1 //! Code that handles "type-outlives" constraints like `T: 'a`. This
2 //! is based on the `push_outlives_components` function defined on the tcx,
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::env::RegionBoundPairs;
63 use crate::infer::outlives::verify::VerifyBoundCx;
64 use crate::infer::{self, GenericKind, InferCtxt, RegionObligation, SubregionOrigin, VerifyBound};
65 use rustc_data_structures::fx::FxHashMap;
67 use crate::traits::ObligationCause;
68 use crate::ty::outlives::Component;
69 use crate::ty::{self, Region, Ty, TyCtxt, TypeFoldable};
70 use crate::ty::subst::UnpackedKind;
72 impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
73 /// Registers that the given region obligation must be resolved
74 /// from within the scope of `body_id`. These regions are enqueued
75 /// and later processed by regionck, when full type information is
76 /// available (see `region_obligations` field for more
78 pub fn register_region_obligation(
81 obligation: RegionObligation<'tcx>,
84 "register_region_obligation(body_id={:?}, obligation={:?})",
88 self.region_obligations
90 .push((body_id, obligation));
93 pub fn register_region_obligation_with_cause(
96 sub_region: Region<'tcx>,
97 cause: &ObligationCause<'tcx>,
99 let origin = SubregionOrigin::from_obligation_cause(cause, || {
100 infer::RelateParamBound(cause.span, sup_type)
103 self.register_region_obligation(
113 /// Trait queries just want to pass back type obligations "as is"
114 pub fn take_registered_region_obligations(&self) -> Vec<(hir::HirId, RegionObligation<'tcx>)> {
115 ::std::mem::replace(&mut *self.region_obligations.borrow_mut(), vec![])
118 /// Process the region obligations that must be proven (during
119 /// `regionck`) for the given `body_id`, given information about
120 /// the region bounds in scope and so forth. This function must be
121 /// invoked for all relevant body-ids before region inference is
122 /// done (or else an assert will fire).
124 /// See the `region_obligations` field of `InferCtxt` for some
125 /// comments about how this function fits into the overall expected
126 /// flow of the inferencer. The key point is that it is
127 /// invoked after all type-inference variables have been bound --
128 /// towards the end of regionck. This also ensures that the
129 /// region-bound-pairs are available (see comments above regarding
134 /// - `region_bound_pairs`: the set of region bounds implied by
135 /// the parameters and where-clauses. In particular, each pair
136 /// `('a, K)` in this list tells us that the bounds in scope
137 /// indicate that `K: 'a`, where `K` is either a generic
138 /// parameter like `T` or a projection like `T::Item`.
139 /// - `implicit_region_bound`: if some, this is a region bound
140 /// that is considered to hold for all type parameters (the
142 /// - `param_env` is the parameter environment for the enclosing function.
143 /// - `body_id` is the body-id whose region obligations are being
148 /// This function may have to perform normalizations, and hence it
149 /// returns an `InferOk` with subobligations that must be
151 pub fn process_registered_region_obligations(
153 region_bound_pairs_map: &FxHashMap<hir::HirId, RegionBoundPairs<'tcx>>,
154 implicit_region_bound: Option<ty::Region<'tcx>>,
155 param_env: ty::ParamEnv<'tcx>,
158 !self.in_snapshot.get(),
159 "cannot process registered region obligations in a snapshot"
162 debug!("process_registered_region_obligations()");
164 let my_region_obligations = self.take_registered_region_obligations();
173 ) in my_region_obligations
176 "process_registered_region_obligations: sup_type={:?} sub_region={:?} origin={:?}",
177 sup_type, sub_region, origin
180 let sup_type = self.resolve_vars_if_possible(&sup_type);
182 if let Some(region_bound_pairs) = region_bound_pairs_map.get(&body_id) {
183 let outlives = &mut TypeOutlives::new(
187 implicit_region_bound,
190 outlives.type_must_outlive(origin, sup_type, sub_region);
192 self.tcx.sess.delay_span_bug(
194 &format!("no region-bound-pairs for {:?}", body_id),
200 /// Processes a single ad-hoc region obligation that was not
201 /// registered in advance.
202 pub fn type_must_outlive(
204 region_bound_pairs: &RegionBoundPairs<'tcx>,
205 implicit_region_bound: Option<ty::Region<'tcx>>,
206 param_env: ty::ParamEnv<'tcx>,
207 origin: infer::SubregionOrigin<'tcx>,
209 region: ty::Region<'tcx>,
211 let outlives = &mut TypeOutlives::new(
215 implicit_region_bound,
218 let ty = self.resolve_vars_if_possible(&ty);
219 outlives.type_must_outlive(origin, ty, region);
223 /// The `TypeOutlives` struct has the job of "lowering" a `T: 'a`
224 /// obligation into a series of `'a: 'b` constraints and "verifys", as
225 /// described on the module comment. The final constraints are emitted
226 /// via a "delegate" of type `D` -- this is usually the `infcx`, which
227 /// accrues them into the `region_obligations` code, but for NLL we
228 /// use something else.
229 pub struct TypeOutlives<'cx, 'tcx, D>
231 D: TypeOutlivesDelegate<'tcx>,
233 // See the comments on `process_registered_region_obligations` for the meaning
237 verify_bound: VerifyBoundCx<'cx, 'tcx>,
240 pub trait TypeOutlivesDelegate<'tcx> {
241 fn push_sub_region_constraint(
243 origin: SubregionOrigin<'tcx>,
250 origin: SubregionOrigin<'tcx>,
251 kind: GenericKind<'tcx>,
253 bound: VerifyBound<'tcx>,
257 impl<'cx, 'tcx, D> TypeOutlives<'cx, 'tcx, D>
259 D: TypeOutlivesDelegate<'tcx>,
264 region_bound_pairs: &'cx RegionBoundPairs<'tcx>,
265 implicit_region_bound: Option<ty::Region<'tcx>>,
266 param_env: ty::ParamEnv<'tcx>,
271 verify_bound: VerifyBoundCx::new(
274 implicit_region_bound,
280 /// Adds constraints to inference such that `T: 'a` holds (or
281 /// reports an error if it cannot).
285 /// - `origin`, the reason we need this constraint
286 /// - `ty`, the type `T`
287 /// - `region`, the region `'a`
288 pub fn type_must_outlive(
290 origin: infer::SubregionOrigin<'tcx>,
292 region: ty::Region<'tcx>,
295 "type_must_outlive(ty={:?}, region={:?}, origin={:?})",
299 assert!(!ty.has_escaping_bound_vars());
301 let mut components = smallvec![];
302 self.tcx.push_outlives_components(ty, &mut components);
303 self.components_must_outlive(origin, &components, region);
306 fn components_must_outlive(
308 origin: infer::SubregionOrigin<'tcx>,
309 components: &[Component<'tcx>],
310 region: ty::Region<'tcx>,
312 for component in components.iter() {
313 let origin = origin.clone();
315 Component::Region(region1) => {
317 .push_sub_region_constraint(origin, region, region1);
319 Component::Param(param_ty) => {
320 self.param_ty_must_outlive(origin, region, *param_ty);
322 Component::Projection(projection_ty) => {
323 self.projection_must_outlive(origin, region, *projection_ty);
325 Component::EscapingProjection(subcomponents) => {
326 self.components_must_outlive(origin, &subcomponents, region);
328 Component::UnresolvedInferenceVariable(v) => {
329 // ignore this, we presume it will yield an error
330 // later, since if a type variable is not resolved by
331 // this point it never will be
332 self.tcx.sess.delay_span_bug(
334 &format!("unresolved inference variable in outlives: {:?}", v),
341 fn param_ty_must_outlive(
343 origin: infer::SubregionOrigin<'tcx>,
344 region: ty::Region<'tcx>,
345 param_ty: ty::ParamTy,
348 "param_ty_must_outlive(region={:?}, param_ty={:?}, origin={:?})",
349 region, param_ty, origin
352 let generic = GenericKind::Param(param_ty);
353 let verify_bound = self.verify_bound.generic_bound(generic);
355 .push_verify(origin, generic, region, verify_bound);
358 fn projection_must_outlive(
360 origin: infer::SubregionOrigin<'tcx>,
361 region: ty::Region<'tcx>,
362 projection_ty: ty::ProjectionTy<'tcx>,
365 "projection_must_outlive(region={:?}, projection_ty={:?}, origin={:?})",
366 region, projection_ty, origin
369 // This case is thorny for inference. The fundamental problem is
370 // that there are many cases where we have choice, and inference
371 // doesn't like choice (the current region inference in
372 // particular). :) First off, we have to choose between using the
373 // OutlivesProjectionEnv, OutlivesProjectionTraitDef, and
374 // OutlivesProjectionComponent rules, any one of which is
375 // sufficient. If there are no inference variables involved, it's
376 // not hard to pick the right rule, but if there are, we're in a
377 // bit of a catch 22: if we picked which rule we were going to
378 // use, we could add constraints to the region inference graph
379 // that make it apply, but if we don't add those constraints, the
380 // rule might not apply (but another rule might). For now, we err
381 // on the side of adding too few edges into the graph.
383 // Compute the bounds we can derive from the trait definition.
384 // These are guaranteed to apply, no matter the inference
386 let trait_bounds: Vec<_> = self.verify_bound
387 .projection_declared_bounds_from_trait(projection_ty)
390 // Compute the bounds we can derive from the environment. This
391 // is an "approximate" match -- in some cases, these bounds
393 let mut approx_env_bounds = self.verify_bound
394 .projection_approx_declared_bounds_from_env(projection_ty);
396 "projection_must_outlive: approx_env_bounds={:?}",
400 // Remove outlives bounds that we get from the environment but
401 // which are also deducable from the trait. This arises (cc
402 // #55756) in cases where you have e.g., `<T as Foo<'a>>::Item:
403 // 'a` in the environment but `trait Foo<'b> { type Item: 'b
404 // }` in the trait definition.
405 approx_env_bounds.retain(|bound| {
407 ty::Projection(projection_ty) => {
408 self.verify_bound.projection_declared_bounds_from_trait(projection_ty)
409 .all(|r| r != bound.1)
412 _ => panic!("expected only projection types from env, not {:?}", bound.0),
416 // If declared bounds list is empty, the only applicable rule is
417 // OutlivesProjectionComponent. If there are inference variables,
418 // then, we can break down the outlives into more primitive
419 // components without adding unnecessary edges.
421 // If there are *no* inference variables, however, we COULD do
422 // this, but we choose not to, because the error messages are less
423 // good. For example, a requirement like `T::Item: 'r` would be
424 // translated to a requirement that `T: 'r`; when this is reported
425 // to the user, it will thus say "T: 'r must hold so that T::Item:
426 // 'r holds". But that makes it sound like the only way to fix
427 // the problem is to add `T: 'r`, which isn't true. So, if there are no
428 // inference variables, we use a verify constraint instead of adding
429 // edges, which winds up enforcing the same condition.
430 let needs_infer = projection_ty.needs_infer();
431 if approx_env_bounds.is_empty() && trait_bounds.is_empty() && needs_infer {
432 debug!("projection_must_outlive: no declared bounds");
434 for k in projection_ty.substs {
436 UnpackedKind::Lifetime(lt) => {
437 self.delegate.push_sub_region_constraint(origin.clone(), region, lt);
439 UnpackedKind::Type(ty) => {
440 self.type_must_outlive(origin.clone(), ty, region);
442 UnpackedKind::Const(_) => {
443 // Const parameters don't impose constraints.
451 // If we found a unique bound `'b` from the trait, and we
452 // found nothing else from the environment, then the best
453 // action is to require that `'b: 'r`, so do that.
455 // This is best no matter what rule we use:
457 // - OutlivesProjectionEnv: these would translate to the requirement that `'b:'r`
458 // - OutlivesProjectionTraitDef: these would translate to the requirement that `'b:'r`
459 // - OutlivesProjectionComponent: this would require `'b:'r`
460 // in addition to other conditions
461 if !trait_bounds.is_empty()
464 .chain(approx_env_bounds.iter().map(|b| &b.1))
465 .all(|b| *b == trait_bounds[0])
467 let unique_bound = trait_bounds[0];
469 "projection_must_outlive: unique trait bound = {:?}",
472 debug!("projection_must_outlive: unique declared bound appears in trait ref");
474 .push_sub_region_constraint(origin, region, unique_bound);
478 // Fallback to verifying after the fact that there exists a
479 // declared bound, or that all the components appearing in the
480 // projection outlive; in some cases, this may add insufficient
481 // edges into the inference graph, leading to inference failures
482 // even though a satisfactory solution exists.
483 let generic = GenericKind::Projection(projection_ty);
484 let verify_bound = self.verify_bound.generic_bound(generic);
486 .push_verify(origin, generic.clone(), region, verify_bound);
490 impl<'cx, 'tcx> TypeOutlivesDelegate<'tcx> for &'cx InferCtxt<'cx, 'tcx> {
491 fn push_sub_region_constraint(
493 origin: SubregionOrigin<'tcx>,
497 self.sub_regions(origin, a, b)
502 origin: SubregionOrigin<'tcx>,
503 kind: GenericKind<'tcx>,
505 bound: VerifyBound<'tcx>,
507 self.verify_generic_bound(origin, kind, a, bound)