src/librustc/middle/infer/coercion.rs

   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 //! # Type Coercion
  12 //!
  13 //! Under certain circumstances we will coerce from one type to another,
  14 //! for example by auto-borrowing.  This occurs in situations where the
  15 //! compiler has a firm 'expected type' that was supplied from the user,
  16 //! and where the actual type is similar to that expected type in purpose
  17 //! but not in representation (so actual subtyping is inappropriate).
  18 //!
  19 //! ## Reborrowing
  20 //!
  21 //! Note that if we are expecting a reference, we will *reborrow*
  22 //! even if the argument provided was already a reference.  This is
  23 //! useful for freezing mut/const things (that is, when the expected is &T
  24 //! but you have &const T or &mut T) and also for avoiding the linearity
  25 //! of mut things (when the expected is &mut T and you have &mut T).  See
  26 //! the various `src/test/run-pass/coerce-reborrow-*.rs` tests for
  27 //! examples of where this is useful.
  28 //!
  29 //! ## Subtle note
  30 //!
  31 //! When deciding what type coercions to consider, we do not attempt to
  32 //! resolve any type variables we may encounter.  This is because `b`
  33 //! represents the expected type "as the user wrote it", meaning that if
  34 //! the user defined a generic function like
  35 //!
  36 //!    fn foo<A>(a: A, b: A) { ... }
  37 //!
  38 //! and then we wrote `foo(&1, @2)`, we will not auto-borrow
  39 //! either argument.  In older code we went to some lengths to
  40 //! resolve the `b` variable, which could mean that we'd
  41 //! auto-borrow later arguments but not earlier ones, which
  42 //! seems very confusing.
  43 //!
  44 //! ## Subtler note
  45 //!
  46 //! However, right now, if the user manually specifies the
  47 //! values for the type variables, as so:
  48 //!
  49 //!    foo::<&int>(@1, @2)
  50 //!
  51 //! then we *will* auto-borrow, because we can't distinguish this from a
  52 //! function that declared `&int`.  This is inconsistent but it's easiest
  53 //! at the moment. The right thing to do, I think, is to consider the
  54 //! *unsubstituted* type when deciding whether to auto-borrow, but the
  55 //! *substituted* type when considering the bounds and so forth. But most
  56 //! of our methods don't give access to the unsubstituted type, and
  57 //! rightly so because they'd be error-prone.  So maybe the thing to do is
  58 //! to actually determine the kind of coercions that should occur
  59 //! separately and pass them in.  Or maybe it's ok as is.  Anyway, it's
  60 //! sort of a minor point so I've opted to leave it for later---after all
  61 //! we may want to adjust precisely when coercions occur.
  62
  63 use super::{CoerceResult, Coercion};
  64 use super::combine::{CombineFields, Combine};
  65 use super::sub::Sub;
  66
  67 use middle::subst;
  68 use middle::ty::{AutoPtr, AutoDerefRef, AdjustDerefRef, AutoUnsize, AutoUnsafe};
  69 use middle::ty::{mt};
  70 use middle::ty::{mod, Ty};
  71 use util::ppaux;
  72 use util::ppaux::Repr;
  73
  74 use syntax::abi;
  75 use syntax::ast;
  76
  77 // Note: Coerce is not actually a combiner, in that it does not
  78 // conform to the same interface, though it performs a similar
  79 // function.
  80 pub struct Coerce<'f, 'tcx: 'f>(pub CombineFields<'f, 'tcx>);
  81
  82 impl<'f, 'tcx> Coerce<'f, 'tcx> {
  83     pub fn get_ref<'a>(&'a self) -> &'a CombineFields<'f, 'tcx> {
  84         let Coerce(ref v) = *self; v
  85     }
  86
  87     fn tcx(&self) -> &ty::ctxt<'tcx> {
  88         self.get_ref().infcx.tcx
  89     }
  90
  91     pub fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
  92         debug!("Coerce.tys({} => {})",
  93                a.repr(self.tcx()),
  94                b.repr(self.tcx()));
  95
  96         // Consider coercing the subtype to a DST
  97         let unsize = self.unpack_actual_value(a, |a| {
  98             self.coerce_unsized(a, b)
  99         });
 100         if unsize.is_ok() {
 101             return unsize;
 102         }
 103
 104         // Examine the supertype and consider auto-borrowing.
 105         //
 106         // Note: does not attempt to resolve type variables we encounter.
 107         // See above for details.
 108         match b.sty {
 109             ty::ty_ptr(mt_b) => {
 110                 match mt_b.ty.sty {
 111                     ty::ty_str => {
 112                         return self.unpack_actual_value(a, |a| {
 113                             self.coerce_unsafe_ptr(a, b, ast::MutImmutable)
 114                         });
 115                     }
 116
 117                     ty::ty_trait(..) => {
 118                         let result = self.unpack_actual_value(a, |a| {
 119                             self.coerce_unsafe_object(a, b, mt_b.mutbl)
 120                         });
 121
 122                         match result {
 123                             Ok(t) => return Ok(t),
 124                             Err(..) => {}
 125                         }
 126                     }
 127
 128                     _ => {
 129                         return self.unpack_actual_value(a, |a| {
 130                             self.coerce_unsafe_ptr(a, b, mt_b.mutbl)
 131                         });
 132                     }
 133                 };
 134             }
 135
 136             ty::ty_rptr(_, mt_b) => {
 137                 match mt_b.ty.sty {
 138                     ty::ty_str => {
 139                         return self.unpack_actual_value(a, |a| {
 140                             self.coerce_borrowed_pointer(a, b, ast::MutImmutable)
 141                         });
 142                     }
 143
 144                     ty::ty_trait(..) => {
 145                         let result = self.unpack_actual_value(a, |a| {
 146                             self.coerce_borrowed_object(a, b, mt_b.mutbl)
 147                         });
 148
 149                         match result {
 150                             Ok(t) => return Ok(t),
 151                             Err(..) => {}
 152                         }
 153                     }
 154
 155                     _ => {
 156                         return self.unpack_actual_value(a, |a| {
 157                             self.coerce_borrowed_pointer(a, b, mt_b.mutbl)
 158                         });
 159                     }
 160                 };
 161             }
 162
 163             ty::ty_closure(box ty::ClosureTy {
 164                     store: ty::RegionTraitStore(..),
 165                     ..
 166                 }) => {
 167                 return self.unpack_actual_value(a, |a| {
 168                     self.coerce_borrowed_fn(a, b)
 169                 });
 170             }
 171
 172             _ => {}
 173         }
 174
 175         self.unpack_actual_value(a, |a| {
 176             match a.sty {
 177                 ty::ty_bare_fn(Some(a_def_id), a_f) => {
 178                     // Function items are coercible to any closure
 179                     // type; function pointers are not (that would
 180                     // require double indirection).
 181                     self.coerce_from_fn_item(a, a_def_id, a_f, b)
 182                 }
 183                 _ => {
 184                     // Otherwise, just use subtyping rules.
 185                     self.subtype(a, b)
 186                 }
 187             }
 188         })
 189     }
 190
 191     pub fn subtype(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
 192         match Sub(self.get_ref().clone()).tys(a, b) {
 193             Ok(_) => Ok(None),         // No coercion required.
 194             Err(ref e) => Err(*e)
 195         }
 196     }
 197
 198     pub fn unpack_actual_value<T, F>(&self, a: Ty<'tcx>, f: F) -> T where
 199         F: FnOnce(Ty<'tcx>) -> T,
 200     {
 201         f(self.get_ref().infcx.shallow_resolve(a))
 202     }
 203
 204     // ~T -> &T or &mut T -> &T (including where T = [U] or str)
 205     pub fn coerce_borrowed_pointer(&self,
 206                                    a: Ty<'tcx>,
 207                                    b: Ty<'tcx>,
 208                                    mutbl_b: ast::Mutability)
 209                                    -> CoerceResult<'tcx> {
 210         debug!("coerce_borrowed_pointer(a={}, b={})",
 211                a.repr(self.tcx()),
 212                b.repr(self.tcx()));
 213
 214         // If we have a parameter of type `&M T_a` and the value
 215         // provided is `expr`, we will be adding an implicit borrow,
 216         // meaning that we convert `f(expr)` to `f(&M *expr)`.  Therefore,
 217         // to type check, we will construct the type that `&M*expr` would
 218         // yield.
 219
 220         let sub = Sub(self.get_ref().clone());
 221         let coercion = Coercion(self.get_ref().trace.clone());
 222         let r_borrow = self.get_ref().infcx.next_region_var(coercion);
 223
 224         let inner_ty = match a.sty {
 225             ty::ty_uniq(_) => return Err(ty::terr_mismatch),
 226             ty::ty_rptr(_, mt_a) => mt_a.ty,
 227             _ => {
 228                 return self.subtype(a, b);
 229             }
 230         };
 231
 232         let a_borrowed = ty::mk_rptr(self.tcx(),
 233                                      self.tcx().mk_region(r_borrow),
 234                                      mt {ty: inner_ty, mutbl: mutbl_b});
 235         try!(sub.tys(a_borrowed, b));
 236
 237         Ok(Some(AdjustDerefRef(AutoDerefRef {
 238             autoderefs: 1,
 239             autoref: Some(AutoPtr(r_borrow, mutbl_b, None))
 240         })))
 241     }
 242
 243
 244     // &[T, ..n] or &mut [T, ..n] -> &[T]
 245     // or &mut [T, ..n] -> &mut [T]
 246     // or &Concrete -> &Trait, etc.
 247     fn coerce_unsized(&self,
 248                       a: Ty<'tcx>,
 249                       b: Ty<'tcx>)
 250                       -> CoerceResult<'tcx> {
 251         debug!("coerce_unsized(a={}, b={})",
 252                a.repr(self.tcx()),
 253                b.repr(self.tcx()));
 254
 255         // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
 256         // a DST unless we have to. This currently comes out in the wash since
 257         // we can't unify [T] with U. But to properly support DST, we need to allow
 258         // that, at which point we will need extra checks on b here.
 259
 260         let sub = Sub(self.get_ref().clone());
 261
 262         match (&a.sty, &b.sty) {
 263             (&ty::ty_rptr(_, ty::mt{ty: t_a, mutbl: mutbl_a}), &ty::ty_rptr(_, mt_b)) => {
 264                 self.unpack_actual_value(t_a, |a| {
 265                     match self.unsize_ty(t_a, a, mt_b.ty) {
 266                         Some((ty, kind)) => {
 267                             if !can_coerce_mutbls(mutbl_a, mt_b.mutbl) {
 268                                 return Err(ty::terr_mutability);
 269                             }
 270
 271                             let coercion = Coercion(self.get_ref().trace.clone());
 272                             let r_borrow = self.get_ref().infcx.next_region_var(coercion);
 273                             let ty = ty::mk_rptr(self.tcx(),
 274                                                  self.tcx().mk_region(r_borrow),
 275                                                  ty::mt{ty: ty, mutbl: mt_b.mutbl});
 276                             try!(self.get_ref().infcx.try(|_| sub.tys(ty, b)));
 277                             debug!("Success, coerced with AutoDerefRef(1, \
 278                                     AutoPtr(AutoUnsize({})))", kind);
 279                             Ok(Some(AdjustDerefRef(AutoDerefRef {
 280                                 autoderefs: 1,
 281                                 autoref: Some(ty::AutoPtr(r_borrow, mt_b.mutbl,
 282                                                           Some(box AutoUnsize(kind))))
 283                             })))
 284                         }
 285                         _ => Err(ty::terr_mismatch)
 286                     }
 287                 })
 288             }
 289             (&ty::ty_rptr(_, ty::mt{ty: t_a, mutbl: mutbl_a}), &ty::ty_ptr(mt_b)) => {
 290                 self.unpack_actual_value(t_a, |a| {
 291                     match self.unsize_ty(t_a, a, mt_b.ty) {
 292                         Some((ty, kind)) => {
 293                             if !can_coerce_mutbls(mutbl_a, mt_b.mutbl) {
 294                                 return Err(ty::terr_mutability);
 295                             }
 296
 297                             let ty = ty::mk_ptr(self.tcx(),
 298                                                  ty::mt{ty: ty, mutbl: mt_b.mutbl});
 299                             try!(self.get_ref().infcx.try(|_| sub.tys(ty, b)));
 300                             debug!("Success, coerced with AutoDerefRef(1, \
 301                                     AutoPtr(AutoUnsize({})))", kind);
 302                             Ok(Some(AdjustDerefRef(AutoDerefRef {
 303                                 autoderefs: 1,
 304                                 autoref: Some(ty::AutoUnsafe(mt_b.mutbl,
 305                                                              Some(box AutoUnsize(kind))))
 306                             })))
 307                         }
 308                         _ => Err(ty::terr_mismatch)
 309                     }
 310                 })
 311             }
 312             (&ty::ty_uniq(t_a), &ty::ty_uniq(t_b)) => {
 313                 self.unpack_actual_value(t_a, |a| {
 314                     match self.unsize_ty(t_a, a, t_b) {
 315                         Some((ty, kind)) => {
 316                             let ty = ty::mk_uniq(self.tcx(), ty);
 317                             try!(self.get_ref().infcx.try(|_| sub.tys(ty, b)));
 318                             debug!("Success, coerced with AutoDerefRef(1, \
 319                                     AutoUnsizeUniq({}))", kind);
 320                             Ok(Some(AdjustDerefRef(AutoDerefRef {
 321                                 autoderefs: 1,
 322                                 autoref: Some(ty::AutoUnsizeUniq(kind))
 323                             })))
 324                         }
 325                         _ => Err(ty::terr_mismatch)
 326                     }
 327                 })
 328             }
 329             _ => Err(ty::terr_mismatch)
 330         }
 331     }
 332
 333     // Takes a type and returns an unsized version along with the adjustment
 334     // performed to unsize it.
 335     // E.g., `[T, ..n]` -> `([T], UnsizeLength(n))`
 336     fn unsize_ty(&self,
 337                  ty_a: Ty<'tcx>,
 338                  a: Ty<'tcx>,
 339                  ty_b: Ty<'tcx>)
 340                  -> Option<(Ty<'tcx>, ty::UnsizeKind<'tcx>)> {
 341         debug!("unsize_ty(a={}, ty_b={})", a, ty_b.repr(self.tcx()));
 342
 343         let tcx = self.tcx();
 344
 345         self.unpack_actual_value(ty_b, |b|
 346             match (&a.sty, &b.sty) {
 347                 (&ty::ty_vec(t_a, Some(len)), &ty::ty_vec(_, None)) => {
 348                     let ty = ty::mk_vec(tcx, t_a, None);
 349                     Some((ty, ty::UnsizeLength(len)))
 350                 }
 351                 (&ty::ty_trait(..), &ty::ty_trait(..)) => {
 352                     None
 353                 }
 354                 (_, &ty::ty_trait(box ty::TyTrait { ref principal, ref bounds })) => {
 355                     // FIXME what is the purpose of `ty`?
 356                     let ty = ty::mk_trait(tcx, principal.clone(), bounds.clone());
 357                     Some((ty, ty::UnsizeVtable(ty::TyTrait { principal: principal.clone(),
 358                                                              bounds: bounds.clone() },
 359                                                ty_a)))
 360                 }
 361                 (&ty::ty_struct(did_a, substs_a), &ty::ty_struct(did_b, substs_b))
 362                   if did_a == did_b => {
 363                     debug!("unsizing a struct");
 364                     // Try unsizing each type param in turn to see if we end up with ty_b.
 365                     let ty_substs_a = substs_a.types.get_slice(subst::TypeSpace);
 366                     let ty_substs_b = substs_b.types.get_slice(subst::TypeSpace);
 367                     assert!(ty_substs_a.len() == ty_substs_b.len());
 368
 369                     let sub = Sub(self.get_ref().clone());
 370
 371                     let mut result = None;
 372                     let mut tps = ty_substs_a.iter().zip(ty_substs_b.iter()).enumerate();
 373                     for (i, (tp_a, tp_b)) in tps {
 374                         if self.get_ref().infcx.try(|_| sub.tys(*tp_a, *tp_b)).is_ok() {
 375                             continue;
 376                         }
 377                         match
 378                             self.unpack_actual_value(
 379                                 *tp_a,
 380                                 |tp| self.unsize_ty(*tp_a, tp, *tp_b))
 381                         {
 382                             Some((new_tp, k)) => {
 383                                 // Check that the whole types match.
 384                                 let mut new_substs = substs_a.clone();
 385                                 new_substs.types.get_mut_slice(subst::TypeSpace)[i] = new_tp;
 386                                 let ty = ty::mk_struct(tcx, did_a, tcx.mk_substs(new_substs));
 387                                 if self.get_ref().infcx.try(|_| sub.tys(ty, ty_b)).is_err() {
 388                                     debug!("Unsized type parameter '{}', but still \
 389                                             could not match types {} and {}",
 390                                            ppaux::ty_to_string(tcx, *tp_a),
 391                                            ppaux::ty_to_string(tcx, ty),
 392                                            ppaux::ty_to_string(tcx, ty_b));
 393                                     // We can only unsize a single type parameter, so
 394                                     // if we unsize one and it doesn't give us the
 395                                     // type we want, then we won't succeed later.
 396                                     break;
 397                                 }
 398
 399                                 result = Some((ty, ty::UnsizeStruct(box k, i)));
 400                                 break;
 401                             }
 402                             None => {}
 403                         }
 404                     }
 405                     result
 406                 }
 407                 _ => None
 408             }
 409         )
 410     }
 411
 412     fn coerce_borrowed_object(&self,
 413                               a: Ty<'tcx>,
 414                               b: Ty<'tcx>,
 415                               b_mutbl: ast::Mutability) -> CoerceResult<'tcx>
 416     {
 417         let tcx = self.tcx();
 418
 419         debug!("coerce_borrowed_object(a={}, b={}, b_mutbl={})",
 420                a.repr(tcx),
 421                b.repr(tcx), b_mutbl);
 422
 423         let coercion = Coercion(self.get_ref().trace.clone());
 424         let r_a = self.get_ref().infcx.next_region_var(coercion);
 425
 426         self.coerce_object(a, b, b_mutbl,
 427                            |tr| ty::mk_rptr(tcx, tcx.mk_region(r_a),
 428                                             ty::mt{ mutbl: b_mutbl, ty: tr }),
 429                            || AutoPtr(r_a, b_mutbl, None))
 430     }
 431
 432     fn coerce_unsafe_object(&self,
 433                             a: Ty<'tcx>,
 434                             b: Ty<'tcx>,
 435                             b_mutbl: ast::Mutability) -> CoerceResult<'tcx>
 436     {
 437         let tcx = self.tcx();
 438
 439         debug!("coerce_unsafe_object(a={}, b={}, b_mutbl={})",
 440                a.repr(tcx),
 441                b.repr(tcx), b_mutbl);
 442
 443         self.coerce_object(a, b, b_mutbl,
 444                            |tr| ty::mk_ptr(tcx, ty::mt{ mutbl: b_mutbl, ty: tr }),
 445                            || AutoUnsafe(b_mutbl, None))
 446     }
 447
 448     fn coerce_object<F, G>(&self,
 449                            a: Ty<'tcx>,
 450                            b: Ty<'tcx>,
 451                            b_mutbl: ast::Mutability,
 452                            mk_ty: F,
 453                            mk_adjust: G) -> CoerceResult<'tcx> where
 454         F: FnOnce(Ty<'tcx>) -> Ty<'tcx>,
 455         G: FnOnce() -> ty::AutoRef<'tcx>,
 456     {
 457         let tcx = self.tcx();
 458
 459         match a.sty {
 460             ty::ty_rptr(_, ty::mt{ty, mutbl}) => match ty.sty {
 461                 ty::ty_trait(box ty::TyTrait { ref principal, ref bounds }) => {
 462                     debug!("mutbl={} b_mutbl={}", mutbl, b_mutbl);
 463                     let tr = ty::mk_trait(tcx, principal.clone(), bounds.clone());
 464                     try!(self.subtype(mk_ty(tr), b));
 465                     Ok(Some(AdjustDerefRef(AutoDerefRef {
 466                         autoderefs: 1,
 467                         autoref: Some(mk_adjust())
 468                     })))
 469                 }
 470                 _ => {
 471                     self.subtype(a, b)
 472                 }
 473             },
 474             _ => {
 475                 self.subtype(a, b)
 476             }
 477         }
 478     }
 479
 480     pub fn coerce_borrowed_fn(&self,
 481                               a: Ty<'tcx>,
 482                               b: Ty<'tcx>)
 483                               -> CoerceResult<'tcx> {
 484         debug!("coerce_borrowed_fn(a={}, b={})",
 485                a.repr(self.tcx()),
 486                b.repr(self.tcx()));
 487
 488         match a.sty {
 489             ty::ty_bare_fn(Some(a_def_id), f) => {
 490                 self.coerce_from_fn_item(a, a_def_id, f, b)
 491             }
 492             _ => {
 493                 self.subtype(a, b)
 494             }
 495         }
 496     }
 497
 498     fn coerce_from_fn_item(&self,
 499                            a: Ty<'tcx>,
 500                            fn_def_id_a: ast::DefId,
 501                            fn_ty_a: &'tcx ty::BareFnTy<'tcx>,
 502                            b: Ty<'tcx>)
 503                            -> CoerceResult<'tcx> {
 504         /*!
 505          * Attempts to coerce from the type of a Rust function item
 506          * into a closure or a `proc`.
 507          */
 508
 509         self.unpack_actual_value(b, |b| {
 510             debug!("coerce_from_fn_item(a={}, b={})",
 511                    a.repr(self.tcx()), b.repr(self.tcx()));
 512
 513             match b.sty {
 514                 ty::ty_closure(ref f) => {
 515                     if fn_ty_a.abi != abi::Rust || fn_ty_a.unsafety != ast::Unsafety::Normal {
 516                         return self.subtype(a, b);
 517                     }
 518
 519                     let fn_ty_b = (*f).clone();
 520                     let adj = ty::AdjustAddEnv(fn_def_id_a, fn_ty_b.store);
 521                     let a_closure = ty::mk_closure(self.tcx(),
 522                                                    ty::ClosureTy {
 523                                                        sig: fn_ty_a.sig.clone(),
 524                                                        .. *fn_ty_b
 525                                                    });
 526                     try!(self.subtype(a_closure, b));
 527                     Ok(Some(adj))
 528                 }
 529                 ty::ty_bare_fn(None, _) => {
 530                     let a_fn_pointer = ty::mk_bare_fn(self.tcx(), None, fn_ty_a);
 531                     try!(self.subtype(a_fn_pointer, b));
 532                     Ok(Some(ty::AdjustReifyFnPointer(fn_def_id_a)))
 533                 }
 534                 _ => {
 535                     return self.subtype(a, b)
 536                 }
 537             }
 538         })
 539     }
 540
 541     pub fn coerce_unsafe_ptr(&self,
 542                              a: Ty<'tcx>,
 543                              b: Ty<'tcx>,
 544                              mutbl_b: ast::Mutability)
 545                              -> CoerceResult<'tcx> {
 546         debug!("coerce_unsafe_ptr(a={}, b={})",
 547                a.repr(self.tcx()),
 548                b.repr(self.tcx()));
 549
 550         let mt_a = match a.sty {
 551             ty::ty_rptr(_, mt) | ty::ty_ptr(mt) => mt,
 552             _ => {
 553                 return self.subtype(a, b);
 554             }
 555         };
 556
 557         // Check that the types which they point at are compatible.
 558         let a_unsafe = ty::mk_ptr(self.tcx(), ty::mt{ mutbl: mutbl_b, ty: mt_a.ty });
 559         try!(self.subtype(a_unsafe, b));
 560         if !can_coerce_mutbls(mt_a.mutbl, mutbl_b) {
 561             return Err(ty::terr_mutability);
 562         }
 563
 564         // Although references and unsafe ptrs have the same
 565         // representation, we still register an AutoDerefRef so that
 566         // regionck knows that the region for `a` must be valid here.
 567         Ok(Some(AdjustDerefRef(AutoDerefRef {
 568             autoderefs: 1,
 569             autoref: Some(ty::AutoUnsafe(mutbl_b, None))
 570         })))
 571     }
 572 }
 573
 574 fn can_coerce_mutbls(from_mutbl: ast::Mutability,
 575                      to_mutbl: ast::Mutability)
 576                      -> bool {
 577     match (from_mutbl, to_mutbl) {
 578         (ast::MutMutable, ast::MutMutable) => true,
 579         (ast::MutImmutable, ast::MutImmutable) => true,
 580         (ast::MutMutable, ast::MutImmutable) => true,
 581         (ast::MutImmutable, ast::MutMutable) => false,
 582     }
 583 }