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.
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.
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.
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.
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.
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".
34 use rustc_data_structures::fx::FxHashMap;
35 use rustc_middle::infer::unify_key::ToType;
36 use rustc_middle::ty::fold::TypeFolder;
37 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitable};
38 use std::collections::hash_map::Entry;
40 pub struct TypeFreshener<'a, 'tcx> {
41 infcx: &'a InferCtxt<'tcx>,
42 ty_freshen_count: u32,
43 const_freshen_count: u32,
44 ty_freshen_map: FxHashMap<ty::InferTy, Ty<'tcx>>,
45 const_freshen_map: FxHashMap<ty::InferConst<'tcx>, ty::Const<'tcx>>,
49 impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
50 pub fn new(infcx: &'a InferCtxt<'tcx>, keep_static: bool) -> TypeFreshener<'a, 'tcx> {
54 const_freshen_count: 0,
55 ty_freshen_map: Default::default(),
56 const_freshen_map: Default::default(),
63 opt_ty: Option<Ty<'tcx>>,
68 F: FnOnce(u32) -> ty::InferTy,
70 if let Some(ty) = opt_ty {
71 return ty.fold_with(self);
74 match self.ty_freshen_map.entry(key) {
75 Entry::Occupied(entry) => *entry.get(),
76 Entry::Vacant(entry) => {
77 let index = self.ty_freshen_count;
78 self.ty_freshen_count += 1;
79 let t = self.infcx.tcx.mk_ty_infer(freshener(index));
88 opt_ct: Option<ty::Const<'tcx>>,
89 key: ty::InferConst<'tcx>,
94 F: FnOnce(u32) -> ty::InferConst<'tcx>,
96 if let Some(ct) = opt_ct {
97 return ct.fold_with(self);
100 match self.const_freshen_map.entry(key) {
101 Entry::Occupied(entry) => *entry.get(),
102 Entry::Vacant(entry) => {
103 let index = self.const_freshen_count;
104 self.const_freshen_count += 1;
105 let ct = self.infcx.tcx.mk_const(freshener(index), ty);
113 impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
114 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
118 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
120 ty::ReLateBound(..) => {
121 // leave bound regions alone
128 | ty::RePlaceholder(..)
130 // replace all free regions with 'erased
131 self.tcx().lifetimes.re_erased
134 if self.keep_static {
137 self.tcx().lifetimes.re_erased
143 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
144 if !t.needs_infer() && !t.has_erasable_regions() {
148 let tcx = self.infcx.tcx;
151 ty::Infer(ty::TyVar(v)) => {
152 let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
153 self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy)
156 ty::Infer(ty::IntVar(v)) => self.freshen_ty(
160 .int_unification_table()
162 .map(|v| v.to_type(tcx)),
167 ty::Infer(ty::FloatVar(v)) => self.freshen_ty(
171 .float_unification_table()
173 .map(|v| v.to_type(tcx)),
178 ty::Infer(ty::FreshTy(ct) | ty::FreshIntTy(ct) | ty::FreshFloatTy(ct)) => {
179 if ct >= self.ty_freshen_count {
181 "Encountered a freshend type with id {} \
182 but our counter is only at {}",
184 self.ty_freshen_count
212 | ty::GeneratorWitness(..)
213 | ty::Opaque(..) => t.super_fold_with(self),
215 ty::Placeholder(..) | ty::Bound(..) => bug!("unexpected type {:?}", t),
219 fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
221 ty::ConstKind::Infer(ty::InferConst::Var(v)) => {
226 .const_unification_table()
230 self.freshen_const(opt_ct, ty::InferConst::Var(v), ty::InferConst::Fresh, ct.ty())
232 ty::ConstKind::Infer(ty::InferConst::Fresh(i)) => {
233 if i >= self.const_freshen_count {
235 "Encountered a freshend const with id {} \
236 but our counter is only at {}",
238 self.const_freshen_count,
244 ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
245 bug!("unexpected const {:?}", ct)
248 ty::ConstKind::Param(_)
249 | ty::ConstKind::Value(_)
250 | ty::ConstKind::Unevaluated(..)
251 | ty::ConstKind::Expr(..)
252 | ty::ConstKind::Error(_) => ct.super_fold_with(self),