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Diffstat (limited to 'compiler/rustc_middle/src/ty/inhabitedness/mod.rs')
| -rw-r--r-- | compiler/rustc_middle/src/ty/inhabitedness/mod.rs | 234 |
1 files changed, 234 insertions, 0 deletions
diff --git a/compiler/rustc_middle/src/ty/inhabitedness/mod.rs b/compiler/rustc_middle/src/ty/inhabitedness/mod.rs new file mode 100644 index 000000000..3d22f5a04 --- /dev/null +++ b/compiler/rustc_middle/src/ty/inhabitedness/mod.rs @@ -0,0 +1,234 @@ +pub use self::def_id_forest::DefIdForest; + +use crate::ty; +use crate::ty::context::TyCtxt; +use crate::ty::{AdtDef, FieldDef, Ty, VariantDef}; +use crate::ty::{AdtKind, Visibility}; +use crate::ty::{DefId, SubstsRef}; + +use rustc_type_ir::sty::TyKind::*; + +mod def_id_forest; + +// The methods in this module calculate `DefIdForest`s of modules in which an +// `AdtDef`/`VariantDef`/`FieldDef` is visibly uninhabited. +// +// # Example +// ```rust +// enum Void {} +// mod a { +// pub mod b { +// pub struct SecretlyUninhabited { +// _priv: !, +// } +// } +// } +// +// mod c { +// pub struct AlsoSecretlyUninhabited { +// _priv: Void, +// } +// mod d { +// } +// } +// +// struct Foo { +// x: a::b::SecretlyUninhabited, +// y: c::AlsoSecretlyUninhabited, +// } +// ``` +// In this code, the type `Foo` will only be visibly uninhabited inside the +// modules `b`, `c` and `d`. Calling `uninhabited_from` on `Foo` or its `AdtDef` will +// return the forest of modules {`b`, `c`->`d`} (represented in a `DefIdForest` by the +// set {`b`, `c`}). +// +// We need this information for pattern-matching on `Foo` or types that contain +// `Foo`. +// +// # Example +// ```rust +// let foo_result: Result<T, Foo> = ... ; +// let Ok(t) = foo_result; +// ``` +// This code should only compile in modules where the uninhabitedness of `Foo` is +// visible. + +impl<'tcx> TyCtxt<'tcx> { + /// Checks whether a type is visibly uninhabited from a particular module. + /// + /// # Example + /// ``` + /// #![feature(never_type)] + /// # fn main() {} + /// enum Void {} + /// mod a { + /// pub mod b { + /// pub struct SecretlyUninhabited { + /// _priv: !, + /// } + /// } + /// } + /// + /// mod c { + /// use super::Void; + /// pub struct AlsoSecretlyUninhabited { + /// _priv: Void, + /// } + /// mod d { + /// } + /// } + /// + /// struct Foo { + /// x: a::b::SecretlyUninhabited, + /// y: c::AlsoSecretlyUninhabited, + /// } + /// ``` + /// In this code, the type `Foo` will only be visibly uninhabited inside the + /// modules b, c and d. This effects pattern-matching on `Foo` or types that + /// contain `Foo`. + /// + /// # Example + /// ```ignore (illustrative) + /// let foo_result: Result<T, Foo> = ... ; + /// let Ok(t) = foo_result; + /// ``` + /// This code should only compile in modules where the uninhabitedness of Foo is + /// visible. + pub fn is_ty_uninhabited_from( + self, + module: DefId, + ty: Ty<'tcx>, + param_env: ty::ParamEnv<'tcx>, + ) -> bool { + // To check whether this type is uninhabited at all (not just from the + // given node), you could check whether the forest is empty. + // ``` + // forest.is_empty() + // ``` + ty.uninhabited_from(self, param_env).contains(self, module) + } +} + +impl<'tcx> AdtDef<'tcx> { + /// Calculates the forest of `DefId`s from which this ADT is visibly uninhabited. + fn uninhabited_from( + self, + tcx: TyCtxt<'tcx>, + substs: SubstsRef<'tcx>, + param_env: ty::ParamEnv<'tcx>, + ) -> DefIdForest<'tcx> { + // Non-exhaustive ADTs from other crates are always considered inhabited. + if self.is_variant_list_non_exhaustive() && !self.did().is_local() { + DefIdForest::empty() + } else { + DefIdForest::intersection( + tcx, + self.variants() + .iter() + .map(|v| v.uninhabited_from(tcx, substs, self.adt_kind(), param_env)), + ) + } + } +} + +impl<'tcx> VariantDef { + /// Calculates the forest of `DefId`s from which this variant is visibly uninhabited. + pub fn uninhabited_from( + &self, + tcx: TyCtxt<'tcx>, + substs: SubstsRef<'tcx>, + adt_kind: AdtKind, + param_env: ty::ParamEnv<'tcx>, + ) -> DefIdForest<'tcx> { + let is_enum = match adt_kind { + // For now, `union`s are never considered uninhabited. + // The precise semantics of inhabitedness with respect to unions is currently undecided. + AdtKind::Union => return DefIdForest::empty(), + AdtKind::Enum => true, + AdtKind::Struct => false, + }; + // Non-exhaustive variants from other crates are always considered inhabited. + if self.is_field_list_non_exhaustive() && !self.def_id.is_local() { + DefIdForest::empty() + } else { + DefIdForest::union( + tcx, + self.fields.iter().map(|f| f.uninhabited_from(tcx, substs, is_enum, param_env)), + ) + } + } +} + +impl<'tcx> FieldDef { + /// Calculates the forest of `DefId`s from which this field is visibly uninhabited. + fn uninhabited_from( + &self, + tcx: TyCtxt<'tcx>, + substs: SubstsRef<'tcx>, + is_enum: bool, + param_env: ty::ParamEnv<'tcx>, + ) -> DefIdForest<'tcx> { + let data_uninhabitedness = move || self.ty(tcx, substs).uninhabited_from(tcx, param_env); + // FIXME(canndrew): Currently enum fields are (incorrectly) stored with + // `Visibility::Invisible` so we need to override `self.vis` if we're + // dealing with an enum. + if is_enum { + data_uninhabitedness() + } else { + match self.vis { + Visibility::Invisible => DefIdForest::empty(), + Visibility::Restricted(from) => { + let forest = DefIdForest::from_id(from); + let iter = Some(forest).into_iter().chain(Some(data_uninhabitedness())); + DefIdForest::intersection(tcx, iter) + } + Visibility::Public => data_uninhabitedness(), + } + } + } +} + +impl<'tcx> Ty<'tcx> { + /// Calculates the forest of `DefId`s from which this type is visibly uninhabited. + fn uninhabited_from( + self, + tcx: TyCtxt<'tcx>, + param_env: ty::ParamEnv<'tcx>, + ) -> DefIdForest<'tcx> { + tcx.type_uninhabited_from(param_env.and(self)) + } +} + +// Query provider for `type_uninhabited_from`. +pub(crate) fn type_uninhabited_from<'tcx>( + tcx: TyCtxt<'tcx>, + key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>, +) -> DefIdForest<'tcx> { + let ty = key.value; + let param_env = key.param_env; + match *ty.kind() { + Adt(def, substs) => def.uninhabited_from(tcx, substs, param_env), + + Never => DefIdForest::full(), + + Tuple(ref tys) => { + DefIdForest::union(tcx, tys.iter().map(|ty| ty.uninhabited_from(tcx, param_env))) + } + + Array(ty, len) => match len.try_eval_usize(tcx, param_env) { + Some(0) | None => DefIdForest::empty(), + // If the array is definitely non-empty, it's uninhabited if + // the type of its elements is uninhabited. + Some(1..) => ty.uninhabited_from(tcx, param_env), + }, + + // References to uninitialised memory are valid for any type, including + // uninhabited types, in unsafe code, so we treat all references as + // inhabited. + // The precise semantics of inhabitedness with respect to references is currently + // undecided. + Ref(..) => DefIdForest::empty(), + + _ => DefIdForest::empty(), + } +} |