1 // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Support code for rustdoc and external tools . You really don't
12 //! want to be using this unless you need to.
16 use std::collections::hash_map::Entry;
17 use std::collections::VecDeque;
19 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
21 use infer::region_constraints::{Constraint, RegionConstraintData};
22 use infer::{InferCtxt, RegionObligation};
24 use ty::fold::TypeFolder;
25 use ty::{Region, RegionVid};
27 // FIXME(twk): this is obviously not nice to duplicate like that
28 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
29 pub enum RegionTarget<'tcx> {
34 #[derive(Default, Debug, Clone)]
35 pub struct RegionDeps<'tcx> {
36 larger: FxHashSet<RegionTarget<'tcx>>,
37 smaller: FxHashSet<RegionTarget<'tcx>>,
40 pub enum AutoTraitResult<A> {
46 impl<A> AutoTraitResult<A> {
47 fn is_auto(&self) -> bool {
49 AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl => true,
55 pub struct AutoTraitInfo<'cx> {
56 pub full_user_env: ty::ParamEnv<'cx>,
57 pub region_data: RegionConstraintData<'cx>,
58 pub names_map: FxHashSet<String>,
59 pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
62 pub struct AutoTraitFinder<'a, 'tcx: 'a> {
63 tcx: &'a TyCtxt<'a, 'tcx, 'tcx>,
66 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
67 pub fn new(tcx: &'a TyCtxt<'a, 'tcx, 'tcx>) -> Self {
68 AutoTraitFinder { tcx }
71 /// Make a best effort to determine whether and under which conditions an auto trait is
72 /// implemented for a type. For example, if you have
75 /// struct Foo<T> { data: Box<T> }
78 /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
79 /// The analysis attempts to account for custom impls as well as other complex cases. This
80 /// result is intended for use by rustdoc and other such consumers.
82 /// (Note that due to the coinductive nature of Send, the full and correct result is actually
83 /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
84 /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
85 /// But this is often not the best way to present to the user.)
87 /// Warning: The API should be considered highly unstable, and it may be refactored or removed
89 pub fn find_auto_trait_generics<A>(
93 generics: &ty::Generics,
94 auto_trait_callback: impl for<'i> Fn(&InferCtxt<'_, 'tcx, 'i>, AutoTraitInfo<'i>) -> A,
95 ) -> AutoTraitResult<A> {
97 let ty = self.tcx.type_of(did);
99 let orig_params = tcx.param_env(did);
101 let trait_ref = ty::TraitRef {
103 substs: tcx.mk_substs_trait(ty, &[]),
106 let trait_pred = ty::Binder::bind(trait_ref);
108 let bail_out = tcx.infer_ctxt().enter(|infcx| {
109 let mut selcx = SelectionContext::with_negative(&infcx, true);
110 let result = selcx.select(&Obligation::new(
111 ObligationCause::dummy(),
113 trait_pred.to_poly_trait_predicate(),
116 Ok(Some(Vtable::VtableImpl(_))) => {
118 "find_auto_trait_generics(did={:?}, trait_did={:?}, generics={:?}): \
119 manual impl found, bailing out",
120 did, trait_did, generics
128 // If an explicit impl exists, it always takes priority over an auto impl
130 return AutoTraitResult::ExplicitImpl;
133 return tcx.infer_ctxt().enter(|mut infcx| {
134 let mut fresh_preds = FxHashSet();
136 // Due to the way projections are handled by SelectionContext, we need to run
137 // evaluate_predicates twice: once on the original param env, and once on the result of
138 // the first evaluate_predicates call.
140 // The problem is this: most of rustc, including SelectionContext and traits::project,
141 // are designed to work with a concrete usage of a type (e.g. Vec<u8>
142 // fn<T>() { Vec<T> }. This information will generally never change - given
143 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
144 // If we're unable to prove that 'T' implements a particular trait, we're done -
145 // there's nothing left to do but error out.
147 // However, synthesizing an auto trait impl works differently. Here, we start out with
148 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
149 // with - and progressively discover the conditions we need to fulfill for it to
150 // implement a certain auto trait. This ends up breaking two assumptions made by trait
151 // selection and projection:
153 // * We can always cache the result of a particular trait selection for the lifetime of
155 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
156 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
158 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
159 // in between calls to SelectionContext.select. This allows us to keep all of the
160 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
161 // them between calls.
163 // We fix the second assumption by reprocessing the result of our first call to
164 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
165 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
166 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
167 // SelectionContext to return it back to us.
169 let (new_env, user_env) = match self.evaluate_predicates(
180 None => return AutoTraitResult::NegativeImpl,
183 let (full_env, full_user_env) = self.evaluate_predicates(
192 ).unwrap_or_else(|| {
194 "Failed to fully process: {:?} {:?} {:?}",
195 ty, trait_did, orig_params
200 "find_auto_trait_generics(did={:?}, trait_did={:?}, generics={:?}): fulfilling \
202 did, trait_did, generics, full_env
204 infcx.clear_caches();
206 // At this point, we already have all of the bounds we need. FulfillmentContext is used
207 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
208 // an additional sanity check.
209 let mut fulfill = FulfillmentContext::new();
210 fulfill.register_bound(
215 ObligationCause::misc(DUMMY_SP, ast::DUMMY_NODE_ID),
217 fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
219 "Unable to fulfill trait {:?} for '{:?}': {:?}",
224 let names_map: FxHashSet<String> = generics
227 .filter_map(|param| {
229 ty::GenericParamDefKind::Lifetime => Some(param.name.to_string()),
235 let body_ids: FxHashSet<_> = infcx
243 infcx.process_registered_region_obligations(&[], None, full_env.clone(), id);
246 let region_data = infcx
247 .borrow_region_constraints()
248 .region_constraint_data()
251 let vid_to_region = self.map_vid_to_region(®ion_data);
253 let info = AutoTraitInfo {
260 return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info));
265 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
266 // The core logic responsible for computing the bounds for our synthesized impl.
268 // To calculate the bounds, we call SelectionContext.select in a loop. Like FulfillmentContext,
269 // we recursively select the nested obligations of predicates we encounter. However, whenver we
270 // encounter an UnimplementedError involving a type parameter, we add it to our ParamEnv. Since
271 // our goal is to determine when a particular type implements an auto trait, Unimplemented
272 // errors tell us what conditions need to be met.
274 // This method ends up working somewhat similary to FulfillmentContext, but with a few key
275 // differences. FulfillmentContext works under the assumption that it's dealing with concrete
276 // user code. According, it considers all possible ways that a Predicate could be met - which
277 // isn't always what we want for a synthesized impl. For example, given the predicate 'T:
278 // Iterator', FulfillmentContext can end up reporting an Unimplemented error for T:
279 // IntoIterator - since there's an implementation of Iteratpr where T: IntoIterator,
280 // FulfillmentContext will drive SelectionContext to consider that impl before giving up. If we
281 // were to rely on FulfillmentContext's decision, we might end up synthesizing an impl like
283 // 'impl<T> Send for Foo<T> where T: IntoIterator'
285 // While it might be technically true that Foo implements Send where T: IntoIterator,
286 // the bound is overly restrictive - it's really only necessary that T: Iterator.
288 // For this reason, evaluate_predicates handles predicates with type variables specially. When
289 // we encounter an Unimplemented error for a bound such as 'T: Iterator', we immediately add it
290 // to our ParamEnv, and add it to our stack for recursive evaluation. When we later select it,
291 // we'll pick up any nested bounds, without ever inferring that 'T: IntoIterator' needs to
294 // One additonal consideration is supertrait bounds. Normally, a ParamEnv is only ever
295 // consutrcted once for a given type. As part of the construction process, the ParamEnv will
296 // have any supertrait bounds normalized - e.g. if we have a type 'struct Foo<T: Copy>', the
297 // ParamEnv will contain 'T: Copy' and 'T: Clone', since 'Copy: Clone'. When we construct our
298 // own ParamEnv, we need to do this outselves, through traits::elaborate_predicates, or else
299 // SelectionContext will choke on the missing predicates. However, this should never show up in
300 // the final synthesized generics: we don't want our generated docs page to contain something
301 // like 'T: Copy + Clone', as that's redundant. Therefore, we keep track of a separate
302 // 'user_env', which only holds the predicates that will actually be displayed to the user.
303 pub fn evaluate_predicates<'b, 'gcx, 'c>(
305 infcx: &InferCtxt<'b, 'tcx, 'c>,
309 param_env: ty::ParamEnv<'c>,
310 user_env: ty::ParamEnv<'c>,
311 fresh_preds: &mut FxHashSet<ty::Predicate<'c>>,
312 only_projections: bool,
313 ) -> Option<(ty::ParamEnv<'c>, ty::ParamEnv<'c>)> {
316 let mut select = SelectionContext::new(&infcx);
318 let mut already_visited = FxHashSet();
319 let mut predicates = VecDeque::new();
320 predicates.push_back(ty::Binder::bind(ty::TraitPredicate {
321 trait_ref: ty::TraitRef {
323 substs: infcx.tcx.mk_substs_trait(ty, &[]),
327 let mut computed_preds: FxHashSet<_> = param_env.caller_bounds.iter().cloned().collect();
328 let mut user_computed_preds: FxHashSet<_> =
329 user_env.caller_bounds.iter().cloned().collect();
331 let mut new_env = param_env.clone();
332 let dummy_cause = ObligationCause::misc(DUMMY_SP, ast::DUMMY_NODE_ID);
334 while let Some(pred) = predicates.pop_front() {
335 infcx.clear_caches();
337 if !already_visited.insert(pred.clone()) {
341 let result = select.select(&Obligation::new(dummy_cause.clone(), new_env, pred));
344 &Ok(Some(ref vtable)) => {
345 let obligations = vtable.clone().nested_obligations().into_iter();
347 if !self.evaluate_nested_obligations(
350 &mut user_computed_preds,
360 &Err(SelectionError::Unimplemented) => {
361 if self.is_of_param(pred.skip_binder().trait_ref.substs) {
362 already_visited.remove(&pred);
363 user_computed_preds.insert(ty::Predicate::Trait(pred.clone()));
364 predicates.push_back(pred);
367 "evaluate_nested_obligations: Unimplemented found, bailing: \
371 pred.skip_binder().trait_ref.substs
376 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
379 computed_preds.extend(user_computed_preds.iter().cloned());
380 let normalized_preds =
381 elaborate_predicates(tcx, computed_preds.clone().into_iter().collect());
382 new_env = ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal);
385 let final_user_env = ty::ParamEnv::new(
386 tcx.mk_predicates(user_computed_preds.into_iter()),
390 "evaluate_nested_obligations(ty_did={:?}, trait_did={:?}): succeeded with '{:?}' \
392 ty_did, trait_did, new_env, final_user_env
395 return Some((new_env, final_user_env));
398 pub fn region_name(&self, region: Region) -> Option<String> {
400 &ty::ReEarlyBound(r) => Some(r.name.to_string()),
405 pub fn get_lifetime(&self, region: Region, names_map: &FxHashMap<String, String>) -> String {
406 self.region_name(region)
408 names_map.get(&name).unwrap_or_else(|| {
409 panic!("Missing lifetime with name {:?} for {:?}", name, region)
412 .unwrap_or(&"'static".to_string())
416 // This is very similar to handle_lifetimes. However, instead of matching ty::Region's
417 // to each other, we match ty::RegionVid's to ty::Region's
418 pub fn map_vid_to_region<'cx>(
420 regions: &RegionConstraintData<'cx>,
421 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
422 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap();
423 let mut finished_map = FxHashMap();
425 for constraint in regions.constraints.keys() {
427 &Constraint::VarSubVar(r1, r2) => {
430 .entry(RegionTarget::RegionVid(r1))
431 .or_insert_with(|| Default::default());
432 deps1.larger.insert(RegionTarget::RegionVid(r2));
436 .entry(RegionTarget::RegionVid(r2))
437 .or_insert_with(|| Default::default());
438 deps2.smaller.insert(RegionTarget::RegionVid(r1));
440 &Constraint::RegSubVar(region, vid) => {
443 .entry(RegionTarget::Region(region))
444 .or_insert_with(|| Default::default());
445 deps1.larger.insert(RegionTarget::RegionVid(vid));
449 .entry(RegionTarget::RegionVid(vid))
450 .or_insert_with(|| Default::default());
451 deps2.smaller.insert(RegionTarget::Region(region));
453 &Constraint::VarSubReg(vid, region) => {
454 finished_map.insert(vid, region);
456 &Constraint::RegSubReg(r1, r2) => {
459 .entry(RegionTarget::Region(r1))
460 .or_insert_with(|| Default::default());
461 deps1.larger.insert(RegionTarget::Region(r2));
465 .entry(RegionTarget::Region(r2))
466 .or_insert_with(|| Default::default());
467 deps2.smaller.insert(RegionTarget::Region(r1));
472 while !vid_map.is_empty() {
473 let target = vid_map.keys().next().expect("Keys somehow empty").clone();
474 let deps = vid_map.remove(&target).expect("Entry somehow missing");
476 for smaller in deps.smaller.iter() {
477 for larger in deps.larger.iter() {
478 match (smaller, larger) {
479 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
480 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
481 let smaller_deps = v.into_mut();
482 smaller_deps.larger.insert(*larger);
483 smaller_deps.larger.remove(&target);
486 if let Entry::Occupied(v) = vid_map.entry(*larger) {
487 let larger_deps = v.into_mut();
488 larger_deps.smaller.insert(*smaller);
489 larger_deps.smaller.remove(&target);
492 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
493 finished_map.insert(v1, r1);
495 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
496 // Do nothing - we don't care about regions that are smaller than vids
498 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
499 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
500 let smaller_deps = v.into_mut();
501 smaller_deps.larger.insert(*larger);
502 smaller_deps.larger.remove(&target);
505 if let Entry::Occupied(v) = vid_map.entry(*larger) {
506 let larger_deps = v.into_mut();
507 larger_deps.smaller.insert(*smaller);
508 larger_deps.smaller.remove(&target);
518 pub fn is_of_param(&self, substs: &Substs) -> bool {
519 if substs.is_noop() {
523 return match substs.type_at(0).sty {
524 ty::TyParam(_) => true,
525 ty::TyProjection(p) => self.is_of_param(p.substs),
530 pub fn evaluate_nested_obligations<
535 T: Iterator<Item = Obligation<'cx, ty::Predicate<'cx>>>,
540 computed_preds: &'b mut FxHashSet<ty::Predicate<'cx>>,
541 fresh_preds: &'b mut FxHashSet<ty::Predicate<'cx>>,
542 predicates: &'b mut VecDeque<ty::PolyTraitPredicate<'cx>>,
543 select: &mut SelectionContext<'c, 'd, 'cx>,
544 only_projections: bool,
546 let dummy_cause = ObligationCause::misc(DUMMY_SP, ast::DUMMY_NODE_ID);
548 for (obligation, predicate) in nested
549 .filter(|o| o.recursion_depth == 1)
550 .map(|o| (o.clone(), o.predicate.clone()))
553 fresh_preds.insert(self.clean_pred(select.infcx(), predicate.clone()));
556 &ty::Predicate::Trait(ref p) => {
557 let substs = &p.skip_binder().trait_ref.substs;
559 if self.is_of_param(substs) && !only_projections && is_new_pred {
560 computed_preds.insert(predicate);
562 predicates.push_back(p.clone());
564 &ty::Predicate::Projection(p) => {
565 // If the projection isn't all type vars, then
566 // we don't want to add it as a bound
567 if self.is_of_param(p.skip_binder().projection_ty.substs) && is_new_pred {
568 computed_preds.insert(predicate);
570 match poly_project_and_unify_type(select, &obligation.with(p.clone())) {
573 "evaluate_nested_obligations: Unable to unify predicate \
574 '{:?}' '{:?}', bailing out",
580 if !self.evaluate_nested_obligations(
582 v.clone().iter().cloned(),
593 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
598 &ty::Predicate::RegionOutlives(ref binder) => {
599 if let Err(_) = select
601 .region_outlives_predicate(&dummy_cause, binder)
606 &ty::Predicate::TypeOutlives(ref binder) => {
608 binder.no_late_bound_regions(),
609 binder.map_bound_ref(|pred| pred.0).no_late_bound_regions(),
611 (None, Some(t_a)) => {
612 select.infcx().register_region_obligation(
616 sub_region: select.infcx().tcx.types.re_static,
617 cause: dummy_cause.clone(),
621 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
622 select.infcx().register_region_obligation(
627 cause: dummy_cause.clone(),
634 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
640 pub fn clean_pred<'c, 'd, 'cx>(
642 infcx: &InferCtxt<'c, 'd, 'cx>,
643 p: ty::Predicate<'cx>,
644 ) -> ty::Predicate<'cx> {
649 // Replaces all ReVars in a type with ty::Region's, using the provided map
650 pub struct RegionReplacer<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
651 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
652 tcx: TyCtxt<'a, 'gcx, 'tcx>,
655 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionReplacer<'a, 'gcx, 'tcx> {
656 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> {
660 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
662 &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
664 }).unwrap_or_else(|| r.super_fold_with(self))