compiler/rustc_middle/src/ty/inhabitedness/mod.rs

   1 pub use self::def_id_forest::DefIdForest;
   2
   3 use crate::ty;
   4 use crate::ty::context::TyCtxt;
   5 use crate::ty::TyKind::*;
   6 use crate::ty::{AdtDef, FieldDef, Ty, TyS, VariantDef};
   7 use crate::ty::{AdtKind, Visibility};
   8 use crate::ty::{DefId, SubstsRef};
   9 use rustc_data_structures::stack::ensure_sufficient_stack;
  10
  11 mod def_id_forest;
  12
  13 // The methods in this module calculate `DefIdForest`s of modules in which a
  14 // `AdtDef`/`VariantDef`/`FieldDef` is visibly uninhabited.
  15 //
  16 // # Example
  17 // ```rust
  18 // enum Void {}
  19 // mod a {
  20 //     pub mod b {
  21 //         pub struct SecretlyUninhabited {
  22 //             _priv: !,
  23 //         }
  24 //     }
  25 // }
  26 //
  27 // mod c {
  28 //     pub struct AlsoSecretlyUninhabited {
  29 //         _priv: Void,
  30 //     }
  31 //     mod d {
  32 //     }
  33 // }
  34 //
  35 // struct Foo {
  36 //     x: a::b::SecretlyUninhabited,
  37 //     y: c::AlsoSecretlyUninhabited,
  38 // }
  39 // ```
  40 // In this code, the type `Foo` will only be visibly uninhabited inside the
  41 // modules `b`, `c` and `d`. Calling `uninhabited_from` on `Foo` or its `AdtDef` will
  42 // return the forest of modules {`b`, `c`->`d`} (represented in a `DefIdForest` by the
  43 // set {`b`, `c`}).
  44 //
  45 // We need this information for pattern-matching on `Foo` or types that contain
  46 // `Foo`.
  47 //
  48 // # Example
  49 // ```rust
  50 // let foo_result: Result<T, Foo> = ... ;
  51 // let Ok(t) = foo_result;
  52 // ```
  53 // This code should only compile in modules where the uninhabitedness of `Foo` is
  54 // visible.
  55
  56 impl<'tcx> TyCtxt<'tcx> {
  57     /// Checks whether a type is visibly uninhabited from a particular module.
  58     ///
  59     /// # Example
  60     /// ```rust
  61     /// enum Void {}
  62     /// mod a {
  63     ///     pub mod b {
  64     ///         pub struct SecretlyUninhabited {
  65     ///             _priv: !,
  66     ///         }
  67     ///     }
  68     /// }
  69     ///
  70     /// mod c {
  71     ///     pub struct AlsoSecretlyUninhabited {
  72     ///         _priv: Void,
  73     ///     }
  74     ///     mod d {
  75     ///     }
  76     /// }
  77     ///
  78     /// struct Foo {
  79     ///     x: a::b::SecretlyUninhabited,
  80     ///     y: c::AlsoSecretlyUninhabited,
  81     /// }
  82     /// ```
  83     /// In this code, the type `Foo` will only be visibly uninhabited inside the
  84     /// modules b, c and d. This effects pattern-matching on `Foo` or types that
  85     /// contain `Foo`.
  86     ///
  87     /// # Example
  88     /// ```rust
  89     /// let foo_result: Result<T, Foo> = ... ;
  90     /// let Ok(t) = foo_result;
  91     /// ```
  92     /// This code should only compile in modules where the uninhabitedness of Foo is
  93     /// visible.
  94     pub fn is_ty_uninhabited_from(
  95         self,
  96         module: DefId,
  97         ty: Ty<'tcx>,
  98         param_env: ty::ParamEnv<'tcx>,
  99     ) -> bool {
 100         // To check whether this type is uninhabited at all (not just from the
 101         // given node), you could check whether the forest is empty.
 102         // ```
 103         // forest.is_empty()
 104         // ```
 105         ty.uninhabited_from(self, param_env).contains(self, module)
 106     }
 107 }
 108
 109 impl<'tcx> AdtDef {
 110     /// Calculates the forest of `DefId`s from which this ADT is visibly uninhabited.
 111     fn uninhabited_from(
 112         &self,
 113         tcx: TyCtxt<'tcx>,
 114         substs: SubstsRef<'tcx>,
 115         param_env: ty::ParamEnv<'tcx>,
 116     ) -> DefIdForest {
 117         // Non-exhaustive ADTs from other crates are always considered inhabited.
 118         if self.is_variant_list_non_exhaustive() && !self.did.is_local() {
 119             DefIdForest::empty()
 120         } else {
 121             DefIdForest::intersection(
 122                 tcx,
 123                 self.variants
 124                     .iter()
 125                     .map(|v| v.uninhabited_from(tcx, substs, self.adt_kind(), param_env)),
 126             )
 127         }
 128     }
 129 }
 130
 131 impl<'tcx> VariantDef {
 132     /// Calculates the forest of `DefId`s from which this variant is visibly uninhabited.
 133     pub fn uninhabited_from(
 134         &self,
 135         tcx: TyCtxt<'tcx>,
 136         substs: SubstsRef<'tcx>,
 137         adt_kind: AdtKind,
 138         param_env: ty::ParamEnv<'tcx>,
 139     ) -> DefIdForest {
 140         let is_enum = match adt_kind {
 141             // For now, `union`s are never considered uninhabited.
 142             // The precise semantics of inhabitedness with respect to unions is currently undecided.
 143             AdtKind::Union => return DefIdForest::empty(),
 144             AdtKind::Enum => true,
 145             AdtKind::Struct => false,
 146         };
 147         // Non-exhaustive variants from other crates are always considered inhabited.
 148         if self.is_field_list_non_exhaustive() && !self.def_id.is_local() {
 149             DefIdForest::empty()
 150         } else {
 151             DefIdForest::union(
 152                 tcx,
 153                 self.fields.iter().map(|f| f.uninhabited_from(tcx, substs, is_enum, param_env)),
 154             )
 155         }
 156     }
 157 }
 158
 159 impl<'tcx> FieldDef {
 160     /// Calculates the forest of `DefId`s from which this field is visibly uninhabited.
 161     fn uninhabited_from(
 162         &self,
 163         tcx: TyCtxt<'tcx>,
 164         substs: SubstsRef<'tcx>,
 165         is_enum: bool,
 166         param_env: ty::ParamEnv<'tcx>,
 167     ) -> DefIdForest {
 168         let data_uninhabitedness = move || self.ty(tcx, substs).uninhabited_from(tcx, param_env);
 169         // FIXME(canndrew): Currently enum fields are (incorrectly) stored with
 170         // `Visibility::Invisible` so we need to override `self.vis` if we're
 171         // dealing with an enum.
 172         if is_enum {
 173             data_uninhabitedness()
 174         } else {
 175             match self.vis {
 176                 Visibility::Invisible => DefIdForest::empty(),
 177                 Visibility::Restricted(from) => {
 178                     let forest = DefIdForest::from_id(from);
 179                     let iter = Some(forest).into_iter().chain(Some(data_uninhabitedness()));
 180                     DefIdForest::intersection(tcx, iter)
 181                 }
 182                 Visibility::Public => data_uninhabitedness(),
 183             }
 184         }
 185     }
 186 }
 187
 188 impl<'tcx> TyS<'tcx> {
 189     /// Calculates the forest of `DefId`s from which this type is visibly uninhabited.
 190     fn uninhabited_from(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> DefIdForest {
 191         match *self.kind() {
 192             Adt(def, substs) => {
 193                 ensure_sufficient_stack(|| def.uninhabited_from(tcx, substs, param_env))
 194             }
 195
 196             Never => DefIdForest::full(tcx),
 197
 198             Tuple(ref tys) => DefIdForest::union(
 199                 tcx,
 200                 tys.iter().map(|ty| ty.expect_ty().uninhabited_from(tcx, param_env)),
 201             ),
 202
 203             Array(ty, len) => match len.try_eval_usize(tcx, param_env) {
 204                 // If the array is definitely non-empty, it's uninhabited if
 205                 // the type of its elements is uninhabited.
 206                 Some(n) if n != 0 => ty.uninhabited_from(tcx, param_env),
 207                 _ => DefIdForest::empty(),
 208             },
 209
 210             // References to uninitialised memory is valid for any type, including
 211             // uninhabited types, in unsafe code, so we treat all references as
 212             // inhabited.
 213             // The precise semantics of inhabitedness with respect to references is currently
 214             // undecided.
 215             Ref(..) => DefIdForest::empty(),
 216
 217             _ => DefIdForest::empty(),
 218         }
 219     }
 220 }