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 `sub_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 #[derive(Copy, Clone, Debug)]
41 pub struct TypeVariableOrigin {
42 pub kind: TypeVariableOriginKind,
46 /// Reasons to create a type inference variable
47 #[derive(Copy, Clone, Debug)]
48 pub enum TypeVariableOriginKind {
50 NormalizeProjectionType,
52 TypeParameterDefinition(InternedString),
54 /// One of the upvars or closure kind parameters in a `ClosureSubsts`
55 /// (before it has been determined).
57 SubstitutionPlaceholder,
64 struct TypeVariableData {
65 origin: TypeVariableOrigin,
69 #[derive(Copy, Clone, Debug)]
70 pub enum TypeVariableValue<'tcx> {
71 Known { value: Ty<'tcx> },
72 Unknown { universe: ty::UniverseIndex },
75 impl<'tcx> TypeVariableValue<'tcx> {
76 /// If this value is known, returns the type it is known to be.
77 /// Otherwise, `None`.
78 pub fn known(&self) -> Option<Ty<'tcx>> {
80 TypeVariableValue::Unknown { .. } => None,
81 TypeVariableValue::Known { value } => Some(value),
85 pub fn is_unknown(&self) -> bool {
87 TypeVariableValue::Unknown { .. } => true,
88 TypeVariableValue::Known { .. } => false,
93 pub struct Snapshot<'tcx> {
94 snapshot: sv::Snapshot,
95 eq_snapshot: ut::Snapshot<ut::InPlace<TyVidEqKey<'tcx>>>,
96 sub_snapshot: ut::Snapshot<ut::InPlace<ty::TyVid>>,
105 impl<'tcx> TypeVariableTable<'tcx> {
106 pub fn new() -> TypeVariableTable<'tcx> {
108 values: sv::SnapshotVec::new(),
109 eq_relations: ut::UnificationTable::new(),
110 sub_relations: ut::UnificationTable::new(),
114 /// Returns the diverges flag given when `vid` was created.
116 /// Note that this function does not return care whether
117 /// `vid` has been unified with something else or not.
118 pub fn var_diverges<'a>(&'a self, vid: ty::TyVid) -> bool {
119 self.values.get(vid.index as usize).diverging
122 /// Returns the origin that was given when `vid` was created.
124 /// Note that this function does not return care whether
125 /// `vid` has been unified with something else or not.
126 pub fn var_origin(&self, vid: ty::TyVid) -> &TypeVariableOrigin {
127 &self.values.get(vid.index as usize).origin
130 /// Records that `a == b`, depending on `dir`.
132 /// Precondition: neither `a` nor `b` are known.
133 pub fn equate(&mut self, a: ty::TyVid, b: ty::TyVid) {
134 debug_assert!(self.probe(a).is_unknown());
135 debug_assert!(self.probe(b).is_unknown());
136 self.eq_relations.union(a, b);
137 self.sub_relations.union(a, b);
140 /// Records that `a <: b`, depending on `dir`.
142 /// Precondition: neither `a` nor `b` are known.
143 pub fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
144 debug_assert!(self.probe(a).is_unknown());
145 debug_assert!(self.probe(b).is_unknown());
146 self.sub_relations.union(a, b);
149 /// Instantiates `vid` with the type `ty`.
151 /// Precondition: `vid` must not have been previously instantiated.
152 pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
153 let vid = self.root_var(vid);
154 debug_assert!(self.probe(vid).is_unknown());
155 debug_assert!(self.eq_relations.probe_value(vid).is_unknown(),
156 "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
157 vid, ty, self.eq_relations.probe_value(vid));
158 self.eq_relations.union_value(vid, TypeVariableValue::Known { value: ty });
160 // Hack: we only need this so that `types_escaping_snapshot`
161 // can see what has been unified; see the Delegate impl for
163 self.values.record(Instantiate { vid });
166 /// Creates a new type variable.
168 /// - `diverging`: indicates if this is a "diverging" type
169 /// variable, e.g., one created as the type of a `return`
170 /// expression. The code in this module doesn't care if a
171 /// variable is diverging, but the main Rust type-checker will
172 /// sometimes "unify" such variables with the `!` or `()` types.
173 /// - `origin`: indicates *why* the type variable was created.
174 /// The code in this module doesn't care, but it can be useful
175 /// for improving error messages.
176 pub fn new_var(&mut self,
177 universe: ty::UniverseIndex,
179 origin: TypeVariableOrigin)
181 let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown { universe });
183 let sub_key = self.sub_relations.new_key(());
184 assert_eq!(eq_key.vid, sub_key);
186 let index = self.values.push(TypeVariableData {
190 assert_eq!(eq_key.vid.index, index as u32);
193 "new_var(index={:?}, universe={:?}, diverging={:?}, origin={:?}",
203 /// Returns the number of type variables created thus far.
204 pub fn num_vars(&self) -> usize {
208 /// Returns the "root" variable of `vid` in the `eq_relations`
209 /// equivalence table. All type variables that have been equated
210 /// will yield the same root variable (per the union-find
211 /// algorithm), so `root_var(a) == root_var(b)` implies that `a ==
212 /// b` (transitively).
213 pub fn root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
214 self.eq_relations.find(vid).vid
217 /// Returns the "root" variable of `vid` in the `sub_relations`
218 /// equivalence table. All type variables that have been are
219 /// related via equality or subtyping will yield the same root
220 /// variable (per the union-find algorithm), so `sub_root_var(a)
221 /// == sub_root_var(b)` implies that:
223 /// exists X. (a <: X || X <: a) && (b <: X || X <: b)
224 pub fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
225 self.sub_relations.find(vid)
228 /// Returns `true` if `a` and `b` have same "sub-root" (i.e., exists some
229 /// type X such that `forall i in {a, b}. (i <: X || X <: i)`.
230 pub fn sub_unified(&mut self, a: ty::TyVid, b: ty::TyVid) -> bool {
231 self.sub_root_var(a) == self.sub_root_var(b)
234 /// Retrieves the type to which `vid` has been instantiated, if
236 pub fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
237 self.eq_relations.probe_value(vid)
240 /// If `t` is a type-inference variable, and it has been
241 /// instantiated, then return the with which it was
242 /// instantiated. Otherwise, returns `t`.
243 pub fn replace_if_possible(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
245 ty::Infer(ty::TyVar(v)) => {
246 match self.probe(v) {
247 TypeVariableValue::Unknown { .. } => t,
248 TypeVariableValue::Known { value } => value,
255 /// Creates a snapshot of the type variable state. This snapshot
256 /// must later be committed (`commit()`) or rolled back
257 /// (`rollback_to()`). Nested snapshots are permitted, but must
258 /// be processed in a stack-like fashion.
259 pub fn snapshot(&mut self) -> Snapshot<'tcx> {
261 snapshot: self.values.start_snapshot(),
262 eq_snapshot: self.eq_relations.snapshot(),
263 sub_snapshot: self.sub_relations.snapshot(),
267 /// Undoes all changes since the snapshot was created. Any
268 /// snapshots created since that point must already have been
269 /// committed or rolled back.
270 pub fn rollback_to(&mut self, s: Snapshot<'tcx>) {
271 debug!("rollback_to{:?}", {
272 for action in self.values.actions_since_snapshot(&s.snapshot) {
273 if let sv::UndoLog::NewElem(index) = *action {
274 debug!("inference variable _#{}t popped", index)
279 let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
280 self.values.rollback_to(snapshot);
281 self.eq_relations.rollback_to(eq_snapshot);
282 self.sub_relations.rollback_to(sub_snapshot);
285 /// Commits all changes since the snapshot was created, making
286 /// them permanent (unless this snapshot was created within
287 /// another snapshot). Any snapshots created since that point
288 /// must already have been committed or rolled back.
289 pub fn commit(&mut self, s: Snapshot<'tcx>) {
290 let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
291 self.values.commit(snapshot);
292 self.eq_relations.commit(eq_snapshot);
293 self.sub_relations.commit(sub_snapshot);
296 /// Returns a range of the type variables created during the snapshot.
297 pub fn vars_since_snapshot(
300 ) -> (Range<TyVid>, Vec<TypeVariableOrigin>) {
301 let range = self.eq_relations.vars_since_snapshot(&s.eq_snapshot);
302 (range.start.vid..range.end.vid, (range.start.vid.index..range.end.vid.index).map(|index| {
303 self.values.get(index as usize).origin.clone()
307 /// Finds the set of type variables that existed *before* `s`
308 /// but which have only been unified since `s` started, and
309 /// return the types with which they were unified. So if we had
310 /// a type variable `V0`, then we started the snapshot, then we
311 /// created a type variable `V1`, unified `V0` with `T0`, and
312 /// unified `V1` with `T1`, this function would return `{T0}`.
313 pub fn types_escaping_snapshot(&mut self, s: &Snapshot<'tcx>) -> Vec<Ty<'tcx>> {
314 let mut new_elem_threshold = u32::MAX;
315 let mut escaping_types = Vec::new();
316 let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot);
317 debug!("actions_since_snapshot.len() = {}", actions_since_snapshot.len());
318 for action in actions_since_snapshot {
320 sv::UndoLog::NewElem(index) => {
321 // if any new variables were created during the
322 // snapshot, remember the lower index (which will
323 // always be the first one we see). Note that this
324 // action must precede those variables being
326 new_elem_threshold = cmp::min(new_elem_threshold, index as u32);
327 debug!("NewElem({}) new_elem_threshold={}", index, new_elem_threshold);
330 sv::UndoLog::Other(Instantiate { vid, .. }) => {
331 if vid.index < new_elem_threshold {
332 // quick check to see if this variable was
333 // created since the snapshot started or not.
334 let escaping_type = match self.eq_relations.probe_value(vid) {
335 TypeVariableValue::Unknown { .. } => bug!(),
336 TypeVariableValue::Known { value } => value,
338 escaping_types.push(escaping_type);
340 debug!("SpecifyVar({:?}) new_elem_threshold={}", vid, new_elem_threshold);
350 /// Returns indices of all variables that are not yet
352 pub fn unsolved_variables(&mut self) -> Vec<ty::TyVid> {
353 (0..self.values.len())
355 let vid = ty::TyVid { index: i as u32 };
356 match self.probe(vid) {
357 TypeVariableValue::Unknown { .. } => Some(vid),
358 TypeVariableValue::Known { .. } => None,
365 impl sv::SnapshotVecDelegate for Delegate {
366 type Value = TypeVariableData;
367 type Undo = Instantiate;
369 fn reverse(_values: &mut Vec<TypeVariableData>, _action: Instantiate) {
370 // We don't actually have to *do* anything to reverse an
371 // instantiation; the value for a variable is stored in the
372 // `eq_relations` and hence its rollback code will handle
373 // it. In fact, we could *almost* just remove the
374 // `SnapshotVec` entirely, except that we would have to
375 // reproduce *some* of its logic, since we want to know which
376 // type variables have been instantiated since the snapshot
377 // was started, so we can implement `types_escaping_snapshot`.
379 // (If we extended the `UnificationTable` to let us see which
380 // values have been unified and so forth, that might also
385 ///////////////////////////////////////////////////////////////////////////
387 /// These structs (a newtyped TyVid) are used as the unification key
388 /// for the `eq_relations`; they carry a `TypeVariableValue` along
390 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
391 struct TyVidEqKey<'tcx> {
394 // in the table, we map each ty-vid to one of these:
395 phantom: PhantomData<TypeVariableValue<'tcx>>,
398 impl<'tcx> From<ty::TyVid> for TyVidEqKey<'tcx> {
399 fn from(vid: ty::TyVid) -> Self {
400 TyVidEqKey { vid, phantom: PhantomData }
404 impl<'tcx> ut::UnifyKey for TyVidEqKey<'tcx> {
405 type Value = TypeVariableValue<'tcx>;
406 fn index(&self) -> u32 { self.vid.index }
407 fn from_index(i: u32) -> Self { TyVidEqKey::from(ty::TyVid { index: i }) }
408 fn tag() -> &'static str { "TyVidEqKey" }
411 impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {
412 type Error = ut::NoError;
414 fn unify_values(value1: &Self, value2: &Self) -> Result<Self, ut::NoError> {
415 match (value1, value2) {
416 // We never equate two type variables, both of which
417 // have known types. Instead, we recursively equate
419 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Known { .. }) => {
420 bug!("equating two type variables, both of which have known types")
423 // If one side is known, prefer that one.
424 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Unknown { .. }) => Ok(*value1),
425 (&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),
427 // If both sides are *unknown*, it hardly matters, does it?
428 (&TypeVariableValue::Unknown { universe: universe1 },
429 &TypeVariableValue::Unknown { universe: universe2 }) => {
430 // If we unify two unbound variables, ?T and ?U, then whatever
431 // value they wind up taking (which must be the same value) must
432 // be nameable by both universes. Therefore, the resulting
433 // universe is the minimum of the two universes, because that is
434 // the one which contains the fewest names in scope.
435 let universe = cmp::min(universe1, universe2);
436 Ok(TypeVariableValue::Unknown { universe })
442 /// Raw `TyVid` are used as the unification key for `sub_relations`;
443 /// they carry no values.
444 impl ut::UnifyKey for ty::TyVid {
446 fn index(&self) -> u32 { self.index }
447 fn from_index(i: u32) -> ty::TyVid { ty::TyVid { index: i } }
448 fn tag() -> &'static str { "TyVid" }