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487cf647 FG |
1 | use crate::errors::DumpVTableEntries; |
2 | use crate::traits::{impossible_predicates, is_vtable_safe_method}; | |
3 | use rustc_hir::def_id::DefId; | |
4 | use rustc_hir::lang_items::LangItem; | |
5 | use rustc_infer::traits::util::PredicateSet; | |
6 | use rustc_infer::traits::ImplSource; | |
7 | use rustc_middle::ty::visit::TypeVisitable; | |
8 | use rustc_middle::ty::InternalSubsts; | |
9 | use rustc_middle::ty::{self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, VtblEntry}; | |
10 | use rustc_span::{sym, Span}; | |
11 | use smallvec::SmallVec; | |
12 | ||
13 | use std::fmt::Debug; | |
14 | use std::ops::ControlFlow; | |
15 | ||
16 | #[derive(Clone, Debug)] | |
17 | pub(super) enum VtblSegment<'tcx> { | |
18 | MetadataDSA, | |
19 | TraitOwnEntries { trait_ref: ty::PolyTraitRef<'tcx>, emit_vptr: bool }, | |
20 | } | |
21 | ||
22 | /// Prepare the segments for a vtable | |
23 | pub(super) fn prepare_vtable_segments<'tcx, T>( | |
24 | tcx: TyCtxt<'tcx>, | |
25 | trait_ref: ty::PolyTraitRef<'tcx>, | |
26 | mut segment_visitor: impl FnMut(VtblSegment<'tcx>) -> ControlFlow<T>, | |
27 | ) -> Option<T> { | |
28 | // The following constraints holds for the final arrangement. | |
29 | // 1. The whole virtual table of the first direct super trait is included as the | |
30 | // the prefix. If this trait doesn't have any super traits, then this step | |
31 | // consists of the dsa metadata. | |
32 | // 2. Then comes the proper pointer metadata(vptr) and all own methods for all | |
33 | // other super traits except those already included as part of the first | |
34 | // direct super trait virtual table. | |
35 | // 3. finally, the own methods of this trait. | |
36 | ||
37 | // This has the advantage that trait upcasting to the first direct super trait on each level | |
38 | // is zero cost, and to another trait includes only replacing the pointer with one level indirection, | |
39 | // while not using too much extra memory. | |
40 | ||
41 | // For a single inheritance relationship like this, | |
42 | // D --> C --> B --> A | |
43 | // The resulting vtable will consists of these segments: | |
44 | // DSA, A, B, C, D | |
45 | ||
46 | // For a multiple inheritance relationship like this, | |
47 | // D --> C --> A | |
48 | // \-> B | |
49 | // The resulting vtable will consists of these segments: | |
50 | // DSA, A, B, B-vptr, C, D | |
51 | ||
52 | // For a diamond inheritance relationship like this, | |
53 | // D --> B --> A | |
54 | // \-> C -/ | |
55 | // The resulting vtable will consists of these segments: | |
56 | // DSA, A, B, C, C-vptr, D | |
57 | ||
58 | // For a more complex inheritance relationship like this: | |
59 | // O --> G --> C --> A | |
60 | // \ \ \-> B | |
61 | // | |-> F --> D | |
62 | // | \-> E | |
63 | // |-> N --> J --> H | |
64 | // \ \-> I | |
65 | // |-> M --> K | |
66 | // \-> L | |
67 | // The resulting vtable will consists of these segments: | |
68 | // DSA, A, B, B-vptr, C, D, D-vptr, E, E-vptr, F, F-vptr, G, | |
69 | // H, H-vptr, I, I-vptr, J, J-vptr, K, K-vptr, L, L-vptr, M, M-vptr, | |
70 | // N, N-vptr, O | |
71 | ||
72 | // emit dsa segment first. | |
73 | if let ControlFlow::Break(v) = (segment_visitor)(VtblSegment::MetadataDSA) { | |
74 | return Some(v); | |
75 | } | |
76 | ||
77 | let mut emit_vptr_on_new_entry = false; | |
78 | let mut visited = PredicateSet::new(tcx); | |
79 | let predicate = trait_ref.without_const().to_predicate(tcx); | |
80 | let mut stack: SmallVec<[(ty::PolyTraitRef<'tcx>, _, _); 5]> = | |
81 | smallvec![(trait_ref, emit_vptr_on_new_entry, None)]; | |
82 | visited.insert(predicate); | |
83 | ||
84 | // the main traversal loop: | |
85 | // basically we want to cut the inheritance directed graph into a few non-overlapping slices of nodes | |
86 | // that each node is emitted after all its descendents have been emitted. | |
87 | // so we convert the directed graph into a tree by skipping all previously visited nodes using a visited set. | |
88 | // this is done on the fly. | |
89 | // Each loop run emits a slice - it starts by find a "childless" unvisited node, backtracking upwards, and it | |
90 | // stops after it finds a node that has a next-sibling node. | |
91 | // This next-sibling node will used as the starting point of next slice. | |
92 | ||
93 | // Example: | |
94 | // For a diamond inheritance relationship like this, | |
95 | // D#1 --> B#0 --> A#0 | |
96 | // \-> C#1 -/ | |
97 | ||
98 | // Starting point 0 stack [D] | |
99 | // Loop run #0: Stack after diving in is [D B A], A is "childless" | |
100 | // after this point, all newly visited nodes won't have a vtable that equals to a prefix of this one. | |
101 | // Loop run #0: Emitting the slice [B A] (in reverse order), B has a next-sibling node, so this slice stops here. | |
102 | // Loop run #0: Stack after exiting out is [D C], C is the next starting point. | |
103 | // Loop run #1: Stack after diving in is [D C], C is "childless", since its child A is skipped(already emitted). | |
104 | // Loop run #1: Emitting the slice [D C] (in reverse order). No one has a next-sibling node. | |
105 | // Loop run #1: Stack after exiting out is []. Now the function exits. | |
106 | ||
107 | loop { | |
108 | // dive deeper into the stack, recording the path | |
109 | 'diving_in: loop { | |
110 | if let Some((inner_most_trait_ref, _, _)) = stack.last() { | |
111 | let inner_most_trait_ref = *inner_most_trait_ref; | |
112 | let mut direct_super_traits_iter = tcx | |
113 | .super_predicates_of(inner_most_trait_ref.def_id()) | |
114 | .predicates | |
115 | .into_iter() | |
116 | .filter_map(move |(pred, _)| { | |
117 | pred.subst_supertrait(tcx, &inner_most_trait_ref).to_opt_poly_trait_pred() | |
118 | }); | |
119 | ||
120 | 'diving_in_skip_visited_traits: loop { | |
121 | if let Some(next_super_trait) = direct_super_traits_iter.next() { | |
122 | if visited.insert(next_super_trait.to_predicate(tcx)) { | |
123 | // We're throwing away potential constness of super traits here. | |
124 | // FIXME: handle ~const super traits | |
125 | let next_super_trait = next_super_trait.map_bound(|t| t.trait_ref); | |
126 | stack.push(( | |
127 | next_super_trait, | |
128 | emit_vptr_on_new_entry, | |
129 | Some(direct_super_traits_iter), | |
130 | )); | |
131 | break 'diving_in_skip_visited_traits; | |
132 | } else { | |
133 | continue 'diving_in_skip_visited_traits; | |
134 | } | |
135 | } else { | |
136 | break 'diving_in; | |
137 | } | |
138 | } | |
139 | } | |
140 | } | |
141 | ||
142 | // Other than the left-most path, vptr should be emitted for each trait. | |
143 | emit_vptr_on_new_entry = true; | |
144 | ||
145 | // emit innermost item, move to next sibling and stop there if possible, otherwise jump to outer level. | |
146 | 'exiting_out: loop { | |
147 | if let Some((inner_most_trait_ref, emit_vptr, siblings_opt)) = stack.last_mut() { | |
148 | if let ControlFlow::Break(v) = (segment_visitor)(VtblSegment::TraitOwnEntries { | |
149 | trait_ref: *inner_most_trait_ref, | |
150 | emit_vptr: *emit_vptr, | |
151 | }) { | |
152 | return Some(v); | |
153 | } | |
154 | ||
155 | 'exiting_out_skip_visited_traits: loop { | |
156 | if let Some(siblings) = siblings_opt { | |
157 | if let Some(next_inner_most_trait_ref) = siblings.next() { | |
158 | if visited.insert(next_inner_most_trait_ref.to_predicate(tcx)) { | |
159 | // We're throwing away potential constness of super traits here. | |
160 | // FIXME: handle ~const super traits | |
161 | let next_inner_most_trait_ref = | |
162 | next_inner_most_trait_ref.map_bound(|t| t.trait_ref); | |
163 | *inner_most_trait_ref = next_inner_most_trait_ref; | |
164 | *emit_vptr = emit_vptr_on_new_entry; | |
165 | break 'exiting_out; | |
166 | } else { | |
167 | continue 'exiting_out_skip_visited_traits; | |
168 | } | |
169 | } | |
170 | } | |
171 | stack.pop(); | |
172 | continue 'exiting_out; | |
173 | } | |
174 | } | |
175 | // all done | |
176 | return None; | |
177 | } | |
178 | } | |
179 | } | |
180 | ||
181 | fn dump_vtable_entries<'tcx>( | |
182 | tcx: TyCtxt<'tcx>, | |
183 | sp: Span, | |
184 | trait_ref: ty::PolyTraitRef<'tcx>, | |
185 | entries: &[VtblEntry<'tcx>], | |
186 | ) { | |
187 | tcx.sess.emit_err(DumpVTableEntries { | |
188 | span: sp, | |
189 | trait_ref, | |
190 | entries: format!("{:#?}", entries), | |
191 | }); | |
192 | } | |
193 | ||
f25598a0 | 194 | fn own_existential_vtable_entries(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &[DefId] { |
487cf647 FG |
195 | let trait_methods = tcx |
196 | .associated_items(trait_def_id) | |
197 | .in_definition_order() | |
198 | .filter(|item| item.kind == ty::AssocKind::Fn); | |
199 | // Now list each method's DefId (for within its trait). | |
200 | let own_entries = trait_methods.filter_map(move |trait_method| { | |
201 | debug!("own_existential_vtable_entry: trait_method={:?}", trait_method); | |
202 | let def_id = trait_method.def_id; | |
203 | ||
204 | // Some methods cannot be called on an object; skip those. | |
205 | if !is_vtable_safe_method(tcx, trait_def_id, &trait_method) { | |
206 | debug!("own_existential_vtable_entry: not vtable safe"); | |
207 | return None; | |
208 | } | |
209 | ||
210 | Some(def_id) | |
211 | }); | |
212 | ||
213 | tcx.arena.alloc_from_iter(own_entries.into_iter()) | |
214 | } | |
215 | ||
216 | /// Given a trait `trait_ref`, iterates the vtable entries | |
217 | /// that come from `trait_ref`, including its supertraits. | |
218 | fn vtable_entries<'tcx>( | |
219 | tcx: TyCtxt<'tcx>, | |
220 | trait_ref: ty::PolyTraitRef<'tcx>, | |
221 | ) -> &'tcx [VtblEntry<'tcx>] { | |
222 | debug!("vtable_entries({:?})", trait_ref); | |
223 | ||
224 | let mut entries = vec![]; | |
225 | ||
226 | let vtable_segment_callback = |segment| -> ControlFlow<()> { | |
227 | match segment { | |
228 | VtblSegment::MetadataDSA => { | |
229 | entries.extend(TyCtxt::COMMON_VTABLE_ENTRIES); | |
230 | } | |
231 | VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => { | |
232 | let existential_trait_ref = trait_ref | |
233 | .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)); | |
234 | ||
235 | // Lookup the shape of vtable for the trait. | |
236 | let own_existential_entries = | |
237 | tcx.own_existential_vtable_entries(existential_trait_ref.def_id()); | |
238 | ||
239 | let own_entries = own_existential_entries.iter().copied().map(|def_id| { | |
240 | debug!("vtable_entries: trait_method={:?}", def_id); | |
241 | ||
242 | // The method may have some early-bound lifetimes; add regions for those. | |
243 | let substs = trait_ref.map_bound(|trait_ref| { | |
244 | InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind { | |
245 | GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(), | |
246 | GenericParamDefKind::Type { .. } | |
247 | | GenericParamDefKind::Const { .. } => { | |
248 | trait_ref.substs[param.index as usize] | |
249 | } | |
250 | }) | |
251 | }); | |
252 | ||
253 | // The trait type may have higher-ranked lifetimes in it; | |
254 | // erase them if they appear, so that we get the type | |
255 | // at some particular call site. | |
256 | let substs = tcx | |
257 | .normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), substs); | |
258 | ||
259 | // It's possible that the method relies on where-clauses that | |
260 | // do not hold for this particular set of type parameters. | |
261 | // Note that this method could then never be called, so we | |
262 | // do not want to try and codegen it, in that case (see #23435). | |
263 | let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs); | |
f25598a0 FG |
264 | if impossible_predicates( |
265 | tcx, | |
266 | predicates.map(|(predicate, _)| predicate).collect(), | |
267 | ) { | |
487cf647 FG |
268 | debug!("vtable_entries: predicates do not hold"); |
269 | return VtblEntry::Vacant; | |
270 | } | |
271 | ||
272 | let instance = ty::Instance::resolve_for_vtable( | |
273 | tcx, | |
274 | ty::ParamEnv::reveal_all(), | |
275 | def_id, | |
276 | substs, | |
277 | ) | |
278 | .expect("resolution failed during building vtable representation"); | |
279 | VtblEntry::Method(instance) | |
280 | }); | |
281 | ||
282 | entries.extend(own_entries); | |
283 | ||
284 | if emit_vptr { | |
285 | entries.push(VtblEntry::TraitVPtr(trait_ref)); | |
286 | } | |
287 | } | |
288 | } | |
289 | ||
290 | ControlFlow::Continue(()) | |
291 | }; | |
292 | ||
293 | let _ = prepare_vtable_segments(tcx, trait_ref, vtable_segment_callback); | |
294 | ||
295 | if tcx.has_attr(trait_ref.def_id(), sym::rustc_dump_vtable) { | |
296 | let sp = tcx.def_span(trait_ref.def_id()); | |
297 | dump_vtable_entries(tcx, sp, trait_ref, &entries); | |
298 | } | |
299 | ||
300 | tcx.arena.alloc_from_iter(entries.into_iter()) | |
301 | } | |
302 | ||
303 | /// Find slot base for trait methods within vtable entries of another trait | |
304 | pub(super) fn vtable_trait_first_method_offset<'tcx>( | |
305 | tcx: TyCtxt<'tcx>, | |
306 | key: ( | |
307 | ty::PolyTraitRef<'tcx>, // trait_to_be_found | |
308 | ty::PolyTraitRef<'tcx>, // trait_owning_vtable | |
309 | ), | |
310 | ) -> usize { | |
311 | let (trait_to_be_found, trait_owning_vtable) = key; | |
312 | ||
313 | // #90177 | |
314 | let trait_to_be_found_erased = tcx.erase_regions(trait_to_be_found); | |
315 | ||
316 | let vtable_segment_callback = { | |
317 | let mut vtable_base = 0; | |
318 | ||
319 | move |segment| { | |
320 | match segment { | |
321 | VtblSegment::MetadataDSA => { | |
322 | vtable_base += TyCtxt::COMMON_VTABLE_ENTRIES.len(); | |
323 | } | |
324 | VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => { | |
325 | if tcx.erase_regions(trait_ref) == trait_to_be_found_erased { | |
326 | return ControlFlow::Break(vtable_base); | |
327 | } | |
328 | vtable_base += count_own_vtable_entries(tcx, trait_ref); | |
329 | if emit_vptr { | |
330 | vtable_base += 1; | |
331 | } | |
332 | } | |
333 | } | |
334 | ControlFlow::Continue(()) | |
335 | } | |
336 | }; | |
337 | ||
338 | if let Some(vtable_base) = | |
339 | prepare_vtable_segments(tcx, trait_owning_vtable, vtable_segment_callback) | |
340 | { | |
341 | vtable_base | |
342 | } else { | |
343 | bug!("Failed to find info for expected trait in vtable"); | |
344 | } | |
345 | } | |
346 | ||
347 | /// Find slot offset for trait vptr within vtable entries of another trait | |
348 | pub(crate) fn vtable_trait_upcasting_coercion_new_vptr_slot<'tcx>( | |
349 | tcx: TyCtxt<'tcx>, | |
350 | key: ( | |
351 | Ty<'tcx>, // trait object type whose trait owning vtable | |
352 | Ty<'tcx>, // trait object for supertrait | |
353 | ), | |
354 | ) -> Option<usize> { | |
355 | let (source, target) = key; | |
356 | assert!(matches!(&source.kind(), &ty::Dynamic(..)) && !source.needs_infer()); | |
357 | assert!(matches!(&target.kind(), &ty::Dynamic(..)) && !target.needs_infer()); | |
358 | ||
359 | // this has been typecked-before, so diagnostics is not really needed. | |
360 | let unsize_trait_did = tcx.require_lang_item(LangItem::Unsize, None); | |
361 | ||
362 | let trait_ref = tcx.mk_trait_ref(unsize_trait_did, [source, target]); | |
363 | ||
364 | match tcx.codegen_select_candidate((ty::ParamEnv::reveal_all(), ty::Binder::dummy(trait_ref))) { | |
365 | Ok(ImplSource::TraitUpcasting(implsrc_traitcasting)) => { | |
366 | implsrc_traitcasting.vtable_vptr_slot | |
367 | } | |
368 | otherwise => bug!("expected TraitUpcasting candidate, got {otherwise:?}"), | |
369 | } | |
370 | } | |
371 | ||
372 | /// Given a trait `trait_ref`, returns the number of vtable entries | |
373 | /// that come from `trait_ref`, excluding its supertraits. Used in | |
374 | /// computing the vtable base for an upcast trait of a trait object. | |
375 | pub(crate) fn count_own_vtable_entries<'tcx>( | |
376 | tcx: TyCtxt<'tcx>, | |
377 | trait_ref: ty::PolyTraitRef<'tcx>, | |
378 | ) -> usize { | |
379 | tcx.own_existential_vtable_entries(trait_ref.def_id()).len() | |
380 | } | |
381 | ||
382 | pub(super) fn provide(providers: &mut ty::query::Providers) { | |
383 | *providers = ty::query::Providers { | |
384 | own_existential_vtable_entries, | |
385 | vtable_entries, | |
386 | vtable_trait_upcasting_coercion_new_vptr_slot, | |
387 | ..*providers | |
388 | }; | |
389 | } |