1 use syntax::symbol::InternedString;
3 use crate::ty::{self, Ty, TyVid};
6 use std::marker::PhantomData;
9 use rustc_data_structures::snapshot_vec as sv;
10 use rustc_data_structures::unify as ut;
12 pub struct TypeVariableTable<'tcx> {
13 values: sv::SnapshotVec<Delegate>,
15 /// Two variables are unified in `eq_relations` when we have a
16 /// constraint `?X == ?Y`. This table also stores, for each key,
18 eq_relations: ut::UnificationTable<ut::InPlace<TyVidEqKey<'tcx>>>,
20 /// Two variables are unified in `eq_relations` when we have a
21 /// constraint `?X <: ?Y` *or* a constraint `?Y <: ?X`. This second
22 /// table exists only to help with the occurs check. In particular,
23 /// we want to report constraints like these as an occurs check
29 /// This works because `?1` and `?3` are unified in the
30 /// `sub_relations` relation (not in `eq_relations`). Then when we
31 /// process the `Box<?3> <: ?1` constraint, we do an occurs check
32 /// on `Box<?3>` and find a potential cycle.
34 /// This is reasonable because, in Rust, subtypes have the same
35 /// "skeleton" and hence there is no possible type such that
36 /// (e.g.) `Box<?3> <: ?3` for any `?3`.
37 sub_relations: ut::UnificationTable<ut::InPlace<ty::TyVid>>,
40 /// Reasons to create a type inference variable
41 #[derive(Copy, Clone, Debug)]
42 pub enum TypeVariableOrigin {
44 NormalizeProjectionType(Span),
46 TypeParameterDefinition(Span, InternedString),
48 /// one of the upvars or closure kind parameters in a `ClosureSubsts`
49 /// (before it has been determined)
50 ClosureSynthetic(Span),
51 SubstitutionPlaceholder(Span),
55 DivergingBlockExpr(Span),
57 LatticeVariable(Span),
58 Generalized(ty::TyVid),
61 struct TypeVariableData {
62 origin: TypeVariableOrigin,
66 #[derive(Copy, Clone, Debug)]
67 pub enum TypeVariableValue<'tcx> {
68 Known { value: Ty<'tcx> },
69 Unknown { universe: ty::UniverseIndex },
72 impl<'tcx> TypeVariableValue<'tcx> {
73 /// If this value is known, returns the type it is known to be.
74 /// Otherwise, `None`.
75 pub fn known(&self) -> Option<Ty<'tcx>> {
77 TypeVariableValue::Unknown { .. } => None,
78 TypeVariableValue::Known { value } => Some(value),
82 pub fn is_unknown(&self) -> bool {
84 TypeVariableValue::Unknown { .. } => true,
85 TypeVariableValue::Known { .. } => false,
90 pub struct Snapshot<'tcx> {
91 snapshot: sv::Snapshot,
92 eq_snapshot: ut::Snapshot<ut::InPlace<TyVidEqKey<'tcx>>>,
93 sub_snapshot: ut::Snapshot<ut::InPlace<ty::TyVid>>,
102 impl<'tcx> TypeVariableTable<'tcx> {
103 pub fn new() -> TypeVariableTable<'tcx> {
105 values: sv::SnapshotVec::new(),
106 eq_relations: ut::UnificationTable::new(),
107 sub_relations: ut::UnificationTable::new(),
111 /// Returns the diverges flag given when `vid` was created.
113 /// Note that this function does not return care whether
114 /// `vid` has been unified with something else or not.
115 pub fn var_diverges<'a>(&'a self, vid: ty::TyVid) -> bool {
116 self.values.get(vid.index as usize).diverging
119 /// Returns the origin that was given when `vid` was created.
121 /// Note that this function does not return care whether
122 /// `vid` has been unified with something else or not.
123 pub fn var_origin(&self, vid: ty::TyVid) -> &TypeVariableOrigin {
124 &self.values.get(vid.index as usize).origin
127 /// Records that `a == b`, depending on `dir`.
129 /// Precondition: neither `a` nor `b` are known.
130 pub fn equate(&mut self, a: ty::TyVid, b: ty::TyVid) {
131 debug_assert!(self.probe(a).is_unknown());
132 debug_assert!(self.probe(b).is_unknown());
133 self.eq_relations.union(a, b);
134 self.sub_relations.union(a, b);
137 /// Records that `a <: b`, depending on `dir`.
139 /// Precondition: neither `a` nor `b` are known.
140 pub fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
141 debug_assert!(self.probe(a).is_unknown());
142 debug_assert!(self.probe(b).is_unknown());
143 self.sub_relations.union(a, b);
146 /// Instantiates `vid` with the type `ty`.
148 /// Precondition: `vid` must not have been previously instantiated.
149 pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
150 let vid = self.root_var(vid);
151 debug_assert!(self.probe(vid).is_unknown());
152 debug_assert!(self.eq_relations.probe_value(vid).is_unknown(),
153 "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
154 vid, ty, self.eq_relations.probe_value(vid));
155 self.eq_relations.union_value(vid, TypeVariableValue::Known { value: ty });
157 // Hack: we only need this so that `types_escaping_snapshot`
158 // can see what has been unified; see the Delegate impl for
160 self.values.record(Instantiate { vid });
163 /// Creates a new type variable.
165 /// - `diverging`: indicates if this is a "diverging" type
166 /// variable, e.g., one created as the type of a `return`
167 /// expression. The code in this module doesn't care if a
168 /// variable is diverging, but the main Rust type-checker will
169 /// sometimes "unify" such variables with the `!` or `()` types.
170 /// - `origin`: indicates *why* the type variable was created.
171 /// The code in this module doesn't care, but it can be useful
172 /// for improving error messages.
173 pub fn new_var(&mut self,
174 universe: ty::UniverseIndex,
176 origin: TypeVariableOrigin)
178 let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown { universe });
180 let sub_key = self.sub_relations.new_key(());
181 assert_eq!(eq_key.vid, sub_key);
183 let index = self.values.push(TypeVariableData {
187 assert_eq!(eq_key.vid.index, index as u32);
190 "new_var(index={:?}, universe={:?}, diverging={:?}, origin={:?}",
200 /// Returns the number of type variables created thus far.
201 pub fn num_vars(&self) -> usize {
205 /// Returns the "root" variable of `vid` in the `eq_relations`
206 /// equivalence table. All type variables that have been equated
207 /// will yield the same root variable (per the union-find
208 /// algorithm), so `root_var(a) == root_var(b)` implies that `a ==
209 /// b` (transitively).
210 pub fn root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
211 self.eq_relations.find(vid).vid
214 /// Returns the "root" variable of `vid` in the `sub_relations`
215 /// equivalence table. All type variables that have been are
216 /// related via equality or subtyping will yield the same root
217 /// variable (per the union-find algorithm), so `sub_root_var(a)
218 /// == sub_root_var(b)` implies that:
220 /// exists X. (a <: X || X <: a) && (b <: X || X <: b)
221 pub fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
222 self.sub_relations.find(vid)
225 /// Returns `true` if `a` and `b` have same "sub-root" (i.e., exists some
226 /// type X such that `forall i in {a, b}. (i <: X || X <: i)`.
227 pub fn sub_unified(&mut self, a: ty::TyVid, b: ty::TyVid) -> bool {
228 self.sub_root_var(a) == self.sub_root_var(b)
231 /// Retrieves the type to which `vid` has been instantiated, if
233 pub fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
234 self.eq_relations.probe_value(vid)
237 /// If `t` is a type-inference variable, and it has been
238 /// instantiated, then return the with which it was
239 /// instantiated. Otherwise, returns `t`.
240 pub fn replace_if_possible(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
242 ty::Infer(ty::TyVar(v)) => {
243 match self.probe(v) {
244 TypeVariableValue::Unknown { .. } => t,
245 TypeVariableValue::Known { value } => value,
252 /// Creates a snapshot of the type variable state. This snapshot
253 /// must later be committed (`commit()`) or rolled back
254 /// (`rollback_to()`). Nested snapshots are permitted, but must
255 /// be processed in a stack-like fashion.
256 pub fn snapshot(&mut self) -> Snapshot<'tcx> {
258 snapshot: self.values.start_snapshot(),
259 eq_snapshot: self.eq_relations.snapshot(),
260 sub_snapshot: self.sub_relations.snapshot(),
264 /// Undoes all changes since the snapshot was created. Any
265 /// snapshots created since that point must already have been
266 /// committed or rolled back.
267 pub fn rollback_to(&mut self, s: Snapshot<'tcx>) {
268 debug!("rollback_to{:?}", {
269 for action in self.values.actions_since_snapshot(&s.snapshot) {
270 if let sv::UndoLog::NewElem(index) = *action {
271 debug!("inference variable _#{}t popped", index)
276 let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
277 self.values.rollback_to(snapshot);
278 self.eq_relations.rollback_to(eq_snapshot);
279 self.sub_relations.rollback_to(sub_snapshot);
282 /// Commits all changes since the snapshot was created, making
283 /// them permanent (unless this snapshot was created within
284 /// another snapshot). Any snapshots created since that point
285 /// must already have been committed or rolled back.
286 pub fn commit(&mut self, s: Snapshot<'tcx>) {
287 let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
288 self.values.commit(snapshot);
289 self.eq_relations.commit(eq_snapshot);
290 self.sub_relations.commit(sub_snapshot);
293 /// Returns a range of the type variables created during the snapshot.
294 pub fn vars_since_snapshot(
297 ) -> (Range<TyVid>, Vec<TypeVariableOrigin>) {
298 let range = self.eq_relations.vars_since_snapshot(&s.eq_snapshot);
299 (range.start.vid..range.end.vid, (range.start.vid.index..range.end.vid.index).map(|index| {
300 self.values.get(index as usize).origin.clone()
304 /// Finds the set of type variables that existed *before* `s`
305 /// but which have only been unified since `s` started, and
306 /// return the types with which they were unified. So if we had
307 /// a type variable `V0`, then we started the snapshot, then we
308 /// created a type variable `V1`, unified `V0` with `T0`, and
309 /// unified `V1` with `T1`, this function would return `{T0}`.
310 pub fn types_escaping_snapshot(&mut self, s: &Snapshot<'tcx>) -> Vec<Ty<'tcx>> {
311 let mut new_elem_threshold = u32::MAX;
312 let mut escaping_types = Vec::new();
313 let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot);
314 debug!("actions_since_snapshot.len() = {}", actions_since_snapshot.len());
315 for action in actions_since_snapshot {
317 sv::UndoLog::NewElem(index) => {
318 // if any new variables were created during the
319 // snapshot, remember the lower index (which will
320 // always be the first one we see). Note that this
321 // action must precede those variables being
323 new_elem_threshold = cmp::min(new_elem_threshold, index as u32);
324 debug!("NewElem({}) new_elem_threshold={}", index, new_elem_threshold);
327 sv::UndoLog::Other(Instantiate { vid, .. }) => {
328 if vid.index < new_elem_threshold {
329 // quick check to see if this variable was
330 // created since the snapshot started or not.
331 let escaping_type = match self.eq_relations.probe_value(vid) {
332 TypeVariableValue::Unknown { .. } => bug!(),
333 TypeVariableValue::Known { value } => value,
335 escaping_types.push(escaping_type);
337 debug!("SpecifyVar({:?}) new_elem_threshold={}", vid, new_elem_threshold);
347 /// Returns indices of all variables that are not yet
349 pub fn unsolved_variables(&mut self) -> Vec<ty::TyVid> {
350 (0..self.values.len())
352 let vid = ty::TyVid { index: i as u32 };
353 match self.probe(vid) {
354 TypeVariableValue::Unknown { .. } => Some(vid),
355 TypeVariableValue::Known { .. } => None,
362 impl sv::SnapshotVecDelegate for Delegate {
363 type Value = TypeVariableData;
364 type Undo = Instantiate;
366 fn reverse(_values: &mut Vec<TypeVariableData>, _action: Instantiate) {
367 // We don't actually have to *do* anything to reverse an
368 // instanation; the value for a variable is stored in the
369 // `eq_relations` and hence its rollback code will handle
370 // it. In fact, we could *almost* just remove the
371 // `SnapshotVec` entirely, except that we would have to
372 // reproduce *some* of its logic, since we want to know which
373 // type variables have been instantiated since the snapshot
374 // was started, so we can implement `types_escaping_snapshot`.
376 // (If we extended the `UnificationTable` to let us see which
377 // values have been unified and so forth, that might also
382 ///////////////////////////////////////////////////////////////////////////
384 /// These structs (a newtyped TyVid) are used as the unification key
385 /// for the `eq_relations`; they carry a `TypeVariableValue` along
387 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
388 struct TyVidEqKey<'tcx> {
391 // in the table, we map each ty-vid to one of these:
392 phantom: PhantomData<TypeVariableValue<'tcx>>,
395 impl<'tcx> From<ty::TyVid> for TyVidEqKey<'tcx> {
396 fn from(vid: ty::TyVid) -> Self {
397 TyVidEqKey { vid, phantom: PhantomData }
401 impl<'tcx> ut::UnifyKey for TyVidEqKey<'tcx> {
402 type Value = TypeVariableValue<'tcx>;
403 fn index(&self) -> u32 { self.vid.index }
404 fn from_index(i: u32) -> Self { TyVidEqKey::from(ty::TyVid { index: i }) }
405 fn tag() -> &'static str { "TyVidEqKey" }
408 impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {
409 type Error = ut::NoError;
411 fn unify_values(value1: &Self, value2: &Self) -> Result<Self, ut::NoError> {
412 match (value1, value2) {
413 // We never equate two type variables, both of which
414 // have known types. Instead, we recursively equate
416 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Known { .. }) => {
417 bug!("equating two type variables, both of which have known types")
420 // If one side is known, prefer that one.
421 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Unknown { .. }) => Ok(*value1),
422 (&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),
424 // If both sides are *unknown*, it hardly matters, does it?
425 (&TypeVariableValue::Unknown { universe: universe1 },
426 &TypeVariableValue::Unknown { universe: universe2 }) => {
427 // If we unify two unbound variables, ?T and ?U, then whatever
428 // value they wind up taking (which must be the same value) must
429 // be nameable by both universes. Therefore, the resulting
430 // universe is the minimum of the two universes, because that is
431 // the one which contains the fewest names in scope.
432 let universe = cmp::min(universe1, universe2);
433 Ok(TypeVariableValue::Unknown { universe })
439 /// Raw `TyVid` are used as the unification key for `sub_relations`;
440 /// they carry no values.
441 impl ut::UnifyKey for ty::TyVid {
443 fn index(&self) -> u32 { self.index }
444 fn from_index(i: u32) -> ty::TyVid { ty::TyVid { index: i } }
445 fn tag() -> &'static str { "TyVid" }