compiler/rustc_infer/src/infer/freshen.rs

   1 //! Freshening is the process of replacing unknown variables with fresh types. The idea is that
   2 //! the type, after freshening, contains no inference variables but instead contains either a
   3 //! value for each variable or fresh "arbitrary" types wherever a variable would have been.
   4 //!
   5 //! Freshening is used primarily to get a good type for inserting into a cache. The result
   6 //! summarizes what the type inferencer knows "so far". The primary place it is used right now is
   7 //! in the trait matching algorithm, which needs to be able to cache whether an `impl` self type
   8 //! matches some other type X -- *without* affecting `X`. That means if that if the type `X` is in
   9 //! fact an unbound type variable, we want the match to be regarded as ambiguous, because depending
  10 //! on what type that type variable is ultimately assigned, the match may or may not succeed.
  11 //!
  12 //! To handle closures, freshened types also have to contain the signature and kind of any
  13 //! closure in the local inference context, as otherwise the cache key might be invalidated.
  14 //! The way this is done is somewhat hacky - the closure signature is appended to the substs,
  15 //! as well as the closure kind "encoded" as a type. Also, special handling is needed when
  16 //! the closure signature contains a reference to the original closure.
  17 //!
  18 //! Note that you should be careful not to allow the output of freshening to leak to the user in
  19 //! error messages or in any other form. Freshening is only really useful as an internal detail.
  20 //!
  21 //! Because of the manipulation required to handle closures, doing arbitrary operations on
  22 //! freshened types is not recommended. However, in addition to doing equality/hash
  23 //! comparisons (for caching), it is possible to do a `ty::_match` operation between
  24 //! 2 freshened types - this works even with the closure encoding.
  25 //!
  26 //! __An important detail concerning regions.__ The freshener also replaces *all* free regions with
  27 //! 'erased. The reason behind this is that, in general, we do not take region relationships into
  28 //! account when making type-overloaded decisions. This is important because of the design of the
  29 //! region inferencer, which is not based on unification but rather on accumulating and then
  30 //! solving a set of constraints. In contrast, the type inferencer assigns a value to each type
  31 //! variable only once, and it does so as soon as it can, so it is reasonable to ask what the type
  32 //! inferencer knows "so far".
  33
  34 use rustc_middle::ty::fold::TypeFolder;
  35 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
  36
  37 use rustc_data_structures::fx::FxHashMap;
  38
  39 use std::collections::hash_map::Entry;
  40
  41 use super::unify_key::ToType;
  42 use super::InferCtxt;
  43
  44 pub struct TypeFreshener<'a, 'tcx> {
  45     infcx: &'a InferCtxt<'a, 'tcx>,
  46     ty_freshen_count: u32,
  47     const_freshen_count: u32,
  48     ty_freshen_map: FxHashMap<ty::InferTy, Ty<'tcx>>,
  49     const_freshen_map: FxHashMap<ty::InferConst<'tcx>, ty::Const<'tcx>>,
  50     keep_static: bool,
  51 }
  52
  53 impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
  54     pub fn new(infcx: &'a InferCtxt<'a, 'tcx>, keep_static: bool) -> TypeFreshener<'a, 'tcx> {
  55         TypeFreshener {
  56             infcx,
  57             ty_freshen_count: 0,
  58             const_freshen_count: 0,
  59             ty_freshen_map: Default::default(),
  60             const_freshen_map: Default::default(),
  61             keep_static,
  62         }
  63     }
  64
  65     fn freshen_ty<F>(
  66         &mut self,
  67         opt_ty: Option<Ty<'tcx>>,
  68         key: ty::InferTy,
  69         freshener: F,
  70     ) -> Ty<'tcx>
  71     where
  72         F: FnOnce(u32) -> ty::InferTy,
  73     {
  74         if let Some(ty) = opt_ty {
  75             return ty.fold_with(self);
  76         }
  77
  78         match self.ty_freshen_map.entry(key) {
  79             Entry::Occupied(entry) => *entry.get(),
  80             Entry::Vacant(entry) => {
  81                 let index = self.ty_freshen_count;
  82                 self.ty_freshen_count += 1;
  83                 let t = self.infcx.tcx.mk_ty_infer(freshener(index));
  84                 entry.insert(t);
  85                 t
  86             }
  87         }
  88     }
  89
  90     fn freshen_const<F>(
  91         &mut self,
  92         opt_ct: Option<ty::Const<'tcx>>,
  93         key: ty::InferConst<'tcx>,
  94         freshener: F,
  95         ty: Ty<'tcx>,
  96     ) -> ty::Const<'tcx>
  97     where
  98         F: FnOnce(u32) -> ty::InferConst<'tcx>,
  99     {
 100         if let Some(ct) = opt_ct {
 101             return ct.fold_with(self);
 102         }
 103
 104         match self.const_freshen_map.entry(key) {
 105             Entry::Occupied(entry) => *entry.get(),
 106             Entry::Vacant(entry) => {
 107                 let index = self.const_freshen_count;
 108                 self.const_freshen_count += 1;
 109                 let ct = self.infcx.tcx.mk_const_infer(freshener(index), ty);
 110                 entry.insert(ct);
 111                 ct
 112             }
 113         }
 114     }
 115 }
 116
 117 impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
 118     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 119         self.infcx.tcx
 120     }
 121
 122     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 123         match *r {
 124             ty::ReLateBound(..) => {
 125                 // leave bound regions alone
 126                 r
 127             }
 128
 129             ty::ReEarlyBound(..)
 130             | ty::ReFree(_)
 131             | ty::ReVar(_)
 132             | ty::RePlaceholder(..)
 133             | ty::ReEmpty(_)
 134             | ty::ReErased => {
 135                 // replace all free regions with 'erased
 136                 self.tcx().lifetimes.re_erased
 137             }
 138             ty::ReStatic => {
 139                 if self.keep_static {
 140                     r
 141                 } else {
 142                     self.tcx().lifetimes.re_erased
 143                 }
 144             }
 145         }
 146     }
 147
 148     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 149         if !t.needs_infer() && !t.has_erasable_regions() {
 150             return t;
 151         }
 152
 153         let tcx = self.infcx.tcx;
 154
 155         match *t.kind() {
 156             ty::Infer(ty::TyVar(v)) => {
 157                 let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
 158                 self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy)
 159             }
 160
 161             ty::Infer(ty::IntVar(v)) => self.freshen_ty(
 162                 self.infcx
 163                     .inner
 164                     .borrow_mut()
 165                     .int_unification_table()
 166                     .probe_value(v)
 167                     .map(|v| v.to_type(tcx)),
 168                 ty::IntVar(v),
 169                 ty::FreshIntTy,
 170             ),
 171
 172             ty::Infer(ty::FloatVar(v)) => self.freshen_ty(
 173                 self.infcx
 174                     .inner
 175                     .borrow_mut()
 176                     .float_unification_table()
 177                     .probe_value(v)
 178                     .map(|v| v.to_type(tcx)),
 179                 ty::FloatVar(v),
 180                 ty::FreshFloatTy,
 181             ),
 182
 183             ty::Infer(ty::FreshTy(ct) | ty::FreshIntTy(ct) | ty::FreshFloatTy(ct)) => {
 184                 if ct >= self.ty_freshen_count {
 185                     bug!(
 186                         "Encountered a freshend type with id {} \
 187                           but our counter is only at {}",
 188                         ct,
 189                         self.ty_freshen_count
 190                     );
 191                 }
 192                 t
 193             }
 194
 195             ty::Generator(..)
 196             | ty::Bool
 197             | ty::Char
 198             | ty::Int(..)
 199             | ty::Uint(..)
 200             | ty::Float(..)
 201             | ty::Adt(..)
 202             | ty::Str
 203             | ty::Error(_)
 204             | ty::Array(..)
 205             | ty::Slice(..)
 206             | ty::RawPtr(..)
 207             | ty::Ref(..)
 208             | ty::FnDef(..)
 209             | ty::FnPtr(_)
 210             | ty::Dynamic(..)
 211             | ty::Never
 212             | ty::Tuple(..)
 213             | ty::Projection(..)
 214             | ty::Foreign(..)
 215             | ty::Param(..)
 216             | ty::Closure(..)
 217             | ty::GeneratorWitness(..)
 218             | ty::Opaque(..) => t.super_fold_with(self),
 219
 220             ty::Placeholder(..) | ty::Bound(..) => bug!("unexpected type {:?}", t),
 221         }
 222     }
 223
 224     fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
 225         match ct.val() {
 226             ty::ConstKind::Infer(ty::InferConst::Var(v)) => {
 227                 let opt_ct = self
 228                     .infcx
 229                     .inner
 230                     .borrow_mut()
 231                     .const_unification_table()
 232                     .probe_value(v)
 233                     .val
 234                     .known();
 235                 return self.freshen_const(
 236                     opt_ct,
 237                     ty::InferConst::Var(v),
 238                     ty::InferConst::Fresh,
 239                     ct.ty(),
 240                 );
 241             }
 242             ty::ConstKind::Infer(ty::InferConst::Fresh(i)) => {
 243                 if i >= self.const_freshen_count {
 244                     bug!(
 245                         "Encountered a freshend const with id {} \
 246                             but our counter is only at {}",
 247                         i,
 248                         self.const_freshen_count,
 249                     );
 250                 }
 251                 return ct;
 252             }
 253
 254             ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
 255                 bug!("unexpected const {:?}", ct)
 256             }
 257
 258             ty::ConstKind::Param(_)
 259             | ty::ConstKind::Value(_)
 260             | ty::ConstKind::Unevaluated(..)
 261             | ty::ConstKind::Error(_) => {}
 262         }
 263
 264         ct.super_fold_with(self)
 265     }
 266 }