id),
115            _ => {}
116        }
117    }
118
119    fn check_impl_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx hir::ImplItem<'tcx>) {
120        match &it.kind {
121            hir::ImplItemKind::Fn(_, _) => check_fn(cx.tcx, it.owner_id.def_id),
122            _ => {}
123        }
124    }
125
126    fn check_trait_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx hir::TraitItem<'tcx>) {
127        match &it.kind {
128            hir::TraitItemKind::Fn(_, _) => check_fn(cx.tcx, it.owner_id.def_id),
129            _ => {}
130        }
131    }
132}
133
134#[derive(PartialEq, Eq, Hash, Debug, Copy, Clone)]
135enum ParamKind {
136    // Early-bound var.
137    Early(Symbol, u32),
138    // Late-bound var on function, not within a binder. We can capture these.
139    Free(DefId),
140    // Late-bound var in a binder. We can't capture these yet.
141    Late,
142}
143
144fn check_fn(tcx: TyCtxt<'_>, parent_def_id: LocalDefId) {
145    let sig = tcx.fn_sig(parent_def_id).instantiate_identity();
146
147    let mut in_scope_parameters = FxIndexMap::default();
148    // Populate the in_scope_parameters list first with all of the generics in scope
149    let mut current_def_id = Some(parent_def_id.to_def_id());
150    while let Some(def_id) = current_def_id {
151        let generics = tcx.generics_of(def_id);
152        for param in &generics.own_params {
153            in_scope_parameters.insert(param.def_id, ParamKind::Early(param.name, param.index));
154        }
155        current_def_id = generics.parent;
156    }
157
158    for bound_var in sig.bound_vars() {
159        let ty::BoundVariableKind::Region(ty::BoundRegionKind::Named(def_id)) = bound_var else {
160            span_bug!(tcx.def_span(parent_def_id), "unexpected non-lifetime binder on fn sig");
161        };
162
163        in_scope_parameters.insert(def_id, ParamKind::Free(def_id));
164    }
165
166    let sig = tcx.liberate_late_bound_regions(parent_def_id.to_def_id(), sig);
167
168    // Then visit the signature to walk through all the binders (incl. the late-bound
169    // vars on the function itself, which we need to count too).
170    sig.visit_with(&mut VisitOpaqueTypes {
171        tcx,
172        parent_def_id,
173        in_scope_parameters,
174        seen: Default::default(),
175        // Lazily compute these two, since they're likely a bit expensive.
176        variances: LazyCell::new(|| {
177            let mut functional_variances = FunctionalVariances {
178                tcx,
179                variances: FxHashMap::default(),
180                ambient_variance: ty::Covariant,
181                generics: tcx.generics_of(parent_def_id),
182            };
183            functional_variances.relate(sig, sig).unwrap();
184            functional_variances.variances
185        }),
186        outlives_env: LazyCell::new(|| {
187            let typing_env = ty::TypingEnv::non_body_analysis(tcx, parent_def_id);
188            let (infcx, param_env) = tcx.infer_ctxt().build_with_typing_env(typing_env);
189            let ocx = ObligationCtxt::new(&infcx);
190            let assumed_wf_tys = ocx.assumed_wf_types(param_env, parent_def_id).unwrap_or_default();
191            OutlivesEnvironment::new(&infcx, parent_def_id, param_env, assumed_wf_tys)
192        }),
193    });
194}
195
196struct VisitOpaqueTypes<'tcx, VarFn, OutlivesFn> {
197    tcx: TyCtxt<'tcx>,
198    parent_def_id: LocalDefId,
199    in_scope_parameters: FxIndexMap<DefId, ParamKind>,
200    variances: LazyCell<FxHashMap<DefId, ty::Variance>, VarFn>,
201    outlives_env: LazyCell<OutlivesEnvironment<'tcx>, OutlivesFn>,
202    seen: FxIndexSet<LocalDefId>,
203}
204
205impl<'tcx, VarFn, OutlivesFn> TypeVisitor<TyCtxt<'tcx>>
206    for VisitOpaqueTypes<'tcx, VarFn, OutlivesFn>
207where
208    VarFn: FnOnce() -> FxHashMap<DefId, ty::Variance>,
209    OutlivesFn: FnOnce() -> OutlivesEnvironment<'tcx>,
210{
211    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(&mut self, t: &ty::Binder<'tcx, T>) {
212        // When we get into a binder, we need to add its own bound vars to the scope.
213        let mut added = vec![];
214        for arg in t.bound_vars() {
215            let arg: ty::BoundVariableKind = arg;
216            match arg {
217                ty::BoundVariableKind::Region(ty::BoundRegionKind::Named(def_id))
218                | ty::BoundVariableKind::Ty(ty::BoundTyKind::Param(def_id)) => {
219                    added.push(def_id);
220                    let unique = self.in_scope_parameters.insert(def_id, ParamKind::Late);
221                    assert_eq!(unique, None);
222                }
223                _ => {
224                    self.tcx.dcx().span_delayed_bug(
225                        self.tcx.def_span(self.parent_def_id),
226                        format!("unsupported bound variable kind: {arg:?}"),
227                    );
228                }
229            }
230        }
231
232        t.super_visit_with(self);
233
234        // And remove them. The `shift_remove` should be `O(1)` since we're popping
235        // them off from the end.
236        for arg in added.into_iter().rev() {
237            self.in_scope_parameters.shift_remove(&arg);
238        }
239    }
240
241    fn visit_ty(&mut self, t: Ty<'tcx>) {
242        if !t.has_aliases() {
243            return;
244        }
245
246        if let ty::Alias(ty::Projection, opaque_ty) = *t.kind()
247            && self.tcx.is_impl_trait_in_trait(opaque_ty.def_id)
248        {
249            // visit the opaque of the RPITIT
250            self.tcx
251                .type_of(opaque_ty.def_id)
252                .instantiate(self.tcx, opaque_ty.args)
253                .visit_with(self)
254        } else if let ty::Alias(ty::Opaque, opaque_ty) = *t.kind()
255            && let Some(opaque_def_id) = opaque_ty.def_id.as_local()
256            // Don't recurse infinitely on an opaque
257            && self.seen.insert(opaque_def_id)
258            // If it's owned by this function
259            && let opaque =
260                self.tcx.hir_node_by_def_id(opaque_def_id).expect_opaque_ty()
261            // We want to recurse into RPITs and async fns, even though the latter
262            // doesn't overcapture on its own, it may mention additional RPITs
263            // in its bounds.
264            && let hir::OpaqueTyOrigin::FnReturn { parent, .. }
265                | hir::OpaqueTyOrigin::AsyncFn { parent, .. } = opaque.origin
266            && parent == self.parent_def_id
267        {
268            let opaque_span = self.tcx.def_span(opaque_def_id);
269            let new_capture_rules = opaque_span.at_least_rust_2024();
270            if !new_capture_rules
271                && !opaque.bounds.iter().any(|bound| matches!(bound, hir::GenericBound::Use(..)))
272            {
273                // Compute the set of args that are captured by the opaque...
274                let mut captured = FxIndexSet::default();
275                let mut captured_regions = FxIndexSet::default();
276                let variances = self.tcx.variances_of(opaque_def_id);
277                let mut current_def_id = Some(opaque_def_id.to_def_id());
278                while let Some(def_id) = current_def_id {
279                    let generics = self.tcx.generics_of(def_id);
280                    for param in &generics.own_params {
281                        // A param is captured if it's invariant.
282                        if variances[param.index as usize] != ty::Invariant {
283                            continue;
284                        }
285
286                        let arg = opaque_ty.args[param.index as usize];
287                        // We need to turn all `ty::Param`/`ConstKind::Param` and
288                        // `ReEarlyParam`/`ReBound` into def ids.
289                        captured.insert(extract_def_id_from_arg(self.tcx, generics, arg));
290
291                        captured_regions.extend(arg.as_region());
292                    }
293                    current_def_id = generics.parent;
294                }
295
296                // Compute the set of in scope params that are not captured.
297                let mut uncaptured_args: FxIndexSet<_> = self
298                    .in_scope_parameters
299                    .iter()
300                    .filter(|&(def_id, _)| !captured.contains(def_id))
301                    .collect();
302                // Remove the set of lifetimes that are in-scope that outlive some other captured
303                // lifetime and are contravariant (i.e. covariant in argument position).
304                uncaptured_args.retain(|&(def_id, kind)| {
305                    let Some(ty::Bivariant | ty::Contravariant) = self.variances.get(def_id) else {
306                        // Keep all covariant/invariant args. Also if variance is `None`,
307                        // then that means it's either not a lifetime, or it didn't show up
308                        // anywhere in the signature.
309                        return true;
310                    };
311                    // We only computed variance of lifetimes...
312                    debug_assert_matches!(self.tcx.def_kind(def_id), DefKind::LifetimeParam);
313                    let uncaptured = match *kind {
314                        ParamKind::Early(name, index) => ty::Region::new_early_param(
315                            self.tcx,
316                            ty::EarlyParamRegion { name, index },
317                        ),
318                        ParamKind::Free(def_id) => ty::Region::new_late_param(
319                            self.tcx,
320                            self.parent_def_id.to_def_id(),
321                            ty::LateParamRegionKind::Named(def_id),
322                        ),
323                        // Totally ignore late bound args from binders.
324                        ParamKind::Late => return true,
325                    };
326                    // Does this region outlive any captured region?
327                    !captured_regions.iter().any(|r| {
328                        self.outlives_env
329                            .free_region_map()
330                            .sub_free_regions(self.tcx, *r, uncaptured)
331                    })
332                });
333
334                // If we have uncaptured args, and if the opaque doesn't already have
335                // `use<>` syntax on it, and we're < edition 2024, then warn the user.
336                if !uncaptured_args.is_empty() {
337                    let suggestion = impl_trait_overcapture_suggestion(
338                        self.tcx,
339                        opaque_def_id,
340                        self.parent_def_id,
341                        captured,
342                    );
343
344                    let uncaptured_spans: Vec<_> = uncaptured_args
345                        .into_iter()
346                        .map(|(def_id, _)| self.tcx.def_span(def_id))
347                        .collect();
348
349                    self.tcx.emit_node_span_lint(
350                        IMPL_TRAIT_OVERCAPTURES,
351                        self.tcx.local_def_id_to_hir_id(opaque_def_id),
352                        opaque_span,
353                        ImplTraitOvercapturesLint {
354                            self_ty: t,
355                            num_captured: uncaptured_spans.len(),
356                            uncaptured_spans,
357                            suggestion,
358                        },
359                    );
360                }
361            }
362
363            // Otherwise, if we are edition 2024, have `use<>` syntax, and
364            // have no uncaptured args, then we should warn to the user that
365            // it's redundant to capture all args explicitly.
366            if new_capture_rules
367                && let Some((captured_args, capturing_span)) =
368                    opaque.bounds.iter().find_map(|bound| match *bound {
369                        hir::GenericBound::Use(a, s) => Some((a, s)),
370                        _ => None,
371                    })
372            {
373                let mut explicitly_captured = UnordSet::default();
374                for arg in captured_args {
375                    match self.tcx.named_bound_var(arg.hir_id()) {
376                        Some(
377                            ResolvedArg::EarlyBound(def_id) | ResolvedArg::LateBound(_, _, def_id),
378                        ) => {
379                            if self.tcx.def_kind(self.tcx.local_parent(def_id)) == DefKind::OpaqueTy
380                            {
381                                let def_id = self
382                                    .tcx
383                                    .map_opaque_lifetime_to_parent_lifetime(def_id)
384                                    .opt_param_def_id(self.tcx, self.parent_def_id.to_def_id())
385                                    .expect("variable should have been duplicated from parent");
386
387                                explicitly_captured.insert(def_id);
388                            } else {
389                                explicitly_captured.insert(def_id.to_def_id());
390                            }
391                        }
392                        _ => {
393                            self.tcx.dcx().span_delayed_bug(
394                                self.tcx.hir_span(arg.hir_id()),
395                                "no valid for captured arg",
396                            );
397                        }
398                    }
399                }
400
401                if self
402                    .in_scope_parameters
403                    .iter()
404                    .all(|(def_id, _)| explicitly_captured.contains(def_id))
405                {
406                    self.tcx.emit_node_span_lint(
407                        IMPL_TRAIT_REDUNDANT_CAPTURES,
408                        self.tcx.local_def_id_to_hir_id(opaque_def_id),
409                        opaque_span,
410                        ImplTraitRedundantCapturesLint { capturing_span },
411                    );
412                }
413            }
414
415            // Walk into the bounds of the opaque, too, since we want to get nested opaques
416            // in this lint as well. Interestingly, one place that I expect this lint to fire
417            // is for `impl for<'a> Bound<Out = impl Other>`, since `impl Other` will begin
418            // to capture `'a` in e2024 (even though late-bound vars in opaques are not allowed).
419            for clause in
420                self.tcx.item_bounds(opaque_ty.def_id).iter_instantiated(self.tcx, opaque_ty.args)
421            {
422                clause.visit_with(self)
423            }
424        }
425
426        t.super_visit_with(self);
427    }
428}
429
430struct ImplTraitOvercapturesLint<'tcx> {
431    uncaptured_spans: Vec<Span>,
432    self_ty: Ty<'tcx>,
433    num_captured: usize,
434    suggestion: Option<AddPreciseCapturingForOvercapture>,
435}
436
437impl<'a> LintDiagnostic<'a, ()> for ImplTraitOvercapturesLint<'_> {
438    fn decorate_lint<'b>(self, diag: &'b mut rustc_errors::Diag<'a, ()>) {
439        diag.primary_message(fluent::lint_impl_trait_overcaptures);
440        diag.arg("self_ty", self.self_ty.to_string())
441            .arg("num_captured", self.num_captured)
442            .span_note(self.uncaptured_spans, fluent::lint_note)
443            .note(fluent::lint_note2);
444        if let Some(suggestion) = self.suggestion {
445            suggestion.add_to_diag(diag);
446        }
447    }
448}
449
450#[derive(LintDiagnostic)]
451#[diag(lint_impl_trait_redundant_captures)]
452struct ImplTraitRedundantCapturesLint {
453    #[suggestion(lint_suggestion, code = "", applicability = "machine-applicable")]
454    capturing_span: Span,
455}
456
457fn extract_def_id_from_arg<'tcx>(
458    tcx: TyCtxt<'tcx>,
459    generics: &'tcx ty::Generics,
460    arg: ty::GenericArg<'tcx>,
461) -> DefId {
462    match arg.kind() {
463        ty::GenericArgKind::Lifetime(re) => match re.kind() {
464            ty::ReEarlyParam(ebr) => generics.region_param(ebr, tcx).def_id,
465            ty::ReBound(_, ty::BoundRegion { kind: ty::BoundRegionKind::Named(def_id), .. })
466            | ty::ReLateParam(ty::LateParamRegion {
467                scope: _,
468                kind: ty::LateParamRegionKind::Named(def_id),
469            }) => def_id,
470            _ => unreachable!(),
471        },
472        ty::GenericArgKind::Type(ty) => {
473            let ty::Param(param_ty) = *ty.kind() else {
474                bug!();
475            };
476            generics.type_param(param_ty, tcx).def_id
477        }
478        ty::GenericArgKind::Const(ct) => {
479            let ty::ConstKind::Param(param_ct) = ct.kind() else {
480                bug!();
481            };
482            generics.const_param(param_ct, tcx).def_id
483        }
484    }
485}
486
487/// Computes the variances of regions that appear in the type, but considering
488/// late-bound regions too, which don't have their variance computed usually.
489///
490/// Like generalization, this is a unary operation implemented on top of the binary
491/// relation infrastructure, mostly because it's much easier to have the relation
492/// track the variance for you, rather than having to do it yourself.
493struct FunctionalVariances<'tcx> {
494    tcx: TyCtxt<'tcx>,
495    variances: FxHashMap<DefId, ty::Variance>,
496    ambient_variance: ty::Variance,
497    generics: &'tcx ty::Generics,
498}
499
500impl<'tcx> TypeRelation<TyCtxt<'tcx>> for FunctionalVariances<'tcx> {
501    fn cx(&self) -> TyCtxt<'tcx> {
502        self.tcx
503    }
504
505    fn relate_with_variance<T: Relate<TyCtxt<'tcx>>>(
506        &mut self,
507        variance: ty::Variance,
508        _: ty::VarianceDiagInfo<TyCtxt<'tcx>>,
509        a: T,
510        b: T,
511    ) -> RelateResult<'tcx, T> {
512        let old_variance = self.ambient_variance;
513        self.ambient_variance = self.ambient_variance.xform(variance);
514        self.relate(a, b).unwrap();
515        self.ambient_variance = old_variance;
516        Ok(a)
517    }
518
519    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
520        structurally_relate_tys(self, a, b).unwrap();
521        Ok(a)
522    }
523
524    fn regions(
525        &mut self,
526        a: ty::Region<'tcx>,
527        _: ty::Region<'tcx>,
528    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
529        let def_id = match a.kind() {
530            ty::ReEarlyParam(ebr) => self.generics.region_param(ebr, self.tcx).def_id,
531            ty::ReBound(_, ty::BoundRegion { kind: ty::BoundRegionKind::Named(def_id), .. })
532            | ty::ReLateParam(ty::LateParamRegion {
533                scope: _,
534                kind: ty::LateParamRegionKind::Named(def_id),
535            }) => def_id,
536            _ => {
537                return Ok(a);
538            }
539        };
540
541        if let Some(variance) = self.variances.get_mut(&def_id) {
542            *variance = unify(*variance, self.ambient_variance);
543        } else {
544            self.variances.insert(def_id, self.ambient_variance);
545        }
546
547        Ok(a)
548    }
549
550    fn consts(
551        &mut self,
552        a: ty::Const<'tcx>,
553        b: ty::Const<'tcx>,
554    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
555        structurally_relate_consts(self, a, b).unwrap();
556        Ok(a)
557    }
558
559    fn binders<T>(
560        &mut self,
561        a: ty::Binder<'tcx, T>,
562        b: ty::Binder<'tcx, T>,
563    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
564    where
565        T: Relate<TyCtxt<'tcx>>,
566    {
567        self.relate(a.skip_binder(), b.skip_binder()).unwrap();
568        Ok(a)
569    }
570}
571
572/// What is the variance that satisfies the two variances?
573fn unify(a: ty::Variance, b: ty::Variance) -> ty::Variance {
574    match (a, b) {
575        // Bivariance is lattice bottom.
576        (ty::Bivariant, other) | (other, ty::Bivariant) => other,
577        // Invariant is lattice top.
578        (ty::Invariant, _) | (_, ty::Invariant) => ty::Invariant,
579        // If type is required to be covariant and contravariant, then it's invariant.
580        (ty::Contravariant, ty::Covariant) | (ty::Covariant, ty::Contravariant) => ty::Invariant,
581        // Otherwise, co + co = co, contra + contra = contra.
582        (ty::Contravariant, ty::Contravariant) => ty::Contravariant,
583        (ty::Covariant, ty::Covariant) => ty::Covariant,
584    }
585}
rustc_lint/impl_trait_overcaptures.rs

rustc_lint/
impl_trait_overcaptures.rs
