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[rustc.git] / compiler / rustc_trait_selection / src / traits / auto_trait.rs
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1//! Support code for rustdoc and external tools.
2//! You really don't want to be using this unless you need to.
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3
4use super::*;
5
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6use crate::infer::region_constraints::{Constraint, RegionConstraintData};
7use crate::infer::InferCtxt;
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8use rustc_middle::ty::fold::TypeFolder;
9use rustc_middle::ty::{Region, RegionVid};
94b46f34 10
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11use rustc_data_structures::fx::{FxHashMap, FxHashSet};
12
13use std::collections::hash_map::Entry;
14use std::collections::VecDeque;
15
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16// FIXME(twk): this is obviously not nice to duplicate like that
17#[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
18pub enum RegionTarget<'tcx> {
19 Region(Region<'tcx>),
20 RegionVid(RegionVid),
21}
22
23#[derive(Default, Debug, Clone)]
24pub struct RegionDeps<'tcx> {
25 larger: FxHashSet<RegionTarget<'tcx>>,
26 smaller: FxHashSet<RegionTarget<'tcx>>,
27}
28
29pub enum AutoTraitResult<A> {
30 ExplicitImpl,
31 PositiveImpl(A),
32 NegativeImpl,
33}
34
1b1a35ee 35#[allow(dead_code)]
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36impl<A> AutoTraitResult<A> {
37 fn is_auto(&self) -> bool {
5869c6ff 38 matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl)
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39 }
40}
41
42pub struct AutoTraitInfo<'cx> {
43 pub full_user_env: ty::ParamEnv<'cx>,
44 pub region_data: RegionConstraintData<'cx>,
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45 pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
46}
47
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48pub struct AutoTraitFinder<'tcx> {
49 tcx: TyCtxt<'tcx>,
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50}
51
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52impl<'tcx> AutoTraitFinder<'tcx> {
53 pub fn new(tcx: TyCtxt<'tcx>) -> Self {
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54 AutoTraitFinder { tcx }
55 }
56
9fa01778 57 /// Makes a best effort to determine whether and under which conditions an auto trait is
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58 /// implemented for a type. For example, if you have
59 ///
60 /// ```
61 /// struct Foo<T> { data: Box<T> }
62 /// ```
0bf4aa26 63 ///
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64 /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
65 /// The analysis attempts to account for custom impls as well as other complex cases. This
66 /// result is intended for use by rustdoc and other such consumers.
0bf4aa26 67 ///
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68 /// (Note that due to the coinductive nature of Send, the full and correct result is actually
69 /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
70 /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
71 /// But this is often not the best way to present to the user.)
0bf4aa26 72 ///
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73 /// Warning: The API should be considered highly unstable, and it may be refactored or removed
74 /// in the future.
75 pub fn find_auto_trait_generics<A>(
76 &self,
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77 ty: Ty<'tcx>,
78 orig_env: ty::ParamEnv<'tcx>,
94b46f34 79 trait_did: DefId,
6a06907d 80 mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A,
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81 ) -> AutoTraitResult<A> {
82 let tcx = self.tcx;
94b46f34 83
dfeec247 84 let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
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85
86 let trait_pred = ty::Binder::bind(trait_ref);
87
88 let bail_out = tcx.infer_ctxt().enter(|infcx| {
89 let mut selcx = SelectionContext::with_negative(&infcx, true);
90 let result = selcx.select(&Obligation::new(
91 ObligationCause::dummy(),
48663c56 92 orig_env,
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93 trait_pred.to_poly_trait_predicate(),
94 ));
0bf4aa26 95
94b46f34 96 match result {
1b1a35ee 97 Ok(Some(ImplSource::UserDefined(_))) => {
94b46f34 98 debug!(
48663c56 99 "find_auto_trait_generics({:?}): \
94b46f34 100 manual impl found, bailing out",
48663c56 101 trait_ref
94b46f34 102 );
0bf4aa26 103 true
94b46f34 104 }
dfeec247 105 _ => false,
0bf4aa26 106 }
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107 });
108
109 // If an explicit impl exists, it always takes priority over an auto impl
110 if bail_out {
111 return AutoTraitResult::ExplicitImpl;
112 }
113
ba9703b0 114 tcx.infer_ctxt().enter(|infcx| {
0bf4aa26 115 let mut fresh_preds = FxHashSet::default();
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116
117 // Due to the way projections are handled by SelectionContext, we need to run
118 // evaluate_predicates twice: once on the original param env, and once on the result of
119 // the first evaluate_predicates call.
120 //
121 // The problem is this: most of rustc, including SelectionContext and traits::project,
0731742a 122 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
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123 // fn<T>() { Vec<T> }. This information will generally never change - given
124 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
125 // If we're unable to prove that 'T' implements a particular trait, we're done -
126 // there's nothing left to do but error out.
127 //
128 // However, synthesizing an auto trait impl works differently. Here, we start out with
129 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
130 // with - and progressively discover the conditions we need to fulfill for it to
131 // implement a certain auto trait. This ends up breaking two assumptions made by trait
132 // selection and projection:
133 //
134 // * We can always cache the result of a particular trait selection for the lifetime of
135 // an InfCtxt
136 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
137 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
138 //
139 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
140 // in between calls to SelectionContext.select. This allows us to keep all of the
141 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
142 // them between calls.
143 //
144 // We fix the second assumption by reprocessing the result of our first call to
145 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
146 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
147 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
148 // SelectionContext to return it back to us.
149
150 let (new_env, user_env) = match self.evaluate_predicates(
74b04a01 151 &infcx,
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152 trait_did,
153 ty,
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154 orig_env,
155 orig_env,
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156 &mut fresh_preds,
157 false,
158 ) {
159 Some(e) => e,
160 None => return AutoTraitResult::NegativeImpl,
161 };
162
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163 let (full_env, full_user_env) = self
164 .evaluate_predicates(
ba9703b0 165 &infcx,
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166 trait_did,
167 ty,
168 new_env,
169 user_env,
170 &mut fresh_preds,
171 true,
94b46f34 172 )
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173 .unwrap_or_else(|| {
174 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
175 });
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176
177 debug!(
48663c56 178 "find_auto_trait_generics({:?}): fulfilling \
94b46f34 179 with {:?}",
48663c56 180 trait_ref, full_env
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181 );
182 infcx.clear_caches();
183
184 // At this point, we already have all of the bounds we need. FulfillmentContext is used
185 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
186 // an additional sanity check.
187 let mut fulfill = FulfillmentContext::new();
ba9703b0 188 fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
94b46f34 189 fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
dfeec247 190 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e)
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191 });
192
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193 let body_id_map: FxHashMap<_, _> = infcx
194 .inner
195 .borrow()
f9f354fc 196 .region_obligations()
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197 .iter()
198 .map(|&(id, _)| (id, vec![]))
199 .collect();
94b46f34 200
48663c56 201 infcx.process_registered_region_obligations(&body_id_map, None, full_env);
94b46f34 202
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203 let region_data = infcx
204 .inner
205 .borrow_mut()
206 .unwrap_region_constraints()
207 .region_constraint_data()
208 .clone();
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209
210 let vid_to_region = self.map_vid_to_region(&region_data);
211
dfeec247 212 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
94b46f34 213
6a06907d 214 AutoTraitResult::PositiveImpl(auto_trait_callback(info))
ba9703b0 215 })
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216 }
217}
218
dc9dc135 219impl AutoTraitFinder<'tcx> {
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220 /// The core logic responsible for computing the bounds for our synthesized impl.
221 ///
222 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
223 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
224 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
225 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
226 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
227 ///
228 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
229 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
230 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
231 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
232 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
233 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
234 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
235 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
236 /// like this:
237 ///
238 /// impl<T> Send for Foo<T> where T: IntoIterator
239 ///
240 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
241 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
242 ///
243 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
244 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
245 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
246 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
247 /// needs to hold.
248 ///
249 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
250 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
251 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
252 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
253 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
254 /// else `SelectionContext` will choke on the missing predicates. However, this should never
255 /// show up in the final synthesized generics: we don't want our generated docs page to contain
256 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
257 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
258 /// user.
dc9dc135 259 fn evaluate_predicates(
94b46f34 260 &self,
dc9dc135 261 infcx: &InferCtxt<'_, 'tcx>,
94b46f34 262 trait_did: DefId,
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263 ty: Ty<'tcx>,
264 param_env: ty::ParamEnv<'tcx>,
265 user_env: ty::ParamEnv<'tcx>,
266 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
94b46f34 267 only_projections: bool,
dc9dc135 268 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
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269 let tcx = infcx.tcx;
270
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271 // Don't try to proess any nested obligations involving predicates
272 // that are already in the `ParamEnv` (modulo regions): we already
273 // know that they must hold.
274 for predicate in param_env.caller_bounds() {
275 fresh_preds.insert(self.clean_pred(infcx, predicate));
276 }
277
a1dfa0c6 278 let mut select = SelectionContext::with_negative(&infcx, true);
94b46f34 279
0bf4aa26 280 let mut already_visited = FxHashSet::default();
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281 let mut predicates = VecDeque::new();
282 predicates.push_back(ty::Binder::bind(ty::TraitPredicate {
283 trait_ref: ty::TraitRef {
284 def_id: trait_did,
285 substs: infcx.tcx.mk_substs_trait(ty, &[]),
286 },
287 }));
288
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289 let computed_preds = param_env.caller_bounds().iter();
290 let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
94b46f34 291
48663c56 292 let mut new_env = param_env;
ba9703b0 293 let dummy_cause = ObligationCause::dummy();
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294
295 while let Some(pred) = predicates.pop_front() {
296 infcx.clear_caches();
297
48663c56 298 if !already_visited.insert(pred) {
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299 continue;
300 }
301
60c5eb7d 302 // Call `infcx.resolve_vars_if_possible` to see if we can
69743fb6 303 // get rid of any inference variables.
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304 let obligation =
305 infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
69743fb6 306 let result = select.select(&obligation);
94b46f34 307
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308 match result {
309 Ok(Some(ref impl_source)) => {
60c5eb7d 310 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
a1dfa0c6 311 // we immediately bail out, since it's impossible for us to continue.
ba9703b0 312
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313 if let ImplSource::UserDefined(ImplSourceUserDefinedData {
314 impl_def_id, ..
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315 }) = impl_source
316 {
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317 // Blame 'tidy' for the weird bracket placement.
318 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
319 debug!(
320 "evaluate_nested_obligations: found explicit negative impl\
dfeec247 321 {:?}, bailing out",
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322 impl_def_id
323 );
324 return None;
dfeec247 325 }
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326 }
327
f035d41b 328 let obligations = impl_source.clone().nested_obligations().into_iter();
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329
330 if !self.evaluate_nested_obligations(
331 ty,
332 obligations,
333 &mut user_computed_preds,
334 fresh_preds,
335 &mut predicates,
336 &mut select,
337 only_projections,
338 ) {
339 return None;
340 }
341 }
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342 Ok(None) => {}
343 Err(SelectionError::Unimplemented) => {
69743fb6 344 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
94b46f34 345 already_visited.remove(&pred);
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346 self.add_user_pred(
347 &mut user_computed_preds,
3dfed10e 348 pred.without_const().to_predicate(self.tcx),
0bf4aa26 349 );
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350 predicates.push_back(pred);
351 } else {
352 debug!(
60c5eb7d 353 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
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354 {:?} {:?} {:?}",
355 ty,
356 pred,
357 pred.skip_binder().trait_ref.substs
358 );
359 return None;
360 }
361 }
362 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
363 };
364
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365 let normalized_preds = elaborate_predicates(
366 tcx,
367 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
368 )
369 .map(|o| o.predicate);
1b1a35ee 370 new_env = ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal());
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371 }
372
373 let final_user_env = ty::ParamEnv::new(
374 tcx.mk_predicates(user_computed_preds.into_iter()),
f035d41b 375 user_env.reveal(),
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376 );
377 debug!(
48663c56 378 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
94b46f34 379 '{:?}'",
48663c56 380 ty, trait_did, new_env, final_user_env
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381 );
382
ba9703b0 383 Some((new_env, final_user_env))
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384 }
385
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386 /// This method is designed to work around the following issue:
387 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
388 /// progressively building a `ParamEnv` based on the results we get.
389 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
390 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
391 ///
392 /// This can lead to a corner case when dealing with region parameters.
393 /// During our selection loop in `evaluate_predicates`, we might end up with
394 /// two trait predicates that differ only in their region parameters:
395 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
396 /// lifetime parameter. For example:
397 ///
398 /// T as MyTrait<'a>
399 /// T as MyTrait<'static>
400 ///
401 /// If we put both of these predicates in our computed `ParamEnv`, we'll
402 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
403 ///
404 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
405 /// Our end goal is to generate a user-visible description of the conditions
406 /// under which a type implements an auto trait. A trait predicate involving
407 /// a HRTB means that the type needs to work with any choice of lifetime,
408 /// not just one specific lifetime (e.g., `'static`).
3dfed10e 409 fn add_user_pred(
0bf4aa26 410 &self,
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411 user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
412 new_pred: ty::Predicate<'tcx>,
0bf4aa26 413 ) {
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414 let mut should_add_new = true;
415 user_computed_preds.retain(|&old_pred| {
f9f354fc 416 if let (
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417 ty::PredicateKind::Trait(new_trait, _),
418 ty::PredicateKind::Trait(old_trait, _),
419 ) = (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
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420 {
421 if new_trait.def_id() == old_trait.def_id() {
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422 let new_substs = new_trait.trait_ref.substs;
423 let old_substs = old_trait.trait_ref.substs;
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424
425 if !new_substs.types().eq(old_substs.types()) {
426 // We can't compare lifetimes if the types are different,
427 // so skip checking `old_pred`.
428 return true;
429 }
b7449926 430
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431 for (new_region, old_region) in new_substs.regions().zip(old_substs.regions()) {
432 match (new_region, old_region) {
433 // If both predicates have an `ReLateBound` (a HRTB) in the
434 // same spot, we do nothing.
435 (
436 ty::RegionKind::ReLateBound(_, _),
437 ty::RegionKind::ReLateBound(_, _),
438 ) => {}
439
440 (ty::RegionKind::ReLateBound(_, _), _)
441 | (_, ty::RegionKind::ReVar(_)) => {
442 // One of these is true:
443 // The new predicate has a HRTB in a spot where the old
444 // predicate does not (if they both had a HRTB, the previous
445 // match arm would have executed). A HRBT is a 'stricter'
446 // bound than anything else, so we want to keep the newer
447 // predicate (with the HRBT) in place of the old predicate.
448 //
449 // OR
450 //
451 // The old predicate has a region variable where the new
452 // predicate has some other kind of region. An region
453 // variable isn't something we can actually display to a user,
454 // so we choose their new predicate (which doesn't have a region
455 // variable).
456 //
457 // In both cases, we want to remove the old predicate,
458 // from `user_computed_preds`, and replace it with the new
459 // one. Having both the old and the new
460 // predicate in a `ParamEnv` would confuse `SelectionContext`.
461 //
462 // We're currently in the predicate passed to 'retain',
463 // so we return `false` to remove the old predicate from
464 // `user_computed_preds`.
465 return false;
b7449926 466 }
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467 (_, ty::RegionKind::ReLateBound(_, _))
468 | (ty::RegionKind::ReVar(_), _) => {
469 // This is the opposite situation as the previous arm.
470 // One of these is true:
471 //
472 // The old predicate has a HRTB lifetime in a place where the
473 // new predicate does not.
474 //
475 // OR
476 //
477 // The new predicate has a region variable where the old
478 // predicate has some other type of region.
479 //
480 // We want to leave the old
481 // predicate in `user_computed_preds`, and skip adding
482 // new_pred to `user_computed_params`.
483 should_add_new = false
484 }
485 _ => {}
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486 }
487 }
0bf4aa26 488 }
b7449926 489 }
ba9703b0 490 true
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491 });
492
493 if should_add_new {
494 user_computed_preds.insert(new_pred);
495 }
496 }
497
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498 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
499 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
48663c56 500 fn map_vid_to_region<'cx>(
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501 &self,
502 regions: &RegionConstraintData<'cx>,
503 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
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504 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
505 let mut finished_map = FxHashMap::default();
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506
507 for constraint in regions.constraints.keys() {
508 match constraint {
509 &Constraint::VarSubVar(r1, r2) => {
510 {
0bf4aa26 511 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
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512 deps1.larger.insert(RegionTarget::RegionVid(r2));
513 }
514
0bf4aa26 515 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
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516 deps2.smaller.insert(RegionTarget::RegionVid(r1));
517 }
518 &Constraint::RegSubVar(region, vid) => {
519 {
0bf4aa26 520 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
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521 deps1.larger.insert(RegionTarget::RegionVid(vid));
522 }
523
0bf4aa26 524 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
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525 deps2.smaller.insert(RegionTarget::Region(region));
526 }
527 &Constraint::VarSubReg(vid, region) => {
528 finished_map.insert(vid, region);
529 }
530 &Constraint::RegSubReg(r1, r2) => {
531 {
0bf4aa26 532 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
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533 deps1.larger.insert(RegionTarget::Region(r2));
534 }
535
0bf4aa26 536 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
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537 deps2.smaller.insert(RegionTarget::Region(r1));
538 }
539 }
540 }
541
542 while !vid_map.is_empty() {
dfeec247 543 let target = *vid_map.keys().next().expect("Keys somehow empty");
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544 let deps = vid_map.remove(&target).expect("Entry somehow missing");
545
546 for smaller in deps.smaller.iter() {
547 for larger in deps.larger.iter() {
548 match (smaller, larger) {
549 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
550 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
551 let smaller_deps = v.into_mut();
552 smaller_deps.larger.insert(*larger);
553 smaller_deps.larger.remove(&target);
554 }
555
556 if let Entry::Occupied(v) = vid_map.entry(*larger) {
557 let larger_deps = v.into_mut();
558 larger_deps.smaller.insert(*smaller);
559 larger_deps.smaller.remove(&target);
560 }
561 }
562 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
563 finished_map.insert(v1, r1);
564 }
565 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
60c5eb7d 566 // Do nothing; we don't care about regions that are smaller than vids.
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567 }
568 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
569 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
570 let smaller_deps = v.into_mut();
571 smaller_deps.larger.insert(*larger);
572 smaller_deps.larger.remove(&target);
573 }
574
575 if let Entry::Occupied(v) = vid_map.entry(*larger) {
576 let larger_deps = v.into_mut();
577 larger_deps.smaller.insert(*smaller);
578 larger_deps.smaller.remove(&target);
579 }
580 }
581 }
582 }
583 }
584 }
585 finished_map
586 }
587
532ac7d7 588 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
ba9703b0 589 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
69743fb6 590 }
94b46f34 591
69743fb6 592 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
1b1a35ee 593 match ty.kind() {
b7449926 594 ty::Param(_) => true,
69743fb6 595 ty::Projection(p) => self.is_of_param(p.self_ty()),
94b46f34 596 _ => false,
ba9703b0 597 }
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598 }
599
69743fb6 600 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
5869c6ff 601 matches!(*p.ty().skip_binder().kind(), ty::Projection(proj) if proj == p.skip_binder().projection_ty)
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602 }
603
dc9dc135 604 fn evaluate_nested_obligations(
94b46f34 605 &self,
48663c56 606 ty: Ty<'_>,
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607 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
608 computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
609 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
610 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
611 select: &mut SelectionContext<'_, 'tcx>,
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612 only_projections: bool,
613 ) -> bool {
ba9703b0 614 let dummy_cause = ObligationCause::dummy();
94b46f34 615
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616 for obligation in nested {
617 let is_new_pred =
618 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
94b46f34 619
69743fb6 620 // Resolve any inference variables that we can, to help selection succeed
fc512014 621 let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
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622
623 // We only add a predicate as a user-displayable bound if
624 // it involves a generic parameter, and doesn't contain
625 // any inference variables.
626 //
627 // Displaying a bound involving a concrete type (instead of a generic
628 // parameter) would be pointless, since it's always true
629 // (e.g. u8: Copy)
630 // Displaying an inference variable is impossible, since they're
631 // an internal compiler detail without a defined visual representation
632 //
633 // We check this by calling is_of_param on the relevant types
634 // from the various possible predicates
3dfed10e 635
5869c6ff 636 let bound_predicate = predicate.kind();
29967ef6 637 match bound_predicate.skip_binder() {
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638 ty::PredicateKind::Trait(p, _) => {
639 // Add this to `predicates` so that we end up calling `select`
640 // with it. If this predicate ends up being unimplemented,
641 // then `evaluate_predicates` will handle adding it the `ParamEnv`
642 // if possible.
29967ef6 643 predicates.push_back(bound_predicate.rebind(p));
94b46f34 644 }
5869c6ff 645 ty::PredicateKind::Projection(p) => {
29967ef6 646 let p = bound_predicate.rebind(p);
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647 debug!(
648 "evaluate_nested_obligations: examining projection predicate {:?}",
649 predicate
650 );
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651
652 // As described above, we only want to display
653 // bounds which include a generic parameter but don't include
654 // an inference variable.
655 // Additionally, we check if we've seen this predicate before,
656 // to avoid rendering duplicate bounds to the user.
657 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
48663c56 658 && !p.ty().skip_binder().has_infer_types()
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659 && is_new_pred
660 {
661 debug!(
662 "evaluate_nested_obligations: adding projection predicate\
663 to computed_preds: {:?}",
664 predicate
665 );
666
667 // Under unusual circumstances, we can end up with a self-refeential
668 // projection predicate. For example:
669 // <T as MyType>::Value == <T as MyType>::Value
670 // Not only is displaying this to the user pointless,
671 // having it in the ParamEnv will cause an issue if we try to call
672 // poly_project_and_unify_type on the predicate, since this kind of
673 // predicate will normally never end up in a ParamEnv.
674 //
675 // For these reasons, we ignore these weird predicates,
676 // ensuring that we're able to properly synthesize an auto trait impl
677 if self.is_self_referential_projection(p) {
678 debug!(
679 "evaluate_nested_obligations: encountered a projection
680 predicate equating a type with itself! Skipping"
681 );
682 } else {
683 self.add_user_pred(computed_preds, predicate);
684 }
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685 }
686
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687 // There are three possible cases when we project a predicate:
688 //
689 // 1. We encounter an error. This means that it's impossible for
690 // our current type to implement the auto trait - there's bound
691 // that we could add to our ParamEnv that would 'fix' this kind
692 // of error, as it's not caused by an unimplemented type.
693 //
60c5eb7d 694 // 2. We successfully project the predicate (Ok(Some(_))), generating
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695 // some subobligations. We then process these subobligations
696 // like any other generated sub-obligations.
697 //
74b04a01 698 // 3. We receive an 'ambiguous' result (Ok(None))
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699 // If we were actually trying to compile a crate,
700 // we would need to re-process this obligation later.
701 // However, all we care about is finding out what bounds
702 // are needed for our type to implement a particular auto trait.
703 // We've already added this obligation to our computed ParamEnv
704 // above (if it was necessary). Therefore, we don't need
705 // to do any further processing of the obligation.
706 //
707 // Note that we *must* try to project *all* projection predicates
708 // we encounter, even ones without inference variable.
709 // This ensures that we detect any projection errors,
710 // which indicate that our type can *never* implement the given
711 // auto trait. In that case, we will generate an explicit negative
712 // impl (e.g. 'impl !Send for MyType'). However, we don't
713 // try to process any of the generated subobligations -
714 // they contain no new information, since we already know
715 // that our type implements the projected-through trait,
716 // and can lead to weird region issues.
717 //
718 // Normally, we'll generate a negative impl as a result of encountering
719 // a type with an explicit negative impl of an auto trait
720 // (for example, raw pointers have !Send and !Sync impls)
721 // However, through some **interesting** manipulations of the type
722 // system, it's actually possible to write a type that never
723 // implements an auto trait due to a projection error, not a normal
724 // negative impl error. To properly handle this case, we need
725 // to ensure that we catch any potential projection errors,
726 // and turn them into an explicit negative impl for our type.
dfeec247 727 debug!("Projecting and unifying projection predicate {:?}", predicate);
dc9dc135 728
f9652781 729 match project::poly_project_and_unify_type(select, &obligation.with(p)) {
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730 Err(e) => {
731 debug!(
732 "evaluate_nested_obligations: Unable to unify predicate \
733 '{:?}' '{:?}', bailing out",
734 ty, e
735 );
736 return false;
737 }
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738 Ok(Err(project::InProgress)) => {
739 debug!("evaluate_nested_obligations: recursive projection predicate");
740 return false;
741 }
742 Ok(Ok(Some(v))) => {
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743 // We only care about sub-obligations
744 // when we started out trying to unify
745 // some inference variables. See the comment above
746 // for more infomration
747 if p.ty().skip_binder().has_infer_types() {
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748 if !self.evaluate_nested_obligations(
749 ty,
ba9703b0 750 v.into_iter(),
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751 computed_preds,
752 fresh_preds,
753 predicates,
754 select,
755 only_projections,
756 ) {
757 return false;
758 }
759 }
dc9dc135 760 }
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761 Ok(Ok(None)) => {
762 // It's ok not to make progress when have no inference variables -
dc9dc135 763 // in that case, we were only performing unifcation to check if an
60c5eb7d 764 // error occurred (which would indicate that it's impossible for our
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765 // type to implement the auto trait).
766 // However, we should always make progress (either by generating
767 // subobligations or getting an error) when we started off with
768 // inference variables
769 if p.ty().skip_binder().has_infer_types() {
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770 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
771 }
772 }
773 }
774 }
5869c6ff 775 ty::PredicateKind::RegionOutlives(binder) => {
29967ef6 776 let binder = bound_predicate.rebind(binder);
dfeec247 777 if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
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778 return false;
779 }
780 }
5869c6ff 781 ty::PredicateKind::TypeOutlives(binder) => {
29967ef6 782 let binder = bound_predicate.rebind(binder);
94b46f34 783 match (
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784 binder.no_bound_vars(),
785 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
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786 ) {
787 (None, Some(t_a)) => {
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788 select.infcx().register_region_obligation_with_cause(
789 t_a,
48663c56 790 select.infcx().tcx.lifetimes.re_static,
0bf4aa26 791 &dummy_cause,
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792 );
793 }
794 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
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795 select.infcx().register_region_obligation_with_cause(
796 t_a,
797 r_b,
798 &dummy_cause,
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799 );
800 }
801 _ => {}
802 };
803 }
5869c6ff 804 ty::PredicateKind::ConstEquate(c1, c2) => {
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805 let evaluate = |c: &'tcx ty::Const<'tcx>| {
806 if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val {
807 match select.infcx().const_eval_resolve(
808 obligation.param_env,
809 def,
810 substs,
811 promoted,
812 Some(obligation.cause.span),
813 ) {
814 Ok(val) => Ok(ty::Const::from_value(select.tcx(), val, c.ty)),
815 Err(err) => Err(err),
816 }
817 } else {
818 Ok(c)
819 }
820 };
821
822 match (evaluate(c1), evaluate(c2)) {
823 (Ok(c1), Ok(c2)) => {
824 match select
825 .infcx()
826 .at(&obligation.cause, obligation.param_env)
827 .eq(c1, c2)
828 {
829 Ok(_) => (),
830 Err(_) => return false,
831 }
832 }
833 _ => return false,
834 }
835 }
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836 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
837 };
838 }
ba9703b0 839 true
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840 }
841
dc9dc135 842 pub fn clean_pred(
94b46f34 843 &self,
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844 infcx: &InferCtxt<'_, 'tcx>,
845 p: ty::Predicate<'tcx>,
846 ) -> ty::Predicate<'tcx> {
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847 infcx.freshen(p)
848 }
849}
850
851// Replaces all ReVars in a type with ty::Region's, using the provided map
dc9dc135 852pub struct RegionReplacer<'a, 'tcx> {
94b46f34 853 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
dc9dc135 854 tcx: TyCtxt<'tcx>,
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855}
856
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857impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
858 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
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859 self.tcx
860 }
861
862 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
863 (match r {
5869c6ff 864 ty::ReVar(vid) => self.vid_to_region.get(vid).cloned(),
94b46f34 865 _ => None,
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866 })
867 .unwrap_or_else(|| r.super_fold_with(self))
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868 }
869}