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
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
use rustc_errors::Diagnostic;
use rustc_span::Span;
use smallvec::smallvec;
use smallvec::SmallVec;

use rustc_data_structures::fx::FxHashSet;
use rustc_hir::def_id::DefId;
use rustc_middle::ty::subst::{GenericArg, Subst, SubstsRef};
use rustc_middle::ty::{self, ImplSubject, ToPredicate, Ty, TyCtxt, TypeVisitable};

use super::{Normalized, Obligation, ObligationCause, PredicateObligation, SelectionContext};
pub use rustc_infer::traits::{self, util::*};

///////////////////////////////////////////////////////////////////////////
// `TraitAliasExpander` iterator
///////////////////////////////////////////////////////////////////////////

/// "Trait alias expansion" is the process of expanding a sequence of trait
/// references into another sequence by transitively following all trait
/// aliases. e.g. If you have bounds like `Foo + Send`, a trait alias
/// `trait Foo = Bar + Sync;`, and another trait alias
/// `trait Bar = Read + Write`, then the bounds would expand to
/// `Read + Write + Sync + Send`.
/// Expansion is done via a DFS (depth-first search), and the `visited` field
/// is used to avoid cycles.
pub struct TraitAliasExpander<'tcx> {
    tcx: TyCtxt<'tcx>,
    stack: Vec<TraitAliasExpansionInfo<'tcx>>,
}

/// Stores information about the expansion of a trait via a path of zero or more trait aliases.
#[derive(Debug, Clone)]
pub struct TraitAliasExpansionInfo<'tcx> {
    pub path: SmallVec<[(ty::PolyTraitRef<'tcx>, Span); 4]>,
}

impl<'tcx> TraitAliasExpansionInfo<'tcx> {
    fn new(trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
        Self { path: smallvec![(trait_ref, span)] }
    }

    /// Adds diagnostic labels to `diag` for the expansion path of a trait through all intermediate
    /// trait aliases.
    pub fn label_with_exp_info(&self, diag: &mut Diagnostic, top_label: &str, use_desc: &str) {
        diag.span_label(self.top().1, top_label);
        if self.path.len() > 1 {
            for (_, sp) in self.path.iter().rev().skip(1).take(self.path.len() - 2) {
                diag.span_label(*sp, format!("referenced here ({})", use_desc));
            }
        }
        if self.top().1 != self.bottom().1 {
            // When the trait object is in a return type these two spans match, we don't want
            // redundant labels.
            diag.span_label(
                self.bottom().1,
                format!("trait alias used in trait object type ({})", use_desc),
            );
        }
    }

    pub fn trait_ref(&self) -> ty::PolyTraitRef<'tcx> {
        self.top().0
    }

    pub fn top(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
        self.path.last().unwrap()
    }

    pub fn bottom(&self) -> &(ty::PolyTraitRef<'tcx>, Span) {
        self.path.first().unwrap()
    }

    fn clone_and_push(&self, trait_ref: ty::PolyTraitRef<'tcx>, span: Span) -> Self {
        let mut path = self.path.clone();
        path.push((trait_ref, span));

        Self { path }
    }
}

pub fn expand_trait_aliases<'tcx>(
    tcx: TyCtxt<'tcx>,
    trait_refs: impl Iterator<Item = (ty::PolyTraitRef<'tcx>, Span)>,
) -> TraitAliasExpander<'tcx> {
    let items: Vec<_> =
        trait_refs.map(|(trait_ref, span)| TraitAliasExpansionInfo::new(trait_ref, span)).collect();
    TraitAliasExpander { tcx, stack: items }
}

impl<'tcx> TraitAliasExpander<'tcx> {
    /// If `item` is a trait alias and its predicate has not yet been visited, then expands `item`
    /// to the definition, pushes the resulting expansion onto `self.stack`, and returns `false`.
    /// Otherwise, immediately returns `true` if `item` is a regular trait, or `false` if it is a
    /// trait alias.
    /// The return value indicates whether `item` should be yielded to the user.
    fn expand(&mut self, item: &TraitAliasExpansionInfo<'tcx>) -> bool {
        let tcx = self.tcx;
        let trait_ref = item.trait_ref();
        let pred = trait_ref.without_const().to_predicate(tcx);

        debug!("expand_trait_aliases: trait_ref={:?}", trait_ref);

        // Don't recurse if this bound is not a trait alias.
        let is_alias = tcx.is_trait_alias(trait_ref.def_id());
        if !is_alias {
            return true;
        }

        // Don't recurse if this trait alias is already on the stack for the DFS search.
        let anon_pred = anonymize_predicate(tcx, pred);
        if item.path.iter().rev().skip(1).any(|&(tr, _)| {
            anonymize_predicate(tcx, tr.without_const().to_predicate(tcx)) == anon_pred
        }) {
            return false;
        }

        // Get components of trait alias.
        let predicates = tcx.super_predicates_of(trait_ref.def_id());
        debug!(?predicates);

        let items = predicates.predicates.iter().rev().filter_map(|(pred, span)| {
            pred.subst_supertrait(tcx, &trait_ref)
                .to_opt_poly_trait_pred()
                .map(|trait_ref| item.clone_and_push(trait_ref.map_bound(|t| t.trait_ref), *span))
        });
        debug!("expand_trait_aliases: items={:?}", items.clone().collect::<Vec<_>>());

        self.stack.extend(items);

        false
    }
}

impl<'tcx> Iterator for TraitAliasExpander<'tcx> {
    type Item = TraitAliasExpansionInfo<'tcx>;

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.stack.len(), None)
    }

    fn next(&mut self) -> Option<TraitAliasExpansionInfo<'tcx>> {
        while let Some(item) = self.stack.pop() {
            if self.expand(&item) {
                return Some(item);
            }
        }
        None
    }
}

///////////////////////////////////////////////////////////////////////////
// Iterator over def-IDs of supertraits
///////////////////////////////////////////////////////////////////////////

pub struct SupertraitDefIds<'tcx> {
    tcx: TyCtxt<'tcx>,
    stack: Vec<DefId>,
    visited: FxHashSet<DefId>,
}

pub fn supertrait_def_ids(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SupertraitDefIds<'_> {
    SupertraitDefIds {
        tcx,
        stack: vec![trait_def_id],
        visited: Some(trait_def_id).into_iter().collect(),
    }
}

impl Iterator for SupertraitDefIds<'_> {
    type Item = DefId;

    fn next(&mut self) -> Option<DefId> {
        let def_id = self.stack.pop()?;
        let predicates = self.tcx.super_predicates_of(def_id);
        let visited = &mut self.visited;
        self.stack.extend(
            predicates
                .predicates
                .iter()
                .filter_map(|(pred, _)| pred.to_opt_poly_trait_pred())
                .map(|trait_ref| trait_ref.def_id())
                .filter(|&super_def_id| visited.insert(super_def_id)),
        );
        Some(def_id)
    }
}

///////////////////////////////////////////////////////////////////////////
// Other
///////////////////////////////////////////////////////////////////////////

/// Instantiate all bound parameters of the impl subject with the given substs,
/// returning the resulting subject and all obligations that arise.
/// The obligations are closed under normalization.
pub fn impl_subject_and_oblig<'a, 'tcx>(
    selcx: &mut SelectionContext<'a, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    impl_def_id: DefId,
    impl_substs: SubstsRef<'tcx>,
) -> (ImplSubject<'tcx>, impl Iterator<Item = PredicateObligation<'tcx>>) {
    let subject = selcx.tcx().bound_impl_subject(impl_def_id);
    let subject = subject.subst(selcx.tcx(), impl_substs);
    let Normalized { value: subject, obligations: normalization_obligations1 } =
        super::normalize(selcx, param_env, ObligationCause::dummy(), subject);

    let predicates = selcx.tcx().predicates_of(impl_def_id);
    let predicates = predicates.instantiate(selcx.tcx(), impl_substs);
    let Normalized { value: predicates, obligations: normalization_obligations2 } =
        super::normalize(selcx, param_env, ObligationCause::dummy(), predicates);
    let impl_obligations =
        super::predicates_for_generics(|_, _| ObligationCause::dummy(), param_env, predicates);

    let impl_obligations = impl_obligations
        .chain(normalization_obligations1.into_iter())
        .chain(normalization_obligations2.into_iter());

    (subject, impl_obligations)
}

pub fn predicate_for_trait_ref<'tcx>(
    tcx: TyCtxt<'tcx>,
    cause: ObligationCause<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    trait_ref: ty::TraitRef<'tcx>,
    recursion_depth: usize,
) -> PredicateObligation<'tcx> {
    Obligation {
        cause,
        param_env,
        recursion_depth,
        predicate: ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx),
    }
}

pub fn predicate_for_trait_def<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    cause: ObligationCause<'tcx>,
    trait_def_id: DefId,
    recursion_depth: usize,
    self_ty: Ty<'tcx>,
    params: &[GenericArg<'tcx>],
) -> PredicateObligation<'tcx> {
    let trait_ref =
        ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(self_ty, params) };
    predicate_for_trait_ref(tcx, cause, param_env, trait_ref, recursion_depth)
}

/// Casts a trait reference into a reference to one of its super
/// traits; returns `None` if `target_trait_def_id` is not a
/// supertrait.
pub fn upcast_choices<'tcx>(
    tcx: TyCtxt<'tcx>,
    source_trait_ref: ty::PolyTraitRef<'tcx>,
    target_trait_def_id: DefId,
) -> Vec<ty::PolyTraitRef<'tcx>> {
    if source_trait_ref.def_id() == target_trait_def_id {
        return vec![source_trait_ref]; // Shortcut the most common case.
    }

    supertraits(tcx, source_trait_ref).filter(|r| r.def_id() == target_trait_def_id).collect()
}

/// Given a trait `trait_ref`, returns the number of vtable entries
/// that come from `trait_ref`, excluding its supertraits. Used in
/// computing the vtable base for an upcast trait of a trait object.
pub fn count_own_vtable_entries<'tcx>(
    tcx: TyCtxt<'tcx>,
    trait_ref: ty::PolyTraitRef<'tcx>,
) -> usize {
    let existential_trait_ref =
        trait_ref.map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
    let existential_trait_ref = tcx.erase_regions(existential_trait_ref);
    tcx.own_existential_vtable_entries(existential_trait_ref).len()
}

/// Given an upcast trait object described by `object`, returns the
/// index of the method `method_def_id` (which should be part of
/// `object.upcast_trait_ref`) within the vtable for `object`.
pub fn get_vtable_index_of_object_method<'tcx, N>(
    tcx: TyCtxt<'tcx>,
    object: &super::ImplSourceObjectData<'tcx, N>,
    method_def_id: DefId,
) -> Option<usize> {
    let existential_trait_ref = object
        .upcast_trait_ref
        .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
    let existential_trait_ref = tcx.erase_regions(existential_trait_ref);

    // Count number of methods preceding the one we are selecting and
    // add them to the total offset.
    if let Some(index) = tcx
        .own_existential_vtable_entries(existential_trait_ref)
        .iter()
        .copied()
        .position(|def_id| def_id == method_def_id)
    {
        Some(object.vtable_base + index)
    } else {
        None
    }
}

pub fn closure_trait_ref_and_return_type<'tcx>(
    tcx: TyCtxt<'tcx>,
    fn_trait_def_id: DefId,
    self_ty: Ty<'tcx>,
    sig: ty::PolyFnSig<'tcx>,
    tuple_arguments: TupleArgumentsFlag,
) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>)> {
    let arguments_tuple = match tuple_arguments {
        TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
        TupleArgumentsFlag::Yes => tcx.intern_tup(sig.skip_binder().inputs()),
    };
    debug_assert!(!self_ty.has_escaping_bound_vars());
    let trait_ref = ty::TraitRef {
        def_id: fn_trait_def_id,
        substs: tcx.mk_substs_trait(self_ty, &[arguments_tuple.into()]),
    };
    sig.map_bound(|sig| (trait_ref, sig.output()))
}

pub fn generator_trait_ref_and_outputs<'tcx>(
    tcx: TyCtxt<'tcx>,
    fn_trait_def_id: DefId,
    self_ty: Ty<'tcx>,
    sig: ty::PolyGenSig<'tcx>,
) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>, Ty<'tcx>)> {
    debug_assert!(!self_ty.has_escaping_bound_vars());
    let trait_ref = ty::TraitRef {
        def_id: fn_trait_def_id,
        substs: tcx.mk_substs_trait(self_ty, &[sig.skip_binder().resume_ty.into()]),
    };
    sig.map_bound(|sig| (trait_ref, sig.yield_ty, sig.return_ty))
}

pub fn impl_item_is_final(tcx: TyCtxt<'_>, assoc_item: &ty::AssocItem) -> bool {
    assoc_item.defaultness(tcx).is_final()
        && tcx.impl_defaultness(assoc_item.container_id(tcx)).is_final()
}

pub enum TupleArgumentsFlag {
    Yes,
    No,
}