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}
;
38 use std
::collections
::hash_map
::Entry
;
40 pub struct TypeFreshener
<'a
, 'tcx
> {
41 infcx
: &'a InferCtxt
<'a
, '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
<'a
, '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_infer(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(..)
131 // replace all free regions with 'erased
132 self.tcx().lifetimes
.re_erased
135 if self.keep_static
{
138 self.tcx().lifetimes
.re_erased
144 fn fold_ty(&mut self, t
: Ty
<'tcx
>) -> Ty
<'tcx
> {
145 if !t
.needs_infer() && !t
.has_erasable_regions() {
149 let tcx
= self.infcx
.tcx
;
152 ty
::Infer(ty
::TyVar(v
)) => {
153 let opt_ty
= self.infcx
.inner
.borrow_mut().type_variables().probe(v
).known();
154 self.freshen_ty(opt_ty
, ty
::TyVar(v
), ty
::FreshTy
)
157 ty
::Infer(ty
::IntVar(v
)) => self.freshen_ty(
161 .int_unification_table()
163 .map(|v
| v
.to_type(tcx
)),
168 ty
::Infer(ty
::FloatVar(v
)) => self.freshen_ty(
172 .float_unification_table()
174 .map(|v
| v
.to_type(tcx
)),
179 ty
::Infer(ty
::FreshTy(ct
) | ty
::FreshIntTy(ct
) | ty
::FreshFloatTy(ct
)) => {
180 if ct
>= self.ty_freshen_count
{
182 "Encountered a freshend type with id {} \
183 but our counter is only at {}",
185 self.ty_freshen_count
213 | ty
::GeneratorWitness(..)
214 | ty
::Opaque(..) => t
.super_fold_with(self),
216 ty
::Placeholder(..) | ty
::Bound(..) => bug
!("unexpected type {:?}", t
),
220 fn fold_const(&mut self, ct
: ty
::Const
<'tcx
>) -> ty
::Const
<'tcx
> {
222 ty
::ConstKind
::Infer(ty
::InferConst
::Var(v
)) => {
227 .const_unification_table()
231 self.freshen_const(opt_ct
, ty
::InferConst
::Var(v
), ty
::InferConst
::Fresh
, ct
.ty())
233 ty
::ConstKind
::Infer(ty
::InferConst
::Fresh(i
)) => {
234 if i
>= self.const_freshen_count
{
236 "Encountered a freshend const with id {} \
237 but our counter is only at {}",
239 self.const_freshen_count
,
245 ty
::ConstKind
::Bound(..) | ty
::ConstKind
::Placeholder(_
) => {
246 bug
!("unexpected const {:?}", ct
)
249 ty
::ConstKind
::Param(_
)
250 | ty
::ConstKind
::Value(_
)
251 | ty
::ConstKind
::Unevaluated(..)
252 | ty
::ConstKind
::Error(_
) => ct
.super_fold_with(self),