1 ///////////////////////////////////////////////////////////////////////////
4 // There are four type combiners: equate, sub, lub, and glb. Each
5 // implements the trait `Combine` and contains methods for combining
6 // two instances of various things and yielding a new instance. These
7 // combiner methods always yield a `Result<T>`. There is a lot of
8 // common code for these operations, implemented as default methods on
9 // the `Combine` trait.
11 // Each operation may have side-effects on the inference context,
12 // though these can be unrolled using snapshots. On success, the
13 // LUB/GLB operations return the appropriate bound. The Eq and Sub
14 // operations generally return the first operand.
18 // When you are relating two things which have a contravariant
19 // relationship, you should use `contratys()` or `contraregions()`,
20 // rather than inversing the order of arguments! This is necessary
21 // because the order of arguments is not relevant for LUB and GLB. It
22 // is also useful to track which value is the "expected" value in
23 // terms of error reporting.
25 use super::equate::Equate;
27 use super::{InferCtxt, MiscVariable, TypeTrace};
30 use super::type_variable::TypeVariableValue;
32 use crate::hir::def_id::DefId;
33 use crate::ty::{IntType, UintType};
34 use crate::ty::{self, Ty, TyCtxt};
35 use crate::ty::error::TypeError;
36 use crate::ty::relate::{self, Relate, RelateResult, TypeRelation};
37 use crate::ty::subst::SubstsRef;
38 use crate::traits::{Obligation, PredicateObligations};
44 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
45 pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
46 pub trace: TypeTrace<'tcx>,
47 pub cause: Option<ty::relate::Cause>,
48 pub param_env: ty::ParamEnv<'tcx>,
49 pub obligations: PredicateObligations<'tcx>,
52 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
53 pub enum RelationDir {
54 SubtypeOf, SupertypeOf, EqTo
57 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
58 pub fn super_combine_tys<R>(&self,
62 -> RelateResult<'tcx, Ty<'tcx>>
63 where R: TypeRelation<'infcx, 'gcx, 'tcx>
65 let a_is_expected = relation.a_is_expected();
67 match (&a.sty, &b.sty) {
68 // Relate integral variables to other types
69 (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
70 self.int_unification_table
72 .unify_var_var(a_id, b_id)
73 .map_err(|e| int_unification_error(a_is_expected, e))?;
76 (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
77 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
79 (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
80 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
82 (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
83 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
85 (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
86 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
89 // Relate floating-point variables to other types
90 (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
91 self.float_unification_table
93 .unify_var_var(a_id, b_id)
94 .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
97 (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
98 self.unify_float_variable(a_is_expected, v_id, v)
100 (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
101 self.unify_float_variable(!a_is_expected, v_id, v)
104 // All other cases of inference are errors
106 (_, &ty::Infer(_)) => {
107 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
112 ty::relate::super_relate_tys(relation, a, b)
117 fn unify_integral_variable(&self,
118 vid_is_expected: bool,
120 val: ty::IntVarValue)
121 -> RelateResult<'tcx, Ty<'tcx>>
123 self.int_unification_table
125 .unify_var_value(vid, Some(val))
126 .map_err(|e| int_unification_error(vid_is_expected, e))?;
128 IntType(v) => Ok(self.tcx.mk_mach_int(v)),
129 UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
133 fn unify_float_variable(&self,
134 vid_is_expected: bool,
137 -> RelateResult<'tcx, Ty<'tcx>>
139 self.float_unification_table
141 .unify_var_value(vid, Some(ty::FloatVarValue(val)))
142 .map_err(|e| float_unification_error(vid_is_expected, e))?;
143 Ok(self.tcx.mk_mach_float(val))
147 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
148 pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
152 pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
153 Equate::new(self, a_is_expected)
156 pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
157 Sub::new(self, a_is_expected)
160 pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
161 Lub::new(self, a_is_expected)
164 pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
165 Glb::new(self, a_is_expected)
168 /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
169 /// The idea is that we should ensure that the type `a_ty` is equal
170 /// to, a subtype of, or a supertype of (respectively) the type
171 /// to which `b_vid` is bound.
173 /// Since `b_vid` has not yet been instantiated with a type, we
174 /// will first instantiate `b_vid` with a *generalized* version
175 /// of `a_ty`. Generalization introduces other inference
176 /// variables wherever subtyping could occur.
177 pub fn instantiate(&mut self,
182 -> RelateResult<'tcx, ()>
184 use self::RelationDir::*;
186 // Get the actual variable that b_vid has been inferred to
187 debug_assert!(self.infcx.type_variables.borrow_mut().probe(b_vid).is_unknown());
189 debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
191 // Generalize type of `a_ty` appropriately depending on the
192 // direction. As an example, assume:
194 // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
195 // inference variable,
196 // - and `dir` == `SubtypeOf`.
198 // Then the generalized form `b_ty` would be `&'?2 ?3`, where
199 // `'?2` and `?3` are fresh region/type inference
200 // variables. (Down below, we will relate `a_ty <: b_ty`,
201 // adding constraints like `'x: '?2` and `?1 <: ?3`.)
202 let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
203 debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
204 a_ty, dir, b_vid, b_ty);
205 self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
208 self.obligations.push(Obligation::new(self.trace.cause.clone(),
210 ty::Predicate::WellFormed(b_ty)));
213 // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
215 // FIXME(#16847): This code is non-ideal because all these subtype
216 // relations wind up attributed to the same spans. We need
217 // to associate causes/spans with each of the relations in
218 // the stack to get this right.
220 EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
221 SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
222 SupertypeOf => self.sub(a_is_expected).relate_with_variance(
223 ty::Contravariant, &a_ty, &b_ty),
229 /// Attempts to generalize `ty` for the type variable `for_vid`.
230 /// This checks for cycle -- that is, whether the type `ty`
231 /// references `for_vid`. The `dir` is the "direction" for which we
232 /// a performing the generalization (i.e., are we producing a type
233 /// that can be used as a supertype etc).
237 /// - `for_vid` is a "root vid"
242 -> RelateResult<'tcx, Generalization<'tcx>>
244 debug!("generalize(ty={:?}, for_vid={:?}, dir={:?}", ty, for_vid, dir);
245 // Determine the ambient variance within which `ty` appears.
246 // The surrounding equation is:
250 // where `op` is either `==`, `<:`, or `:>`. This maps quite
252 let ambient_variance = match dir {
253 RelationDir::EqTo => ty::Invariant,
254 RelationDir::SubtypeOf => ty::Covariant,
255 RelationDir::SupertypeOf => ty::Contravariant,
258 debug!("generalize: ambient_variance = {:?}", ambient_variance);
260 let for_universe = match self.infcx.type_variables.borrow_mut().probe(for_vid) {
261 v @ TypeVariableValue::Known { .. } => panic!(
262 "instantiating {:?} which has a known value {:?}",
266 TypeVariableValue::Unknown { universe } => universe,
269 debug!("generalize: for_universe = {:?}", for_universe);
271 let mut generalize = Generalizer {
273 span: self.trace.cause.span,
274 for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
281 let ty = match generalize.relate(&ty, &ty) {
284 debug!("generalize: failure {:?}", e);
288 let needs_wf = generalize.needs_wf;
289 debug!("generalize: success {{ {:?}, {:?} }}", ty, needs_wf);
290 Ok(Generalization { ty, needs_wf })
294 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
295 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
297 /// The span, used when creating new type variables and things.
300 /// The vid of the type variable that is in the process of being
301 /// instantiated; if we find this within the type we are folding,
302 /// that means we would have created a cyclic type.
303 for_vid_sub_root: ty::TyVid,
305 /// The universe of the type variable that is in the process of
306 /// being instantiated. Any fresh variables that we create in this
307 /// process should be in that same universe.
308 for_universe: ty::UniverseIndex,
310 /// Track the variance as we descend into the type.
311 ambient_variance: ty::Variance,
313 /// See the field `needs_wf` in `Generalization`.
316 /// The root type that we are generalizing. Used when reporting cycles.
320 /// Result from a generalization operation. This includes
321 /// not only the generalized type, but also a bool flag
322 /// indicating whether further WF checks are needed.
323 struct Generalization<'tcx> {
326 /// If true, then the generalized type may not be well-formed,
327 /// even if the source type is well-formed, so we should add an
328 /// additional check to enforce that it is. This arises in
329 /// particular around 'bivariant' type parameters that are only
330 /// constrained by a where-clause. As an example, imagine a type:
332 /// struct Foo<A, B> where A: Iterator<Item = B> {
336 /// here, `A` will be covariant, but `B` is
337 /// unconstrained. However, whatever it is, for `Foo` to be WF, it
338 /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
339 /// then after generalization we will wind up with a type like
340 /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
341 /// ?D>` (or `>:`), we will wind up with the requirement that `?A
342 /// <: ?C`, but no particular relationship between `?B` and `?D`
343 /// (after all, we do not know the variance of the normalized form
344 /// of `A::Item` with respect to `A`). If we do nothing else, this
345 /// may mean that `?D` goes unconstrained (as in #41677). So, in
346 /// this scenario where we create a new type variable in a
347 /// bivariant context, we set the `needs_wf` flag to true. This
348 /// will force the calling code to check that `WF(Foo<?C, ?D>)`
349 /// holds, which in turn implies that `?C::Item == ?D`. So once
350 /// `?C` is constrained, that should suffice to restrict `?D`.
354 impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
355 fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
359 fn tag(&self) -> &'static str {
363 fn a_is_expected(&self) -> bool {
367 fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
368 -> RelateResult<'tcx, ty::Binder<T>>
369 where T: Relate<'tcx>
371 Ok(ty::Binder::bind(self.relate(a.skip_binder(), b.skip_binder())?))
374 fn relate_item_substs(&mut self,
376 a_subst: SubstsRef<'tcx>,
377 b_subst: SubstsRef<'tcx>)
378 -> RelateResult<'tcx, SubstsRef<'tcx>>
380 if self.ambient_variance == ty::Variance::Invariant {
381 // Avoid fetching the variance if we are in an invariant
382 // context; no need, and it can induce dependency cycles
384 relate::relate_substs(self, None, a_subst, b_subst)
386 let opt_variances = self.tcx().variances_of(item_def_id);
387 relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
391 fn relate_with_variance<T: Relate<'tcx>>(&mut self,
392 variance: ty::Variance,
395 -> RelateResult<'tcx, T>
397 let old_ambient_variance = self.ambient_variance;
398 self.ambient_variance = self.ambient_variance.xform(variance);
400 let result = self.relate(a, b);
401 self.ambient_variance = old_ambient_variance;
405 fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
406 assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
408 debug!("generalize: t={:?}", t);
410 // Check to see whether the type we are genealizing references
411 // any other type variable related to `vid` via
412 // subtyping. This is basically our "occurs check", preventing
413 // us from creating infinitely sized types.
415 ty::Infer(ty::TyVar(vid)) => {
416 let mut variables = self.infcx.type_variables.borrow_mut();
417 let vid = variables.root_var(vid);
418 let sub_vid = variables.sub_root_var(vid);
419 if sub_vid == self.for_vid_sub_root {
420 // If sub-roots are equal, then `for_vid` and
421 // `vid` are related via subtyping.
422 return Err(TypeError::CyclicTy(self.root_ty));
424 match variables.probe(vid) {
425 TypeVariableValue::Known { value: u } => {
427 debug!("generalize: known value {:?}", u);
430 TypeVariableValue::Unknown { universe } => {
431 match self.ambient_variance {
432 // Invariant: no need to make a fresh type variable.
434 if self.for_universe.can_name(universe) {
439 // Bivariant: make a fresh var, but we
440 // may need a WF predicate. See
441 // comment on `needs_wf` field for
443 ty::Bivariant => self.needs_wf = true,
445 // Co/contravariant: this will be
446 // sufficiently constrained later on.
447 ty::Covariant | ty::Contravariant => (),
450 let origin = *variables.var_origin(vid);
451 let new_var_id = variables.new_var(self.for_universe, false, origin);
452 let u = self.tcx().mk_ty_var(new_var_id);
453 debug!("generalize: replacing original vid={:?} with new={:?}",
460 ty::Infer(ty::IntVar(_)) |
461 ty::Infer(ty::FloatVar(_)) => {
462 // No matter what mode we are in,
463 // integer/floating-point types must be equal to be
468 relate::super_relate_tys(self, t, t)
473 fn regions(&mut self, r: ty::Region<'tcx>, r2: ty::Region<'tcx>)
474 -> RelateResult<'tcx, ty::Region<'tcx>> {
475 assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
477 debug!("generalize: regions r={:?}", r);
480 // Never make variables for regions bound within the type itself,
481 // nor for erased regions.
482 ty::ReLateBound(..) |
487 ty::ReClosureBound(..) => {
490 "encountered unexpected ReClosureBound: {:?}",
495 ty::RePlaceholder(..) |
500 ty::ReEarlyBound(..) |
502 // see common code below
506 // If we are in an invariant context, we can re-use the region
507 // as is, unless it happens to be in some universe that we
508 // can't name. (In the case of a region *variable*, we could
509 // use it if we promoted it into our universe, but we don't
511 if let ty::Invariant = self.ambient_variance {
512 let r_universe = self.infcx.universe_of_region(r);
513 if self.for_universe.can_name(r_universe) {
518 // FIXME: This is non-ideal because we don't give a
519 // very descriptive origin for this region variable.
520 Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
524 pub trait RelateResultCompare<'tcx, T> {
525 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
526 F: FnOnce() -> TypeError<'tcx>;
529 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
530 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
531 F: FnOnce() -> TypeError<'tcx>,
533 self.clone().and_then(|s| {
543 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
547 TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
550 fn float_unification_error<'tcx>(a_is_expected: bool,
551 v: (ty::FloatVarValue, ty::FloatVarValue))
554 let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
555 TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))