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60c5eb7d XL |
1 | //! Support code for rustdoc and external tools. |
2 | //! You really don't want to be using this unless you need to. | |
94b46f34 XL |
3 | |
4 | use super::*; | |
5 | ||
9fa01778 XL |
6 | use crate::infer::region_constraints::{Constraint, RegionConstraintData}; |
7 | use crate::infer::InferCtxt; | |
9fa01778 XL |
8 | use crate::ty::fold::TypeFolder; |
9 | use crate::ty::{Region, RegionVid}; | |
94b46f34 | 10 | |
60c5eb7d XL |
11 | use rustc_data_structures::fx::{FxHashMap, FxHashSet}; |
12 | ||
13 | use std::collections::hash_map::Entry; | |
14 | use std::collections::VecDeque; | |
15 | ||
94b46f34 XL |
16 | // FIXME(twk): this is obviously not nice to duplicate like that |
17 | #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)] | |
18 | pub enum RegionTarget<'tcx> { | |
19 | Region(Region<'tcx>), | |
20 | RegionVid(RegionVid), | |
21 | } | |
22 | ||
23 | #[derive(Default, Debug, Clone)] | |
24 | pub struct RegionDeps<'tcx> { | |
25 | larger: FxHashSet<RegionTarget<'tcx>>, | |
26 | smaller: FxHashSet<RegionTarget<'tcx>>, | |
27 | } | |
28 | ||
29 | pub enum AutoTraitResult<A> { | |
30 | ExplicitImpl, | |
31 | PositiveImpl(A), | |
32 | NegativeImpl, | |
33 | } | |
34 | ||
35 | impl<A> AutoTraitResult<A> { | |
36 | fn is_auto(&self) -> bool { | |
37 | match *self { | |
38 | AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl => true, | |
39 | _ => false, | |
40 | } | |
41 | } | |
42 | } | |
43 | ||
44 | pub struct AutoTraitInfo<'cx> { | |
45 | pub full_user_env: ty::ParamEnv<'cx>, | |
46 | pub region_data: RegionConstraintData<'cx>, | |
94b46f34 XL |
47 | pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>, |
48 | } | |
49 | ||
dc9dc135 XL |
50 | pub struct AutoTraitFinder<'tcx> { |
51 | tcx: TyCtxt<'tcx>, | |
94b46f34 XL |
52 | } |
53 | ||
dc9dc135 XL |
54 | impl<'tcx> AutoTraitFinder<'tcx> { |
55 | pub fn new(tcx: TyCtxt<'tcx>) -> Self { | |
94b46f34 XL |
56 | AutoTraitFinder { tcx } |
57 | } | |
58 | ||
9fa01778 | 59 | /// Makes a best effort to determine whether and under which conditions an auto trait is |
94b46f34 XL |
60 | /// implemented for a type. For example, if you have |
61 | /// | |
62 | /// ``` | |
63 | /// struct Foo<T> { data: Box<T> } | |
64 | /// ``` | |
0bf4aa26 | 65 | /// |
94b46f34 XL |
66 | /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type). |
67 | /// The analysis attempts to account for custom impls as well as other complex cases. This | |
68 | /// result is intended for use by rustdoc and other such consumers. | |
0bf4aa26 | 69 | /// |
94b46f34 XL |
70 | /// (Note that due to the coinductive nature of Send, the full and correct result is actually |
71 | /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field | |
72 | /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send. | |
73 | /// But this is often not the best way to present to the user.) | |
0bf4aa26 | 74 | /// |
94b46f34 XL |
75 | /// Warning: The API should be considered highly unstable, and it may be refactored or removed |
76 | /// in the future. | |
77 | pub fn find_auto_trait_generics<A>( | |
78 | &self, | |
48663c56 XL |
79 | ty: Ty<'tcx>, |
80 | orig_env: ty::ParamEnv<'tcx>, | |
94b46f34 | 81 | trait_did: DefId, |
dc9dc135 | 82 | auto_trait_callback: impl Fn(&InferCtxt<'_, 'tcx>, AutoTraitInfo<'tcx>) -> A, |
94b46f34 XL |
83 | ) -> AutoTraitResult<A> { |
84 | let tcx = self.tcx; | |
94b46f34 | 85 | |
dfeec247 | 86 | let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) }; |
94b46f34 XL |
87 | |
88 | let trait_pred = ty::Binder::bind(trait_ref); | |
89 | ||
90 | let bail_out = tcx.infer_ctxt().enter(|infcx| { | |
91 | let mut selcx = SelectionContext::with_negative(&infcx, true); | |
92 | let result = selcx.select(&Obligation::new( | |
93 | ObligationCause::dummy(), | |
48663c56 | 94 | orig_env, |
94b46f34 XL |
95 | trait_pred.to_poly_trait_predicate(), |
96 | )); | |
0bf4aa26 | 97 | |
94b46f34 XL |
98 | match result { |
99 | Ok(Some(Vtable::VtableImpl(_))) => { | |
100 | debug!( | |
48663c56 | 101 | "find_auto_trait_generics({:?}): \ |
94b46f34 | 102 | manual impl found, bailing out", |
48663c56 | 103 | trait_ref |
94b46f34 | 104 | ); |
0bf4aa26 | 105 | true |
94b46f34 | 106 | } |
dfeec247 | 107 | _ => false, |
0bf4aa26 | 108 | } |
94b46f34 XL |
109 | }); |
110 | ||
111 | // If an explicit impl exists, it always takes priority over an auto impl | |
112 | if bail_out { | |
113 | return AutoTraitResult::ExplicitImpl; | |
114 | } | |
115 | ||
116 | return tcx.infer_ctxt().enter(|mut infcx| { | |
0bf4aa26 | 117 | let mut fresh_preds = FxHashSet::default(); |
94b46f34 XL |
118 | |
119 | // Due to the way projections are handled by SelectionContext, we need to run | |
120 | // evaluate_predicates twice: once on the original param env, and once on the result of | |
121 | // the first evaluate_predicates call. | |
122 | // | |
123 | // The problem is this: most of rustc, including SelectionContext and traits::project, | |
0731742a | 124 | // are designed to work with a concrete usage of a type (e.g., Vec<u8> |
94b46f34 XL |
125 | // fn<T>() { Vec<T> }. This information will generally never change - given |
126 | // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'. | |
127 | // If we're unable to prove that 'T' implements a particular trait, we're done - | |
128 | // there's nothing left to do but error out. | |
129 | // | |
130 | // However, synthesizing an auto trait impl works differently. Here, we start out with | |
131 | // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing | |
132 | // with - and progressively discover the conditions we need to fulfill for it to | |
133 | // implement a certain auto trait. This ends up breaking two assumptions made by trait | |
134 | // selection and projection: | |
135 | // | |
136 | // * We can always cache the result of a particular trait selection for the lifetime of | |
137 | // an InfCtxt | |
138 | // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T: | |
139 | // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K' | |
140 | // | |
141 | // We fix the first assumption by manually clearing out all of the InferCtxt's caches | |
142 | // in between calls to SelectionContext.select. This allows us to keep all of the | |
143 | // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift | |
144 | // them between calls. | |
145 | // | |
146 | // We fix the second assumption by reprocessing the result of our first call to | |
147 | // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first | |
148 | // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass, | |
149 | // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing | |
150 | // SelectionContext to return it back to us. | |
151 | ||
152 | let (new_env, user_env) = match self.evaluate_predicates( | |
153 | &mut infcx, | |
94b46f34 XL |
154 | trait_did, |
155 | ty, | |
48663c56 XL |
156 | orig_env, |
157 | orig_env, | |
94b46f34 XL |
158 | &mut fresh_preds, |
159 | false, | |
160 | ) { | |
161 | Some(e) => e, | |
162 | None => return AutoTraitResult::NegativeImpl, | |
163 | }; | |
164 | ||
dfeec247 XL |
165 | let (full_env, full_user_env) = self |
166 | .evaluate_predicates( | |
167 | &mut infcx, | |
168 | trait_did, | |
169 | ty, | |
170 | new_env, | |
171 | user_env, | |
172 | &mut fresh_preds, | |
173 | true, | |
94b46f34 | 174 | ) |
dfeec247 XL |
175 | .unwrap_or_else(|| { |
176 | panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env) | |
177 | }); | |
94b46f34 XL |
178 | |
179 | debug!( | |
48663c56 | 180 | "find_auto_trait_generics({:?}): fulfilling \ |
94b46f34 | 181 | with {:?}", |
48663c56 | 182 | trait_ref, full_env |
94b46f34 XL |
183 | ); |
184 | infcx.clear_caches(); | |
185 | ||
186 | // At this point, we already have all of the bounds we need. FulfillmentContext is used | |
187 | // to store all of the necessary region/lifetime bounds in the InferContext, as well as | |
188 | // an additional sanity check. | |
189 | let mut fulfill = FulfillmentContext::new(); | |
190 | fulfill.register_bound( | |
191 | &infcx, | |
192 | full_env, | |
193 | ty, | |
194 | trait_did, | |
9fa01778 | 195 | ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID), |
94b46f34 XL |
196 | ); |
197 | fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| { | |
dfeec247 | 198 | panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e) |
94b46f34 XL |
199 | }); |
200 | ||
dfeec247 XL |
201 | let body_id_map: FxHashMap<_, _> = |
202 | infcx.region_obligations.borrow().iter().map(|&(id, _)| (id, vec![])).collect(); | |
94b46f34 | 203 | |
48663c56 | 204 | infcx.process_registered_region_obligations(&body_id_map, None, full_env); |
94b46f34 | 205 | |
dfeec247 | 206 | let region_data = infcx.borrow_region_constraints().region_constraint_data().clone(); |
94b46f34 XL |
207 | |
208 | let vid_to_region = self.map_vid_to_region(®ion_data); | |
209 | ||
dfeec247 | 210 | let info = AutoTraitInfo { full_user_env, region_data, vid_to_region }; |
94b46f34 XL |
211 | |
212 | return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info)); | |
213 | }); | |
214 | } | |
215 | } | |
216 | ||
dc9dc135 | 217 | impl AutoTraitFinder<'tcx> { |
60c5eb7d XL |
218 | /// The core logic responsible for computing the bounds for our synthesized impl. |
219 | /// | |
220 | /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like | |
221 | /// `FulfillmentContext`, we recursively select the nested obligations of predicates we | |
222 | /// encounter. However, whenever we encounter an `UnimplementedError` involving a type | |
223 | /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular | |
224 | /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met. | |
225 | /// | |
226 | /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key | |
227 | /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete | |
228 | /// user code. According, it considers all possible ways that a `Predicate` could be met, which | |
229 | /// isn't always what we want for a synthesized impl. For example, given the predicate `T: | |
230 | /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T: | |
231 | /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`, | |
232 | /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up. | |
233 | /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl | |
234 | /// like this: | |
235 | /// | |
236 | /// impl<T> Send for Foo<T> where T: IntoIterator | |
237 | /// | |
238 | /// While it might be technically true that Foo implements Send where `T: IntoIterator`, | |
239 | /// the bound is overly restrictive - it's really only necessary that `T: Iterator`. | |
240 | /// | |
241 | /// For this reason, `evaluate_predicates` handles predicates with type variables specially. | |
242 | /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately | |
243 | /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later | |
244 | /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator` | |
245 | /// needs to hold. | |
246 | /// | |
247 | /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever | |
248 | /// constructed once for a given type. As part of the construction process, the `ParamEnv` will | |
249 | /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the | |
250 | /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our | |
251 | /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or | |
252 | /// else `SelectionContext` will choke on the missing predicates. However, this should never | |
253 | /// show up in the final synthesized generics: we don't want our generated docs page to contain | |
254 | /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a | |
255 | /// separate `user_env`, which only holds the predicates that will actually be displayed to the | |
256 | /// user. | |
dc9dc135 | 257 | fn evaluate_predicates( |
94b46f34 | 258 | &self, |
dc9dc135 | 259 | infcx: &InferCtxt<'_, 'tcx>, |
94b46f34 | 260 | trait_did: DefId, |
dc9dc135 XL |
261 | ty: Ty<'tcx>, |
262 | param_env: ty::ParamEnv<'tcx>, | |
263 | user_env: ty::ParamEnv<'tcx>, | |
264 | fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, | |
94b46f34 | 265 | only_projections: bool, |
dc9dc135 | 266 | ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> { |
94b46f34 XL |
267 | let tcx = infcx.tcx; |
268 | ||
a1dfa0c6 | 269 | let mut select = SelectionContext::with_negative(&infcx, true); |
94b46f34 | 270 | |
0bf4aa26 | 271 | let mut already_visited = FxHashSet::default(); |
94b46f34 XL |
272 | let mut predicates = VecDeque::new(); |
273 | predicates.push_back(ty::Binder::bind(ty::TraitPredicate { | |
274 | trait_ref: ty::TraitRef { | |
275 | def_id: trait_did, | |
276 | substs: infcx.tcx.mk_substs_trait(ty, &[]), | |
277 | }, | |
278 | })); | |
279 | ||
280 | let mut computed_preds: FxHashSet<_> = param_env.caller_bounds.iter().cloned().collect(); | |
281 | let mut user_computed_preds: FxHashSet<_> = | |
282 | user_env.caller_bounds.iter().cloned().collect(); | |
283 | ||
48663c56 | 284 | let mut new_env = param_env; |
9fa01778 | 285 | let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID); |
94b46f34 XL |
286 | |
287 | while let Some(pred) = predicates.pop_front() { | |
288 | infcx.clear_caches(); | |
289 | ||
48663c56 | 290 | if !already_visited.insert(pred) { |
94b46f34 XL |
291 | continue; |
292 | } | |
293 | ||
60c5eb7d | 294 | // Call `infcx.resolve_vars_if_possible` to see if we can |
69743fb6 | 295 | // get rid of any inference variables. |
dfeec247 XL |
296 | let obligation = infcx.resolve_vars_if_possible(&Obligation::new( |
297 | dummy_cause.clone(), | |
298 | new_env, | |
299 | pred, | |
300 | )); | |
69743fb6 | 301 | let result = select.select(&obligation); |
94b46f34 XL |
302 | |
303 | match &result { | |
304 | &Ok(Some(ref vtable)) => { | |
60c5eb7d | 305 | // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`), |
a1dfa0c6 XL |
306 | // we immediately bail out, since it's impossible for us to continue. |
307 | match vtable { | |
308 | Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) => { | |
60c5eb7d | 309 | // Blame 'tidy' for the weird bracket placement. |
dfeec247 XL |
310 | if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative { |
311 | debug!( | |
312 | "evaluate_nested_obligations: found explicit negative impl\ | |
313 | {:?}, bailing out", | |
314 | impl_def_id | |
315 | ); | |
a1dfa0c6 XL |
316 | return None; |
317 | } | |
dfeec247 | 318 | } |
a1dfa0c6 XL |
319 | _ => {} |
320 | } | |
321 | ||
94b46f34 XL |
322 | let obligations = vtable.clone().nested_obligations().into_iter(); |
323 | ||
324 | if !self.evaluate_nested_obligations( | |
325 | ty, | |
326 | obligations, | |
327 | &mut user_computed_preds, | |
328 | fresh_preds, | |
329 | &mut predicates, | |
330 | &mut select, | |
331 | only_projections, | |
332 | ) { | |
333 | return None; | |
334 | } | |
335 | } | |
336 | &Ok(None) => {} | |
337 | &Err(SelectionError::Unimplemented) => { | |
69743fb6 | 338 | if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) { |
94b46f34 | 339 | already_visited.remove(&pred); |
0bf4aa26 XL |
340 | self.add_user_pred( |
341 | &mut user_computed_preds, | |
dfeec247 | 342 | ty::Predicate::Trait(pred, ast::Constness::NotConst), |
0bf4aa26 | 343 | ); |
94b46f34 XL |
344 | predicates.push_back(pred); |
345 | } else { | |
346 | debug!( | |
60c5eb7d | 347 | "evaluate_nested_obligations: `Unimplemented` found, bailing: \ |
94b46f34 XL |
348 | {:?} {:?} {:?}", |
349 | ty, | |
350 | pred, | |
351 | pred.skip_binder().trait_ref.substs | |
352 | ); | |
353 | return None; | |
354 | } | |
355 | } | |
356 | _ => panic!("Unexpected error for '{:?}': {:?}", ty, result), | |
357 | }; | |
358 | ||
359 | computed_preds.extend(user_computed_preds.iter().cloned()); | |
360 | let normalized_preds = | |
48663c56 | 361 | elaborate_predicates(tcx, computed_preds.iter().cloned().collect()); |
dfeec247 XL |
362 | new_env = |
363 | ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal, None); | |
94b46f34 XL |
364 | } |
365 | ||
366 | let final_user_env = ty::ParamEnv::new( | |
367 | tcx.mk_predicates(user_computed_preds.into_iter()), | |
368 | user_env.reveal, | |
dfeec247 | 369 | None, |
94b46f34 XL |
370 | ); |
371 | debug!( | |
48663c56 | 372 | "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \ |
94b46f34 | 373 | '{:?}'", |
48663c56 | 374 | ty, trait_did, new_env, final_user_env |
94b46f34 XL |
375 | ); |
376 | ||
377 | return Some((new_env, final_user_env)); | |
378 | } | |
379 | ||
60c5eb7d XL |
380 | /// This method is designed to work around the following issue: |
381 | /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`, | |
382 | /// progressively building a `ParamEnv` based on the results we get. | |
383 | /// However, our usage of `SelectionContext` differs from its normal use within the compiler, | |
384 | /// in that we capture and re-reprocess predicates from `Unimplemented` errors. | |
385 | /// | |
386 | /// This can lead to a corner case when dealing with region parameters. | |
387 | /// During our selection loop in `evaluate_predicates`, we might end up with | |
388 | /// two trait predicates that differ only in their region parameters: | |
389 | /// one containing a HRTB lifetime parameter, and one containing a 'normal' | |
390 | /// lifetime parameter. For example: | |
391 | /// | |
392 | /// T as MyTrait<'a> | |
393 | /// T as MyTrait<'static> | |
394 | /// | |
395 | /// If we put both of these predicates in our computed `ParamEnv`, we'll | |
396 | /// confuse `SelectionContext`, since it will (correctly) view both as being applicable. | |
397 | /// | |
398 | /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB | |
399 | /// Our end goal is to generate a user-visible description of the conditions | |
400 | /// under which a type implements an auto trait. A trait predicate involving | |
401 | /// a HRTB means that the type needs to work with any choice of lifetime, | |
402 | /// not just one specific lifetime (e.g., `'static`). | |
0bf4aa26 XL |
403 | fn add_user_pred<'c>( |
404 | &self, | |
405 | user_computed_preds: &mut FxHashSet<ty::Predicate<'c>>, | |
406 | new_pred: ty::Predicate<'c>, | |
407 | ) { | |
b7449926 XL |
408 | let mut should_add_new = true; |
409 | user_computed_preds.retain(|&old_pred| { | |
410 | match (&new_pred, old_pred) { | |
dfeec247 | 411 | (&ty::Predicate::Trait(new_trait, _), ty::Predicate::Trait(old_trait, _)) => { |
b7449926 XL |
412 | if new_trait.def_id() == old_trait.def_id() { |
413 | let new_substs = new_trait.skip_binder().trait_ref.substs; | |
414 | let old_substs = old_trait.skip_binder().trait_ref.substs; | |
0bf4aa26 | 415 | |
b7449926 XL |
416 | if !new_substs.types().eq(old_substs.types()) { |
417 | // We can't compare lifetimes if the types are different, | |
60c5eb7d | 418 | // so skip checking `old_pred`. |
0bf4aa26 | 419 | return true; |
b7449926 XL |
420 | } |
421 | ||
0bf4aa26 XL |
422 | for (new_region, old_region) in |
423 | new_substs.regions().zip(old_substs.regions()) | |
424 | { | |
b7449926 | 425 | match (new_region, old_region) { |
60c5eb7d XL |
426 | // If both predicates have an `ReLateBound` (a HRTB) in the |
427 | // same spot, we do nothing. | |
b7449926 XL |
428 | ( |
429 | ty::RegionKind::ReLateBound(_, _), | |
0bf4aa26 XL |
430 | ty::RegionKind::ReLateBound(_, _), |
431 | ) => {} | |
b7449926 | 432 | |
dfeec247 XL |
433 | (ty::RegionKind::ReLateBound(_, _), _) |
434 | | (_, ty::RegionKind::ReVar(_)) => { | |
a1dfa0c6 | 435 | // One of these is true: |
b7449926 XL |
436 | // The new predicate has a HRTB in a spot where the old |
437 | // predicate does not (if they both had a HRTB, the previous | |
a1dfa0c6 XL |
438 | // match arm would have executed). A HRBT is a 'stricter' |
439 | // bound than anything else, so we want to keep the newer | |
440 | // predicate (with the HRBT) in place of the old predicate. | |
441 | // | |
442 | // OR | |
443 | // | |
444 | // The old predicate has a region variable where the new | |
445 | // predicate has some other kind of region. An region | |
446 | // variable isn't something we can actually display to a user, | |
60c5eb7d | 447 | // so we choose their new predicate (which doesn't have a region |
a1dfa0c6 | 448 | // varaible). |
b7449926 | 449 | // |
a1dfa0c6 | 450 | // In both cases, we want to remove the old predicate, |
60c5eb7d | 451 | // from `user_computed_preds`, and replace it with the new |
a1dfa0c6 | 452 | // one. Having both the old and the new |
60c5eb7d | 453 | // predicate in a `ParamEnv` would confuse `SelectionContext`. |
a1dfa0c6 | 454 | // |
b7449926 | 455 | // We're currently in the predicate passed to 'retain', |
60c5eb7d XL |
456 | // so we return `false` to remove the old predicate from |
457 | // `user_computed_preds`. | |
b7449926 | 458 | return false; |
0bf4aa26 | 459 | } |
dfeec247 XL |
460 | (_, ty::RegionKind::ReLateBound(_, _)) |
461 | | (ty::RegionKind::ReVar(_), _) => { | |
a1dfa0c6 XL |
462 | // This is the opposite situation as the previous arm. |
463 | // One of these is true: | |
464 | // | |
465 | // The old predicate has a HRTB lifetime in a place where the | |
466 | // new predicate does not. | |
467 | // | |
468 | // OR | |
469 | // | |
470 | // The new predicate has a region variable where the old | |
471 | // predicate has some other type of region. | |
472 | // | |
473 | // We want to leave the old | |
60c5eb7d XL |
474 | // predicate in `user_computed_preds`, and skip adding |
475 | // new_pred to `user_computed_params`. | |
b7449926 | 476 | should_add_new = false |
dfeec247 | 477 | } |
b7449926 XL |
478 | _ => {} |
479 | } | |
480 | } | |
481 | } | |
0bf4aa26 | 482 | } |
b7449926 XL |
483 | _ => {} |
484 | } | |
0bf4aa26 | 485 | return true; |
b7449926 XL |
486 | }); |
487 | ||
488 | if should_add_new { | |
489 | user_computed_preds.insert(new_pred); | |
490 | } | |
491 | } | |
492 | ||
60c5eb7d XL |
493 | /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s |
494 | /// to each other, we match `ty::RegionVid`s to `ty::Region`s. | |
48663c56 | 495 | fn map_vid_to_region<'cx>( |
94b46f34 XL |
496 | &self, |
497 | regions: &RegionConstraintData<'cx>, | |
498 | ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> { | |
0bf4aa26 XL |
499 | let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default(); |
500 | let mut finished_map = FxHashMap::default(); | |
94b46f34 XL |
501 | |
502 | for constraint in regions.constraints.keys() { | |
503 | match constraint { | |
504 | &Constraint::VarSubVar(r1, r2) => { | |
505 | { | |
0bf4aa26 | 506 | let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default(); |
94b46f34 XL |
507 | deps1.larger.insert(RegionTarget::RegionVid(r2)); |
508 | } | |
509 | ||
0bf4aa26 | 510 | let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default(); |
94b46f34 XL |
511 | deps2.smaller.insert(RegionTarget::RegionVid(r1)); |
512 | } | |
513 | &Constraint::RegSubVar(region, vid) => { | |
514 | { | |
0bf4aa26 | 515 | let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default(); |
94b46f34 XL |
516 | deps1.larger.insert(RegionTarget::RegionVid(vid)); |
517 | } | |
518 | ||
0bf4aa26 | 519 | let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default(); |
94b46f34 XL |
520 | deps2.smaller.insert(RegionTarget::Region(region)); |
521 | } | |
522 | &Constraint::VarSubReg(vid, region) => { | |
523 | finished_map.insert(vid, region); | |
524 | } | |
525 | &Constraint::RegSubReg(r1, r2) => { | |
526 | { | |
0bf4aa26 | 527 | let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default(); |
94b46f34 XL |
528 | deps1.larger.insert(RegionTarget::Region(r2)); |
529 | } | |
530 | ||
0bf4aa26 | 531 | let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default(); |
94b46f34 XL |
532 | deps2.smaller.insert(RegionTarget::Region(r1)); |
533 | } | |
534 | } | |
535 | } | |
536 | ||
537 | while !vid_map.is_empty() { | |
dfeec247 | 538 | let target = *vid_map.keys().next().expect("Keys somehow empty"); |
94b46f34 XL |
539 | let deps = vid_map.remove(&target).expect("Entry somehow missing"); |
540 | ||
541 | for smaller in deps.smaller.iter() { | |
542 | for larger in deps.larger.iter() { | |
543 | match (smaller, larger) { | |
544 | (&RegionTarget::Region(_), &RegionTarget::Region(_)) => { | |
545 | if let Entry::Occupied(v) = vid_map.entry(*smaller) { | |
546 | let smaller_deps = v.into_mut(); | |
547 | smaller_deps.larger.insert(*larger); | |
548 | smaller_deps.larger.remove(&target); | |
549 | } | |
550 | ||
551 | if let Entry::Occupied(v) = vid_map.entry(*larger) { | |
552 | let larger_deps = v.into_mut(); | |
553 | larger_deps.smaller.insert(*smaller); | |
554 | larger_deps.smaller.remove(&target); | |
555 | } | |
556 | } | |
557 | (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => { | |
558 | finished_map.insert(v1, r1); | |
559 | } | |
560 | (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => { | |
60c5eb7d | 561 | // Do nothing; we don't care about regions that are smaller than vids. |
94b46f34 XL |
562 | } |
563 | (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => { | |
564 | if let Entry::Occupied(v) = vid_map.entry(*smaller) { | |
565 | let smaller_deps = v.into_mut(); | |
566 | smaller_deps.larger.insert(*larger); | |
567 | smaller_deps.larger.remove(&target); | |
568 | } | |
569 | ||
570 | if let Entry::Occupied(v) = vid_map.entry(*larger) { | |
571 | let larger_deps = v.into_mut(); | |
572 | larger_deps.smaller.insert(*smaller); | |
573 | larger_deps.smaller.remove(&target); | |
574 | } | |
575 | } | |
576 | } | |
577 | } | |
578 | } | |
579 | } | |
580 | finished_map | |
581 | } | |
582 | ||
532ac7d7 | 583 | fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool { |
dfeec247 | 584 | return self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types()); |
69743fb6 | 585 | } |
94b46f34 | 586 | |
69743fb6 | 587 | pub fn is_of_param(&self, ty: Ty<'_>) -> bool { |
e74abb32 | 588 | return match ty.kind { |
b7449926 | 589 | ty::Param(_) => true, |
69743fb6 | 590 | ty::Projection(p) => self.is_of_param(p.self_ty()), |
94b46f34 XL |
591 | _ => false, |
592 | }; | |
593 | } | |
594 | ||
69743fb6 | 595 | fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool { |
e74abb32 | 596 | match p.ty().skip_binder().kind { |
dfeec247 XL |
597 | ty::Projection(proj) if proj == p.skip_binder().projection_ty => true, |
598 | _ => false, | |
69743fb6 XL |
599 | } |
600 | } | |
601 | ||
dc9dc135 | 602 | fn evaluate_nested_obligations( |
94b46f34 | 603 | &self, |
48663c56 | 604 | ty: Ty<'_>, |
dc9dc135 XL |
605 | nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>, |
606 | computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>, | |
607 | fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, | |
608 | predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>, | |
609 | select: &mut SelectionContext<'_, 'tcx>, | |
94b46f34 XL |
610 | only_projections: bool, |
611 | ) -> bool { | |
9fa01778 | 612 | let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID); |
94b46f34 | 613 | |
dfeec247 XL |
614 | for (obligation, mut predicate) in nested.map(|o| (o.clone(), o.predicate)) { |
615 | let is_new_pred = fresh_preds.insert(self.clean_pred(select.infcx(), predicate)); | |
94b46f34 | 616 | |
69743fb6 | 617 | // Resolve any inference variables that we can, to help selection succeed |
dc9dc135 | 618 | predicate = select.infcx().resolve_vars_if_possible(&predicate); |
69743fb6 XL |
619 | |
620 | // We only add a predicate as a user-displayable bound if | |
621 | // it involves a generic parameter, and doesn't contain | |
622 | // any inference variables. | |
623 | // | |
624 | // Displaying a bound involving a concrete type (instead of a generic | |
625 | // parameter) would be pointless, since it's always true | |
626 | // (e.g. u8: Copy) | |
627 | // Displaying an inference variable is impossible, since they're | |
628 | // an internal compiler detail without a defined visual representation | |
629 | // | |
630 | // We check this by calling is_of_param on the relevant types | |
631 | // from the various possible predicates | |
94b46f34 | 632 | match &predicate { |
dfeec247 | 633 | &ty::Predicate::Trait(p, _) => { |
69743fb6 XL |
634 | if self.is_param_no_infer(p.skip_binder().trait_ref.substs) |
635 | && !only_projections | |
dfeec247 XL |
636 | && is_new_pred |
637 | { | |
b7449926 | 638 | self.add_user_pred(computed_preds, predicate); |
94b46f34 | 639 | } |
48663c56 | 640 | predicates.push_back(p); |
94b46f34 XL |
641 | } |
642 | &ty::Predicate::Projection(p) => { | |
dfeec247 XL |
643 | debug!( |
644 | "evaluate_nested_obligations: examining projection predicate {:?}", | |
645 | predicate | |
646 | ); | |
69743fb6 XL |
647 | |
648 | // As described above, we only want to display | |
649 | // bounds which include a generic parameter but don't include | |
650 | // an inference variable. | |
651 | // Additionally, we check if we've seen this predicate before, | |
652 | // to avoid rendering duplicate bounds to the user. | |
653 | if self.is_param_no_infer(p.skip_binder().projection_ty.substs) | |
48663c56 | 654 | && !p.ty().skip_binder().has_infer_types() |
dfeec247 XL |
655 | && is_new_pred |
656 | { | |
657 | debug!( | |
658 | "evaluate_nested_obligations: adding projection predicate\ | |
659 | to computed_preds: {:?}", | |
660 | predicate | |
661 | ); | |
662 | ||
663 | // Under unusual circumstances, we can end up with a self-refeential | |
664 | // projection predicate. For example: | |
665 | // <T as MyType>::Value == <T as MyType>::Value | |
666 | // Not only is displaying this to the user pointless, | |
667 | // having it in the ParamEnv will cause an issue if we try to call | |
668 | // poly_project_and_unify_type on the predicate, since this kind of | |
669 | // predicate will normally never end up in a ParamEnv. | |
670 | // | |
671 | // For these reasons, we ignore these weird predicates, | |
672 | // ensuring that we're able to properly synthesize an auto trait impl | |
673 | if self.is_self_referential_projection(p) { | |
674 | debug!( | |
675 | "evaluate_nested_obligations: encountered a projection | |
676 | predicate equating a type with itself! Skipping" | |
677 | ); | |
678 | } else { | |
679 | self.add_user_pred(computed_preds, predicate); | |
680 | } | |
69743fb6 XL |
681 | } |
682 | ||
dc9dc135 XL |
683 | // There are three possible cases when we project a predicate: |
684 | // | |
685 | // 1. We encounter an error. This means that it's impossible for | |
686 | // our current type to implement the auto trait - there's bound | |
687 | // that we could add to our ParamEnv that would 'fix' this kind | |
688 | // of error, as it's not caused by an unimplemented type. | |
689 | // | |
60c5eb7d | 690 | // 2. We successfully project the predicate (Ok(Some(_))), generating |
dc9dc135 XL |
691 | // some subobligations. We then process these subobligations |
692 | // like any other generated sub-obligations. | |
693 | // | |
694 | // 3. We receieve an 'ambiguous' result (Ok(None)) | |
695 | // If we were actually trying to compile a crate, | |
696 | // we would need to re-process this obligation later. | |
697 | // However, all we care about is finding out what bounds | |
698 | // are needed for our type to implement a particular auto trait. | |
699 | // We've already added this obligation to our computed ParamEnv | |
700 | // above (if it was necessary). Therefore, we don't need | |
701 | // to do any further processing of the obligation. | |
702 | // | |
703 | // Note that we *must* try to project *all* projection predicates | |
704 | // we encounter, even ones without inference variable. | |
705 | // This ensures that we detect any projection errors, | |
706 | // which indicate that our type can *never* implement the given | |
707 | // auto trait. In that case, we will generate an explicit negative | |
708 | // impl (e.g. 'impl !Send for MyType'). However, we don't | |
709 | // try to process any of the generated subobligations - | |
710 | // they contain no new information, since we already know | |
711 | // that our type implements the projected-through trait, | |
712 | // and can lead to weird region issues. | |
713 | // | |
714 | // Normally, we'll generate a negative impl as a result of encountering | |
715 | // a type with an explicit negative impl of an auto trait | |
716 | // (for example, raw pointers have !Send and !Sync impls) | |
717 | // However, through some **interesting** manipulations of the type | |
718 | // system, it's actually possible to write a type that never | |
719 | // implements an auto trait due to a projection error, not a normal | |
720 | // negative impl error. To properly handle this case, we need | |
721 | // to ensure that we catch any potential projection errors, | |
722 | // and turn them into an explicit negative impl for our type. | |
dfeec247 | 723 | debug!("Projecting and unifying projection predicate {:?}", predicate); |
dc9dc135 XL |
724 | |
725 | match poly_project_and_unify_type(select, &obligation.with(p)) { | |
726 | Err(e) => { | |
727 | debug!( | |
728 | "evaluate_nested_obligations: Unable to unify predicate \ | |
729 | '{:?}' '{:?}', bailing out", | |
730 | ty, e | |
731 | ); | |
732 | return false; | |
733 | } | |
734 | Ok(Some(v)) => { | |
735 | // We only care about sub-obligations | |
736 | // when we started out trying to unify | |
737 | // some inference variables. See the comment above | |
738 | // for more infomration | |
739 | if p.ty().skip_binder().has_infer_types() { | |
94b46f34 XL |
740 | if !self.evaluate_nested_obligations( |
741 | ty, | |
742 | v.clone().iter().cloned(), | |
743 | computed_preds, | |
744 | fresh_preds, | |
745 | predicates, | |
746 | select, | |
747 | only_projections, | |
748 | ) { | |
749 | return false; | |
750 | } | |
751 | } | |
dc9dc135 XL |
752 | } |
753 | Ok(None) => { | |
754 | // It's ok not to make progress when hvave no inference variables - | |
755 | // in that case, we were only performing unifcation to check if an | |
60c5eb7d | 756 | // error occurred (which would indicate that it's impossible for our |
dc9dc135 XL |
757 | // type to implement the auto trait). |
758 | // However, we should always make progress (either by generating | |
759 | // subobligations or getting an error) when we started off with | |
760 | // inference variables | |
761 | if p.ty().skip_binder().has_infer_types() { | |
94b46f34 XL |
762 | panic!("Unexpected result when selecting {:?} {:?}", ty, obligation) |
763 | } | |
764 | } | |
765 | } | |
766 | } | |
767 | &ty::Predicate::RegionOutlives(ref binder) => { | |
dfeec247 | 768 | if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() { |
94b46f34 XL |
769 | return false; |
770 | } | |
771 | } | |
772 | &ty::Predicate::TypeOutlives(ref binder) => { | |
773 | match ( | |
a1dfa0c6 XL |
774 | binder.no_bound_vars(), |
775 | binder.map_bound_ref(|pred| pred.0).no_bound_vars(), | |
94b46f34 XL |
776 | ) { |
777 | (None, Some(t_a)) => { | |
0bf4aa26 XL |
778 | select.infcx().register_region_obligation_with_cause( |
779 | t_a, | |
48663c56 | 780 | select.infcx().tcx.lifetimes.re_static, |
0bf4aa26 | 781 | &dummy_cause, |
94b46f34 XL |
782 | ); |
783 | } | |
784 | (Some(ty::OutlivesPredicate(t_a, r_b)), _) => { | |
0bf4aa26 XL |
785 | select.infcx().register_region_obligation_with_cause( |
786 | t_a, | |
787 | r_b, | |
788 | &dummy_cause, | |
94b46f34 XL |
789 | ); |
790 | } | |
791 | _ => {} | |
792 | }; | |
793 | } | |
794 | _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate), | |
795 | }; | |
796 | } | |
797 | return true; | |
798 | } | |
799 | ||
dc9dc135 | 800 | pub fn clean_pred( |
94b46f34 | 801 | &self, |
dc9dc135 XL |
802 | infcx: &InferCtxt<'_, 'tcx>, |
803 | p: ty::Predicate<'tcx>, | |
804 | ) -> ty::Predicate<'tcx> { | |
94b46f34 XL |
805 | infcx.freshen(p) |
806 | } | |
807 | } | |
808 | ||
809 | // Replaces all ReVars in a type with ty::Region's, using the provided map | |
dc9dc135 | 810 | pub struct RegionReplacer<'a, 'tcx> { |
94b46f34 | 811 | vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>, |
dc9dc135 | 812 | tcx: TyCtxt<'tcx>, |
94b46f34 XL |
813 | } |
814 | ||
dc9dc135 XL |
815 | impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> { |
816 | fn tcx<'b>(&'b self) -> TyCtxt<'tcx> { | |
94b46f34 XL |
817 | self.tcx |
818 | } | |
819 | ||
820 | fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> { | |
821 | (match r { | |
822 | &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(), | |
823 | _ => None, | |
dfeec247 XL |
824 | }) |
825 | .unwrap_or_else(|| r.super_fold_with(self)) | |
94b46f34 XL |
826 | } |
827 | } |