rustc_ty_utils/
representability.rs

1use rustc_hir::def::DefKind;
2use rustc_index::bit_set::DenseBitSet;
3use rustc_middle::bug;
4use rustc_middle::query::Providers;
5use rustc_middle::ty::{self, Ty, TyCtxt};
6use rustc_span::def_id::LocalDefId;
7
8pub(crate) fn provide(providers: &mut Providers) {
9    *providers = Providers {
10        check_representability,
11        check_representability_adt_ty,
12        params_in_repr,
13        ..*providers
14    };
15}
16
17fn check_representability(tcx: TyCtxt<'_>, def_id: LocalDefId) {
18    match tcx.def_kind(def_id) {
19        DefKind::Struct | DefKind::Union | DefKind::Enum => {
20            for variant in tcx.adt_def(def_id).variants() {
21                for field in variant.fields.iter() {
22                    tcx.ensure_ok().check_representability(field.did.expect_local());
23                }
24            }
25        }
26        DefKind::Field => {
27            check_representability_ty(tcx, tcx.type_of(def_id).instantiate_identity());
28        }
29        def_kind => ::rustc_middle::util::bug::bug_fmt(format_args!("unexpected {0:?}", def_kind))bug!("unexpected {def_kind:?}"),
30    }
31}
32
33fn check_representability_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) {
34    match *ty.kind() {
35        // This one must be a query rather than a vanilla `check_representability_adt_ty` call. See
36        // the comment on `check_representability_adt_ty` below for why.
37        ty::Adt(..) => {
38            tcx.ensure_ok().check_representability_adt_ty(ty);
39        }
40        // FIXME(#11924) allow zero-length arrays?
41        ty::Array(ty, _) => {
42            check_representability_ty(tcx, ty);
43        }
44        ty::Tuple(tys) => {
45            for ty in tys {
46                check_representability_ty(tcx, ty);
47            }
48        }
49        _ => {}
50    }
51}
52
53// The reason for this being a separate query is very subtle. Consider this
54// infinitely sized struct: `struct Foo(Box<Foo>, Bar<Foo>)`. When calling
55// check_representability(Foo), a query cycle will occur:
56//
57//   check_representability(Foo)
58//     -> check_representability_adt_ty(Bar<Foo>)
59//     -> check_representability(Foo)
60//
61// For the diagnostic output (in `check_representability`), we want to detect
62// that the `Foo` in the *second* field of the struct is culpable. This
63// requires traversing the HIR of the struct and calling `params_in_repr(Bar)`.
64// But we can't call params_in_repr for a given type unless it is known to be
65// representable. params_in_repr will cycle/panic on infinitely sized types.
66// Looking at the query cycle above, we know that `Bar` is representable
67// because `check_representability_adt_ty(Bar<..>)` is in the cycle and
68// `check_representability(Bar)` is *not* in the cycle.
69fn check_representability_adt_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) {
70    let ty::Adt(adt, args) = ty.kind() else { ::rustc_middle::util::bug::bug_fmt(format_args!("expected adt"))bug!("expected adt") };
71    if let Some(def_id) = adt.did().as_local() {
72        tcx.ensure_ok().check_representability(def_id);
73    }
74    // At this point, we know that the item of the ADT type is representable;
75    // but the type parameters may cause a cycle with an upstream type
76    let params_in_repr = tcx.params_in_repr(adt.did());
77    for (i, arg) in args.iter().enumerate() {
78        if let ty::GenericArgKind::Type(ty) = arg.kind() {
79            if params_in_repr.contains(i as u32) {
80                check_representability_ty(tcx, ty);
81            }
82        }
83    }
84}
85
86fn params_in_repr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> DenseBitSet<u32> {
87    let adt_def = tcx.adt_def(def_id);
88    let generics = tcx.generics_of(def_id);
89    let mut params_in_repr = DenseBitSet::new_empty(generics.own_params.len());
90    for variant in adt_def.variants() {
91        for field in variant.fields.iter() {
92            params_in_repr_ty(
93                tcx,
94                tcx.type_of(field.did).instantiate_identity(),
95                &mut params_in_repr,
96            );
97        }
98    }
99    params_in_repr
100}
101
102fn params_in_repr_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, params_in_repr: &mut DenseBitSet<u32>) {
103    match *ty.kind() {
104        ty::Adt(adt, args) => {
105            let inner_params_in_repr = tcx.params_in_repr(adt.did());
106            for (i, arg) in args.iter().enumerate() {
107                if let ty::GenericArgKind::Type(ty) = arg.kind() {
108                    if inner_params_in_repr.contains(i as u32) {
109                        params_in_repr_ty(tcx, ty, params_in_repr);
110                    }
111                }
112            }
113        }
114        ty::Array(ty, _) => params_in_repr_ty(tcx, ty, params_in_repr),
115        ty::Tuple(tys) => tys.iter().for_each(|ty| params_in_repr_ty(tcx, ty, params_in_repr)),
116        ty::Param(param) => {
117            params_in_repr.insert(param.index);
118        }
119        _ => {}
120    }
121}
rustc_ty_utils/representability.rs

rustc_ty_utils/
representability.rs
