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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
85aaf69f 35use super::bivariate::Bivariate;
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36use super::equate::Equate;
37use super::glb::Glb;
38use super::lub::Lub;
39use super::sub::Sub;
c34b1796 40use super::{InferCtxt};
1a4d82fc 41use super::{MiscVariable, TypeTrace};
85aaf69f 42use super::type_variable::{RelationDir, BiTo, EqTo, SubtypeOf, SupertypeOf};
1a4d82fc 43
1a4d82fc 44use middle::ty::{IntType, UintType};
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45use middle::ty::{self, Ty};
46use middle::ty::error::TypeError;
47use middle::ty::fold::{TypeFolder, TypeFoldable};
48use middle::ty::relate::{Relate, RelateResult, TypeRelation};
1a4d82fc 49
b039eaaf 50use syntax::ast;
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51use syntax::codemap::Span;
52
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53#[derive(Clone)]
54pub struct CombineFields<'a, 'tcx: 'a> {
55 pub infcx: &'a InferCtxt<'a, 'tcx>,
56 pub a_is_expected: bool,
57 pub trace: TypeTrace<'tcx>,
e9174d1e 58 pub cause: Option<ty::relate::Cause>,
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59}
60
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61pub fn super_combine_tys<'a,'tcx:'a,R>(infcx: &InferCtxt<'a, 'tcx>,
62 relation: &mut R,
63 a: Ty<'tcx>,
64 b: Ty<'tcx>)
65 -> RelateResult<'tcx, Ty<'tcx>>
66 where R: TypeRelation<'a,'tcx>
1a4d82fc 67{
c34b1796 68 let a_is_expected = relation.a_is_expected();
1a4d82fc 69
c34b1796 70 match (&a.sty, &b.sty) {
1a4d82fc 71 // Relate integral variables to other types
62682a34 72 (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => {
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73 try!(infcx.int_unification_table
74 .borrow_mut()
75 .unify_var_var(a_id, b_id)
76 .map_err(|e| int_unification_error(a_is_expected, e)));
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77 Ok(a)
78 }
62682a34 79 (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
c34b1796 80 unify_integral_variable(infcx, a_is_expected, v_id, IntType(v))
1a4d82fc 81 }
62682a34 82 (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
c34b1796 83 unify_integral_variable(infcx, !a_is_expected, v_id, IntType(v))
1a4d82fc 84 }
62682a34 85 (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
c34b1796 86 unify_integral_variable(infcx, a_is_expected, v_id, UintType(v))
1a4d82fc 87 }
62682a34 88 (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
c34b1796 89 unify_integral_variable(infcx, !a_is_expected, v_id, UintType(v))
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90 }
91
92 // Relate floating-point variables to other types
62682a34 93 (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => {
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94 try!(infcx.float_unification_table
95 .borrow_mut()
96 .unify_var_var(a_id, b_id)
97 .map_err(|e| float_unification_error(relation.a_is_expected(), e)));
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98 Ok(a)
99 }
62682a34 100 (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
c34b1796 101 unify_float_variable(infcx, a_is_expected, v_id, v)
1a4d82fc 102 }
62682a34 103 (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
c34b1796 104 unify_float_variable(infcx, !a_is_expected, v_id, v)
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105 }
106
c34b1796 107 // All other cases of inference are errors
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108 (&ty::TyInfer(_), _) |
109 (_, &ty::TyInfer(_)) => {
e9174d1e 110 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
1a4d82fc 111 }
1a4d82fc 112
1a4d82fc 113
c34b1796 114 _ => {
e9174d1e 115 ty::relate::super_relate_tys(relation, a, b)
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116 }
117 }
c34b1796 118}
1a4d82fc 119
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120fn unify_integral_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
121 vid_is_expected: bool,
122 vid: ty::IntVid,
123 val: ty::IntVarValue)
124 -> RelateResult<'tcx, Ty<'tcx>>
125{
126 try!(infcx
127 .int_unification_table
128 .borrow_mut()
129 .unify_var_value(vid, val)
130 .map_err(|e| int_unification_error(vid_is_expected, e)));
131 match val {
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132 IntType(v) => Ok(infcx.tcx.mk_mach_int(v)),
133 UintType(v) => Ok(infcx.tcx.mk_mach_uint(v)),
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134 }
135}
136
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137fn unify_float_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
138 vid_is_expected: bool,
139 vid: ty::FloatVid,
b039eaaf 140 val: ast::FloatTy)
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141 -> RelateResult<'tcx, Ty<'tcx>>
142{
143 try!(infcx
144 .float_unification_table
145 .borrow_mut()
146 .unify_var_value(vid, val)
147 .map_err(|e| float_unification_error(vid_is_expected, e)));
c1a9b12d 148 Ok(infcx.tcx.mk_mach_float(val))
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149}
150
151impl<'a, 'tcx> CombineFields<'a, 'tcx> {
152 pub fn tcx(&self) -> &'a ty::ctxt<'tcx> {
153 self.infcx.tcx
154 }
155
156 pub fn switch_expected(&self) -> CombineFields<'a, 'tcx> {
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157 CombineFields {
158 a_is_expected: !self.a_is_expected,
159 ..(*self).clone()
160 }
161 }
162
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163 pub fn equate(&self) -> Equate<'a, 'tcx> {
164 Equate::new(self.clone())
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165 }
166
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167 pub fn bivariate(&self) -> Bivariate<'a, 'tcx> {
168 Bivariate::new(self.clone())
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169 }
170
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171 pub fn sub(&self) -> Sub<'a, 'tcx> {
172 Sub::new(self.clone())
173 }
174
175 pub fn lub(&self) -> Lub<'a, 'tcx> {
176 Lub::new(self.clone())
177 }
178
179 pub fn glb(&self) -> Glb<'a, 'tcx> {
180 Glb::new(self.clone())
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181 }
182
183 pub fn instantiate(&self,
184 a_ty: Ty<'tcx>,
185 dir: RelationDir,
186 b_vid: ty::TyVid)
c34b1796 187 -> RelateResult<'tcx, ()>
1a4d82fc 188 {
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189 let mut stack = Vec::new();
190 stack.push((a_ty, dir, b_vid));
191 loop {
192 // For each turn of the loop, we extract a tuple
193 //
194 // (a_ty, dir, b_vid)
195 //
196 // to relate. Here dir is either SubtypeOf or
197 // SupertypeOf. The idea is that we should ensure that
198 // the type `a_ty` is a subtype or supertype (respectively) of the
199 // type to which `b_vid` is bound.
200 //
201 // If `b_vid` has not yet been instantiated with a type
202 // (which is always true on the first iteration, but not
203 // necessarily true on later iterations), we will first
204 // instantiate `b_vid` with a *generalized* version of
205 // `a_ty`. Generalization introduces other inference
206 // variables wherever subtyping could occur (at time of
207 // this writing, this means replacing free regions with
208 // region variables).
209 let (a_ty, dir, b_vid) = match stack.pop() {
210 None => break,
211 Some(e) => e,
212 };
213
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214 debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})",
215 a_ty,
1a4d82fc 216 dir,
62682a34 217 b_vid);
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218
219 // Check whether `vid` has been instantiated yet. If not,
220 // make a generalized form of `ty` and instantiate with
221 // that.
222 let b_ty = self.infcx.type_variables.borrow().probe(b_vid);
223 let b_ty = match b_ty {
224 Some(t) => t, // ...already instantiated.
225 None => { // ...not yet instantiated:
226 // Generalize type if necessary.
227 let generalized_ty = try!(match dir {
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228 EqTo => self.generalize(a_ty, b_vid, false),
229 BiTo | SupertypeOf | SubtypeOf => self.generalize(a_ty, b_vid, true),
1a4d82fc 230 });
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231 debug!("instantiate(a_ty={:?}, dir={:?}, \
232 b_vid={:?}, generalized_ty={:?})",
233 a_ty, dir, b_vid,
234 generalized_ty);
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235 self.infcx.type_variables
236 .borrow_mut()
237 .instantiate_and_push(
238 b_vid, generalized_ty, &mut stack);
239 generalized_ty
240 }
241 };
242
243 // The original triple was `(a_ty, dir, b_vid)` -- now we have
244 // resolved `b_vid` to `b_ty`, so apply `(a_ty, dir, b_ty)`:
245 //
246 // FIXME(#16847): This code is non-ideal because all these subtype
247 // relations wind up attributed to the same spans. We need
248 // to associate causes/spans with each of the relations in
249 // the stack to get this right.
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250 try!(match dir {
251 BiTo => self.bivariate().relate(&a_ty, &b_ty),
252 EqTo => self.equate().relate(&a_ty, &b_ty),
253 SubtypeOf => self.sub().relate(&a_ty, &b_ty),
254 SupertypeOf => self.sub().relate_with_variance(ty::Contravariant, &a_ty, &b_ty),
255 });
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256 }
257
258 Ok(())
259 }
260
261 /// Attempts to generalize `ty` for the type variable `for_vid`. This checks for cycle -- that
262 /// is, whether the type `ty` references `for_vid`. If `make_region_vars` is true, it will also
62682a34 263 /// replace all regions with fresh variables. Returns `TyError` in the case of a cycle, `Ok`
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264 /// otherwise.
265 fn generalize(&self,
266 ty: Ty<'tcx>,
267 for_vid: ty::TyVid,
268 make_region_vars: bool)
c34b1796 269 -> RelateResult<'tcx, Ty<'tcx>>
1a4d82fc 270 {
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271 let mut generalize = Generalizer {
272 infcx: self.infcx,
273 span: self.trace.origin.span(),
274 for_vid: for_vid,
275 make_region_vars: make_region_vars,
276 cycle_detected: false
277 };
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278 let u = ty.fold_with(&mut generalize);
279 if generalize.cycle_detected {
c1a9b12d 280 Err(TypeError::CyclicTy)
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281 } else {
282 Ok(u)
283 }
284 }
285}
286
287struct Generalizer<'cx, 'tcx:'cx> {
288 infcx: &'cx InferCtxt<'cx, 'tcx>,
289 span: Span,
290 for_vid: ty::TyVid,
291 make_region_vars: bool,
292 cycle_detected: bool,
293}
294
e9174d1e 295impl<'cx, 'tcx> ty::fold::TypeFolder<'tcx> for Generalizer<'cx, 'tcx> {
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296 fn tcx(&self) -> &ty::ctxt<'tcx> {
297 self.infcx.tcx
298 }
299
300 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
301 // Check to see whether the type we are genealizing references
302 // `vid`. At the same time, also update any type variables to
303 // the values that they are bound to. This is needed to truly
304 // check for cycles, but also just makes things readable.
305 //
306 // (In particular, you could have something like `$0 = Box<$1>`
307 // where `$1` has already been instantiated with `Box<$0>`)
308 match t.sty {
62682a34 309 ty::TyInfer(ty::TyVar(vid)) => {
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310 if vid == self.for_vid {
311 self.cycle_detected = true;
312 self.tcx().types.err
313 } else {
314 match self.infcx.type_variables.borrow().probe(vid) {
315 Some(u) => self.fold_ty(u),
316 None => t,
317 }
318 }
319 }
320 _ => {
9cc50fc6 321 t.super_fold_with(self)
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322 }
323 }
324 }
325
326 fn fold_region(&mut self, r: ty::Region) -> ty::Region {
327 match r {
328 // Never make variables for regions bound within the type itself.
329 ty::ReLateBound(..) => { return r; }
330
331 // Early-bound regions should really have been substituted away before
332 // we get to this point.
333 ty::ReEarlyBound(..) => {
334 self.tcx().sess.span_bug(
335 self.span,
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336 &format!("Encountered early bound region when generalizing: {:?}",
337 r));
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338 }
339
340 // Always make a fresh region variable for skolemized regions;
341 // the higher-ranked decision procedures rely on this.
e9174d1e 342 ty::ReSkolemized(..) => { }
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343
344 // For anything else, we make a region variable, unless we
345 // are *equating*, in which case it's just wasteful.
346 ty::ReEmpty |
347 ty::ReStatic |
348 ty::ReScope(..) |
e9174d1e 349 ty::ReVar(..) |
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350 ty::ReFree(..) => {
351 if !self.make_region_vars {
352 return r;
353 }
354 }
355 }
356
357 // FIXME: This is non-ideal because we don't give a
358 // very descriptive origin for this region variable.
359 self.infcx.next_region_var(MiscVariable(self.span))
360 }
361}
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362
363pub trait RelateResultCompare<'tcx, T> {
364 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
e9174d1e 365 F: FnOnce() -> TypeError<'tcx>;
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366}
367
368impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
369 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
e9174d1e 370 F: FnOnce() -> TypeError<'tcx>,
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371 {
372 self.clone().and_then(|s| {
373 if s == t {
374 self.clone()
375 } else {
376 Err(f())
377 }
378 })
379 }
380}
381
382fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
e9174d1e 383 -> TypeError<'tcx>
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384{
385 let (a, b) = v;
e9174d1e 386 TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
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387}
388
389fn float_unification_error<'tcx>(a_is_expected: bool,
b039eaaf 390 v: (ast::FloatTy, ast::FloatTy))
e9174d1e 391 -> TypeError<'tcx>
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392{
393 let (a, b) = v;
e9174d1e 394 TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
c34b1796 395}