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.
44 use middle::ty::adjustment;
45 use middle::ty::{self, Binder, Ty, TyCtxt, TypeFlags};
48 use util::nodemap::{FnvHashMap, FnvHashSet};
50 /// The TypeFoldable trait is implemented for every type that can be folded.
51 /// Basically, every type that has a corresponding method in TypeFolder.
52 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
53 fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self;
54 fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
55 self.super_fold_with(folder)
58 fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool;
59 fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
60 self.super_visit_with(visitor)
63 fn has_regions_escaping_depth(&self, depth: u32) -> bool {
64 self.visit_with(&mut HasEscapingRegionsVisitor { depth: depth })
66 fn has_escaping_regions(&self) -> bool {
67 self.has_regions_escaping_depth(0)
70 fn has_type_flags(&self, flags: TypeFlags) -> bool {
71 self.visit_with(&mut HasTypeFlagsVisitor { flags: flags })
73 fn has_projection_types(&self) -> bool {
74 self.has_type_flags(TypeFlags::HAS_PROJECTION)
76 fn references_error(&self) -> bool {
77 self.has_type_flags(TypeFlags::HAS_TY_ERR)
79 fn has_param_types(&self) -> bool {
80 self.has_type_flags(TypeFlags::HAS_PARAMS)
82 fn has_self_ty(&self) -> bool {
83 self.has_type_flags(TypeFlags::HAS_SELF)
85 fn has_infer_types(&self) -> bool {
86 self.has_type_flags(TypeFlags::HAS_TY_INFER)
88 fn needs_infer(&self) -> bool {
89 self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_RE_INFER)
91 fn needs_subst(&self) -> bool {
92 self.has_type_flags(TypeFlags::NEEDS_SUBST)
94 fn has_closure_types(&self) -> bool {
95 self.has_type_flags(TypeFlags::HAS_TY_CLOSURE)
97 fn has_erasable_regions(&self) -> bool {
98 self.has_type_flags(TypeFlags::HAS_RE_EARLY_BOUND |
99 TypeFlags::HAS_RE_INFER |
100 TypeFlags::HAS_FREE_REGIONS)
102 /// Indicates whether this value references only 'global'
103 /// types/lifetimes that are the same regardless of what fn we are
104 /// in. This is used for caching. Errs on the side of returning
106 fn is_global(&self) -> bool {
107 !self.has_type_flags(TypeFlags::HAS_LOCAL_NAMES)
111 /// The TypeFolder trait defines the actual *folding*. There is a
112 /// method defined for every foldable type. Each of these has a
113 /// default implementation that does an "identity" fold. Within each
114 /// identity fold, it should invoke `foo.fold_with(self)` to fold each
116 pub trait TypeFolder<'tcx> : Sized {
117 fn tcx<'a>(&'a self) -> &'a TyCtxt<'tcx>;
119 /// Invoked by the `super_*` routines when we enter a region
120 /// binding level (for example, when entering a function
121 /// signature). This is used by clients that want to track the
122 /// Debruijn index nesting level.
123 fn enter_region_binder(&mut self) { }
125 /// Invoked by the `super_*` routines when we exit a region
126 /// binding level. This is used by clients that want to
127 /// track the Debruijn index nesting level.
128 fn exit_region_binder(&mut self) { }
130 fn fold_binder<T>(&mut self, t: &Binder<T>) -> Binder<T>
131 where T : TypeFoldable<'tcx>
133 // FIXME(#20526) this should replace `enter_region_binder`/`exit_region_binder`.
134 t.super_fold_with(self)
137 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
138 t.super_fold_with(self)
141 fn fold_mt(&mut self, t: &ty::TypeAndMut<'tcx>) -> ty::TypeAndMut<'tcx> {
142 t.super_fold_with(self)
145 fn fold_trait_ref(&mut self, t: &ty::TraitRef<'tcx>) -> ty::TraitRef<'tcx> {
146 t.super_fold_with(self)
149 fn fold_impl_header(&mut self, imp: &ty::ImplHeader<'tcx>) -> ty::ImplHeader<'tcx> {
150 imp.super_fold_with(self)
153 fn fold_substs(&mut self,
154 substs: &subst::Substs<'tcx>)
155 -> subst::Substs<'tcx> {
156 substs.super_fold_with(self)
159 fn fold_fn_sig(&mut self,
160 sig: &ty::FnSig<'tcx>)
162 sig.super_fold_with(self)
165 fn fold_output(&mut self,
166 output: &ty::FnOutput<'tcx>)
167 -> ty::FnOutput<'tcx> {
168 output.super_fold_with(self)
171 fn fold_bare_fn_ty(&mut self,
172 fty: &ty::BareFnTy<'tcx>)
173 -> ty::BareFnTy<'tcx>
175 fty.super_fold_with(self)
178 fn fold_closure_ty(&mut self,
179 fty: &ty::ClosureTy<'tcx>)
180 -> ty::ClosureTy<'tcx> {
181 fty.super_fold_with(self)
184 fn fold_region(&mut self, r: ty::Region) -> ty::Region {
185 r.super_fold_with(self)
188 fn fold_existential_bounds(&mut self, s: &ty::ExistentialBounds<'tcx>)
189 -> ty::ExistentialBounds<'tcx> {
190 s.super_fold_with(self)
193 fn fold_autoref(&mut self, ar: &adjustment::AutoRef<'tcx>)
194 -> adjustment::AutoRef<'tcx> {
195 ar.super_fold_with(self)
199 pub trait TypeVisitor<'tcx> : Sized {
200 fn enter_region_binder(&mut self) { }
201 fn exit_region_binder(&mut self) { }
203 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
204 t.super_visit_with(self)
207 fn visit_region(&mut self, r: ty::Region) -> bool {
208 r.super_visit_with(self)
212 ///////////////////////////////////////////////////////////////////////////
213 // Some sample folders
215 pub struct BottomUpFolder<'a, 'tcx: 'a, F> where F: FnMut(Ty<'tcx>) -> Ty<'tcx> {
216 pub tcx: &'a TyCtxt<'tcx>,
220 impl<'a, 'tcx, F> TypeFolder<'tcx> for BottomUpFolder<'a, 'tcx, F> where
221 F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
223 fn tcx(&self) -> &TyCtxt<'tcx> { self.tcx }
225 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
226 let t1 = ty.super_fold_with(self);
231 ///////////////////////////////////////////////////////////////////////////
234 impl<'tcx> TyCtxt<'tcx> {
235 /// Collects the free and escaping regions in `value` into `region_set`. Returns
236 /// whether any late-bound regions were skipped
237 pub fn collect_regions<T>(&self,
239 region_set: &mut FnvHashSet<ty::Region>)
241 where T : TypeFoldable<'tcx>
243 let mut have_bound_regions = false;
244 self.fold_regions(value, &mut have_bound_regions,
245 |r, d| { region_set.insert(r.from_depth(d)); r });
249 /// Folds the escaping and free regions in `value` using `f`, and
250 /// sets `skipped_regions` to true if any late-bound region was found
252 pub fn fold_regions<T,F>(&self,
254 skipped_regions: &mut bool,
257 where F : FnMut(ty::Region, u32) -> ty::Region,
258 T : TypeFoldable<'tcx>,
260 value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
264 /// Folds over the substructure of a type, visiting its component
265 /// types and all regions that occur *free* within it.
267 /// That is, `Ty` can contain function or method types that bind
268 /// regions at the call site (`ReLateBound`), and occurrences of
269 /// regions (aka "lifetimes") that are bound within a type are not
270 /// visited by this folder; only regions that occur free will be
271 /// visited by `fld_r`.
273 pub struct RegionFolder<'a, 'tcx: 'a> {
274 tcx: &'a TyCtxt<'tcx>,
275 skipped_regions: &'a mut bool,
277 fld_r: &'a mut (FnMut(ty::Region, u32) -> ty::Region + 'a),
280 impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
281 pub fn new<F>(tcx: &'a TyCtxt<'tcx>,
282 skipped_regions: &'a mut bool,
283 fld_r: &'a mut F) -> RegionFolder<'a, 'tcx>
284 where F : FnMut(ty::Region, u32) -> ty::Region
288 skipped_regions: skipped_regions,
295 impl<'a, 'tcx> TypeFolder<'tcx> for RegionFolder<'a, 'tcx>
297 fn tcx(&self) -> &TyCtxt<'tcx> { self.tcx }
299 fn enter_region_binder(&mut self) {
300 self.current_depth += 1;
303 fn exit_region_binder(&mut self) {
304 self.current_depth -= 1;
307 fn fold_region(&mut self, r: ty::Region) -> ty::Region {
309 ty::ReLateBound(debruijn, _) if debruijn.depth < self.current_depth => {
310 debug!("RegionFolder.fold_region({:?}) skipped bound region (current depth={})",
311 r, self.current_depth);
312 *self.skipped_regions = true;
316 debug!("RegionFolder.fold_region({:?}) folding free region (current_depth={})",
317 r, self.current_depth);
318 (self.fld_r)(r, self.current_depth)
324 ///////////////////////////////////////////////////////////////////////////
325 // Late-bound region replacer
327 // Replaces the escaping regions in a type.
329 struct RegionReplacer<'a, 'tcx: 'a> {
330 tcx: &'a TyCtxt<'tcx>,
332 fld_r: &'a mut (FnMut(ty::BoundRegion) -> ty::Region + 'a),
333 map: FnvHashMap<ty::BoundRegion, ty::Region>
336 impl<'tcx> TyCtxt<'tcx> {
337 pub fn replace_late_bound_regions<T,F>(&self,
340 -> (T, FnvHashMap<ty::BoundRegion, ty::Region>)
341 where F : FnMut(ty::BoundRegion) -> ty::Region,
342 T : TypeFoldable<'tcx>,
344 debug!("replace_late_bound_regions({:?})", value);
345 let mut replacer = RegionReplacer::new(self, &mut f);
346 let result = value.skip_binder().fold_with(&mut replacer);
347 (result, replacer.map)
351 /// Replace any late-bound regions bound in `value` with free variants attached to scope-id
353 pub fn liberate_late_bound_regions<T>(&self,
354 all_outlive_scope: region::CodeExtent,
357 where T : TypeFoldable<'tcx>
359 self.replace_late_bound_regions(value, |br| {
360 ty::ReFree(ty::FreeRegion{scope: all_outlive_scope, bound_region: br})
364 /// Flattens two binding levels into one. So `for<'a> for<'b> Foo`
365 /// becomes `for<'a,'b> Foo`.
366 pub fn flatten_late_bound_regions<T>(&self, bound2_value: &Binder<Binder<T>>)
368 where T: TypeFoldable<'tcx>
370 let bound0_value = bound2_value.skip_binder().skip_binder();
371 let value = self.fold_regions(bound0_value, &mut false,
372 |region, current_depth| {
374 ty::ReLateBound(debruijn, br) if debruijn.depth >= current_depth => {
375 // should be true if no escaping regions from bound2_value
376 assert!(debruijn.depth - current_depth <= 1);
377 ty::ReLateBound(ty::DebruijnIndex::new(current_depth), br)
387 pub fn no_late_bound_regions<T>(&self, value: &Binder<T>) -> Option<T>
388 where T : TypeFoldable<'tcx>
390 if value.0.has_escaping_regions() {
393 Some(value.0.clone())
397 /// Replace any late-bound regions bound in `value` with `'static`. Useful in trans but also
398 /// method lookup and a few other places where precise region relationships are not required.
399 pub fn erase_late_bound_regions<T>(&self, value: &Binder<T>) -> T
400 where T : TypeFoldable<'tcx>
402 self.replace_late_bound_regions(value, |_| ty::ReStatic).0
405 /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
406 /// assigned starting at 1 and increasing monotonically in the order traversed
407 /// by the fold operation.
409 /// The chief purpose of this function is to canonicalize regions so that two
410 /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
411 /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
412 /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
413 pub fn anonymize_late_bound_regions<T>(&self, sig: &Binder<T>) -> Binder<T>
414 where T : TypeFoldable<'tcx>,
417 Binder(self.replace_late_bound_regions(sig, |_| {
419 ty::ReLateBound(ty::DebruijnIndex::new(1), ty::BrAnon(counter))
424 impl<'a, 'tcx> RegionReplacer<'a, 'tcx> {
425 fn new<F>(tcx: &'a TyCtxt<'tcx>, fld_r: &'a mut F) -> RegionReplacer<'a, 'tcx>
426 where F : FnMut(ty::BoundRegion) -> ty::Region
437 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx>
439 fn tcx(&self) -> &TyCtxt<'tcx> { self.tcx }
441 fn enter_region_binder(&mut self) {
442 self.current_depth += 1;
445 fn exit_region_binder(&mut self) {
446 self.current_depth -= 1;
449 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
450 if !t.has_regions_escaping_depth(self.current_depth-1) {
454 t.super_fold_with(self)
457 fn fold_region(&mut self, r: ty::Region) -> ty::Region {
459 ty::ReLateBound(debruijn, br) if debruijn.depth == self.current_depth => {
460 debug!("RegionReplacer.fold_region({:?}) folding region (current_depth={})",
461 r, self.current_depth);
462 let fld_r = &mut self.fld_r;
463 let region = *self.map.entry(br).or_insert_with(|| fld_r(br));
464 if let ty::ReLateBound(debruijn1, br) = region {
465 // If the callback returns a late-bound region,
466 // that region should always use depth 1. Then we
467 // adjust it to the correct depth.
468 assert_eq!(debruijn1.depth, 1);
469 ty::ReLateBound(debruijn, br)
479 ///////////////////////////////////////////////////////////////////////////
482 impl<'tcx> TyCtxt<'tcx> {
483 /// Returns an equivalent value with all free regions removed (note
484 /// that late-bound regions remain, because they are important for
485 /// subtyping, but they are anonymized and normalized as well)..
486 pub fn erase_regions<T>(&self, value: &T) -> T
487 where T : TypeFoldable<'tcx>
489 let value1 = value.fold_with(&mut RegionEraser(self));
490 debug!("erase_regions({:?}) = {:?}",
494 struct RegionEraser<'a, 'tcx: 'a>(&'a TyCtxt<'tcx>);
496 impl<'a, 'tcx> TypeFolder<'tcx> for RegionEraser<'a, 'tcx> {
497 fn tcx(&self) -> &TyCtxt<'tcx> { self.0 }
499 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
500 match self.tcx().normalized_cache.borrow().get(&ty).cloned() {
505 let t_norm = ty.super_fold_with(self);
506 self.tcx().normalized_cache.borrow_mut().insert(ty, t_norm);
510 fn fold_binder<T>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T>
511 where T : TypeFoldable<'tcx>
513 let u = self.tcx().anonymize_late_bound_regions(t);
514 u.super_fold_with(self)
517 fn fold_region(&mut self, r: ty::Region) -> ty::Region {
518 // because late-bound regions affect subtyping, we can't
519 // erase the bound/free distinction, but we can replace
520 // all free regions with 'static.
522 // Note that we *CAN* replace early-bound regions -- the
523 // type system never "sees" those, they get substituted
524 // away. In trans, they will always be erased to 'static
525 // whenever a substitution occurs.
527 ty::ReLateBound(..) => r,
532 fn fold_substs(&mut self,
533 substs: &subst::Substs<'tcx>)
534 -> subst::Substs<'tcx> {
535 subst::Substs { regions: subst::ErasedRegions,
536 types: substs.types.fold_with(self) }
542 ///////////////////////////////////////////////////////////////////////////
545 // Shifts the De Bruijn indices on all escaping bound regions by a
546 // fixed amount. Useful in substitution or when otherwise introducing
547 // a binding level that is not intended to capture the existing bound
548 // regions. See comment on `shift_regions_through_binders` method in
549 // `subst.rs` for more details.
551 pub fn shift_region(region: ty::Region, amount: u32) -> ty::Region {
553 ty::ReLateBound(debruijn, br) => {
554 ty::ReLateBound(debruijn.shifted(amount), br)
562 pub fn shift_regions<'tcx, T:TypeFoldable<'tcx>>(tcx: &TyCtxt<'tcx>,
563 amount: u32, value: &T) -> T {
564 debug!("shift_regions(value={:?}, amount={})",
567 value.fold_with(&mut RegionFolder::new(tcx, &mut false, &mut |region, _current_depth| {
568 shift_region(region, amount)
572 /// An "escaping region" is a bound region whose binder is not part of `t`.
574 /// So, for example, consider a type like the following, which has two binders:
576 /// for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
577 /// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
578 /// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~ inner scope
580 /// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
581 /// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
582 /// fn type*, that type has an escaping region: `'a`.
584 /// Note that what I'm calling an "escaping region" is often just called a "free region". However,
585 /// we already use the term "free region". It refers to the regions that we use to represent bound
586 /// regions on a fn definition while we are typechecking its body.
588 /// To clarify, conceptually there is no particular difference between an "escaping" region and a
589 /// "free" region. However, there is a big difference in practice. Basically, when "entering" a
590 /// binding level, one is generally required to do some sort of processing to a bound region, such
591 /// as replacing it with a fresh/skolemized region, or making an entry in the environment to
592 /// represent the scope to which it is attached, etc. An escaping region represents a bound region
593 /// for which this processing has not yet been done.
594 struct HasEscapingRegionsVisitor {
598 impl<'tcx> TypeVisitor<'tcx> for HasEscapingRegionsVisitor {
599 fn enter_region_binder(&mut self) {
603 fn exit_region_binder(&mut self) {
607 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
608 t.region_depth > self.depth
611 fn visit_region(&mut self, r: ty::Region) -> bool {
612 r.escapes_depth(self.depth)
616 struct HasTypeFlagsVisitor {
617 flags: ty::TypeFlags,
620 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
621 fn visit_ty(&mut self, t: Ty) -> bool {
622 t.flags.get().intersects(self.flags)
625 fn visit_region(&mut self, r: ty::Region) -> bool {
626 if self.flags.intersects(ty::TypeFlags::HAS_LOCAL_NAMES) {
627 // does this represent a region that cannot be named
628 // in a global way? used in fulfillment caching.
630 ty::ReStatic | ty::ReEmpty => {}
634 if self.flags.intersects(ty::TypeFlags::HAS_RE_INFER) {
636 ty::ReVar(_) | ty::ReSkolemized(..) => { return true }