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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. | |
4 | // | |
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. | |
10 | ||
11 | /////////////////////////////////////////////////////////////////////////// | |
12 | // # Type combining | |
13 | // | |
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. | |
20 | // | |
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. | |
25 | // | |
26 | // ## Contravariance | |
27 | // | |
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. | |
34 | ||
35 | use super::bivariate::Bivariate; | |
36 | use super::equate::Equate; | |
37 | use super::glb::Glb; | |
38 | use super::lub::Lub; | |
39 | use super::sub::Sub; | |
40 | use super::InferCtxt; | |
41 | use super::{MiscVariable, TypeTrace}; | |
42 | use super::type_variable::{RelationDir, BiTo, EqTo, SubtypeOf, SupertypeOf}; | |
43 | ||
44 | use ty::{IntType, UintType}; | |
45 | use ty::{self, Ty, TyCtxt}; | |
46 | use ty::error::TypeError; | |
47 | use ty::fold::TypeFoldable; | |
48 | use ty::relate::{RelateResult, TypeRelation}; | |
49 | use traits::PredicateObligations; | |
50 | ||
51 | use syntax::ast; | |
52 | use syntax::util::small_vector::SmallVector; | |
53 | use syntax_pos::Span; | |
54 | ||
55 | #[derive(Clone)] | |
56 | pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> { | |
57 | pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>, | |
58 | pub trace: TypeTrace<'tcx>, | |
59 | pub cause: Option<ty::relate::Cause>, | |
60 | pub obligations: PredicateObligations<'tcx>, | |
61 | } | |
62 | ||
63 | impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> { | |
64 | pub fn super_combine_tys<R>(&self, | |
65 | relation: &mut R, | |
66 | a: Ty<'tcx>, | |
67 | b: Ty<'tcx>) | |
68 | -> RelateResult<'tcx, Ty<'tcx>> | |
69 | where R: TypeRelation<'infcx, 'gcx, 'tcx> | |
70 | { | |
71 | let a_is_expected = relation.a_is_expected(); | |
72 | ||
73 | match (&a.sty, &b.sty) { | |
74 | // Relate integral variables to other types | |
75 | (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => { | |
76 | self.int_unification_table | |
77 | .borrow_mut() | |
78 | .unify_var_var(a_id, b_id) | |
79 | .map_err(|e| int_unification_error(a_is_expected, e))?; | |
80 | Ok(a) | |
81 | } | |
82 | (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => { | |
83 | self.unify_integral_variable(a_is_expected, v_id, IntType(v)) | |
84 | } | |
85 | (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => { | |
86 | self.unify_integral_variable(!a_is_expected, v_id, IntType(v)) | |
87 | } | |
88 | (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => { | |
89 | self.unify_integral_variable(a_is_expected, v_id, UintType(v)) | |
90 | } | |
91 | (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => { | |
92 | self.unify_integral_variable(!a_is_expected, v_id, UintType(v)) | |
93 | } | |
94 | ||
95 | // Relate floating-point variables to other types | |
96 | (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => { | |
97 | self.float_unification_table | |
98 | .borrow_mut() | |
99 | .unify_var_var(a_id, b_id) | |
100 | .map_err(|e| float_unification_error(relation.a_is_expected(), e))?; | |
101 | Ok(a) | |
102 | } | |
103 | (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => { | |
104 | self.unify_float_variable(a_is_expected, v_id, v) | |
105 | } | |
106 | (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => { | |
107 | self.unify_float_variable(!a_is_expected, v_id, v) | |
108 | } | |
109 | ||
110 | // All other cases of inference are errors | |
111 | (&ty::TyInfer(_), _) | | |
112 | (_, &ty::TyInfer(_)) => { | |
113 | Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b))) | |
114 | } | |
115 | ||
116 | ||
117 | _ => { | |
118 | ty::relate::super_relate_tys(relation, a, b) | |
119 | } | |
120 | } | |
121 | } | |
122 | ||
123 | fn unify_integral_variable(&self, | |
124 | vid_is_expected: bool, | |
125 | vid: ty::IntVid, | |
126 | val: ty::IntVarValue) | |
127 | -> RelateResult<'tcx, Ty<'tcx>> | |
128 | { | |
129 | self.int_unification_table | |
130 | .borrow_mut() | |
131 | .unify_var_value(vid, val) | |
132 | .map_err(|e| int_unification_error(vid_is_expected, e))?; | |
133 | match val { | |
134 | IntType(v) => Ok(self.tcx.mk_mach_int(v)), | |
135 | UintType(v) => Ok(self.tcx.mk_mach_uint(v)), | |
136 | } | |
137 | } | |
138 | ||
139 | fn unify_float_variable(&self, | |
140 | vid_is_expected: bool, | |
141 | vid: ty::FloatVid, | |
142 | val: ast::FloatTy) | |
143 | -> RelateResult<'tcx, Ty<'tcx>> | |
144 | { | |
145 | self.float_unification_table | |
146 | .borrow_mut() | |
147 | .unify_var_value(vid, val) | |
148 | .map_err(|e| float_unification_error(vid_is_expected, e))?; | |
149 | Ok(self.tcx.mk_mach_float(val)) | |
150 | } | |
151 | } | |
152 | ||
153 | impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> { | |
154 | pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> { | |
155 | self.infcx.tcx | |
156 | } | |
157 | ||
158 | pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> { | |
159 | Equate::new(self, a_is_expected) | |
160 | } | |
161 | ||
162 | pub fn bivariate<'a>(&'a mut self, a_is_expected: bool) -> Bivariate<'a, 'infcx, 'gcx, 'tcx> { | |
163 | Bivariate::new(self, a_is_expected) | |
164 | } | |
165 | ||
166 | pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> { | |
167 | Sub::new(self, a_is_expected) | |
168 | } | |
169 | ||
170 | pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> { | |
171 | Lub::new(self, a_is_expected) | |
172 | } | |
173 | ||
174 | pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> { | |
175 | Glb::new(self, a_is_expected) | |
176 | } | |
177 | ||
178 | pub fn instantiate(&mut self, | |
179 | a_ty: Ty<'tcx>, | |
180 | dir: RelationDir, | |
181 | b_vid: ty::TyVid, | |
182 | a_is_expected: bool) | |
183 | -> RelateResult<'tcx, ()> | |
184 | { | |
185 | // We use SmallVector here instead of Vec because this code is hot and | |
186 | // it's rare that the stack length exceeds 1. | |
187 | let mut stack = SmallVector::zero(); | |
188 | stack.push((a_ty, dir, b_vid)); | |
189 | loop { | |
190 | // For each turn of the loop, we extract a tuple | |
191 | // | |
192 | // (a_ty, dir, b_vid) | |
193 | // | |
194 | // to relate. Here dir is either SubtypeOf or | |
195 | // SupertypeOf. The idea is that we should ensure that | |
196 | // the type `a_ty` is a subtype or supertype (respectively) of the | |
197 | // type to which `b_vid` is bound. | |
198 | // | |
199 | // If `b_vid` has not yet been instantiated with a type | |
200 | // (which is always true on the first iteration, but not | |
201 | // necessarily true on later iterations), we will first | |
202 | // instantiate `b_vid` with a *generalized* version of | |
203 | // `a_ty`. Generalization introduces other inference | |
204 | // variables wherever subtyping could occur (at time of | |
205 | // this writing, this means replacing free regions with | |
206 | // region variables). | |
207 | let (a_ty, dir, b_vid) = match stack.pop() { | |
208 | None => break, | |
209 | Some(e) => e, | |
210 | }; | |
211 | // Get the actual variable that b_vid has been inferred to | |
212 | let (b_vid, b_ty) = { | |
213 | let mut variables = self.infcx.type_variables.borrow_mut(); | |
214 | let b_vid = variables.root_var(b_vid); | |
215 | (b_vid, variables.probe_root(b_vid)) | |
216 | }; | |
217 | ||
218 | debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", | |
219 | a_ty, | |
220 | dir, | |
221 | b_vid); | |
222 | ||
223 | // Check whether `vid` has been instantiated yet. If not, | |
224 | // make a generalized form of `ty` and instantiate with | |
225 | // that. | |
226 | let b_ty = match b_ty { | |
227 | Some(t) => t, // ...already instantiated. | |
228 | None => { // ...not yet instantiated: | |
229 | // Generalize type if necessary. | |
230 | let generalized_ty = match dir { | |
231 | EqTo => self.generalize(a_ty, b_vid, false), | |
232 | BiTo | SupertypeOf | SubtypeOf => self.generalize(a_ty, b_vid, true), | |
233 | }?; | |
234 | debug!("instantiate(a_ty={:?}, dir={:?}, \ | |
235 | b_vid={:?}, generalized_ty={:?})", | |
236 | a_ty, dir, b_vid, | |
237 | generalized_ty); | |
238 | self.infcx.type_variables | |
239 | .borrow_mut() | |
240 | .instantiate_and_push( | |
241 | b_vid, generalized_ty, &mut stack); | |
242 | generalized_ty | |
243 | } | |
244 | }; | |
245 | ||
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)`: | |
248 | // | |
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. | |
253 | match dir { | |
254 | BiTo => self.bivariate(a_is_expected).relate(&a_ty, &b_ty), | |
255 | EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty), | |
256 | SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty), | |
257 | SupertypeOf => self.sub(a_is_expected).relate_with_variance( | |
258 | ty::Contravariant, &a_ty, &b_ty), | |
259 | }?; | |
260 | } | |
261 | ||
262 | Ok(()) | |
263 | } | |
264 | ||
265 | /// Attempts to generalize `ty` for the type variable `for_vid`. This checks for cycle -- that | |
266 | /// is, whether the type `ty` references `for_vid`. If `make_region_vars` is true, it will also | |
267 | /// replace all regions with fresh variables. Returns `TyError` in the case of a cycle, `Ok` | |
268 | /// otherwise. | |
269 | fn generalize(&self, | |
270 | ty: Ty<'tcx>, | |
271 | for_vid: ty::TyVid, | |
272 | make_region_vars: bool) | |
273 | -> RelateResult<'tcx, Ty<'tcx>> | |
274 | { | |
275 | let mut generalize = Generalizer { | |
276 | infcx: self.infcx, | |
277 | span: self.trace.origin.span(), | |
278 | for_vid: for_vid, | |
279 | make_region_vars: make_region_vars, | |
280 | cycle_detected: false | |
281 | }; | |
282 | let u = ty.fold_with(&mut generalize); | |
283 | if generalize.cycle_detected { | |
284 | Err(TypeError::CyclicTy) | |
285 | } else { | |
286 | Ok(u) | |
287 | } | |
288 | } | |
289 | } | |
290 | ||
291 | struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> { | |
292 | infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>, | |
293 | span: Span, | |
294 | for_vid: ty::TyVid, | |
295 | make_region_vars: bool, | |
296 | cycle_detected: bool, | |
297 | } | |
298 | ||
299 | impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> { | |
300 | fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx> { | |
301 | self.infcx.tcx | |
302 | } | |
303 | ||
304 | fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> { | |
305 | // Check to see whether the type we are genealizing references | |
306 | // `vid`. At the same time, also update any type variables to | |
307 | // the values that they are bound to. This is needed to truly | |
308 | // check for cycles, but also just makes things readable. | |
309 | // | |
310 | // (In particular, you could have something like `$0 = Box<$1>` | |
311 | // where `$1` has already been instantiated with `Box<$0>`) | |
312 | match t.sty { | |
313 | ty::TyInfer(ty::TyVar(vid)) => { | |
314 | let mut variables = self.infcx.type_variables.borrow_mut(); | |
315 | let vid = variables.root_var(vid); | |
316 | if vid == self.for_vid { | |
317 | self.cycle_detected = true; | |
318 | self.tcx().types.err | |
319 | } else { | |
320 | match variables.probe_root(vid) { | |
321 | Some(u) => { | |
322 | drop(variables); | |
323 | self.fold_ty(u) | |
324 | } | |
325 | None => t, | |
326 | } | |
327 | } | |
328 | } | |
329 | _ => { | |
330 | t.super_fold_with(self) | |
331 | } | |
332 | } | |
333 | } | |
334 | ||
335 | fn fold_region(&mut self, r: &'tcx ty::Region) -> &'tcx ty::Region { | |
336 | match *r { | |
337 | // Never make variables for regions bound within the type itself, | |
338 | // nor for erased regions. | |
339 | ty::ReLateBound(..) | | |
340 | ty::ReErased => { return r; } | |
341 | ||
342 | // Early-bound regions should really have been substituted away before | |
343 | // we get to this point. | |
344 | ty::ReEarlyBound(..) => { | |
345 | span_bug!( | |
346 | self.span, | |
347 | "Encountered early bound region when generalizing: {:?}", | |
348 | r); | |
349 | } | |
350 | ||
351 | // Always make a fresh region variable for skolemized regions; | |
352 | // the higher-ranked decision procedures rely on this. | |
353 | ty::ReSkolemized(..) => { } | |
354 | ||
355 | // For anything else, we make a region variable, unless we | |
356 | // are *equating*, in which case it's just wasteful. | |
357 | ty::ReEmpty | | |
358 | ty::ReStatic | | |
359 | ty::ReScope(..) | | |
360 | ty::ReVar(..) | | |
361 | ty::ReFree(..) => { | |
362 | if !self.make_region_vars { | |
363 | return r; | |
364 | } | |
365 | } | |
366 | } | |
367 | ||
368 | // FIXME: This is non-ideal because we don't give a | |
369 | // very descriptive origin for this region variable. | |
370 | self.infcx.next_region_var(MiscVariable(self.span)) | |
371 | } | |
372 | } | |
373 | ||
374 | pub trait RelateResultCompare<'tcx, T> { | |
375 | fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where | |
376 | F: FnOnce() -> TypeError<'tcx>; | |
377 | } | |
378 | ||
379 | impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> { | |
380 | fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where | |
381 | F: FnOnce() -> TypeError<'tcx>, | |
382 | { | |
383 | self.clone().and_then(|s| { | |
384 | if s == t { | |
385 | self.clone() | |
386 | } else { | |
387 | Err(f()) | |
388 | } | |
389 | }) | |
390 | } | |
391 | } | |
392 | ||
393 | fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue)) | |
394 | -> TypeError<'tcx> | |
395 | { | |
396 | let (a, b) = v; | |
397 | TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b)) | |
398 | } | |
399 | ||
400 | fn float_unification_error<'tcx>(a_is_expected: bool, | |
401 | v: (ast::FloatTy, ast::FloatTy)) | |
402 | -> TypeError<'tcx> | |
403 | { | |
404 | let (a, b) = v; | |
405 | TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b)) | |
406 | } |