1 use rustc_hir::def_id::DefId;
2 use rustc_middle::ty::{self, Ty, TyVid};
3 use rustc_span::symbol::Symbol;
6 use crate::infer::InferCtxtUndoLogs;
8 use rustc_data_structures::snapshot_vec as sv;
9 use rustc_data_structures::unify as ut;
11 use std::marker::PhantomData;
14 use rustc_data_structures::undo_log::{Rollback, UndoLogs};
16 /// Represents a single undo-able action that affects a type inference variable.
18 pub(crate) enum UndoLog<'tcx> {
19 EqRelation(sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>),
20 SubRelation(sv::UndoLog<ut::Delegate<ty::TyVid>>),
21 Values(sv::UndoLog<Delegate>),
24 /// Convert from a specific kind of undo to the more general UndoLog
25 impl<'tcx> From<sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>> for UndoLog<'tcx> {
26 fn from(l: sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>) -> Self {
27 UndoLog::EqRelation(l)
31 /// Convert from a specific kind of undo to the more general UndoLog
32 impl<'tcx> From<sv::UndoLog<ut::Delegate<ty::TyVid>>> for UndoLog<'tcx> {
33 fn from(l: sv::UndoLog<ut::Delegate<ty::TyVid>>) -> Self {
34 UndoLog::SubRelation(l)
38 /// Convert from a specific kind of undo to the more general UndoLog
39 impl<'tcx> From<sv::UndoLog<Delegate>> for UndoLog<'tcx> {
40 fn from(l: sv::UndoLog<Delegate>) -> Self {
45 /// Convert from a specific kind of undo to the more general UndoLog
46 impl<'tcx> From<Instantiate> for UndoLog<'tcx> {
47 fn from(l: Instantiate) -> Self {
48 UndoLog::Values(sv::UndoLog::Other(l))
52 impl<'tcx> Rollback<UndoLog<'tcx>> for TypeVariableStorage<'tcx> {
53 fn reverse(&mut self, undo: UndoLog<'tcx>) {
55 UndoLog::EqRelation(undo) => self.eq_relations.reverse(undo),
56 UndoLog::SubRelation(undo) => self.sub_relations.reverse(undo),
57 UndoLog::Values(undo) => self.values.reverse(undo),
63 pub struct TypeVariableStorage<'tcx> {
64 values: sv::SnapshotVecStorage<Delegate>,
66 /// Two variables are unified in `eq_relations` when we have a
67 /// constraint `?X == ?Y`. This table also stores, for each key,
69 eq_relations: ut::UnificationTableStorage<TyVidEqKey<'tcx>>,
71 /// Two variables are unified in `sub_relations` when we have a
72 /// constraint `?X <: ?Y` *or* a constraint `?Y <: ?X`. This second
73 /// table exists only to help with the occurs check. In particular,
74 /// we want to report constraints like these as an occurs check
80 /// Without this second table, what would happen in a case like
81 /// this is that we would instantiate `?1` with a generalized
82 /// type like `Box<?6>`. We would then relate `Box<?3> <: Box<?6>`
83 /// and infer that `?3 <: ?6`. Next, since `?1` was instantiated,
84 /// we would process `?1 <: ?3`, generalize `?1 = Box<?6>` to `Box<?9>`,
85 /// and instantiate `?3` with `Box<?9>`. Finally, we would relate
86 /// `?6 <: ?9`. But now that we instantiated `?3`, we can process
87 /// `?3 <: ?6`, which gives us `Box<?9> <: ?6`... and the cycle
88 /// continues. (This is `occurs-check-2.rs`.)
90 /// What prevents this cycle is that when we generalize
91 /// `Box<?3>` to `Box<?6>`, we also sub-unify `?3` and `?6`
92 /// (in the generalizer). When we then process `Box<?6> <: ?3`,
93 /// the occurs check then fails because `?6` and `?3` are sub-unified,
94 /// and hence generalization fails.
96 /// This is reasonable because, in Rust, subtypes have the same
97 /// "skeleton" and hence there is no possible type such that
98 /// (e.g.) `Box<?3> <: ?3` for any `?3`.
100 /// In practice, we sometimes sub-unify variables in other spots, such
101 /// as when processing subtype predicates. This is not necessary but is
102 /// done to aid diagnostics, as it allows us to be more effective when
103 /// we guide the user towards where they should insert type hints.
104 sub_relations: ut::UnificationTableStorage<ty::TyVid>,
107 pub struct TypeVariableTable<'a, 'tcx> {
108 storage: &'a mut TypeVariableStorage<'tcx>,
110 undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
113 #[derive(Copy, Clone, Debug)]
114 pub struct TypeVariableOrigin {
115 pub kind: TypeVariableOriginKind,
119 /// Reasons to create a type inference variable
120 #[derive(Copy, Clone, Debug)]
121 pub enum TypeVariableOriginKind {
123 NormalizeProjectionType,
125 TypeParameterDefinition(Symbol, Option<DefId>),
127 /// One of the upvars or closure kind parameters in a `ClosureSubsts`
128 /// (before it has been determined).
129 // FIXME(eddyb) distinguish upvar inference variables from the rest.
131 SubstitutionPlaceholder,
135 /// In type check, when we are type checking a function that
136 /// returns `-> dyn Foo`, we substitute a type variable for the
137 /// return type for diagnostic purposes.
143 pub(crate) struct TypeVariableData {
144 origin: TypeVariableOrigin,
147 #[derive(Copy, Clone, Debug)]
148 pub enum TypeVariableValue<'tcx> {
149 Known { value: Ty<'tcx> },
150 Unknown { universe: ty::UniverseIndex },
153 impl<'tcx> TypeVariableValue<'tcx> {
154 /// If this value is known, returns the type it is known to be.
155 /// Otherwise, `None`.
156 pub fn known(&self) -> Option<Ty<'tcx>> {
158 TypeVariableValue::Unknown { .. } => None,
159 TypeVariableValue::Known { value } => Some(value),
163 pub fn is_unknown(&self) -> bool {
165 TypeVariableValue::Unknown { .. } => true,
166 TypeVariableValue::Known { .. } => false,
172 pub(crate) struct Instantiate;
174 pub(crate) struct Delegate;
176 impl<'tcx> TypeVariableStorage<'tcx> {
177 pub fn new() -> TypeVariableStorage<'tcx> {
178 TypeVariableStorage {
179 values: sv::SnapshotVecStorage::new(),
180 eq_relations: ut::UnificationTableStorage::new(),
181 sub_relations: ut::UnificationTableStorage::new(),
186 pub(crate) fn with_log<'a>(
188 undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
189 ) -> TypeVariableTable<'a, 'tcx> {
190 TypeVariableTable { storage: self, undo_log }
194 impl<'tcx> TypeVariableTable<'_, 'tcx> {
195 /// Returns the origin that was given when `vid` was created.
197 /// Note that this function does not return care whether
198 /// `vid` has been unified with something else or not.
199 pub fn var_origin(&self, vid: ty::TyVid) -> &TypeVariableOrigin {
200 &self.storage.values.get(vid.as_usize()).origin
203 /// Records that `a == b`, depending on `dir`.
205 /// Precondition: neither `a` nor `b` are known.
206 pub fn equate(&mut self, a: ty::TyVid, b: ty::TyVid) {
207 debug_assert!(self.probe(a).is_unknown());
208 debug_assert!(self.probe(b).is_unknown());
209 self.eq_relations().union(a, b);
210 self.sub_relations().union(a, b);
213 /// Records that `a <: b`, depending on `dir`.
215 /// Precondition: neither `a` nor `b` are known.
216 pub fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
217 debug_assert!(self.probe(a).is_unknown());
218 debug_assert!(self.probe(b).is_unknown());
219 self.sub_relations().union(a, b);
222 /// Instantiates `vid` with the type `ty`.
224 /// Precondition: `vid` must not have been previously instantiated.
225 pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
226 let vid = self.root_var(vid);
227 debug_assert!(self.probe(vid).is_unknown());
229 self.eq_relations().probe_value(vid).is_unknown(),
230 "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
233 self.eq_relations().probe_value(vid)
235 self.eq_relations().union_value(vid, TypeVariableValue::Known { value: ty });
237 // Hack: we only need this so that `types_escaping_snapshot`
238 // can see what has been unified; see the Delegate impl for
240 self.undo_log.push(Instantiate);
243 /// Creates a new type variable.
245 /// - `diverging`: indicates if this is a "diverging" type
246 /// variable, e.g., one created as the type of a `return`
247 /// expression. The code in this module doesn't care if a
248 /// variable is diverging, but the main Rust type-checker will
249 /// sometimes "unify" such variables with the `!` or `()` types.
250 /// - `origin`: indicates *why* the type variable was created.
251 /// The code in this module doesn't care, but it can be useful
252 /// for improving error messages.
255 universe: ty::UniverseIndex,
256 origin: TypeVariableOrigin,
258 let eq_key = self.eq_relations().new_key(TypeVariableValue::Unknown { universe });
260 let sub_key = self.sub_relations().new_key(());
261 assert_eq!(eq_key.vid, sub_key);
263 let index = self.values().push(TypeVariableData { origin });
264 assert_eq!(eq_key.vid.as_u32(), index as u32);
266 debug!("new_var(index={:?}, universe={:?}, origin={:?})", eq_key.vid, universe, origin);
271 /// Returns the number of type variables created thus far.
272 pub fn num_vars(&self) -> usize {
273 self.storage.values.len()
276 /// Returns the "root" variable of `vid` in the `eq_relations`
277 /// equivalence table. All type variables that have been equated
278 /// will yield the same root variable (per the union-find
279 /// algorithm), so `root_var(a) == root_var(b)` implies that `a ==
280 /// b` (transitively).
281 pub fn root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
282 self.eq_relations().find(vid).vid
285 /// Returns the "root" variable of `vid` in the `sub_relations`
286 /// equivalence table. All type variables that have been are
287 /// related via equality or subtyping will yield the same root
288 /// variable (per the union-find algorithm), so `sub_root_var(a)
289 /// == sub_root_var(b)` implies that:
291 /// exists X. (a <: X || X <: a) && (b <: X || X <: b)
293 pub fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
294 self.sub_relations().find(vid)
297 /// Returns `true` if `a` and `b` have same "sub-root" (i.e., exists some
298 /// type X such that `forall i in {a, b}. (i <: X || X <: i)`.
299 pub fn sub_unified(&mut self, a: ty::TyVid, b: ty::TyVid) -> bool {
300 self.sub_root_var(a) == self.sub_root_var(b)
303 /// Retrieves the type to which `vid` has been instantiated, if
305 pub fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
306 self.inlined_probe(vid)
309 /// An always-inlined variant of `probe`, for very hot call sites.
311 pub fn inlined_probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
312 self.eq_relations().inlined_probe_value(vid)
315 /// If `t` is a type-inference variable, and it has been
316 /// instantiated, then return the with which it was
317 /// instantiated. Otherwise, returns `t`.
318 pub fn replace_if_possible(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
320 ty::Infer(ty::TyVar(v)) => match self.probe(v) {
321 TypeVariableValue::Unknown { .. } => t,
322 TypeVariableValue::Known { value } => value,
331 ) -> sv::SnapshotVec<Delegate, &mut Vec<TypeVariableData>, &mut InferCtxtUndoLogs<'tcx>> {
332 self.storage.values.with_log(self.undo_log)
336 fn eq_relations(&mut self) -> super::UnificationTable<'_, 'tcx, TyVidEqKey<'tcx>> {
337 self.storage.eq_relations.with_log(self.undo_log)
341 fn sub_relations(&mut self) -> super::UnificationTable<'_, 'tcx, ty::TyVid> {
342 self.storage.sub_relations.with_log(self.undo_log)
345 /// Returns a range of the type variables created during the snapshot.
346 pub fn vars_since_snapshot(
349 ) -> (Range<TyVid>, Vec<TypeVariableOrigin>) {
350 let range = TyVid::from_usize(value_count)..TyVid::from_usize(self.num_vars());
352 range.start..range.end,
353 (range.start.as_usize()..range.end.as_usize())
354 .map(|index| self.storage.values.get(index).origin)
359 /// Returns indices of all variables that are not yet
361 pub fn unsolved_variables(&mut self) -> Vec<ty::TyVid> {
362 (0..self.storage.values.len())
364 let vid = ty::TyVid::from_usize(i);
365 match self.probe(vid) {
366 TypeVariableValue::Unknown { .. } => Some(vid),
367 TypeVariableValue::Known { .. } => None,
374 impl sv::SnapshotVecDelegate for Delegate {
375 type Value = TypeVariableData;
376 type Undo = Instantiate;
378 fn reverse(_values: &mut Vec<TypeVariableData>, _action: Instantiate) {
379 // We don't actually have to *do* anything to reverse an
380 // instantiation; the value for a variable is stored in the
381 // `eq_relations` and hence its rollback code will handle
382 // it. In fact, we could *almost* just remove the
383 // `SnapshotVec` entirely, except that we would have to
384 // reproduce *some* of its logic, since we want to know which
385 // type variables have been instantiated since the snapshot
386 // was started, so we can implement `types_escaping_snapshot`.
388 // (If we extended the `UnificationTable` to let us see which
389 // values have been unified and so forth, that might also
394 ///////////////////////////////////////////////////////////////////////////
396 /// These structs (a newtyped TyVid) are used as the unification key
397 /// for the `eq_relations`; they carry a `TypeVariableValue` along
399 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
400 pub(crate) struct TyVidEqKey<'tcx> {
403 // in the table, we map each ty-vid to one of these:
404 phantom: PhantomData<TypeVariableValue<'tcx>>,
407 impl<'tcx> From<ty::TyVid> for TyVidEqKey<'tcx> {
408 #[inline] // make this function eligible for inlining - it is quite hot.
409 fn from(vid: ty::TyVid) -> Self {
410 TyVidEqKey { vid, phantom: PhantomData }
414 impl<'tcx> ut::UnifyKey for TyVidEqKey<'tcx> {
415 type Value = TypeVariableValue<'tcx>;
417 fn index(&self) -> u32 {
421 fn from_index(i: u32) -> Self {
422 TyVidEqKey::from(ty::TyVid::from_u32(i))
424 fn tag() -> &'static str {
429 impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {
430 type Error = ut::NoError;
432 fn unify_values(value1: &Self, value2: &Self) -> Result<Self, ut::NoError> {
433 match (value1, value2) {
434 // We never equate two type variables, both of which
435 // have known types. Instead, we recursively equate
437 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Known { .. }) => {
438 bug!("equating two type variables, both of which have known types")
441 // If one side is known, prefer that one.
442 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Unknown { .. }) => Ok(*value1),
443 (&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),
445 // If both sides are *unknown*, it hardly matters, does it?
447 &TypeVariableValue::Unknown { universe: universe1 },
448 &TypeVariableValue::Unknown { universe: universe2 },
450 // If we unify two unbound variables, ?T and ?U, then whatever
451 // value they wind up taking (which must be the same value) must
452 // be nameable by both universes. Therefore, the resulting
453 // universe is the minimum of the two universes, because that is
454 // the one which contains the fewest names in scope.
455 let universe = cmp::min(universe1, universe2);
456 Ok(TypeVariableValue::Unknown { universe })