1 //! Code that handles "type-outlives" constraints like `T: 'a`. This
2 //! is based on the `push_outlives_components` function defined in rustc_infer,
3 //! but it adds a bit of heuristics on top, in particular to deal with
4 //! associated types and projections.
6 //! When we process a given `T: 'a` obligation, we may produce two
7 //! kinds of constraints for the region inferencer:
9 //! - Relationships between inference variables and other regions.
10 //! For example, if we have `&'?0 u32: 'a`, then we would produce
11 //! a constraint that `'a <= '?0`.
12 //! - "Verifys" that must be checked after inferencing is done.
13 //! For example, if we know that, for some type parameter `T`,
14 //! `T: 'a + 'b`, and we have a requirement that `T: '?1`,
15 //! then we add a "verify" that checks that `'?1 <= 'a || '?1 <= 'b`.
16 //! - Note the difference with the previous case: here, the region
17 //! variable must be less than something else, so this doesn't
18 //! affect how inference works (it finds the smallest region that
19 //! will do); it's just a post-condition that we have to check.
21 //! **The key point is that once this function is done, we have
22 //! reduced all of our "type-region outlives" obligations into relationships
23 //! between individual regions.**
25 //! One key input to this function is the set of "region-bound pairs".
26 //! These are basically the relationships between type parameters and
27 //! regions that are in scope at the point where the outlives
28 //! obligation was incurred. **When type-checking a function,
29 //! particularly in the face of closures, this is not known until
30 //! regionck runs!** This is because some of those bounds come
31 //! from things we have yet to infer.
36 //! fn bar<T>(a: T, b: impl for<'a> Fn(&'a T)) {}
38 //! bar(x, |y| { /* ... */})
43 //! Here, the type of `y` may involve inference variables and the
44 //! like, and it may also contain implied bounds that are needed to
45 //! type-check the closure body (e.g., here it informs us that `T`
46 //! outlives the late-bound region `'a`).
48 //! Note that by delaying the gathering of implied bounds until all
49 //! inference information is known, we may find relationships between
50 //! bound regions and other regions in the environment. For example,
51 //! when we first check a closure like the one expected as argument
55 //! fn foo<U, F: for<'a> FnMut(&'a U)>(_f: F) {}
58 //! the type of the closure's first argument would be `&'a ?U`. We
59 //! might later infer `?U` to something like `&'b u32`, which would
60 //! imply that `'b: 'a`.
62 use crate::infer::outlives::components::{push_outlives_components, Component};
63 use crate::infer::outlives::env::OutlivesEnvironment;
64 use crate::infer::outlives::env::RegionBoundPairs;
65 use crate::infer::outlives::verify::VerifyBoundCx;
67 self, GenericKind, InferCtxt, RegionObligation, SubregionOrigin, UndoLog, VerifyBound,
69 use crate::traits::{ObligationCause, ObligationCauseCode};
70 use rustc_data_structures::undo_log::UndoLogs;
71 use rustc_hir::def_id::LocalDefId;
72 use rustc_middle::ty::subst::GenericArgKind;
73 use rustc_middle::ty::{self, Region, Ty, TyCtxt, TypeVisitable};
74 use smallvec::smallvec;
76 impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
77 /// Registers that the given region obligation must be resolved
78 /// from within the scope of `body_id`. These regions are enqueued
79 /// and later processed by regionck, when full type information is
80 /// available (see `region_obligations` field for more
82 #[instrument(level = "debug", skip(self))]
83 pub fn register_region_obligation(&self, obligation: RegionObligation<'tcx>) {
84 let mut inner = self.inner.borrow_mut();
85 inner.undo_log.push(UndoLog::PushRegionObligation);
86 inner.region_obligations.push(obligation);
89 pub fn register_region_obligation_with_cause(
92 sub_region: Region<'tcx>,
93 cause: &ObligationCause<'tcx>,
95 let origin = SubregionOrigin::from_obligation_cause(cause, || {
96 infer::RelateParamBound(
99 match cause.code().peel_derives() {
100 ObligationCauseCode::BindingObligation(_, span) => Some(*span),
106 self.register_region_obligation(RegionObligation { sup_type, sub_region, origin });
109 /// Trait queries just want to pass back type obligations "as is"
110 pub fn take_registered_region_obligations(&self) -> Vec<RegionObligation<'tcx>> {
111 std::mem::take(&mut self.inner.borrow_mut().region_obligations)
114 /// Process the region obligations that must be proven (during
115 /// `regionck`) for the given `body_id`, given information about
116 /// the region bounds in scope and so forth. This function must be
117 /// invoked for all relevant body-ids before region inference is
118 /// done (or else an assert will fire).
120 /// See the `region_obligations` field of `InferCtxt` for some
121 /// comments about how this function fits into the overall expected
122 /// flow of the inferencer. The key point is that it is
123 /// invoked after all type-inference variables have been bound --
124 /// towards the end of regionck. This also ensures that the
125 /// region-bound-pairs are available (see comments above regarding
130 /// - `region_bound_pairs_map`: the set of region bounds implied by
131 /// the parameters and where-clauses. In particular, each pair
132 /// `('a, K)` in this list tells us that the bounds in scope
133 /// indicate that `K: 'a`, where `K` is either a generic
134 /// parameter like `T` or a projection like `T::Item`.
135 /// - `param_env` is the parameter environment for the enclosing function.
136 /// - `body_id` is the body-id whose region obligations are being
138 #[instrument(level = "debug", skip(self, region_bound_pairs))]
139 pub fn process_registered_region_obligations(
141 region_bound_pairs: &RegionBoundPairs<'tcx>,
142 param_env: ty::ParamEnv<'tcx>,
145 !self.in_snapshot.get(),
146 "cannot process registered region obligations in a snapshot"
149 let my_region_obligations = self.take_registered_region_obligations();
151 for RegionObligation { sup_type, sub_region, origin } in my_region_obligations {
153 "process_registered_region_obligations: sup_type={:?} sub_region={:?} origin={:?}",
154 sup_type, sub_region, origin
157 let sup_type = self.resolve_vars_if_possible(sup_type);
160 &mut TypeOutlives::new(self, self.tcx, ®ion_bound_pairs, None, param_env);
161 outlives.type_must_outlive(origin, sup_type, sub_region);
165 pub fn check_region_obligations_and_report_errors(
167 generic_param_scope: LocalDefId,
168 outlives_env: &OutlivesEnvironment<'tcx>,
170 self.process_registered_region_obligations(
171 outlives_env.region_bound_pairs(),
172 outlives_env.param_env,
175 self.resolve_regions_and_report_errors(generic_param_scope, outlives_env)
179 /// The `TypeOutlives` struct has the job of "lowering" a `T: 'a`
180 /// obligation into a series of `'a: 'b` constraints and "verify"s, as
181 /// described on the module comment. The final constraints are emitted
182 /// via a "delegate" of type `D` -- this is usually the `infcx`, which
183 /// accrues them into the `region_obligations` code, but for NLL we
184 /// use something else.
185 pub struct TypeOutlives<'cx, 'tcx, D>
187 D: TypeOutlivesDelegate<'tcx>,
189 // See the comments on `process_registered_region_obligations` for the meaning
193 verify_bound: VerifyBoundCx<'cx, 'tcx>,
196 pub trait TypeOutlivesDelegate<'tcx> {
197 fn push_sub_region_constraint(
199 origin: SubregionOrigin<'tcx>,
206 origin: SubregionOrigin<'tcx>,
207 kind: GenericKind<'tcx>,
209 bound: VerifyBound<'tcx>,
213 impl<'cx, 'tcx, D> TypeOutlives<'cx, 'tcx, D>
215 D: TypeOutlivesDelegate<'tcx>,
220 region_bound_pairs: &'cx RegionBoundPairs<'tcx>,
221 implicit_region_bound: Option<ty::Region<'tcx>>,
222 param_env: ty::ParamEnv<'tcx>,
227 verify_bound: VerifyBoundCx::new(
230 implicit_region_bound,
236 /// Adds constraints to inference such that `T: 'a` holds (or
237 /// reports an error if it cannot).
241 /// - `origin`, the reason we need this constraint
242 /// - `ty`, the type `T`
243 /// - `region`, the region `'a`
244 pub fn type_must_outlive(
246 origin: infer::SubregionOrigin<'tcx>,
248 region: ty::Region<'tcx>,
250 debug!("type_must_outlive(ty={:?}, region={:?}, origin={:?})", ty, region, origin);
252 assert!(!ty.has_escaping_bound_vars());
254 let mut components = smallvec![];
255 push_outlives_components(self.tcx, ty, &mut components);
256 self.components_must_outlive(origin, &components, region);
259 fn components_must_outlive(
261 origin: infer::SubregionOrigin<'tcx>,
262 components: &[Component<'tcx>],
263 region: ty::Region<'tcx>,
265 for component in components.iter() {
266 let origin = origin.clone();
268 Component::Region(region1) => {
269 self.delegate.push_sub_region_constraint(origin, region, *region1);
271 Component::Param(param_ty) => {
272 self.param_ty_must_outlive(origin, region, *param_ty);
274 Component::Projection(projection_ty) => {
275 self.projection_must_outlive(origin, region, *projection_ty);
277 Component::EscapingProjection(subcomponents) => {
278 self.components_must_outlive(origin, &subcomponents, region);
280 Component::UnresolvedInferenceVariable(v) => {
281 // ignore this, we presume it will yield an error
282 // later, since if a type variable is not resolved by
283 // this point it never will be
284 self.tcx.sess.delay_span_bug(
286 &format!("unresolved inference variable in outlives: {:?}", v),
293 fn param_ty_must_outlive(
295 origin: infer::SubregionOrigin<'tcx>,
296 region: ty::Region<'tcx>,
297 param_ty: ty::ParamTy,
300 "param_ty_must_outlive(region={:?}, param_ty={:?}, origin={:?})",
301 region, param_ty, origin
304 let generic = GenericKind::Param(param_ty);
305 let verify_bound = self.verify_bound.generic_bound(generic);
306 self.delegate.push_verify(origin, generic, region, verify_bound);
309 #[tracing::instrument(level = "debug", skip(self))]
310 fn projection_must_outlive(
312 origin: infer::SubregionOrigin<'tcx>,
313 region: ty::Region<'tcx>,
314 projection_ty: ty::ProjectionTy<'tcx>,
316 // This case is thorny for inference. The fundamental problem is
317 // that there are many cases where we have choice, and inference
318 // doesn't like choice (the current region inference in
319 // particular). :) First off, we have to choose between using the
320 // OutlivesProjectionEnv, OutlivesProjectionTraitDef, and
321 // OutlivesProjectionComponent rules, any one of which is
322 // sufficient. If there are no inference variables involved, it's
323 // not hard to pick the right rule, but if there are, we're in a
324 // bit of a catch 22: if we picked which rule we were going to
325 // use, we could add constraints to the region inference graph
326 // that make it apply, but if we don't add those constraints, the
327 // rule might not apply (but another rule might). For now, we err
328 // on the side of adding too few edges into the graph.
330 // Compute the bounds we can derive from the trait definition.
331 // These are guaranteed to apply, no matter the inference
333 let trait_bounds: Vec<_> =
334 self.verify_bound.projection_declared_bounds_from_trait(projection_ty).collect();
336 debug!(?trait_bounds);
338 // Compute the bounds we can derive from the environment. This
339 // is an "approximate" match -- in some cases, these bounds
341 let mut approx_env_bounds =
342 self.verify_bound.projection_approx_declared_bounds_from_env(projection_ty);
343 debug!("projection_must_outlive: approx_env_bounds={:?}", approx_env_bounds);
345 // Remove outlives bounds that we get from the environment but
346 // which are also deducible from the trait. This arises (cc
347 // #55756) in cases where you have e.g., `<T as Foo<'a>>::Item:
348 // 'a` in the environment but `trait Foo<'b> { type Item: 'b
349 // }` in the trait definition.
350 approx_env_bounds.retain(|bound_outlives| {
351 // OK to skip binder because we only manipulate and compare against other
352 // values from the same binder. e.g. if we have (e.g.) `for<'a> <T as Trait<'a>>::Item: 'a`
353 // in `bound`, the `'a` will be a `^1` (bound, debruijn index == innermost) region.
354 // If the declaration is `trait Trait<'b> { type Item: 'b; }`, then `projection_declared_bounds_from_trait`
355 // will be invoked with `['b => ^1]` and so we will get `^1` returned.
356 let bound = bound_outlives.skip_binder();
357 match *bound.0.kind() {
358 ty::Projection(projection_ty) => self
360 .projection_declared_bounds_from_trait(projection_ty)
361 .all(|r| r != bound.1),
363 _ => panic!("expected only projection types from env, not {:?}", bound.0),
367 // If declared bounds list is empty, the only applicable rule is
368 // OutlivesProjectionComponent. If there are inference variables,
369 // then, we can break down the outlives into more primitive
370 // components without adding unnecessary edges.
372 // If there are *no* inference variables, however, we COULD do
373 // this, but we choose not to, because the error messages are less
374 // good. For example, a requirement like `T::Item: 'r` would be
375 // translated to a requirement that `T: 'r`; when this is reported
376 // to the user, it will thus say "T: 'r must hold so that T::Item:
377 // 'r holds". But that makes it sound like the only way to fix
378 // the problem is to add `T: 'r`, which isn't true. So, if there are no
379 // inference variables, we use a verify constraint instead of adding
380 // edges, which winds up enforcing the same condition.
381 let needs_infer = projection_ty.needs_infer();
382 if approx_env_bounds.is_empty() && trait_bounds.is_empty() && needs_infer {
383 debug!("projection_must_outlive: no declared bounds");
385 for k in projection_ty.substs {
387 GenericArgKind::Lifetime(lt) => {
388 self.delegate.push_sub_region_constraint(origin.clone(), region, lt);
390 GenericArgKind::Type(ty) => {
391 self.type_must_outlive(origin.clone(), ty, region);
393 GenericArgKind::Const(_) => {
394 // Const parameters don't impose constraints.
402 // If we found a unique bound `'b` from the trait, and we
403 // found nothing else from the environment, then the best
404 // action is to require that `'b: 'r`, so do that.
406 // This is best no matter what rule we use:
408 // - OutlivesProjectionEnv: these would translate to the requirement that `'b:'r`
409 // - OutlivesProjectionTraitDef: these would translate to the requirement that `'b:'r`
410 // - OutlivesProjectionComponent: this would require `'b:'r`
411 // in addition to other conditions
412 if !trait_bounds.is_empty()
417 // NB: The environment may contain `for<'a> T: 'a` style bounds.
418 // In that case, we don't know if they are equal to the trait bound
419 // or not (since we don't *know* whether the environment bound even applies),
420 // so just map to `None` here if there are bound vars, ensuring that
421 // the call to `all` will fail below.
422 approx_env_bounds.iter().map(|b| b.map_bound(|b| b.1).no_bound_vars()),
424 .all(|b| b == Some(trait_bounds[0]))
426 let unique_bound = trait_bounds[0];
427 debug!("projection_must_outlive: unique trait bound = {:?}", unique_bound);
428 debug!("projection_must_outlive: unique declared bound appears in trait ref");
429 self.delegate.push_sub_region_constraint(origin, region, unique_bound);
433 // Fallback to verifying after the fact that there exists a
434 // declared bound, or that all the components appearing in the
435 // projection outlive; in some cases, this may add insufficient
436 // edges into the inference graph, leading to inference failures
437 // even though a satisfactory solution exists.
438 let generic = GenericKind::Projection(projection_ty);
439 let verify_bound = self.verify_bound.generic_bound(generic);
440 debug!("projection_must_outlive: pushing {:?}", verify_bound);
441 self.delegate.push_verify(origin, generic, region, verify_bound);
445 impl<'cx, 'tcx> TypeOutlivesDelegate<'tcx> for &'cx InferCtxt<'cx, 'tcx> {
446 fn push_sub_region_constraint(
448 origin: SubregionOrigin<'tcx>,
452 self.sub_regions(origin, a, b)
457 origin: SubregionOrigin<'tcx>,
458 kind: GenericKind<'tcx>,
460 bound: VerifyBound<'tcx>,
462 self.verify_generic_bound(origin, kind, a, bound)