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
use super::combine::CombineFields;
use super::{DefineOpaqueTypes, ObligationEmittingRelation, SubregionOrigin};

use crate::traits::{Obligation, PredicateObligations};
use rustc_middle::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
use rustc_middle::ty::visit::TypeVisitableExt;
use rustc_middle::ty::TyVar;
use rustc_middle::ty::{self, Ty, TyCtxt};
use std::mem;

/// Ensures `a` is made a subtype of `b`. Returns `a` on success.
pub struct Sub<'combine, 'a, 'tcx> {
    fields: &'combine mut CombineFields<'a, 'tcx>,
    a_is_expected: bool,
}

impl<'combine, 'infcx, 'tcx> Sub<'combine, 'infcx, 'tcx> {
    pub fn new(
        f: &'combine mut CombineFields<'infcx, 'tcx>,
        a_is_expected: bool,
    ) -> Sub<'combine, 'infcx, 'tcx> {
        Sub { fields: f, a_is_expected }
    }

    fn with_expected_switched<R, F: FnOnce(&mut Self) -> R>(&mut self, f: F) -> R {
        self.a_is_expected = !self.a_is_expected;
        let result = f(self);
        self.a_is_expected = !self.a_is_expected;
        result
    }
}

impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
    fn tag(&self) -> &'static str {
        "Sub"
    }

    fn tcx(&self) -> TyCtxt<'tcx> {
        self.fields.infcx.tcx
    }

    fn param_env(&self) -> ty::ParamEnv<'tcx> {
        self.fields.param_env
    }

    fn a_is_expected(&self) -> bool {
        self.a_is_expected
    }

    fn with_cause<F, R>(&mut self, cause: Cause, f: F) -> R
    where
        F: FnOnce(&mut Self) -> R,
    {
        debug!("sub with_cause={:?}", cause);
        let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
        let r = f(self);
        debug!("sub old_cause={:?}", old_cause);
        self.fields.cause = old_cause;
        r
    }

    fn relate_with_variance<T: Relate<'tcx>>(
        &mut self,
        variance: ty::Variance,
        _info: ty::VarianceDiagInfo<'tcx>,
        a: T,
        b: T,
    ) -> RelateResult<'tcx, T> {
        match variance {
            ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
            ty::Covariant => self.relate(a, b),
            ty::Bivariant => Ok(a),
            ty::Contravariant => self.with_expected_switched(|this| this.relate(b, a)),
        }
    }

    #[instrument(skip(self), level = "debug")]
    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
        if a == b {
            return Ok(a);
        }

        let infcx = self.fields.infcx;
        let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
        let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);

        match (a.kind(), b.kind()) {
            (&ty::Infer(TyVar(_)), &ty::Infer(TyVar(_))) => {
                // Shouldn't have any LBR here, so we can safely put
                // this under a binder below without fear of accidental
                // capture.
                assert!(!a.has_escaping_bound_vars());
                assert!(!b.has_escaping_bound_vars());

                // can't make progress on `A <: B` if both A and B are
                // type variables, so record an obligation.
                self.fields.obligations.push(Obligation::new(
                    self.tcx(),
                    self.fields.trace.cause.clone(),
                    self.fields.param_env,
                    ty::Binder::dummy(ty::PredicateKind::Subtype(ty::SubtypePredicate {
                        a_is_expected: self.a_is_expected,
                        a,
                        b,
                    })),
                ));

                Ok(a)
            }
            (&ty::Infer(TyVar(a_id)), _) => {
                self.fields.instantiate(b, ty::Contravariant, a_id, !self.a_is_expected)?;
                Ok(a)
            }
            (_, &ty::Infer(TyVar(b_id))) => {
                self.fields.instantiate(a, ty::Covariant, b_id, self.a_is_expected)?;
                Ok(a)
            }

            (&ty::Error(e), _) | (_, &ty::Error(e)) => {
                infcx.set_tainted_by_errors(e);
                Ok(Ty::new_error(self.tcx(), e))
            }

            (
                &ty::Alias(ty::Opaque, ty::AliasTy { def_id: a_def_id, .. }),
                &ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }),
            ) if a_def_id == b_def_id => {
                self.fields.infcx.super_combine_tys(self, a, b)?;
                Ok(a)
            }
            (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
            | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
                if self.fields.define_opaque_types == DefineOpaqueTypes::Yes
                    && def_id.is_local()
                    && !self.fields.infcx.next_trait_solver() =>
            {
                self.fields.obligations.extend(
                    infcx
                        .handle_opaque_type(
                            a,
                            b,
                            self.a_is_expected,
                            &self.fields.trace.cause,
                            self.param_env(),
                        )?
                        .obligations,
                );
                Ok(a)
            }
            _ => {
                self.fields.infcx.super_combine_tys(self, a, b)?;
                Ok(a)
            }
        }
    }

    fn regions(
        &mut self,
        a: ty::Region<'tcx>,
        b: ty::Region<'tcx>,
    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
        debug!("{}.regions({:?}, {:?}) self.cause={:?}", self.tag(), a, b, self.fields.cause);

        // FIXME -- we have more fine-grained information available
        // from the "cause" field, we could perhaps give more tailored
        // error messages.
        let origin = SubregionOrigin::Subtype(Box::new(self.fields.trace.clone()));
        // Subtype(&'a u8, &'b u8) => Outlives('a: 'b) => SubRegion('b, 'a)
        self.fields
            .infcx
            .inner
            .borrow_mut()
            .unwrap_region_constraints()
            .make_subregion(origin, b, a);

        Ok(a)
    }

    fn consts(
        &mut self,
        a: ty::Const<'tcx>,
        b: ty::Const<'tcx>,
    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
        self.fields.infcx.super_combine_consts(self, a, b)
    }

    fn binders<T>(
        &mut self,
        a: ty::Binder<'tcx, T>,
        b: ty::Binder<'tcx, T>,
    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
    where
        T: Relate<'tcx>,
    {
        // A binder is always a subtype of itself if it's structurally equal to itself
        if a == b {
            return Ok(a);
        }

        self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
        Ok(a)
    }
}

impl<'tcx> ObligationEmittingRelation<'tcx> for Sub<'_, '_, 'tcx> {
    fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ty::ToPredicate<'tcx>>) {
        self.fields.register_predicates(obligations);
    }

    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
        self.fields.register_obligations(obligations);
    }

    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
        ty::AliasRelationDirection::Subtype
    }
}