1 // Copyright 2012 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 ///////////////////////////////////////////////////////////////////////////
14 // There are four type combiners: equate, sub, lub, and glb. Each
15 // implements the trait `Combine` and contains methods for combining
16 // two instances of various things and yielding a new instance. These
17 // combiner methods always yield a `Result<T>`. There is a lot of
18 // common code for these operations, implemented as default methods on
19 // the `Combine` trait.
21 // Each operation may have side-effects on the inference context,
22 // though these can be unrolled using snapshots. On success, the
23 // LUB/GLB operations return the appropriate bound. The Eq and Sub
24 // operations generally return the first operand.
28 // When you are relating two things which have a contravariant
29 // relationship, you should use `contratys()` or `contraregions()`,
30 // rather than inversing the order of arguments! This is necessary
31 // because the order of arguments is not relevant for LUB and GLB. It
32 // is also useful to track which value is the "expected" value in
33 // terms of error reporting.
35 use super::equate::Equate;
37 use super::{InferCtxt, MiscVariable, TypeTrace};
40 use super::type_variable::TypeVariableValue;
42 use hir::def_id::DefId;
43 use ty::{IntType, UintType};
44 use ty::{self, Ty, TyCtxt};
45 use ty::error::TypeError;
46 use ty::relate::{self, Relate, RelateResult, TypeRelation};
47 use ty::subst::Substs;
48 use traits::{Obligation, PredicateObligations};
54 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
55 pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
56 pub trace: TypeTrace<'tcx>,
57 pub cause: Option<ty::relate::Cause>,
58 pub param_env: ty::ParamEnv<'tcx>,
59 pub obligations: PredicateObligations<'tcx>,
62 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
63 pub enum RelationDir {
64 SubtypeOf, SupertypeOf, EqTo
67 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
68 pub fn super_combine_tys<R>(&self,
72 -> RelateResult<'tcx, Ty<'tcx>>
73 where R: TypeRelation<'infcx, 'gcx, 'tcx>
75 let a_is_expected = relation.a_is_expected();
77 match (&a.sty, &b.sty) {
78 // Relate integral variables to other types
79 (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
80 self.int_unification_table
82 .unify_var_var(a_id, b_id)
83 .map_err(|e| int_unification_error(a_is_expected, e))?;
86 (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
87 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
89 (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
90 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
92 (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
93 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
95 (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
96 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
99 // Relate floating-point variables to other types
100 (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
101 self.float_unification_table
103 .unify_var_var(a_id, b_id)
104 .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
107 (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
108 self.unify_float_variable(a_is_expected, v_id, v)
110 (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
111 self.unify_float_variable(!a_is_expected, v_id, v)
114 // All other cases of inference are errors
116 (_, &ty::Infer(_)) => {
117 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
122 ty::relate::super_relate_tys(relation, a, b)
127 fn unify_integral_variable(&self,
128 vid_is_expected: bool,
130 val: ty::IntVarValue)
131 -> RelateResult<'tcx, Ty<'tcx>>
133 self.int_unification_table
135 .unify_var_value(vid, Some(val))
136 .map_err(|e| int_unification_error(vid_is_expected, e))?;
138 IntType(v) => Ok(self.tcx.mk_mach_int(v)),
139 UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
143 fn unify_float_variable(&self,
144 vid_is_expected: bool,
147 -> RelateResult<'tcx, Ty<'tcx>>
149 self.float_unification_table
151 .unify_var_value(vid, Some(ty::FloatVarValue(val)))
152 .map_err(|e| float_unification_error(vid_is_expected, e))?;
153 Ok(self.tcx.mk_mach_float(val))
157 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
158 pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
162 pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
163 Equate::new(self, a_is_expected)
166 pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
167 Sub::new(self, a_is_expected)
170 pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
171 Lub::new(self, a_is_expected)
174 pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
175 Glb::new(self, a_is_expected)
178 /// Here dir is either EqTo, SubtypeOf, or SupertypeOf. The
179 /// idea is that we should ensure that the type `a_ty` is equal
180 /// to, a subtype of, or a supertype of (respectively) the type
181 /// to which `b_vid` is bound.
183 /// Since `b_vid` has not yet been instantiated with a type, we
184 /// will first instantiate `b_vid` with a *generalized* version
185 /// of `a_ty`. Generalization introduces other inference
186 /// variables wherever subtyping could occur.
187 pub fn instantiate(&mut self,
192 -> RelateResult<'tcx, ()>
194 use self::RelationDir::*;
196 // Get the actual variable that b_vid has been inferred to
197 debug_assert!(self.infcx.type_variables.borrow_mut().probe(b_vid).is_unknown());
199 debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
201 // Generalize type of `a_ty` appropriately depending on the
202 // direction. As an example, assume:
204 // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
205 // inference variable,
206 // - and `dir` == `SubtypeOf`.
208 // Then the generalized form `b_ty` would be `&'?2 ?3`, where
209 // `'?2` and `?3` are fresh region/type inference
210 // variables. (Down below, we will relate `a_ty <: b_ty`,
211 // adding constraints like `'x: '?2` and `?1 <: ?3`.)
212 let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
213 debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
214 a_ty, dir, b_vid, b_ty);
215 self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
218 self.obligations.push(Obligation::new(self.trace.cause.clone(),
220 ty::Predicate::WellFormed(b_ty)));
223 // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
225 // FIXME(#16847): This code is non-ideal because all these subtype
226 // relations wind up attributed to the same spans. We need
227 // to associate causes/spans with each of the relations in
228 // the stack to get this right.
230 EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
231 SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
232 SupertypeOf => self.sub(a_is_expected).relate_with_variance(
233 ty::Contravariant, &a_ty, &b_ty),
239 /// Attempts to generalize `ty` for the type variable `for_vid`.
240 /// This checks for cycle -- that is, whether the type `ty`
241 /// references `for_vid`. The `dir` is the "direction" for which we
242 /// a performing the generalization (i.e., are we producing a type
243 /// that can be used as a supertype etc).
247 /// - `for_vid` is a "root vid"
252 -> RelateResult<'tcx, Generalization<'tcx>>
254 // Determine the ambient variance within which `ty` appears.
255 // The surrounding equation is:
259 // where `op` is either `==`, `<:`, or `:>`. This maps quite
261 let ambient_variance = match dir {
262 RelationDir::EqTo => ty::Invariant,
263 RelationDir::SubtypeOf => ty::Covariant,
264 RelationDir::SupertypeOf => ty::Contravariant,
267 let mut generalize = Generalizer {
269 span: self.trace.cause.span,
270 for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
276 let ty = generalize.relate(&ty, &ty)?;
277 let needs_wf = generalize.needs_wf;
278 Ok(Generalization { ty, needs_wf })
282 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
283 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
285 /// Span, used when creating new type variables and things.
288 /// The vid of the type variable that is in the process of being
289 /// instantiated; if we find this within the type we are folding,
290 /// that means we would have created a cyclic type.
291 for_vid_sub_root: ty::TyVid,
293 /// Track the variance as we descend into the type.
294 ambient_variance: ty::Variance,
296 /// See the field `needs_wf` in `Generalization`.
299 /// The root type that we are generalizing. Used when reporting cycles.
303 /// Result from a generalization operation. This includes
304 /// not only the generalized type, but also a bool flag
305 /// indicating whether further WF checks are needed.
306 struct Generalization<'tcx> {
309 /// If true, then the generalized type may not be well-formed,
310 /// even if the source type is well-formed, so we should add an
311 /// additional check to enforce that it is. This arises in
312 /// particular around 'bivariant' type parameters that are only
313 /// constrained by a where-clause. As an example, imagine a type:
315 /// struct Foo<A, B> where A: Iterator<Item=B> {
319 /// here, `A` will be covariant, but `B` is
320 /// unconstrained. However, whatever it is, for `Foo` to be WF, it
321 /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
322 /// then after generalization we will wind up with a type like
323 /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
324 /// ?D>` (or `>:`), we will wind up with the requirement that `?A
325 /// <: ?C`, but no particular relationship between `?B` and `?D`
326 /// (after all, we do not know the variance of the normalized form
327 /// of `A::Item` with respect to `A`). If we do nothing else, this
328 /// may mean that `?D` goes unconstrained (as in #41677). So, in
329 /// this scenario where we create a new type variable in a
330 /// bivariant context, we set the `needs_wf` flag to true. This
331 /// will force the calling code to check that `WF(Foo<?C, ?D>)`
332 /// holds, which in turn implies that `?C::Item == ?D`. So once
333 /// `?C` is constrained, that should suffice to restrict `?D`.
337 impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
338 fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
342 fn tag(&self) -> &'static str {
346 fn a_is_expected(&self) -> bool {
350 fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
351 -> RelateResult<'tcx, ty::Binder<T>>
352 where T: Relate<'tcx>
354 Ok(ty::Binder::bind(self.relate(a.skip_binder(), b.skip_binder())?))
357 fn relate_item_substs(&mut self,
359 a_subst: &'tcx Substs<'tcx>,
360 b_subst: &'tcx Substs<'tcx>)
361 -> RelateResult<'tcx, &'tcx Substs<'tcx>>
363 if self.ambient_variance == ty::Variance::Invariant {
364 // Avoid fetching the variance if we are in an invariant
365 // context; no need, and it can induce dependency cycles
367 relate::relate_substs(self, None, a_subst, b_subst)
369 let opt_variances = self.tcx().variances_of(item_def_id);
370 relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
374 fn relate_with_variance<T: Relate<'tcx>>(&mut self,
375 variance: ty::Variance,
378 -> RelateResult<'tcx, T>
380 let old_ambient_variance = self.ambient_variance;
381 self.ambient_variance = self.ambient_variance.xform(variance);
383 let result = self.relate(a, b);
384 self.ambient_variance = old_ambient_variance;
388 fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
389 assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
391 // Check to see whether the type we are genealizing references
392 // any other type variable related to `vid` via
393 // subtyping. This is basically our "occurs check", preventing
394 // us from creating infinitely sized types.
396 ty::Infer(ty::TyVar(vid)) => {
397 let mut variables = self.infcx.type_variables.borrow_mut();
398 let vid = variables.root_var(vid);
399 let sub_vid = variables.sub_root_var(vid);
400 if sub_vid == self.for_vid_sub_root {
401 // If sub-roots are equal, then `for_vid` and
402 // `vid` are related via subtyping.
403 return Err(TypeError::CyclicTy(self.root_ty));
405 match variables.probe(vid) {
406 TypeVariableValue::Known { value: u } => {
410 TypeVariableValue::Unknown { universe } => {
411 match self.ambient_variance {
412 // Invariant: no need to make a fresh type variable.
413 ty::Invariant => return Ok(t),
415 // Bivariant: make a fresh var, but we
416 // may need a WF predicate. See
417 // comment on `needs_wf` field for
419 ty::Bivariant => self.needs_wf = true,
421 // Co/contravariant: this will be
422 // sufficiently constrained later on.
423 ty::Covariant | ty::Contravariant => (),
426 let origin = *variables.var_origin(vid);
427 let new_var_id = variables.new_var(universe, false, origin);
428 let u = self.tcx().mk_var(new_var_id);
429 debug!("generalize: replacing original vid={:?} with new={:?}",
436 ty::Infer(ty::IntVar(_)) |
437 ty::Infer(ty::FloatVar(_)) => {
438 // No matter what mode we are in,
439 // integer/floating-point types must be equal to be
444 relate::super_relate_tys(self, t, t)
449 fn regions(&mut self, r: ty::Region<'tcx>, r2: ty::Region<'tcx>)
450 -> RelateResult<'tcx, ty::Region<'tcx>> {
451 assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
454 // Never make variables for regions bound within the type itself,
455 // nor for erased regions.
456 ty::ReLateBound(..) |
461 // Always make a fresh region variable for placeholder
462 // regions; the higher-ranked decision procedures rely on
464 ty::RePlaceholder(..) => { }
466 // For anything else, we make a region variable, unless we
467 // are *equating*, in which case it's just wasteful.
472 ty::ReEarlyBound(..) |
474 match self.ambient_variance {
475 ty::Invariant => return Ok(r),
476 ty::Bivariant | ty::Covariant | ty::Contravariant => (),
480 ty::ReCanonical(..) |
481 ty::ReClosureBound(..) => {
484 "encountered unexpected ReClosureBound: {:?}",
490 // FIXME: This is non-ideal because we don't give a
491 // very descriptive origin for this region variable.
492 Ok(self.infcx.next_region_var(MiscVariable(self.span)))
496 pub trait RelateResultCompare<'tcx, T> {
497 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
498 F: FnOnce() -> TypeError<'tcx>;
501 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
502 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
503 F: FnOnce() -> TypeError<'tcx>,
505 self.clone().and_then(|s| {
515 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
519 TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
522 fn float_unification_error<'tcx>(a_is_expected: bool,
523 v: (ty::FloatVarValue, ty::FloatVarValue))
526 let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
527 TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))