1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
//! Freshening is the process of replacing unknown variables with fresh types. The idea is that
//! the type, after freshening, contains no inference variables but instead contains either a
//! value for each variable or fresh "arbitrary" types wherever a variable would have been.
//!
//! Freshening is used primarily to get a good type for inserting into a cache. The result
//! summarizes what the type inferencer knows "so far". The primary place it is used right now is
//! in the trait matching algorithm, which needs to be able to cache whether an `impl` self type
//! matches some other type X -- *without* affecting `X`. That means if that if the type `X` is in
//! fact an unbound type variable, we want the match to be regarded as ambiguous, because depending
//! on what type that type variable is ultimately assigned, the match may or may not succeed.
//!
//! To handle closures, freshened types also have to contain the signature and kind of any
//! closure in the local inference context, as otherwise the cache key might be invalidated.
//! The way this is done is somewhat hacky - the closure signature is appended to the substs,
//! as well as the closure kind "encoded" as a type. Also, special handling is needed when
//! the closure signature contains a reference to the original closure.
//!
//! Note that you should be careful not to allow the output of freshening to leak to the user in
//! error messages or in any other form. Freshening is only really useful as an internal detail.
//!
//! Because of the manipulation required to handle closures, doing arbitrary operations on
//! freshened types is not recommended. However, in addition to doing equality/hash
//! comparisons (for caching), it is possible to do a `ty::_match` operation between
//! 2 freshened types - this works even with the closure encoding.
//!
//! __An important detail concerning regions.__ The freshener also replaces *all* free regions with
//! 'erased. The reason behind this is that, in general, we do not take region relationships into
//! account when making type-overloaded decisions. This is important because of the design of the
//! region inferencer, which is not based on unification but rather on accumulating and then
//! solving a set of constraints. In contrast, the type inferencer assigns a value to each type
//! variable only once, and it does so as soon as it can, so it is reasonable to ask what the type
//! inferencer knows "so far".
use super::InferCtxt;
use rustc_data_structures::fx::FxHashMap;
use rustc_middle::infer::unify_key::ToType;
use rustc_middle::ty::fold::TypeFolder;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitable};
use std::collections::hash_map::Entry;

pub struct TypeFreshener<'a, 'tcx> {
    infcx: &'a InferCtxt<'a, 'tcx>,
    ty_freshen_count: u32,
    const_freshen_count: u32,
    ty_freshen_map: FxHashMap<ty::InferTy, Ty<'tcx>>,
    const_freshen_map: FxHashMap<ty::InferConst<'tcx>, ty::Const<'tcx>>,
    keep_static: bool,
}

impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
    pub fn new(infcx: &'a InferCtxt<'a, 'tcx>, keep_static: bool) -> TypeFreshener<'a, 'tcx> {
        TypeFreshener {
            infcx,
            ty_freshen_count: 0,
            const_freshen_count: 0,
            ty_freshen_map: Default::default(),
            const_freshen_map: Default::default(),
            keep_static,
        }
    }

    fn freshen_ty<F>(
        &mut self,
        opt_ty: Option<Ty<'tcx>>,
        key: ty::InferTy,
        freshener: F,
    ) -> Ty<'tcx>
    where
        F: FnOnce(u32) -> ty::InferTy,
    {
        if let Some(ty) = opt_ty {
            return ty.fold_with(self);
        }

        match self.ty_freshen_map.entry(key) {
            Entry::Occupied(entry) => *entry.get(),
            Entry::Vacant(entry) => {
                let index = self.ty_freshen_count;
                self.ty_freshen_count += 1;
                let t = self.infcx.tcx.mk_ty_infer(freshener(index));
                entry.insert(t);
                t
            }
        }
    }

    fn freshen_const<F>(
        &mut self,
        opt_ct: Option<ty::Const<'tcx>>,
        key: ty::InferConst<'tcx>,
        freshener: F,
        ty: Ty<'tcx>,
    ) -> ty::Const<'tcx>
    where
        F: FnOnce(u32) -> ty::InferConst<'tcx>,
    {
        if let Some(ct) = opt_ct {
            return ct.fold_with(self);
        }

        match self.const_freshen_map.entry(key) {
            Entry::Occupied(entry) => *entry.get(),
            Entry::Vacant(entry) => {
                let index = self.const_freshen_count;
                self.const_freshen_count += 1;
                let ct = self.infcx.tcx.mk_const_infer(freshener(index), ty);
                entry.insert(ct);
                ct
            }
        }
    }
}

impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
        self.infcx.tcx
    }

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        match *r {
            ty::ReLateBound(..) => {
                // leave bound regions alone
                r
            }

            ty::ReEarlyBound(..)
            | ty::ReFree(_)
            | ty::ReVar(_)
            | ty::RePlaceholder(..)
            | ty::ReErased => {
                // replace all free regions with 'erased
                self.tcx().lifetimes.re_erased
            }
            ty::ReStatic => {
                if self.keep_static {
                    r
                } else {
                    self.tcx().lifetimes.re_erased
                }
            }
        }
    }

    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
        if !t.needs_infer() && !t.has_erasable_regions() {
            return t;
        }

        let tcx = self.infcx.tcx;

        match *t.kind() {
            ty::Infer(ty::TyVar(v)) => {
                let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
                self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy)
            }

            ty::Infer(ty::IntVar(v)) => self.freshen_ty(
                self.infcx
                    .inner
                    .borrow_mut()
                    .int_unification_table()
                    .probe_value(v)
                    .map(|v| v.to_type(tcx)),
                ty::IntVar(v),
                ty::FreshIntTy,
            ),

            ty::Infer(ty::FloatVar(v)) => self.freshen_ty(
                self.infcx
                    .inner
                    .borrow_mut()
                    .float_unification_table()
                    .probe_value(v)
                    .map(|v| v.to_type(tcx)),
                ty::FloatVar(v),
                ty::FreshFloatTy,
            ),

            ty::Infer(ty::FreshTy(ct) | ty::FreshIntTy(ct) | ty::FreshFloatTy(ct)) => {
                if ct >= self.ty_freshen_count {
                    bug!(
                        "Encountered a freshend type with id {} \
                          but our counter is only at {}",
                        ct,
                        self.ty_freshen_count
                    );
                }
                t
            }

            ty::Generator(..)
            | ty::Bool
            | ty::Char
            | ty::Int(..)
            | ty::Uint(..)
            | ty::Float(..)
            | ty::Adt(..)
            | ty::Str
            | ty::Error(_)
            | ty::Array(..)
            | ty::Slice(..)
            | ty::RawPtr(..)
            | ty::Ref(..)
            | ty::FnDef(..)
            | ty::FnPtr(_)
            | ty::Dynamic(..)
            | ty::Never
            | ty::Tuple(..)
            | ty::Projection(..)
            | ty::Foreign(..)
            | ty::Param(..)
            | ty::Closure(..)
            | ty::GeneratorWitness(..)
            | ty::Opaque(..) => t.super_fold_with(self),

            ty::Placeholder(..) | ty::Bound(..) => bug!("unexpected type {:?}", t),
        }
    }

    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
        match ct.kind() {
            ty::ConstKind::Infer(ty::InferConst::Var(v)) => {
                let opt_ct = self
                    .infcx
                    .inner
                    .borrow_mut()
                    .const_unification_table()
                    .probe_value(v)
                    .val
                    .known();
                self.freshen_const(opt_ct, ty::InferConst::Var(v), ty::InferConst::Fresh, ct.ty())
            }
            ty::ConstKind::Infer(ty::InferConst::Fresh(i)) => {
                if i >= self.const_freshen_count {
                    bug!(
                        "Encountered a freshend const with id {} \
                            but our counter is only at {}",
                        i,
                        self.const_freshen_count,
                    );
                }
                ct
            }

            ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
                bug!("unexpected const {:?}", ct)
            }

            ty::ConstKind::Param(_)
            | ty::ConstKind::Value(_)
            | ty::ConstKind::Unevaluated(..)
            | ty::ConstKind::Error(_) => ct.super_fold_with(self),
        }
    }
}