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 ty::{self, Binder, Ty, TyCtxt, TypeFlags};
46 use util::nodemap::{FxHashMap, FxHashSet};
48 /// The TypeFoldable trait is implemented for every type that can be folded.
49 /// Basically, every type that has a corresponding method in TypeFolder.
50 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
51 fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self;
52 fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
53 self.super_fold_with(folder)
56 fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool;
57 fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
58 self.super_visit_with(visitor)
61 fn has_regions_escaping_depth(&self, depth: u32) -> bool {
62 self.visit_with(&mut HasEscapingRegionsVisitor { depth: depth })
64 fn has_escaping_regions(&self) -> bool {
65 self.has_regions_escaping_depth(0)
68 fn has_type_flags(&self, flags: TypeFlags) -> bool {
69 self.visit_with(&mut HasTypeFlagsVisitor { flags: flags })
71 fn has_projections(&self) -> bool {
72 self.has_type_flags(TypeFlags::HAS_PROJECTION)
74 fn references_error(&self) -> bool {
75 self.has_type_flags(TypeFlags::HAS_TY_ERR)
77 fn has_param_types(&self) -> bool {
78 self.has_type_flags(TypeFlags::HAS_PARAMS)
80 fn has_self_ty(&self) -> bool {
81 self.has_type_flags(TypeFlags::HAS_SELF)
83 fn has_infer_types(&self) -> bool {
84 self.has_type_flags(TypeFlags::HAS_TY_INFER)
86 fn needs_infer(&self) -> bool {
87 self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_RE_INFER)
89 fn needs_subst(&self) -> bool {
90 self.has_type_flags(TypeFlags::NEEDS_SUBST)
92 fn has_re_skol(&self) -> bool {
93 self.has_type_flags(TypeFlags::HAS_RE_SKOL)
95 fn has_closure_types(&self) -> bool {
96 self.has_type_flags(TypeFlags::HAS_TY_CLOSURE)
98 fn has_erasable_regions(&self) -> bool {
99 self.has_type_flags(TypeFlags::HAS_RE_EARLY_BOUND |
100 TypeFlags::HAS_RE_INFER |
101 TypeFlags::HAS_FREE_REGIONS)
103 fn is_normalized_for_trans(&self) -> bool {
104 !self.has_type_flags(TypeFlags::HAS_RE_EARLY_BOUND |
105 TypeFlags::HAS_RE_INFER |
106 TypeFlags::HAS_FREE_REGIONS |
107 TypeFlags::HAS_TY_INFER |
108 TypeFlags::HAS_PARAMS |
109 TypeFlags::HAS_NORMALIZABLE_PROJECTION |
110 TypeFlags::HAS_TY_ERR |
113 /// Indicates whether this value references only 'global'
114 /// types/lifetimes that are the same regardless of what fn we are
115 /// in. This is used for caching. Errs on the side of returning
117 fn is_global(&self) -> bool {
118 !self.has_type_flags(TypeFlags::HAS_LOCAL_NAMES)
122 /// The TypeFolder trait defines the actual *folding*. There is a
123 /// method defined for every foldable type. Each of these has a
124 /// default implementation that does an "identity" fold. Within each
125 /// identity fold, it should invoke `foo.fold_with(self)` to fold each
127 pub trait TypeFolder<'gcx: 'tcx, 'tcx> : Sized {
128 fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx>;
130 fn fold_binder<T>(&mut self, t: &Binder<T>) -> Binder<T>
131 where T : TypeFoldable<'tcx>
133 t.super_fold_with(self)
136 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
137 t.super_fold_with(self)
140 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
141 r.super_fold_with(self)
144 fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
145 c.super_fold_with(self)
149 pub trait TypeVisitor<'tcx> : Sized {
150 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
151 t.super_visit_with(self)
154 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
155 t.super_visit_with(self)
158 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
159 r.super_visit_with(self)
162 fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
163 c.super_visit_with(self)
167 ///////////////////////////////////////////////////////////////////////////
168 // Some sample folders
170 pub struct BottomUpFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a, F>
171 where F: FnMut(Ty<'tcx>) -> Ty<'tcx>
173 pub tcx: TyCtxt<'a, 'gcx, 'tcx>,
177 impl<'a, 'gcx, 'tcx, F> TypeFolder<'gcx, 'tcx> for BottomUpFolder<'a, 'gcx, 'tcx, F>
178 where F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
180 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
182 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
183 let t1 = ty.super_fold_with(self);
188 ///////////////////////////////////////////////////////////////////////////
191 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
192 /// Collects the free and escaping regions in `value` into `region_set`. Returns
193 /// whether any late-bound regions were skipped
194 pub fn collect_regions<T>(self,
196 region_set: &mut FxHashSet<ty::Region<'tcx>>)
198 where T : TypeFoldable<'tcx>
200 let mut have_bound_regions = false;
201 self.fold_regions(value, &mut have_bound_regions, |r, d| {
202 region_set.insert(self.mk_region(r.from_depth(d)));
208 /// Folds the escaping and free regions in `value` using `f`, and
209 /// sets `skipped_regions` to true if any late-bound region was found
211 pub fn fold_regions<T,F>(self,
213 skipped_regions: &mut bool,
216 where F : FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx>,
217 T : TypeFoldable<'tcx>,
219 value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
223 /// Folds over the substructure of a type, visiting its component
224 /// types and all regions that occur *free* within it.
226 /// That is, `Ty` can contain function or method types that bind
227 /// regions at the call site (`ReLateBound`), and occurrences of
228 /// regions (aka "lifetimes") that are bound within a type are not
229 /// visited by this folder; only regions that occur free will be
230 /// visited by `fld_r`.
232 pub struct RegionFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
233 tcx: TyCtxt<'a, 'gcx, 'tcx>,
234 skipped_regions: &'a mut bool,
236 fld_r: &'a mut (FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx> + 'a),
239 impl<'a, 'gcx, 'tcx> RegionFolder<'a, 'gcx, 'tcx> {
240 pub fn new<F>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
241 skipped_regions: &'a mut bool,
242 fld_r: &'a mut F) -> RegionFolder<'a, 'gcx, 'tcx>
243 where F : FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx>
254 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionFolder<'a, 'gcx, 'tcx> {
255 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
257 fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
258 self.current_depth += 1;
259 let t = t.super_fold_with(self);
260 self.current_depth -= 1;
264 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
266 ty::ReLateBound(debruijn, _) if debruijn.depth < self.current_depth => {
267 debug!("RegionFolder.fold_region({:?}) skipped bound region (current depth={})",
268 r, self.current_depth);
269 *self.skipped_regions = true;
273 debug!("RegionFolder.fold_region({:?}) folding free region (current_depth={})",
274 r, self.current_depth);
275 (self.fld_r)(r, self.current_depth)
281 ///////////////////////////////////////////////////////////////////////////
282 // Late-bound region replacer
284 // Replaces the escaping regions in a type.
286 struct RegionReplacer<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
287 tcx: TyCtxt<'a, 'gcx, 'tcx>,
289 fld_r: &'a mut (FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
290 map: FxHashMap<ty::BoundRegion, ty::Region<'tcx>>
293 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
294 pub fn replace_late_bound_regions<T,F>(self,
297 -> (T, FxHashMap<ty::BoundRegion, ty::Region<'tcx>>)
298 where F : FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
299 T : TypeFoldable<'tcx>,
301 let mut replacer = RegionReplacer::new(self, &mut f);
302 let result = value.skip_binder().fold_with(&mut replacer);
303 (result, replacer.map)
306 /// Flattens two binding levels into one. So `for<'a> for<'b> Foo`
307 /// becomes `for<'a,'b> Foo`.
308 pub fn flatten_late_bound_regions<T>(self, bound2_value: &Binder<Binder<T>>)
310 where T: TypeFoldable<'tcx>
312 let bound0_value = bound2_value.skip_binder().skip_binder();
313 let value = self.fold_regions(bound0_value, &mut false,
314 |region, current_depth| {
316 ty::ReLateBound(debruijn, br) if debruijn.depth >= current_depth => {
317 // should be true if no escaping regions from bound2_value
318 assert!(debruijn.depth - current_depth <= 1);
319 self.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(current_depth), br))
329 pub fn no_late_bound_regions<T>(self, value: &Binder<T>) -> Option<T>
330 where T : TypeFoldable<'tcx>
332 if value.0.has_escaping_regions() {
335 Some(value.0.clone())
339 /// Returns a set of all late-bound regions that are constrained
340 /// by `value`, meaning that if we instantiate those LBR with
341 /// variables and equate `value` with something else, those
342 /// variables will also be equated.
343 pub fn collect_constrained_late_bound_regions<T>(&self, value: &Binder<T>)
344 -> FxHashSet<ty::BoundRegion>
345 where T : TypeFoldable<'tcx>
347 self.collect_late_bound_regions(value, true)
350 /// Returns a set of all late-bound regions that appear in `value` anywhere.
351 pub fn collect_referenced_late_bound_regions<T>(&self, value: &Binder<T>)
352 -> FxHashSet<ty::BoundRegion>
353 where T : TypeFoldable<'tcx>
355 self.collect_late_bound_regions(value, false)
358 fn collect_late_bound_regions<T>(&self, value: &Binder<T>, just_constraint: bool)
359 -> FxHashSet<ty::BoundRegion>
360 where T : TypeFoldable<'tcx>
362 let mut collector = LateBoundRegionsCollector::new(just_constraint);
363 let result = value.skip_binder().visit_with(&mut collector);
364 assert!(!result); // should never have stopped early
368 /// Replace any late-bound regions bound in `value` with `'erased`. Useful in trans but also
369 /// method lookup and a few other places where precise region relationships are not required.
370 pub fn erase_late_bound_regions<T>(self, value: &Binder<T>) -> T
371 where T : TypeFoldable<'tcx>
373 self.replace_late_bound_regions(value, |_| self.types.re_erased).0
376 /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
377 /// assigned starting at 1 and increasing monotonically in the order traversed
378 /// by the fold operation.
380 /// The chief purpose of this function is to canonicalize regions so that two
381 /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
382 /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
383 /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
384 pub fn anonymize_late_bound_regions<T>(self, sig: &Binder<T>) -> Binder<T>
385 where T : TypeFoldable<'tcx>,
388 Binder(self.replace_late_bound_regions(sig, |_| {
390 self.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1), ty::BrAnon(counter)))
395 impl<'a, 'gcx, 'tcx> RegionReplacer<'a, 'gcx, 'tcx> {
396 fn new<F>(tcx: TyCtxt<'a, 'gcx, 'tcx>, fld_r: &'a mut F)
397 -> RegionReplacer<'a, 'gcx, 'tcx>
398 where F : FnMut(ty::BoundRegion) -> ty::Region<'tcx>
409 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionReplacer<'a, 'gcx, 'tcx> {
410 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
412 fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
413 self.current_depth += 1;
414 let t = t.super_fold_with(self);
415 self.current_depth -= 1;
419 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
420 if !t.has_regions_escaping_depth(self.current_depth-1) {
424 t.super_fold_with(self)
427 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
429 ty::ReLateBound(debruijn, br) if debruijn.depth == self.current_depth => {
430 let fld_r = &mut self.fld_r;
431 let region = *self.map.entry(br).or_insert_with(|| fld_r(br));
432 if let ty::ReLateBound(debruijn1, br) = *region {
433 // If the callback returns a late-bound region,
434 // that region should always use depth 1. Then we
435 // adjust it to the correct depth.
436 assert_eq!(debruijn1.depth, 1);
437 self.tcx.mk_region(ty::ReLateBound(debruijn, br))
447 ///////////////////////////////////////////////////////////////////////////
450 // Shifts the De Bruijn indices on all escaping bound regions by a
451 // fixed amount. Useful in substitution or when otherwise introducing
452 // a binding level that is not intended to capture the existing bound
453 // regions. See comment on `shift_regions_through_binders` method in
454 // `subst.rs` for more details.
456 pub fn shift_region(region: ty::RegionKind, amount: u32) -> ty::RegionKind {
458 ty::ReLateBound(debruijn, br) => {
459 ty::ReLateBound(debruijn.shifted(amount), br)
467 pub fn shift_region_ref<'a, 'gcx, 'tcx>(
468 tcx: TyCtxt<'a, 'gcx, 'tcx>,
469 region: ty::Region<'tcx>,
474 &ty::ReLateBound(debruijn, br) if amount > 0 => {
475 tcx.mk_region(ty::ReLateBound(debruijn.shifted(amount), br))
483 pub fn shift_regions<'a, 'gcx, 'tcx, T>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
486 where T: TypeFoldable<'tcx>
488 debug!("shift_regions(value={:?}, amount={})",
491 value.fold_with(&mut RegionFolder::new(tcx, &mut false, &mut |region, _current_depth| {
492 shift_region_ref(tcx, region, amount)
496 /// An "escaping region" is a bound region whose binder is not part of `t`.
498 /// So, for example, consider a type like the following, which has two binders:
500 /// for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
501 /// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
502 /// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~ inner scope
504 /// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
505 /// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
506 /// fn type*, that type has an escaping region: `'a`.
508 /// Note that what I'm calling an "escaping region" is often just called a "free region". However,
509 /// we already use the term "free region". It refers to the regions that we use to represent bound
510 /// regions on a fn definition while we are typechecking its body.
512 /// To clarify, conceptually there is no particular difference between an "escaping" region and a
513 /// "free" region. However, there is a big difference in practice. Basically, when "entering" a
514 /// binding level, one is generally required to do some sort of processing to a bound region, such
515 /// as replacing it with a fresh/skolemized region, or making an entry in the environment to
516 /// represent the scope to which it is attached, etc. An escaping region represents a bound region
517 /// for which this processing has not yet been done.
518 struct HasEscapingRegionsVisitor {
522 impl<'tcx> TypeVisitor<'tcx> for HasEscapingRegionsVisitor {
523 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
525 let result = t.super_visit_with(self);
530 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
531 t.region_depth > self.depth
534 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
535 r.escapes_depth(self.depth)
539 struct HasTypeFlagsVisitor {
540 flags: ty::TypeFlags,
543 impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
544 fn visit_ty(&mut self, t: Ty) -> bool {
545 debug!("HasTypeFlagsVisitor: t={:?} t.flags={:?} self.flags={:?}", t, t.flags, self.flags);
546 t.flags.intersects(self.flags)
549 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
550 let flags = r.type_flags();
551 debug!("HasTypeFlagsVisitor: r={:?} r.flags={:?} self.flags={:?}", r, flags, self.flags);
552 flags.intersects(self.flags)
555 fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
556 if let ConstVal::Unevaluated(..) = c.val {
557 let projection_flags = TypeFlags::HAS_NORMALIZABLE_PROJECTION |
558 TypeFlags::HAS_PROJECTION;
559 if projection_flags.intersects(self.flags) {
563 c.super_visit_with(self)
567 /// Collects all the late-bound regions it finds into a hash set.
568 struct LateBoundRegionsCollector {
570 regions: FxHashSet<ty::BoundRegion>,
571 just_constrained: bool,
574 impl LateBoundRegionsCollector {
575 fn new(just_constrained: bool) -> Self {
576 LateBoundRegionsCollector {
578 regions: FxHashSet(),
584 impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
585 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
586 self.current_depth += 1;
587 let result = t.super_visit_with(self);
588 self.current_depth -= 1;
592 fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
593 // if we are only looking for "constrained" region, we have to
594 // ignore the inputs to a projection, as they may not appear
595 // in the normalized form
596 if self.just_constrained {
598 ty::TyProjection(..) | ty::TyAnon(..) => { return false; }
603 t.super_visit_with(self)
606 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
608 ty::ReLateBound(debruijn, br) if debruijn.depth == self.current_depth => {
609 self.regions.insert(br);