1 // Copyright 2012-2013 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 //! Generalized type folding mechanism. The setup is a bit convoluted
12 //! but allows for convenient usage. Let T be an instance of some
13 //! "foldable type" (one which implements `TypeFoldable`) and F be an
14 //! instance of a "folder" (a type which implements `TypeFolder`). Then
15 //! the setup is intended to be:
17 //! T.fold_with(F) --calls--> F.fold_T(T) --calls--> T.super_fold_with(F)
19 //! This way, when you define a new folder F, you can override
20 //! `fold_T()` to customize the behavior, and invoke `T.super_fold_with()`
21 //! to get the original behavior. Meanwhile, to actually fold
22 //! something, you can just write `T.fold_with(F)`, which is
23 //! convenient. (Note that `fold_with` will also transparently handle
24 //! things like a `Vec<T>` where T is foldable and so on.)
26 //! In this ideal setup, the only function that actually *does*
27 //! anything is `T.super_fold_with()`, which traverses the type `T`.
28 //! Moreover, `T.super_fold_with()` should only ever call `T.fold_with()`.
30 //! In some cases, we follow a degenerate pattern where we do not have
31 //! a `fold_T` method. Instead, `T.fold_with` traverses the structure directly.
32 //! This is suboptimal because the behavior cannot be overridden, but it's
33 //! much less work to implement. If you ever *do* need an override that
34 //! doesn't exist, it's not hard to convert the degenerate pattern into the
37 //! A `TypeFoldable` T can also be visited by a `TypeVisitor` V using similar setup:
38 //! T.visit_with(V) --calls--> V.visit_T(T) --calls--> T.super_visit_with(V).
39 //! These methods return true to indicate that the visitor has found what it is looking for
40 //! and does not need to visit anything else.
42 use middle::const_val::ConstVal;
43 use hir::def_id::DefId;
44 use ty::{self, Binder, Ty, TyCtxt, TypeFlags};
46 use rustc_data_structures::lazy_btree_map::LazyBTreeMap;
48 use util::nodemap::FxHashSet;
50 /// The TypeFoldable trait is implemented for every type that can be folded.
51 /// Basically, every type that has a corresponding method in TypeFolder.
53 /// To implement this conveniently, use the
54 /// `BraceStructTypeFoldableImpl` etc macros found in `macros.rs`.
55 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
56 fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self;
57 fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
58 self.super_fold_with(folder)
61 fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool;
62 fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
63 self.super_visit_with(visitor)
66 fn has_regions_escaping_depth(&self, depth: u32) -> bool {
67 self.visit_with(&mut HasEscapingRegionsVisitor { depth: depth })
69 fn has_escaping_regions(&self) -> bool {
70 self.has_regions_escaping_depth(0)
73 fn has_type_flags(&self, flags: TypeFlags) -> bool {
74 self.visit_with(&mut HasTypeFlagsVisitor { flags: flags })
76 fn has_projections(&self) -> bool {
77 self.has_type_flags(TypeFlags::HAS_PROJECTION)
79 fn references_error(&self) -> bool {
80 self.has_type_flags(TypeFlags::HAS_TY_ERR)
82 fn has_param_types(&self) -> bool {
83 self.has_type_flags(TypeFlags::HAS_PARAMS)
85 fn has_self_ty(&self) -> bool {
86 self.has_type_flags(TypeFlags::HAS_SELF)
88 fn has_infer_types(&self) -> bool {
89 self.has_type_flags(TypeFlags::HAS_TY_INFER)
91 fn needs_infer(&self) -> bool {
92 self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_RE_INFER)
94 fn has_skol(&self) -> bool {
95 self.has_type_flags(TypeFlags::HAS_RE_SKOL)
97 fn needs_subst(&self) -> bool {
98 self.has_type_flags(TypeFlags::NEEDS_SUBST)
100 fn has_re_skol(&self) -> bool {
101 self.has_type_flags(TypeFlags::HAS_RE_SKOL)
103 fn has_closure_types(&self) -> bool {
104 self.has_type_flags(TypeFlags::HAS_TY_CLOSURE)
106 /// "Free" regions in this context means that it has any region
107 /// that is not (a) erased or (b) late-bound.
108 fn has_free_regions(&self) -> bool {
109 self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
112 /// True if there any any un-erased free regions.
113 fn has_erasable_regions(&self) -> bool {
114 self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
117 /// Indicates whether this value references only 'global'
118 /// types/lifetimes that are the same regardless of what fn we are
119 /// in. This is used for caching.
120 fn is_global(&self) -> bool {
121 !self.has_type_flags(TypeFlags::HAS_FREE_LOCAL_NAMES)
124 /// True if there are any late-bound regions
125 fn has_late_bound_regions(&self) -> bool {
126 self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
130 /// The TypeFolder trait defines the actual *folding*. There is a
131 /// method defined for every foldable type. Each of these has a
132 /// default implementation that does an "identity" fold. Within each
133 /// identity fold, it should invoke `foo.fold_with(self)` to fold each
135 pub trait TypeFolder<'gcx: 'tcx, 'tcx> : Sized {
136 fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx>;
138 fn fold_binder<T>(&mut self, t: &Binder<T>) -> Binder<T>
139 where T : TypeFoldable<'tcx>
141 t.super_fold_with(self)
144 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
145 t.super_fold_with(self)
148 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
149 r.super_fold_with(self)
152 fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
153 c.super_fold_with(self)
157 pub trait TypeVisitor<'tcx> : Sized {
158 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
159 t.super_visit_with(self)
162 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
163 t.super_visit_with(self)
166 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
167 r.super_visit_with(self)
170 fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
171 c.super_visit_with(self)
175 ///////////////////////////////////////////////////////////////////////////
176 // Some sample folders
178 pub struct BottomUpFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a, F>
179 where F: FnMut(Ty<'tcx>) -> Ty<'tcx>
181 pub tcx: TyCtxt<'a, 'gcx, 'tcx>,
185 impl<'a, 'gcx, 'tcx, F> TypeFolder<'gcx, 'tcx> for BottomUpFolder<'a, 'gcx, 'tcx, F>
186 where F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
188 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
190 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
191 let t1 = ty.super_fold_with(self);
196 ///////////////////////////////////////////////////////////////////////////
199 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
200 /// Collects the free and escaping regions in `value` into `region_set`. Returns
201 /// whether any late-bound regions were skipped
202 pub fn collect_regions<T>(self,
204 region_set: &mut FxHashSet<ty::Region<'tcx>>)
206 where T : TypeFoldable<'tcx>
208 let mut have_bound_regions = false;
209 self.fold_regions(value, &mut have_bound_regions, |r, d| {
210 region_set.insert(self.mk_region(r.from_depth(d)));
216 /// Folds the escaping and free regions in `value` using `f`, and
217 /// sets `skipped_regions` to true if any late-bound region was found
219 pub fn fold_regions<T,F>(self,
221 skipped_regions: &mut bool,
224 where F : FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx>,
225 T : TypeFoldable<'tcx>,
227 value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
230 pub fn for_each_free_region<T,F>(self,
233 where F: FnMut(ty::Region<'tcx>),
234 T: TypeFoldable<'tcx>,
236 value.visit_with(&mut RegionVisitor { current_depth: 0, callback });
238 struct RegionVisitor<F> {
243 impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<F>
244 where F : FnMut(ty::Region<'tcx>)
246 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
247 self.current_depth += 1;
248 t.skip_binder().visit_with(self);
249 self.current_depth -= 1;
251 false // keep visiting
254 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
256 ty::ReLateBound(debruijn, _) if debruijn.depth <= self.current_depth => {
257 /* ignore bound regions */
259 _ => (self.callback)(r),
262 false // keep visiting
268 /// Folds over the substructure of a type, visiting its component
269 /// types and all regions that occur *free* within it.
271 /// That is, `Ty` can contain function or method types that bind
272 /// regions at the call site (`ReLateBound`), and occurrences of
273 /// regions (aka "lifetimes") that are bound within a type are not
274 /// visited by this folder; only regions that occur free will be
275 /// visited by `fld_r`.
277 pub struct RegionFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
278 tcx: TyCtxt<'a, 'gcx, 'tcx>,
279 skipped_regions: &'a mut bool,
281 fld_r: &'a mut (dyn FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx> + 'a),
284 impl<'a, 'gcx, 'tcx> RegionFolder<'a, 'gcx, 'tcx> {
285 pub fn new<F>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
286 skipped_regions: &'a mut bool,
287 fld_r: &'a mut F) -> RegionFolder<'a, 'gcx, 'tcx>
288 where F : FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx>
299 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionFolder<'a, 'gcx, 'tcx> {
300 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
302 fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
303 self.current_depth += 1;
304 let t = t.super_fold_with(self);
305 self.current_depth -= 1;
309 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
311 ty::ReLateBound(debruijn, _) if debruijn.depth < self.current_depth => {
312 debug!("RegionFolder.fold_region({:?}) skipped bound region (current depth={})",
313 r, self.current_depth);
314 *self.skipped_regions = true;
318 debug!("RegionFolder.fold_region({:?}) folding free region (current_depth={})",
319 r, self.current_depth);
320 (self.fld_r)(r, self.current_depth)
326 ///////////////////////////////////////////////////////////////////////////
327 // Late-bound region replacer
329 // Replaces the escaping regions in a type.
331 struct RegionReplacer<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
332 tcx: TyCtxt<'a, 'gcx, 'tcx>,
334 fld_r: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
335 map: LazyBTreeMap<ty::BoundRegion, ty::Region<'tcx>>
338 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
339 /// Replace all regions bound by the given `Binder` with the
340 /// results returned by the closure; the closure is expected to
341 /// return a free region (relative to this binder), and hence the
342 /// binder is removed in the return type. The closure is invoked
343 /// once for each unique `BoundRegion`; multiple references to the
344 /// same `BoundRegion` will reuse the previous result. A map is
345 /// returned at the end with each bound region and the free region
346 /// that replaced it.
347 pub fn replace_late_bound_regions<T,F>(self,
350 -> (T, LazyBTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
351 where F : FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
352 T : TypeFoldable<'tcx>,
354 let mut replacer = RegionReplacer::new(self, &mut f);
355 let result = value.skip_binder().fold_with(&mut replacer);
356 (result, replacer.map)
359 /// Replace any late-bound regions bound in `value` with
360 /// free variants attached to `all_outlive_scope`.
361 pub fn liberate_late_bound_regions<T>(
363 all_outlive_scope: DefId,
364 value: &ty::Binder<T>
366 where T: TypeFoldable<'tcx> {
367 self.replace_late_bound_regions(value, |br| {
368 self.mk_region(ty::ReFree(ty::FreeRegion {
369 scope: all_outlive_scope,
375 /// Flattens two binding levels into one. So `for<'a> for<'b> Foo`
376 /// becomes `for<'a,'b> Foo`.
377 pub fn flatten_late_bound_regions<T>(self, bound2_value: &Binder<Binder<T>>)
379 where T: TypeFoldable<'tcx>
381 let bound0_value = bound2_value.skip_binder().skip_binder();
382 let value = self.fold_regions(bound0_value, &mut false,
383 |region, current_depth| {
385 ty::ReLateBound(debruijn, br) if debruijn.depth >= current_depth => {
386 // should be true if no escaping regions from bound2_value
387 assert!(debruijn.depth - current_depth <= 1);
388 self.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(current_depth), br))
398 /// Returns a set of all late-bound regions that are constrained
399 /// by `value`, meaning that if we instantiate those LBR with
400 /// variables and equate `value` with something else, those
401 /// variables will also be equated.
402 pub fn collect_constrained_late_bound_regions<T>(&self, value: &Binder<T>)
403 -> FxHashSet<ty::BoundRegion>
404 where T : TypeFoldable<'tcx>
406 self.collect_late_bound_regions(value, true)
409 /// Returns a set of all late-bound regions that appear in `value` anywhere.
410 pub fn collect_referenced_late_bound_regions<T>(&self, value: &Binder<T>)
411 -> FxHashSet<ty::BoundRegion>
412 where T : TypeFoldable<'tcx>
414 self.collect_late_bound_regions(value, false)
417 fn collect_late_bound_regions<T>(&self, value: &Binder<T>, just_constraint: bool)
418 -> FxHashSet<ty::BoundRegion>
419 where T : TypeFoldable<'tcx>
421 let mut collector = LateBoundRegionsCollector::new(just_constraint);
422 let result = value.skip_binder().visit_with(&mut collector);
423 assert!(!result); // should never have stopped early
427 /// Replace any late-bound regions bound in `value` with `'erased`. Useful in trans but also
428 /// method lookup and a few other places where precise region relationships are not required.
429 pub fn erase_late_bound_regions<T>(self, value: &Binder<T>) -> T
430 where T : TypeFoldable<'tcx>
432 self.replace_late_bound_regions(value, |_| self.types.re_erased).0
435 /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
436 /// assigned starting at 1 and increasing monotonically in the order traversed
437 /// by the fold operation.
439 /// The chief purpose of this function is to canonicalize regions so that two
440 /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
441 /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
442 /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
443 pub fn anonymize_late_bound_regions<T>(self, sig: &Binder<T>) -> Binder<T>
444 where T : TypeFoldable<'tcx>,
447 Binder::bind(self.replace_late_bound_regions(sig, |_| {
449 self.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1), ty::BrAnon(counter)))
454 impl<'a, 'gcx, 'tcx> RegionReplacer<'a, 'gcx, 'tcx> {
455 fn new<F>(tcx: TyCtxt<'a, 'gcx, 'tcx>, fld_r: &'a mut F)
456 -> RegionReplacer<'a, 'gcx, 'tcx>
457 where F : FnMut(ty::BoundRegion) -> ty::Region<'tcx>
463 map: LazyBTreeMap::default()
468 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionReplacer<'a, 'gcx, 'tcx> {
469 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
471 fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
472 self.current_depth += 1;
473 let t = t.super_fold_with(self);
474 self.current_depth -= 1;
478 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
479 if !t.has_regions_escaping_depth(self.current_depth-1) {
483 t.super_fold_with(self)
486 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
488 ty::ReLateBound(debruijn, br) if debruijn.depth == self.current_depth => {
489 let fld_r = &mut self.fld_r;
490 let region = *self.map.entry(br).or_insert_with(|| fld_r(br));
491 if let ty::ReLateBound(debruijn1, br) = *region {
492 // If the callback returns a late-bound region,
493 // that region should always use depth 1. Then we
494 // adjust it to the correct depth.
495 assert_eq!(debruijn1.depth, 1);
496 self.tcx.mk_region(ty::ReLateBound(debruijn, br))
506 ///////////////////////////////////////////////////////////////////////////
509 // Shifts the De Bruijn indices on all escaping bound regions by a
510 // fixed amount. Useful in substitution or when otherwise introducing
511 // a binding level that is not intended to capture the existing bound
512 // regions. See comment on `shift_regions_through_binders` method in
513 // `subst.rs` for more details.
515 pub fn shift_region(region: ty::RegionKind, amount: u32) -> ty::RegionKind {
517 ty::ReLateBound(debruijn, br) => {
518 ty::ReLateBound(debruijn.shifted(amount), br)
526 pub fn shift_region_ref<'a, 'gcx, 'tcx>(
527 tcx: TyCtxt<'a, 'gcx, 'tcx>,
528 region: ty::Region<'tcx>,
533 &ty::ReLateBound(debruijn, br) if amount > 0 => {
534 tcx.mk_region(ty::ReLateBound(debruijn.shifted(amount), br))
542 pub fn shift_regions<'a, 'gcx, 'tcx, T>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
545 where T: TypeFoldable<'tcx>
547 debug!("shift_regions(value={:?}, amount={})",
550 value.fold_with(&mut RegionFolder::new(tcx, &mut false, &mut |region, _current_depth| {
551 shift_region_ref(tcx, region, amount)
555 /// An "escaping region" is a bound region whose binder is not part of `t`.
557 /// So, for example, consider a type like the following, which has two binders:
559 /// for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
560 /// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
561 /// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~ inner scope
563 /// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
564 /// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
565 /// fn type*, that type has an escaping region: `'a`.
567 /// Note that what I'm calling an "escaping region" is often just called a "free region". However,
568 /// we already use the term "free region". It refers to the regions that we use to represent bound
569 /// regions on a fn definition while we are typechecking its body.
571 /// To clarify, conceptually there is no particular difference between an "escaping" region and a
572 /// "free" region. However, there is a big difference in practice. Basically, when "entering" a
573 /// binding level, one is generally required to do some sort of processing to a bound region, such
574 /// as replacing it with a fresh/skolemized region, or making an entry in the environment to
575 /// represent the scope to which it is attached, etc. An escaping region represents a bound region
576 /// for which this processing has not yet been done.
577 struct HasEscapingRegionsVisitor {
581 impl<'tcx> TypeVisitor<'tcx> for HasEscapingRegionsVisitor {
582 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
584 let result = t.super_visit_with(self);
589 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
590 t.region_depth > self.depth
593 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
594 r.escapes_depth(self.depth)
598 struct HasTypeFlagsVisitor {
599 flags: ty::TypeFlags,
602 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
603 fn visit_ty(&mut self, t: Ty) -> bool {
604 debug!("HasTypeFlagsVisitor: t={:?} t.flags={:?} self.flags={:?}", t, t.flags, self.flags);
605 t.flags.intersects(self.flags)
608 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
609 let flags = r.type_flags();
610 debug!("HasTypeFlagsVisitor: r={:?} r.flags={:?} self.flags={:?}", r, flags, self.flags);
611 flags.intersects(self.flags)
614 fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
615 if let ConstVal::Unevaluated(..) = c.val {
616 let projection_flags = TypeFlags::HAS_NORMALIZABLE_PROJECTION |
617 TypeFlags::HAS_PROJECTION;
618 if projection_flags.intersects(self.flags) {
622 c.super_visit_with(self)
626 /// Collects all the late-bound regions it finds into a hash set.
627 struct LateBoundRegionsCollector {
629 regions: FxHashSet<ty::BoundRegion>,
630 just_constrained: bool,
633 impl LateBoundRegionsCollector {
634 fn new(just_constrained: bool) -> Self {
635 LateBoundRegionsCollector {
637 regions: FxHashSet(),
643 impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
644 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
645 self.current_depth += 1;
646 let result = t.super_visit_with(self);
647 self.current_depth -= 1;
651 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
652 // if we are only looking for "constrained" region, we have to
653 // ignore the inputs to a projection, as they may not appear
654 // in the normalized form
655 if self.just_constrained {
657 ty::TyProjection(..) | ty::TyAnon(..) => { return false; }
662 t.super_visit_with(self)
665 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
667 ty::ReLateBound(debruijn, br) if debruijn.depth == self.current_depth => {
668 self.regions.insert(br);