1 // Copyright 2014 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 //! Helper routines for higher-ranked things. See the `doc` module at
12 //! the end of the file for details.
14 use super::{CombinedSnapshot, InferCtxt, HigherRankedType, SkolemizationMap};
15 use super::combine::CombineFields;
17 use middle::ty::{self, TyCtxt, Binder, TypeFoldable};
18 use middle::ty::error::TypeError;
19 use middle::ty::relate::{Relate, RelateResult, TypeRelation};
20 use syntax::codemap::Span;
21 use util::nodemap::{FnvHashMap, FnvHashSet};
23 pub trait HigherRankedRelations<'a,'tcx> {
24 fn higher_ranked_sub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
25 where T: Relate<'a,'tcx>;
27 fn higher_ranked_lub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
28 where T: Relate<'a,'tcx>;
30 fn higher_ranked_glb<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
31 where T: Relate<'a,'tcx>;
35 fn tainted_regions(&self, snapshot: &CombinedSnapshot, r: ty::Region) -> Vec<ty::Region>;
37 fn region_vars_confined_to_snapshot(&self,
38 snapshot: &CombinedSnapshot)
39 -> Vec<ty::RegionVid>;
42 impl<'a,'tcx> HigherRankedRelations<'a,'tcx> for CombineFields<'a,'tcx> {
43 fn higher_ranked_sub<T>(&self, a: &Binder<T>, b: &Binder<T>)
44 -> RelateResult<'tcx, Binder<T>>
45 where T: Relate<'a,'tcx>
47 debug!("higher_ranked_sub(a={:?}, b={:?})",
50 // Rather than checking the subtype relationship between `a` and `b`
51 // as-is, we need to do some extra work here in order to make sure
52 // that function subtyping works correctly with respect to regions
54 // Note: this is a subtle algorithm. For a full explanation,
55 // please see the large comment at the end of the file in the (inlined) module
58 // Start a snapshot so we can examine "all bindings that were
59 // created as part of this type comparison".
60 return self.infcx.commit_if_ok(|snapshot| {
61 // First, we instantiate each bound region in the subtype with a fresh
64 self.infcx.replace_late_bound_regions_with_fresh_var(
65 self.trace.origin.span(),
69 // Second, we instantiate each bound region in the supertype with a
70 // fresh concrete region.
71 let (b_prime, skol_map) =
72 self.infcx.skolemize_late_bound_regions(b, snapshot);
74 debug!("a_prime={:?}", a_prime);
75 debug!("b_prime={:?}", b_prime);
77 // Compare types now that bound regions have been replaced.
78 let result = try!(self.sub().relate(&a_prime, &b_prime));
80 // Presuming type comparison succeeds, we need to check
81 // that the skolemized regions do not "leak".
82 match leak_check(self.infcx, &skol_map, snapshot) {
84 Err((skol_br, tainted_region)) => {
85 if self.a_is_expected {
86 debug!("Not as polymorphic!");
87 return Err(TypeError::RegionsInsufficientlyPolymorphic(skol_br,
90 debug!("Overly polymorphic!");
91 return Err(TypeError::RegionsOverlyPolymorphic(skol_br,
97 debug!("higher_ranked_sub: OK result={:?}",
100 Ok(ty::Binder(result))
104 fn higher_ranked_lub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
105 where T: Relate<'a,'tcx>
107 // Start a snapshot so we can examine "all bindings that were
108 // created as part of this type comparison".
109 return self.infcx.commit_if_ok(|snapshot| {
110 // Instantiate each bound region with a fresh region variable.
111 let span = self.trace.origin.span();
112 let (a_with_fresh, a_map) =
113 self.infcx.replace_late_bound_regions_with_fresh_var(
114 span, HigherRankedType, a);
115 let (b_with_fresh, _) =
116 self.infcx.replace_late_bound_regions_with_fresh_var(
117 span, HigherRankedType, b);
119 // Collect constraints.
121 try!(self.lub().relate(&a_with_fresh, &b_with_fresh));
123 self.infcx.resolve_type_vars_if_possible(&result0);
124 debug!("lub result0 = {:?}", result0);
126 // Generalize the regions appearing in result0 if possible
127 let new_vars = self.infcx.region_vars_confined_to_snapshot(snapshot);
128 let span = self.trace.origin.span();
133 |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
134 &new_vars, &a_map, r));
136 debug!("lub({:?},{:?}) = {:?}",
141 Ok(ty::Binder(result1))
144 fn generalize_region(infcx: &InferCtxt,
146 snapshot: &CombinedSnapshot,
147 debruijn: ty::DebruijnIndex,
148 new_vars: &[ty::RegionVid],
149 a_map: &FnvHashMap<ty::BoundRegion, ty::Region>,
152 // Regions that pre-dated the LUB computation stay as they are.
153 if !is_var_in_set(new_vars, r0) {
154 assert!(!r0.is_bound());
155 debug!("generalize_region(r0={:?}): not new variable", r0);
159 let tainted = infcx.tainted_regions(snapshot, r0);
161 // Variables created during LUB computation which are
162 // *related* to regions that pre-date the LUB computation
164 if !tainted.iter().all(|r| is_var_in_set(new_vars, *r)) {
165 debug!("generalize_region(r0={:?}): \
166 non-new-variables found in {:?}",
168 assert!(!r0.is_bound());
172 // Otherwise, the variable must be associated with at
173 // least one of the variables representing bound regions
174 // in both A and B. Replace the variable with the "first"
175 // bound region from A that we find it to be associated
177 for (a_br, a_r) in a_map {
178 if tainted.iter().any(|x| x == a_r) {
179 debug!("generalize_region(r0={:?}): \
180 replacing with {:?}, tainted={:?}",
182 return ty::ReLateBound(debruijn, *a_br);
186 infcx.tcx.sess.span_bug(
188 &format!("region {:?} is not associated with \
189 any bound region from A!",
194 fn higher_ranked_glb<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
195 where T: Relate<'a,'tcx>
197 debug!("higher_ranked_glb({:?}, {:?})",
200 // Make a snapshot so we can examine "all bindings that were
201 // created as part of this type comparison".
202 return self.infcx.commit_if_ok(|snapshot| {
203 // Instantiate each bound region with a fresh region variable.
204 let (a_with_fresh, a_map) =
205 self.infcx.replace_late_bound_regions_with_fresh_var(
206 self.trace.origin.span(), HigherRankedType, a);
207 let (b_with_fresh, b_map) =
208 self.infcx.replace_late_bound_regions_with_fresh_var(
209 self.trace.origin.span(), HigherRankedType, b);
210 let a_vars = var_ids(self, &a_map);
211 let b_vars = var_ids(self, &b_map);
213 // Collect constraints.
215 try!(self.glb().relate(&a_with_fresh, &b_with_fresh));
217 self.infcx.resolve_type_vars_if_possible(&result0);
218 debug!("glb result0 = {:?}", result0);
220 // Generalize the regions appearing in result0 if possible
221 let new_vars = self.infcx.region_vars_confined_to_snapshot(snapshot);
222 let span = self.trace.origin.span();
227 |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
229 &a_map, &a_vars, &b_vars,
232 debug!("glb({:?},{:?}) = {:?}",
237 Ok(ty::Binder(result1))
240 fn generalize_region(infcx: &InferCtxt,
242 snapshot: &CombinedSnapshot,
243 debruijn: ty::DebruijnIndex,
244 new_vars: &[ty::RegionVid],
245 a_map: &FnvHashMap<ty::BoundRegion, ty::Region>,
246 a_vars: &[ty::RegionVid],
247 b_vars: &[ty::RegionVid],
248 r0: ty::Region) -> ty::Region {
249 if !is_var_in_set(new_vars, r0) {
250 assert!(!r0.is_bound());
254 let tainted = infcx.tainted_regions(snapshot, r0);
258 let mut only_new_vars = true;
260 if is_var_in_set(a_vars, *r) {
262 return fresh_bound_variable(infcx, debruijn);
266 } else if is_var_in_set(b_vars, *r) {
268 return fresh_bound_variable(infcx, debruijn);
272 } else if !is_var_in_set(new_vars, *r) {
273 only_new_vars = false;
277 // NB---I do not believe this algorithm computes
278 // (necessarily) the GLB. As written it can
279 // spuriously fail. In particular, if there is a case
280 // like: |fn(&a)| and fn(fn(&b)), where a and b are
281 // free, it will return fn(&c) where c = GLB(a,b). If
282 // however this GLB is not defined, then the result is
283 // an error, even though something like
284 // "fn<X>(fn(&X))" where X is bound would be a
285 // subtype of both of those.
287 // The problem is that if we were to return a bound
288 // variable, we'd be computing a lower-bound, but not
289 // necessarily the *greatest* lower-bound.
291 // Unfortunately, this problem is non-trivial to solve,
292 // because we do not know at the time of computing the GLB
293 // whether a GLB(a,b) exists or not, because we haven't
294 // run region inference (or indeed, even fully computed
295 // the region hierarchy!). The current algorithm seems to
296 // works ok in practice.
298 if a_r.is_some() && b_r.is_some() && only_new_vars {
299 // Related to exactly one bound variable from each fn:
300 return rev_lookup(infcx, span, a_map, a_r.unwrap());
301 } else if a_r.is_none() && b_r.is_none() {
302 // Not related to bound variables from either fn:
303 assert!(!r0.is_bound());
307 return fresh_bound_variable(infcx, debruijn);
311 fn rev_lookup(infcx: &InferCtxt,
313 a_map: &FnvHashMap<ty::BoundRegion, ty::Region>,
314 r: ty::Region) -> ty::Region
316 for (a_br, a_r) in a_map {
318 return ty::ReLateBound(ty::DebruijnIndex::new(1), *a_br);
321 infcx.tcx.sess.span_bug(
323 &format!("could not find original bound region for {:?}", r));
326 fn fresh_bound_variable(infcx: &InferCtxt, debruijn: ty::DebruijnIndex) -> ty::Region {
327 infcx.region_vars.new_bound(debruijn)
332 fn var_ids<'a, 'tcx>(fields: &CombineFields<'a, 'tcx>,
333 map: &FnvHashMap<ty::BoundRegion, ty::Region>)
334 -> Vec<ty::RegionVid> {
336 .map(|(_, r)| match *r {
337 ty::ReVar(r) => { r }
339 fields.tcx().sess.span_bug(
340 fields.trace.origin.span(),
341 &format!("found non-region-vid: {:?}", r));
347 fn is_var_in_set(new_vars: &[ty::RegionVid], r: ty::Region) -> bool {
349 ty::ReVar(ref v) => new_vars.iter().any(|x| x == v),
354 fn fold_regions_in<'tcx, T, F>(tcx: &TyCtxt<'tcx>,
358 where T: TypeFoldable<'tcx>,
359 F: FnMut(ty::Region, ty::DebruijnIndex) -> ty::Region,
361 tcx.fold_regions(unbound_value, &mut false, |region, current_depth| {
362 // we should only be encountering "escaping" late-bound regions here,
363 // because the ones at the current level should have been replaced
364 // with fresh variables
365 assert!(match region {
366 ty::ReLateBound(..) => false,
370 fldr(region, ty::DebruijnIndex::new(current_depth))
374 impl<'a,'tcx> InferCtxtExt for InferCtxt<'a,'tcx> {
375 fn tainted_regions(&self, snapshot: &CombinedSnapshot, r: ty::Region) -> Vec<ty::Region> {
376 self.region_vars.tainted(&snapshot.region_vars_snapshot, r)
379 fn region_vars_confined_to_snapshot(&self,
380 snapshot: &CombinedSnapshot)
381 -> Vec<ty::RegionVid>
384 * Returns the set of region variables that do not affect any
385 * types/regions which existed before `snapshot` was
386 * started. This is used in the sub/lub/glb computations. The
387 * idea here is that when we are computing lub/glb of two
388 * regions, we sometimes create intermediate region variables.
389 * Those region variables may touch some of the skolemized or
390 * other "forbidden" regions we created to replace bound
391 * regions, but they don't really represent an "external"
394 * However, sometimes fresh variables are created for other
395 * purposes too, and those *may* represent an external
396 * constraint. In particular, when a type variable is
397 * instantiated, we create region variables for all the
398 * regions that appear within, and if that type variable
399 * pre-existed the snapshot, then those region variables
400 * represent external constraints.
402 * An example appears in the unit test
403 * `sub_free_bound_false_infer`. In this test, we want to
407 * fn(_#0t) <: for<'a> fn(&'a int)
410 * Note that the subtype has a type variable. Because the type
411 * variable can't be instantiated with a region that is bound
412 * in the fn signature, this comparison ought to fail. But if
413 * we're not careful, it will succeed.
415 * The reason is that when we walk through the subtyping
416 * algorith, we begin by replacing `'a` with a skolemized
417 * variable `'1`. We then have `fn(_#0t) <: fn(&'1 int)`. This
418 * can be made true by unifying `_#0t` with `&'1 int`. In the
419 * process, we create a fresh variable for the skolemized
420 * region, `'$2`, and hence we have that `_#0t == &'$2
421 * int`. However, because `'$2` was created during the sub
422 * computation, if we're not careful we will erroneously
423 * assume it is one of the transient region variables
424 * representing a lub/glb internally. Not good.
426 * To prevent this, we check for type variables which were
427 * unified during the snapshot, and say that any region
428 * variable created during the snapshot but which finds its
429 * way into a type variable is considered to "escape" the
433 let mut region_vars =
434 self.region_vars.vars_created_since_snapshot(&snapshot.region_vars_snapshot);
437 self.type_variables.borrow().types_escaping_snapshot(&snapshot.type_snapshot);
439 let mut escaping_region_vars = FnvHashSet();
440 for ty in &escaping_types {
441 self.tcx.collect_regions(ty, &mut escaping_region_vars);
444 region_vars.retain(|®ion_vid| {
445 let r = ty::ReVar(region_vid);
446 !escaping_region_vars.contains(&r)
449 debug!("region_vars_confined_to_snapshot: region_vars={:?} escaping_types={:?}",
457 pub fn skolemize_late_bound_regions<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
458 binder: &ty::Binder<T>,
459 snapshot: &CombinedSnapshot)
460 -> (T, SkolemizationMap)
461 where T : TypeFoldable<'tcx>
464 * Replace all regions bound by `binder` with skolemized regions and
465 * return a map indicating which bound-region was replaced with what
466 * skolemized region. This is the first step of checking subtyping
467 * when higher-ranked things are involved. See `README.md` for more
471 let (result, map) = infcx.tcx.replace_late_bound_regions(binder, |br| {
472 infcx.region_vars.new_skolemized(br, &snapshot.region_vars_snapshot)
475 debug!("skolemize_bound_regions(binder={:?}, result={:?}, map={:?})",
483 pub fn leak_check<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
484 skol_map: &SkolemizationMap,
485 snapshot: &CombinedSnapshot)
486 -> Result<(),(ty::BoundRegion,ty::Region)>
489 * Searches the region constriants created since `snapshot` was started
490 * and checks to determine whether any of the skolemized regions created
491 * in `skol_map` would "escape" -- meaning that they are related to
492 * other regions in some way. If so, the higher-ranked subtyping doesn't
493 * hold. See `README.md` for more details.
496 debug!("leak_check: skol_map={:?}",
499 let new_vars = infcx.region_vars_confined_to_snapshot(snapshot);
500 for (&skol_br, &skol) in skol_map {
501 let tainted = infcx.tainted_regions(snapshot, skol);
502 for &tainted_region in &tainted {
503 // Each skolemized should only be relatable to itself
505 match tainted_region {
507 if new_vars.iter().any(|&x| x == vid) { continue; }
510 if tainted_region == skol { continue; }
514 debug!("{:?} (which replaced {:?}) is tainted by {:?}",
519 // A is not as polymorphic as B:
520 return Err((skol_br, tainted_region));
526 /// This code converts from skolemized regions back to late-bound
527 /// regions. It works by replacing each region in the taint set of a
528 /// skolemized region with a bound-region. The bound region will be bound
529 /// by the outer-most binder in `value`; the caller must ensure that there is
530 /// such a binder and it is the right place.
532 /// This routine is only intended to be used when the leak-check has
533 /// passed; currently, it's used in the trait matching code to create
534 /// a set of nested obligations frmo an impl that matches against
535 /// something higher-ranked. More details can be found in
536 /// `librustc/middle/traits/README.md`.
538 /// As a brief example, consider the obligation `for<'a> Fn(&'a int)
539 /// -> &'a int`, and the impl:
541 /// impl<A,R> Fn<A,R> for SomethingOrOther
545 /// Here we will have replaced `'a` with a skolemized region
546 /// `'0`. This means that our substitution will be `{A=>&'0
547 /// int, R=>&'0 int}`.
549 /// When we apply the substitution to the bounds, we will wind up with
550 /// `&'0 int : Clone` as a predicate. As a last step, we then go and
551 /// replace `'0` with a late-bound region `'a`. The depth is matched
552 /// to the depth of the predicate, in this case 1, so that the final
553 /// predicate is `for<'a> &'a int : Clone`.
554 pub fn plug_leaks<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
555 skol_map: SkolemizationMap,
556 snapshot: &CombinedSnapshot,
559 where T : TypeFoldable<'tcx>
561 debug_assert!(leak_check(infcx, &skol_map, snapshot).is_ok());
563 debug!("plug_leaks(skol_map={:?}, value={:?})",
567 // Compute a mapping from the "taint set" of each skolemized
568 // region back to the `ty::BoundRegion` that it originally
569 // represented. Because `leak_check` passed, we know that
570 // these taint sets are mutually disjoint.
571 let inv_skol_map: FnvHashMap<ty::Region, ty::BoundRegion> =
574 .flat_map(|(skol_br, skol)| {
575 infcx.tainted_regions(snapshot, skol)
577 .map(move |tainted_region| (tainted_region, skol_br))
581 debug!("plug_leaks: inv_skol_map={:?}",
584 // Remove any instantiated type variables from `value`; those can hide
585 // references to regions from the `fold_regions` code below.
586 let value = infcx.resolve_type_vars_if_possible(value);
588 // Map any skolemization byproducts back to a late-bound
589 // region. Put that late-bound region at whatever the outermost
590 // binder is that we encountered in `value`. The caller is
591 // responsible for ensuring that (a) `value` contains at least one
592 // binder and (b) that binder is the one we want to use.
593 let result = infcx.tcx.fold_regions(&value, &mut false, |r, current_depth| {
594 match inv_skol_map.get(&r) {
597 // It is the responsibility of the caller to ensure
598 // that each skolemized region appears within a
599 // binder. In practice, this routine is only used by
600 // trait checking, and all of the skolemized regions
601 // appear inside predicates, which always have
602 // binders, so this assert is satisfied.
603 assert!(current_depth > 1);
605 ty::ReLateBound(ty::DebruijnIndex::new(current_depth - 1), br.clone())
610 debug!("plug_leaks: result={:?}",