1 // Copyright 2014 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 use syntax::symbol::InternedString;
16 use std::marker::PhantomData;
18 use rustc_data_structures::fx::FxHashMap;
19 use rustc_data_structures::snapshot_vec as sv;
20 use rustc_data_structures::unify as ut;
22 pub struct TypeVariableTable<'tcx> {
23 values: sv::SnapshotVec<Delegate>,
25 /// Two variables are unified in `eq_relations` when we have a
26 /// constraint `?X == ?Y`. This table also stores, for each key,
28 eq_relations: ut::UnificationTable<ut::InPlace<TyVidEqKey<'tcx>>>,
30 /// Two variables are unified in `eq_relations` when we have a
31 /// constraint `?X <: ?Y` *or* a constraint `?Y <: ?X`. This second
32 /// table exists only to help with the occurs check. In particular,
33 /// we want to report constraints like these as an occurs check
39 /// This works because `?1` and `?3` are unified in the
40 /// `sub_relations` relation (not in `eq_relations`). Then when we
41 /// process the `Box<?3> <: ?1` constraint, we do an occurs check
42 /// on `Box<?3>` and find a potential cycle.
44 /// This is reasonable because, in Rust, subtypes have the same
45 /// "skeleton" and hence there is no possible type such that
46 /// (e.g.) `Box<?3> <: ?3` for any `?3`.
47 sub_relations: ut::UnificationTable<ut::InPlace<ty::TyVid>>,
50 /// Reasons to create a type inference variable
51 #[derive(Copy, Clone, Debug)]
52 pub enum TypeVariableOrigin {
54 NormalizeProjectionType(Span),
56 TypeParameterDefinition(Span, InternedString),
58 /// one of the upvars or closure kind parameters in a `ClosureSubsts`
59 /// (before it has been determined)
60 ClosureSynthetic(Span),
61 SubstitutionPlaceholder(Span),
65 DivergingBlockExpr(Span),
67 LatticeVariable(Span),
68 Generalized(ty::TyVid),
71 pub type TypeVariableMap = FxHashMap<ty::TyVid, TypeVariableOrigin>;
73 struct TypeVariableData {
74 origin: TypeVariableOrigin,
78 #[derive(Copy, Clone, Debug)]
79 pub enum TypeVariableValue<'tcx> {
80 Known { value: Ty<'tcx> },
81 Unknown { universe: ty::UniverseIndex },
84 impl<'tcx> TypeVariableValue<'tcx> {
85 /// If this value is known, returns the type it is known to be.
86 /// Otherwise, `None`.
87 pub fn known(&self) -> Option<Ty<'tcx>> {
89 TypeVariableValue::Unknown { .. } => None,
90 TypeVariableValue::Known { value } => Some(value),
94 pub fn is_unknown(&self) -> bool {
96 TypeVariableValue::Unknown { .. } => true,
97 TypeVariableValue::Known { .. } => false,
102 pub struct Snapshot<'tcx> {
103 snapshot: sv::Snapshot,
104 eq_snapshot: ut::Snapshot<ut::InPlace<TyVidEqKey<'tcx>>>,
105 sub_snapshot: ut::Snapshot<ut::InPlace<ty::TyVid>>,
114 impl<'tcx> TypeVariableTable<'tcx> {
115 pub fn new() -> TypeVariableTable<'tcx> {
117 values: sv::SnapshotVec::new(),
118 eq_relations: ut::UnificationTable::new(),
119 sub_relations: ut::UnificationTable::new(),
123 /// Returns the diverges flag given when `vid` was created.
125 /// Note that this function does not return care whether
126 /// `vid` has been unified with something else or not.
127 pub fn var_diverges<'a>(&'a self, vid: ty::TyVid) -> bool {
128 self.values.get(vid.index as usize).diverging
131 /// Returns the origin that was given when `vid` was created.
133 /// Note that this function does not return care whether
134 /// `vid` has been unified with something else or not.
135 pub fn var_origin(&self, vid: ty::TyVid) -> &TypeVariableOrigin {
136 &self.values.get(vid.index as usize).origin
139 /// Records that `a == b`, depending on `dir`.
141 /// Precondition: neither `a` nor `b` are known.
142 pub fn equate(&mut self, a: ty::TyVid, b: ty::TyVid) {
143 debug_assert!(self.probe(a).is_unknown());
144 debug_assert!(self.probe(b).is_unknown());
145 self.eq_relations.union(a, b);
146 self.sub_relations.union(a, b);
149 /// Records that `a <: b`, depending on `dir`.
151 /// Precondition: neither `a` nor `b` are known.
152 pub fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
153 debug_assert!(self.probe(a).is_unknown());
154 debug_assert!(self.probe(b).is_unknown());
155 self.sub_relations.union(a, b);
158 /// Instantiates `vid` with the type `ty`.
160 /// Precondition: `vid` must not have been previously instantiated.
161 pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
162 let vid = self.root_var(vid);
163 debug_assert!(self.probe(vid).is_unknown());
164 debug_assert!(self.eq_relations.probe_value(vid).is_unknown(),
165 "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
166 vid, ty, self.eq_relations.probe_value(vid));
167 self.eq_relations.union_value(vid, TypeVariableValue::Known { value: ty });
169 // Hack: we only need this so that `types_escaping_snapshot`
170 // can see what has been unified; see the Delegate impl for
172 self.values.record(Instantiate { vid });
175 /// Creates a new type variable.
177 /// - `diverging`: indicates if this is a "diverging" type
178 /// variable, e.g., one created as the type of a `return`
179 /// expression. The code in this module doesn't care if a
180 /// variable is diverging, but the main Rust type-checker will
181 /// sometimes "unify" such variables with the `!` or `()` types.
182 /// - `origin`: indicates *why* the type variable was created.
183 /// The code in this module doesn't care, but it can be useful
184 /// for improving error messages.
185 pub fn new_var(&mut self,
186 universe: ty::UniverseIndex,
188 origin: TypeVariableOrigin)
190 let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown { universe });
192 let sub_key = self.sub_relations.new_key(());
193 assert_eq!(eq_key.vid, sub_key);
195 let index = self.values.push(TypeVariableData {
199 assert_eq!(eq_key.vid.index, index as u32);
201 debug!("new_var(index={:?}, diverging={:?}, origin={:?}", eq_key.vid, diverging, origin);
206 /// Returns the number of type variables created thus far.
207 pub fn num_vars(&self) -> usize {
211 /// Returns the "root" variable of `vid` in the `eq_relations`
212 /// equivalence table. All type variables that have been equated
213 /// will yield the same root variable (per the union-find
214 /// algorithm), so `root_var(a) == root_var(b)` implies that `a ==
215 /// b` (transitively).
216 pub fn root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
217 self.eq_relations.find(vid).vid
220 /// Returns the "root" variable of `vid` in the `sub_relations`
221 /// equivalence table. All type variables that have been are
222 /// related via equality or subtyping will yield the same root
223 /// variable (per the union-find algorithm), so `sub_root_var(a)
224 /// == sub_root_var(b)` implies that:
226 /// exists X. (a <: X || X <: a) && (b <: X || X <: b)
227 pub fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
228 self.sub_relations.find(vid)
231 /// True if `a` and `b` have same "sub-root" (i.e., exists some
232 /// type X such that `forall i in {a, b}. (i <: X || X <: i)`.
233 pub fn sub_unified(&mut self, a: ty::TyVid, b: ty::TyVid) -> bool {
234 self.sub_root_var(a) == self.sub_root_var(b)
237 /// Retrieves the type to which `vid` has been instantiated, if
239 pub fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
240 self.eq_relations.probe_value(vid)
243 /// If `t` is a type-inference variable, and it has been
244 /// instantiated, then return the with which it was
245 /// instantiated. Otherwise, returns `t`.
246 pub fn replace_if_possible(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
248 ty::Infer(ty::TyVar(v)) => {
249 match self.probe(v) {
250 TypeVariableValue::Unknown { .. } => t,
251 TypeVariableValue::Known { value } => value,
258 /// Creates a snapshot of the type variable state. This snapshot
259 /// must later be committed (`commit()`) or rolled back
260 /// (`rollback_to()`). Nested snapshots are permitted, but must
261 /// be processed in a stack-like fashion.
262 pub fn snapshot(&mut self) -> Snapshot<'tcx> {
264 snapshot: self.values.start_snapshot(),
265 eq_snapshot: self.eq_relations.snapshot(),
266 sub_snapshot: self.sub_relations.snapshot(),
270 /// Undoes all changes since the snapshot was created. Any
271 /// snapshots created since that point must already have been
272 /// committed or rolled back.
273 pub fn rollback_to(&mut self, s: Snapshot<'tcx>) {
274 debug!("rollback_to{:?}", {
275 for action in self.values.actions_since_snapshot(&s.snapshot) {
276 if let sv::UndoLog::NewElem(index) = *action {
277 debug!("inference variable _#{}t popped", index)
282 let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
283 self.values.rollback_to(snapshot);
284 self.eq_relations.rollback_to(eq_snapshot);
285 self.sub_relations.rollback_to(sub_snapshot);
288 /// Commits all changes since the snapshot was created, making
289 /// them permanent (unless this snapshot was created within
290 /// another snapshot). Any snapshots created since that point
291 /// must already have been committed or rolled back.
292 pub fn commit(&mut self, s: Snapshot<'tcx>) {
293 let Snapshot { snapshot, eq_snapshot, sub_snapshot } = s;
294 self.values.commit(snapshot);
295 self.eq_relations.commit(eq_snapshot);
296 self.sub_relations.commit(sub_snapshot);
299 /// Returns a map `{V1 -> V2}`, where the keys `{V1}` are
300 /// ty-variables created during the snapshot, and the values
301 /// `{V2}` are the root variables that they were unified with,
302 /// along with their origin.
303 pub fn types_created_since_snapshot(&mut self, s: &Snapshot<'tcx>) -> TypeVariableMap {
304 let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot);
306 actions_since_snapshot
308 .filter_map(|action| match action {
309 &sv::UndoLog::NewElem(index) => Some(ty::TyVid { index: index as u32 }),
313 let origin = self.values.get(vid.index as usize).origin.clone();
319 /// Find the set of type variables that existed *before* `s`
320 /// but which have only been unified since `s` started, and
321 /// return the types with which they were unified. So if we had
322 /// a type variable `V0`, then we started the snapshot, then we
323 /// created a type variable `V1`, unified `V0` with `T0`, and
324 /// unified `V1` with `T1`, this function would return `{T0}`.
325 pub fn types_escaping_snapshot(&mut self, s: &Snapshot<'tcx>) -> Vec<Ty<'tcx>> {
326 let mut new_elem_threshold = u32::MAX;
327 let mut escaping_types = Vec::new();
328 let actions_since_snapshot = self.values.actions_since_snapshot(&s.snapshot);
329 debug!("actions_since_snapshot.len() = {}", actions_since_snapshot.len());
330 for action in actions_since_snapshot {
332 sv::UndoLog::NewElem(index) => {
333 // if any new variables were created during the
334 // snapshot, remember the lower index (which will
335 // always be the first one we see). Note that this
336 // action must precede those variables being
338 new_elem_threshold = cmp::min(new_elem_threshold, index as u32);
339 debug!("NewElem({}) new_elem_threshold={}", index, new_elem_threshold);
342 sv::UndoLog::Other(Instantiate { vid, .. }) => {
343 if vid.index < new_elem_threshold {
344 // quick check to see if this variable was
345 // created since the snapshot started or not.
346 let escaping_type = match self.eq_relations.probe_value(vid) {
347 TypeVariableValue::Unknown { .. } => bug!(),
348 TypeVariableValue::Known { value } => value,
350 escaping_types.push(escaping_type);
352 debug!("SpecifyVar({:?}) new_elem_threshold={}", vid, new_elem_threshold);
362 /// Returns indices of all variables that are not yet
364 pub fn unsolved_variables(&mut self) -> Vec<ty::TyVid> {
365 (0..self.values.len())
367 let vid = ty::TyVid { index: i as u32 };
368 match self.probe(vid) {
369 TypeVariableValue::Unknown { .. } => Some(vid),
370 TypeVariableValue::Known { .. } => None,
377 impl sv::SnapshotVecDelegate for Delegate {
378 type Value = TypeVariableData;
379 type Undo = Instantiate;
381 fn reverse(_values: &mut Vec<TypeVariableData>, _action: Instantiate) {
382 // We don't actually have to *do* anything to reverse an
383 // instanation; the value for a variable is stored in the
384 // `eq_relations` and hence its rollback code will handle
385 // it. In fact, we could *almost* just remove the
386 // `SnapshotVec` entirely, except that we would have to
387 // reproduce *some* of its logic, since we want to know which
388 // type variables have been instantiated since the snapshot
389 // was started, so we can implement `types_escaping_snapshot`.
391 // (If we extended the `UnificationTable` to let us see which
392 // values have been unified and so forth, that might also
397 ///////////////////////////////////////////////////////////////////////////
399 /// These structs (a newtyped TyVid) are used as the unification key
400 /// for the `eq_relations`; they carry a `TypeVariableValue` along
402 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
403 struct TyVidEqKey<'tcx> {
406 // in the table, we map each ty-vid to one of these:
407 phantom: PhantomData<TypeVariableValue<'tcx>>,
410 impl<'tcx> From<ty::TyVid> for TyVidEqKey<'tcx> {
411 fn from(vid: ty::TyVid) -> Self {
412 TyVidEqKey { vid, phantom: PhantomData }
416 impl<'tcx> ut::UnifyKey for TyVidEqKey<'tcx> {
417 type Value = TypeVariableValue<'tcx>;
418 fn index(&self) -> u32 { self.vid.index }
419 fn from_index(i: u32) -> Self { TyVidEqKey::from(ty::TyVid { index: i }) }
420 fn tag() -> &'static str { "TyVidEqKey" }
423 impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {
424 type Error = ut::NoError;
426 fn unify_values(value1: &Self, value2: &Self) -> Result<Self, ut::NoError> {
427 match (value1, value2) {
428 // We never equate two type variables, both of which
429 // have known types. Instead, we recursively equate
431 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Known { .. }) => {
432 bug!("equating two type variables, both of which have known types")
435 // If one side is known, prefer that one.
436 (&TypeVariableValue::Known { .. }, &TypeVariableValue::Unknown { .. }) => Ok(*value1),
437 (&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),
439 // If both sides are *unknown*, it hardly matters, does it?
440 (&TypeVariableValue::Unknown { universe: universe1 },
441 &TypeVariableValue::Unknown { universe: universe2 }) => {
442 // If we unify two unbound variables, ?T and ?U, then whatever
443 // value they wind up taking (which must be the same value) must
444 // be nameable by both universes. Therefore, the resulting
445 // universe is the minimum of the two universes, because that is
446 // the one which contains the fewest names in scope.
447 let universe = cmp::min(universe1, universe2);
448 Ok(TypeVariableValue::Unknown { universe })
454 /// Raw `TyVid` are used as the unification key for `sub_relations`;
455 /// they carry no values.
456 impl ut::UnifyKey for ty::TyVid {
458 fn index(&self) -> u32 { self.index }
459 fn from_index(i: u32) -> ty::TyVid { ty::TyVid { index: i } }
460 fn tag() -> &'static str { "TyVid" }