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
use super::{
    ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote, PredicateObligation,
};
use crate::infer::InferCtxt;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_middle::ty::subst::{Subst, SubstsRef};
use rustc_middle::ty::{self, GenericParamDefKind};
use rustc_span::symbol::sym;
use std::iter;

use super::InferCtxtPrivExt;

pub trait InferCtxtExt<'tcx> {
    /*private*/
    fn impl_similar_to(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        obligation: &PredicateObligation<'tcx>,
    ) -> Option<(DefId, SubstsRef<'tcx>)>;

    /*private*/
    fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str>;

    fn on_unimplemented_note(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        obligation: &PredicateObligation<'tcx>,
    ) -> OnUnimplementedNote;
}

impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
    fn impl_similar_to(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        obligation: &PredicateObligation<'tcx>,
    ) -> Option<(DefId, SubstsRef<'tcx>)> {
        let tcx = self.tcx;
        let param_env = obligation.param_env;
        let trait_ref = tcx.erase_late_bound_regions(trait_ref);
        let trait_self_ty = trait_ref.self_ty();

        let mut self_match_impls = vec![];
        let mut fuzzy_match_impls = vec![];

        self.tcx.for_each_relevant_impl(trait_ref.def_id, trait_self_ty, |def_id| {
            let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
            let impl_trait_ref = tcx.bound_impl_trait_ref(def_id).unwrap().subst(tcx, impl_substs);

            let impl_self_ty = impl_trait_ref.self_ty();

            if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
                self_match_impls.push((def_id, impl_substs));

                if iter::zip(
                    trait_ref.substs.types().skip(1),
                    impl_trait_ref.substs.types().skip(1),
                )
                .all(|(u, v)| self.fuzzy_match_tys(u, v, false).is_some())
                {
                    fuzzy_match_impls.push((def_id, impl_substs));
                }
            }
        });

        let impl_def_id_and_substs = if self_match_impls.len() == 1 {
            self_match_impls[0]
        } else if fuzzy_match_impls.len() == 1 {
            fuzzy_match_impls[0]
        } else {
            return None;
        };

        tcx.has_attr(impl_def_id_and_substs.0, sym::rustc_on_unimplemented)
            .then_some(impl_def_id_and_substs)
    }

    /// Used to set on_unimplemented's `ItemContext`
    /// to be the enclosing (async) block/function/closure
    fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str> {
        let hir = self.tcx.hir();
        let node = hir.find(hir_id)?;
        match &node {
            hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. }) => {
                self.describe_generator(*body_id).or_else(|| {
                    Some(match sig.header {
                        hir::FnHeader { asyncness: hir::IsAsync::Async, .. } => "an async function",
                        _ => "a function",
                    })
                })
            }
            hir::Node::TraitItem(hir::TraitItem {
                kind: hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)),
                ..
            }) => self.describe_generator(*body_id).or_else(|| Some("a trait method")),
            hir::Node::ImplItem(hir::ImplItem {
                kind: hir::ImplItemKind::Fn(sig, body_id),
                ..
            }) => self.describe_generator(*body_id).or_else(|| {
                Some(match sig.header {
                    hir::FnHeader { asyncness: hir::IsAsync::Async, .. } => "an async method",
                    _ => "a method",
                })
            }),
            hir::Node::Expr(hir::Expr {
                kind: hir::ExprKind::Closure(hir::Closure { body, movability, .. }),
                ..
            }) => self.describe_generator(*body).or_else(|| {
                Some(if movability.is_some() { "an async closure" } else { "a closure" })
            }),
            hir::Node::Expr(hir::Expr { .. }) => {
                let parent_hid = hir.get_parent_node(hir_id);
                if parent_hid != hir_id { self.describe_enclosure(parent_hid) } else { None }
            }
            _ => None,
        }
    }

    fn on_unimplemented_note(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        obligation: &PredicateObligation<'tcx>,
    ) -> OnUnimplementedNote {
        let (def_id, substs) = self
            .impl_similar_to(trait_ref, obligation)
            .unwrap_or_else(|| (trait_ref.def_id(), trait_ref.skip_binder().substs));
        let trait_ref = trait_ref.skip_binder();

        let mut flags = vec![(
            sym::ItemContext,
            self.describe_enclosure(obligation.cause.body_id).map(|s| s.to_owned()),
        )];

        match obligation.cause.code() {
            ObligationCauseCode::BuiltinDerivedObligation(..)
            | ObligationCauseCode::ImplDerivedObligation(..)
            | ObligationCauseCode::DerivedObligation(..) => {}
            _ => {
                // this is a "direct", user-specified, rather than derived,
                // obligation.
                flags.push((sym::direct, None));
            }
        }

        if let ObligationCauseCode::ItemObligation(item)
        | ObligationCauseCode::BindingObligation(item, _)
        | ObligationCauseCode::ExprItemObligation(item, ..)
        | ObligationCauseCode::ExprBindingObligation(item, ..) = *obligation.cause.code()
        {
            // FIXME: maybe also have some way of handling methods
            // from other traits? That would require name resolution,
            // which we might want to be some sort of hygienic.
            //
            // Currently I'm leaving it for what I need for `try`.
            if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
                let method = self.tcx.item_name(item);
                flags.push((sym::from_method, None));
                flags.push((sym::from_method, Some(method.to_string())));
            }
        }

        if let Some(k) = obligation.cause.span.desugaring_kind() {
            flags.push((sym::from_desugaring, None));
            flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
        }

        // Add all types without trimmed paths.
        ty::print::with_no_trimmed_paths!({
            let generics = self.tcx.generics_of(def_id);
            let self_ty = trait_ref.self_ty();
            // This is also included through the generics list as `Self`,
            // but the parser won't allow you to use it
            flags.push((sym::_Self, Some(self_ty.to_string())));
            if let Some(def) = self_ty.ty_adt_def() {
                // We also want to be able to select self's original
                // signature with no type arguments resolved
                flags.push((sym::_Self, Some(self.tcx.type_of(def.did()).to_string())));
            }

            for param in generics.params.iter() {
                let value = match param.kind {
                    GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
                        substs[param.index as usize].to_string()
                    }
                    GenericParamDefKind::Lifetime => continue,
                };
                let name = param.name;
                flags.push((name, Some(value)));

                if let GenericParamDefKind::Type { .. } = param.kind {
                    let param_ty = substs[param.index as usize].expect_ty();
                    if let Some(def) = param_ty.ty_adt_def() {
                        // We also want to be able to select the parameter's
                        // original signature with no type arguments resolved
                        flags.push((name, Some(self.tcx.type_of(def.did()).to_string())));
                    }
                }
            }

            if let Some(true) = self_ty.ty_adt_def().map(|def| def.did().is_local()) {
                flags.push((sym::crate_local, None));
            }

            // Allow targeting all integers using `{integral}`, even if the exact type was resolved
            if self_ty.is_integral() {
                flags.push((sym::_Self, Some("{integral}".to_owned())));
            }

            if self_ty.is_array_slice() {
                flags.push((sym::_Self, Some("&[]".to_owned())));
            }

            if self_ty.is_fn() {
                let fn_sig = self_ty.fn_sig(self.tcx);
                let shortname = match fn_sig.unsafety() {
                    hir::Unsafety::Normal => "fn",
                    hir::Unsafety::Unsafe => "unsafe fn",
                };
                flags.push((sym::_Self, Some(shortname.to_owned())));
            }

            // Slices give us `[]`, `[{ty}]`
            if let ty::Slice(aty) = self_ty.kind() {
                flags.push((sym::_Self, Some("[]".to_string())));
                if let Some(def) = aty.ty_adt_def() {
                    // We also want to be able to select the slice's type's original
                    // signature with no type arguments resolved
                    flags.push((sym::_Self, Some(format!("[{}]", self.tcx.type_of(def.did())))));
                }
                if aty.is_integral() {
                    flags.push((sym::_Self, Some("[{integral}]".to_string())));
                }
            }

            // Arrays give us `[]`, `[{ty}; _]` and `[{ty}; N]`
            if let ty::Array(aty, len) = self_ty.kind() {
                flags.push((sym::_Self, Some("[]".to_string())));
                let len = len.kind().try_to_value().and_then(|v| v.try_to_machine_usize(self.tcx));
                flags.push((sym::_Self, Some(format!("[{}; _]", aty))));
                if let Some(n) = len {
                    flags.push((sym::_Self, Some(format!("[{}; {}]", aty, n))));
                }
                if let Some(def) = aty.ty_adt_def() {
                    // We also want to be able to select the array's type's original
                    // signature with no type arguments resolved
                    let def_ty = self.tcx.type_of(def.did());
                    flags.push((sym::_Self, Some(format!("[{def_ty}; _]"))));
                    if let Some(n) = len {
                        flags.push((sym::_Self, Some(format!("[{def_ty}; {n}]"))));
                    }
                }
                if aty.is_integral() {
                    flags.push((sym::_Self, Some("[{integral}; _]".to_string())));
                    if let Some(n) = len {
                        flags.push((sym::_Self, Some(format!("[{{integral}}; {n}]"))));
                    }
                }
            }
            if let ty::Dynamic(traits, _, _) = self_ty.kind() {
                for t in traits.iter() {
                    if let ty::ExistentialPredicate::Trait(trait_ref) = t.skip_binder() {
                        flags.push((sym::_Self, Some(self.tcx.def_path_str(trait_ref.def_id))))
                    }
                }
            }
        });

        if let Ok(Some(command)) = OnUnimplementedDirective::of_item(self.tcx, def_id) {
            command.evaluate(self.tcx, trait_ref, &flags)
        } else {
            OnUnimplementedNote::default()
        }
    }
}