rustc_hir_analysis/
hir_wf_check.rs

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

rustc_hir_analysis/
hir_wf_check.rs
