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;
40 use super::{MiscVariable, TypeTrace};
42 use ty::{IntType, UintType};
43 use ty::{self, Ty, TyCtxt};
44 use ty::error::TypeError;
45 use ty::fold::TypeFoldable;
46 use ty::relate::{RelateResult, TypeRelation};
47 use traits::PredicateObligations;
50 use syntax::util::small_vector::SmallVector;
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 obligations: PredicateObligations<'tcx>,
61 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
62 pub enum RelationDir {
63 SubtypeOf, SupertypeOf, EqTo
66 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
67 pub fn super_combine_tys<R>(&self,
71 -> RelateResult<'tcx, Ty<'tcx>>
72 where R: TypeRelation<'infcx, 'gcx, 'tcx>
74 let a_is_expected = relation.a_is_expected();
76 match (&a.sty, &b.sty) {
77 // Relate integral variables to other types
78 (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => {
79 self.int_unification_table
81 .unify_var_var(a_id, b_id)
82 .map_err(|e| int_unification_error(a_is_expected, e))?;
85 (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
86 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
88 (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
89 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
91 (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
92 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
94 (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
95 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
98 // Relate floating-point variables to other types
99 (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => {
100 self.float_unification_table
102 .unify_var_var(a_id, b_id)
103 .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
106 (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
107 self.unify_float_variable(a_is_expected, v_id, v)
109 (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
110 self.unify_float_variable(!a_is_expected, v_id, v)
113 // All other cases of inference are errors
114 (&ty::TyInfer(_), _) |
115 (_, &ty::TyInfer(_)) => {
116 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
121 ty::relate::super_relate_tys(relation, a, b)
126 fn unify_integral_variable(&self,
127 vid_is_expected: bool,
129 val: ty::IntVarValue)
130 -> RelateResult<'tcx, Ty<'tcx>>
132 self.int_unification_table
134 .unify_var_value(vid, val)
135 .map_err(|e| int_unification_error(vid_is_expected, e))?;
137 IntType(v) => Ok(self.tcx.mk_mach_int(v)),
138 UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
142 fn unify_float_variable(&self,
143 vid_is_expected: bool,
146 -> RelateResult<'tcx, Ty<'tcx>>
148 self.float_unification_table
150 .unify_var_value(vid, val)
151 .map_err(|e| float_unification_error(vid_is_expected, e))?;
152 Ok(self.tcx.mk_mach_float(val))
156 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
157 pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
161 pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
162 Equate::new(self, a_is_expected)
165 pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
166 Sub::new(self, a_is_expected)
169 pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
170 Lub::new(self, a_is_expected)
173 pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
174 Glb::new(self, a_is_expected)
177 pub fn instantiate(&mut self,
182 -> RelateResult<'tcx, ()>
184 use self::RelationDir::*;
186 // We use SmallVector here instead of Vec because this code is hot and
187 // it's rare that the stack length exceeds 1.
188 let mut stack = SmallVector::new();
189 stack.push((a_ty, dir, b_vid));
191 // For each turn of the loop, we extract a tuple
193 // (a_ty, dir, b_vid)
195 // to relate. Here dir is either SubtypeOf or
196 // SupertypeOf. The idea is that we should ensure that
197 // the type `a_ty` is a subtype or supertype (respectively) of the
198 // type to which `b_vid` is bound.
200 // If `b_vid` has not yet been instantiated with a type
201 // (which is always true on the first iteration, but not
202 // necessarily true on later iterations), we will first
203 // instantiate `b_vid` with a *generalized* version of
204 // `a_ty`. Generalization introduces other inference
205 // variables wherever subtyping could occur (at time of
206 // this writing, this means replacing free regions with
207 // region variables).
208 let (a_ty, dir, b_vid) = match stack.pop() {
212 // Get the actual variable that b_vid has been inferred to
213 let (b_vid, b_ty) = {
214 let mut variables = self.infcx.type_variables.borrow_mut();
215 let b_vid = variables.root_var(b_vid);
216 (b_vid, variables.probe_root(b_vid))
219 debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})",
224 // Check whether `vid` has been instantiated yet. If not,
225 // make a generalized form of `ty` and instantiate with
227 let b_ty = match b_ty {
228 Some(t) => t, // ...already instantiated.
229 None => { // ...not yet instantiated:
230 // Generalize type if necessary.
231 let generalized_ty = match dir {
232 EqTo => self.generalize(a_ty, b_vid, false),
233 SupertypeOf | SubtypeOf => self.generalize(a_ty, b_vid, true),
235 debug!("instantiate(a_ty={:?}, dir={:?}, \
236 b_vid={:?}, generalized_ty={:?})",
239 self.infcx.type_variables
241 .instantiate(b_vid, generalized_ty);
246 // The original triple was `(a_ty, dir, b_vid)` -- now we have
247 // resolved `b_vid` to `b_ty`, so apply `(a_ty, dir, b_ty)`:
249 // FIXME(#16847): This code is non-ideal because all these subtype
250 // relations wind up attributed to the same spans. We need
251 // to associate causes/spans with each of the relations in
252 // the stack to get this right.
254 EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
255 SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
256 SupertypeOf => self.sub(a_is_expected).relate_with_variance(
257 ty::Contravariant, &a_ty, &b_ty),
264 /// Attempts to generalize `ty` for the type variable `for_vid`. This checks for cycle -- that
265 /// is, whether the type `ty` references `for_vid`. If `make_region_vars` is true, it will also
266 /// replace all regions with fresh variables. Returns `TyError` in the case of a cycle, `Ok`
271 make_region_vars: bool)
272 -> RelateResult<'tcx, Ty<'tcx>>
274 let mut generalize = Generalizer {
276 span: self.trace.cause.span,
278 make_region_vars: make_region_vars,
279 cycle_detected: false
281 let u = ty.fold_with(&mut generalize);
282 if generalize.cycle_detected {
283 Err(TypeError::CyclicTy)
290 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
291 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
294 make_region_vars: bool,
295 cycle_detected: bool,
298 impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
299 fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx> {
303 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
304 // Check to see whether the type we are genealizing references
305 // `vid`. At the same time, also update any type variables to
306 // the values that they are bound to. This is needed to truly
307 // check for cycles, but also just makes things readable.
309 // (In particular, you could have something like `$0 = Box<$1>`
310 // where `$1` has already been instantiated with `Box<$0>`)
312 ty::TyInfer(ty::TyVar(vid)) => {
313 let mut variables = self.infcx.type_variables.borrow_mut();
314 let vid = variables.root_var(vid);
315 if vid == self.for_vid {
316 self.cycle_detected = true;
319 match variables.probe_root(vid) {
329 t.super_fold_with(self)
334 fn fold_region(&mut self, r: &'tcx ty::Region) -> &'tcx ty::Region {
336 // Never make variables for regions bound within the type itself,
337 // nor for erased regions.
338 ty::ReLateBound(..) |
339 ty::ReErased => { return r; }
341 // Early-bound regions should really have been substituted away before
342 // we get to this point.
343 ty::ReEarlyBound(..) => {
346 "Encountered early bound region when generalizing: {:?}",
350 // Always make a fresh region variable for skolemized regions;
351 // the higher-ranked decision procedures rely on this.
352 ty::ReSkolemized(..) => { }
354 // For anything else, we make a region variable, unless we
355 // are *equating*, in which case it's just wasteful.
361 if !self.make_region_vars {
367 // FIXME: This is non-ideal because we don't give a
368 // very descriptive origin for this region variable.
369 self.infcx.next_region_var(MiscVariable(self.span))
373 pub trait RelateResultCompare<'tcx, T> {
374 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
375 F: FnOnce() -> TypeError<'tcx>;
378 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
379 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
380 F: FnOnce() -> TypeError<'tcx>,
382 self.clone().and_then(|s| {
392 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
396 TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
399 fn float_unification_error<'tcx>(a_is_expected: bool,
400 v: (ast::FloatTy, ast::FloatTy))
404 TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))