rustc_hir_analysis/
hir_wf_check.rs

1use rustc_hir::def::DefKind;
2use rustc_hir::intravisit::{self, Visitor, VisitorExt};
3use rustc_hir::{self as hir, AmbigArg, ForeignItem, ForeignItemKind};
4use rustc_infer::infer::TyCtxtInferExt;
5use rustc_infer::traits::{ObligationCause, ObligationCauseCode, WellFormedLoc};
6use rustc_middle::bug;
7use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt, TypingMode, fold_regions};
8use rustc_span::def_id::LocalDefId;
9use rustc_trait_selection::traits::{self, ObligationCtxt};
10use tracing::debug;
11
12use crate::collect::ItemCtxt;
13
14// Ideally, this would be in `rustc_trait_selection`, but we
15// need access to `ItemCtxt`
16pub(super) fn diagnostic_hir_wf_check<'tcx>(
17    tcx: TyCtxt<'tcx>,
18    (predicate, loc): (ty::Predicate<'tcx>, WellFormedLoc),
19) -> Option<ObligationCause<'tcx>> {
20    let def_id = match loc {
21        WellFormedLoc::Ty(def_id) => def_id,
22        WellFormedLoc::Param { function, param_idx: _ } => function,
23    };
24    let hir_id = tcx.local_def_id_to_hir_id(def_id);
25
26    // HIR wfcheck should only ever happen as part of improving an existing error
27    tcx.dcx()
28        .span_delayed_bug(tcx.def_span(def_id), "Performed HIR wfcheck without an existing error!");
29
30    let icx = ItemCtxt::new(tcx, def_id);
31
32    // To perform HIR-based WF checking, we iterate over all HIR types
33    // that occur 'inside' the item we're checking. For example,
34    // given the type `Option<MyStruct<u8>>`, we will check
35    // `Option<MyStruct<u8>>`, `MyStruct<u8>`, and `u8`.
36    // For each type, we perform a well-formed check, and see if we get
37    // an error that matches our expected predicate. We save
38    // the `ObligationCause` corresponding to the *innermost* type,
39    // which is the most specific type that we can point to.
40    // In general, the different components of an `hir::Ty` may have
41    // completely different spans due to macro invocations. Pointing
42    // to the most accurate part of the type can be the difference
43    // between a useless span (e.g. the macro invocation site)
44    // and a useful span (e.g. a user-provided type passed into the macro).
45    //
46    // This approach is quite inefficient - we redo a lot of work done
47    // by the normal WF checker. However, this code is run at most once
48    // per reported error - it will have no impact when compilation succeeds,
49    // and should only have an impact if a very large number of errors is
50    // displayed to the user.
51    struct HirWfCheck<'tcx> {
52        tcx: TyCtxt<'tcx>,
53        predicate: ty::Predicate<'tcx>,
54        cause: Option<ObligationCause<'tcx>>,
55        cause_depth: usize,
56        icx: ItemCtxt<'tcx>,
57        def_id: LocalDefId,
58        param_env: ty::ParamEnv<'tcx>,
59        depth: usize,
60    }
61
62    impl<'tcx> Visitor<'tcx> for HirWfCheck<'tcx> {
63        fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx, AmbigArg>) {
64            let infcx = self.tcx.infer_ctxt().build(TypingMode::non_body_analysis());
65            let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
66
67            // We don't handle infer vars but we wouldn't handle them anyway as we're creating a
68            // fresh `InferCtxt` in this function.
69            let tcx_ty = self.icx.lower_ty(ty.as_unambig_ty());
70            // This visitor can walk into binders, resulting in the `tcx_ty` to
71            // potentially reference escaping bound variables. We simply erase
72            // those here.
73            let tcx_ty = fold_regions(self.tcx, tcx_ty, |r, _| {
74                if r.is_bound() { self.tcx.lifetimes.re_erased } else { r }
75            });
76
77            // We may be checking the WFness of a type in an opaque with a non-lifetime bound.
78            // Perhaps we could rebind all the escaping bound vars, but they're coming from
79            // arbitrary debruijn indices and aren't particularly important anyways, since they
80            // are only coming from `feature(non_lifetime_binders)` anyways.
81            if tcx_ty.has_escaping_bound_vars() {
82                return;
83            }
84
85            let cause = traits::ObligationCause::new(
86                ty.span,
87                self.def_id,
88                traits::ObligationCauseCode::WellFormed(None),
89            );
90
91            ocx.register_obligation(traits::Obligation::new(
92                self.tcx,
93                cause,
94                self.param_env,
95                ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(tcx_ty.into())),
96            ));
97
98            for error in ocx.select_all_or_error() {
99                debug!("Wf-check got error for {:?}: {:?}", ty, error);
100                if error.obligation.predicate == self.predicate {
101                    // Save the cause from the greatest depth - this corresponds
102                    // to picking more-specific types (e.g. `MyStruct<u8>`)
103                    // over less-specific types (e.g. `Option<MyStruct<u8>>`)
104                    if self.depth >= self.cause_depth {
105                        self.cause = Some(error.obligation.cause);
106                        if let hir::TyKind::TraitObject(..) = ty.kind
107                            && let DefKind::AssocTy | DefKind::AssocConst | DefKind::AssocFn =
108                                self.tcx.def_kind(self.def_id)
109                        {
110                            self.cause = Some(ObligationCause::new(
111                                ty.span,
112                                self.def_id,
113                                ObligationCauseCode::DynCompatible(ty.span),
114                            ));
115                        }
116                        self.cause_depth = self.depth
117                    }
118                }
119            }
120
121            self.depth += 1;
122            intravisit::walk_ty(self, ty);
123            self.depth -= 1;
124        }
125    }
126
127    let mut visitor = HirWfCheck {
128        tcx,
129        predicate,
130        cause: None,
131        cause_depth: 0,
132        icx,
133        def_id,
134        param_env: tcx.param_env(def_id.to_def_id()),
135        depth: 0,
136    };
137
138    // Get the starting `hir::Ty` using our `WellFormedLoc`.
139    // We will walk 'into' this type to try to find
140    // a more precise span for our predicate.
141    let tys = match loc {
142        WellFormedLoc::Ty(_) => match tcx.hir_node(hir_id) {
143            hir::Node::ImplItem(item) => match item.kind {
144                hir::ImplItemKind::Type(ty) => vec![ty],
145                hir::ImplItemKind::Const(ty, _) => vec![ty],
146                ref item => bug!("Unexpected ImplItem {:?}", item),
147            },
148            hir::Node::TraitItem(item) => match item.kind {
149                hir::TraitItemKind::Type(_, ty) => ty.into_iter().collect(),
150                hir::TraitItemKind::Const(ty, _) => vec![ty],
151                ref item => bug!("Unexpected TraitItem {:?}", item),
152            },
153            hir::Node::Item(item) => match item.kind {
154                hir::ItemKind::TyAlias(_, _, ty)
155                | hir::ItemKind::Static(_, _, ty, _)
156                | hir::ItemKind::Const(_, _, ty, _) => vec![ty],
157                hir::ItemKind::Impl(impl_) => match &impl_.of_trait {
158                    Some(t) => t
159                        .path
160                        .segments
161                        .last()
162                        .iter()
163                        .flat_map(|seg| seg.args().args)
164                        .filter_map(|arg| {
165                            if let hir::GenericArg::Type(ty) = arg {
166                                Some(ty.as_unambig_ty())
167                            } else {
168                                None
169                            }
170                        })
171                        .chain([impl_.self_ty])
172                        .collect(),
173                    None => {
174                        vec![impl_.self_ty]
175                    }
176                },
177                ref item => bug!("Unexpected item {:?}", item),
178            },
179            hir::Node::Field(field) => vec![field.ty],
180            hir::Node::ForeignItem(ForeignItem {
181                kind: ForeignItemKind::Static(ty, _, _), ..
182            }) => vec![*ty],
183            hir::Node::GenericParam(hir::GenericParam {
184                kind: hir::GenericParamKind::Type { default: Some(ty), .. },
185                ..
186            }) => vec![*ty],
187            hir::Node::AnonConst(_) => {
188                if let Some(const_param_id) = tcx.hir_opt_const_param_default_param_def_id(hir_id)
189                    && let hir::Node::GenericParam(hir::GenericParam {
190                        kind: hir::GenericParamKind::Const { ty, .. },
191                        ..
192                    }) = tcx.hir_node_by_def_id(const_param_id)
193                {
194                    vec![*ty]
195                } else {
196                    vec![]
197                }
198            }
199            ref node => bug!("Unexpected node {:?}", node),
200        },
201        WellFormedLoc::Param { function: _, param_idx } => {
202            let fn_decl = tcx.hir_fn_decl_by_hir_id(hir_id).unwrap();
203            // Get return type
204            if param_idx as usize == fn_decl.inputs.len() {
205                match fn_decl.output {
206                    hir::FnRetTy::Return(ty) => vec![ty],
207                    // The unit type `()` is always well-formed
208                    hir::FnRetTy::DefaultReturn(_span) => vec![],
209                }
210            } else {
211                vec![&fn_decl.inputs[param_idx as usize]]
212            }
213        }
214    };
215    for ty in tys {
216        visitor.visit_ty_unambig(ty);
217    }
218    visitor.cause
219}
rustc_hir_analysis/hir_wf_check.rs

rustc_hir_analysis/
hir_wf_check.rs
