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//! This module contains logic for determining whether a type is inhabited or
//! uninhabited. The [`InhabitedPredicate`] type captures the minimum
//! information needed to determine whether a type is inhabited given a
//! `ParamEnv` and module ID.
//!
//! # Example
//! ```rust
//! #![feature(never_type)]
//! mod a {
//! pub mod b {
//! pub struct SecretlyUninhabited {
//! _priv: !,
//! }
//! }
//! }
//!
//! mod c {
//! enum 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`. Calling `inhabited_predicate` on `Foo` will
//! return `NotInModule(b) AND NotInModule(c)`.
//!
//! We need this information for 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.
use crate::query::Providers;
use crate::ty::context::TyCtxt;
use crate::ty::{self, DefId, Ty, VariantDef, Visibility};
use rustc_type_ir::sty::TyKind::*;
pub mod inhabited_predicate;
pub use inhabited_predicate::InhabitedPredicate;
pub(crate) fn provide(providers: &mut Providers) {
*providers = Providers { inhabited_predicate_adt, inhabited_predicate_type, ..*providers };
}
/// Returns an `InhabitedPredicate` that is generic over type parameters and
/// requires calling [`InhabitedPredicate::instantiate`]
fn inhabited_predicate_adt(tcx: TyCtxt<'_>, def_id: DefId) -> InhabitedPredicate<'_> {
if let Some(def_id) = def_id.as_local() {
if matches!(tcx.representability(def_id), ty::Representability::Infinite) {
return InhabitedPredicate::True;
}
}
let adt = tcx.adt_def(def_id);
InhabitedPredicate::any(
tcx,
adt.variants().iter().map(|variant| variant.inhabited_predicate(tcx, adt)),
)
}
impl<'tcx> VariantDef {
/// Calculates the forest of `DefId`s from which this variant is visibly uninhabited.
pub fn inhabited_predicate(
&self,
tcx: TyCtxt<'tcx>,
adt: ty::AdtDef<'_>,
) -> InhabitedPredicate<'tcx> {
debug_assert!(!adt.is_union());
if self.is_field_list_non_exhaustive() && !self.def_id.is_local() {
// Non-exhaustive variants from other crates are always considered inhabited.
return InhabitedPredicate::True;
}
InhabitedPredicate::all(
tcx,
self.fields.iter().map(|field| {
let pred = tcx.type_of(field.did).instantiate_identity().inhabited_predicate(tcx);
if adt.is_enum() {
return pred;
}
match field.vis {
Visibility::Public => pred,
Visibility::Restricted(from) => {
pred.or(tcx, InhabitedPredicate::NotInModule(from))
}
}
}),
)
}
}
impl<'tcx> Ty<'tcx> {
pub fn inhabited_predicate(self, tcx: TyCtxt<'tcx>) -> InhabitedPredicate<'tcx> {
match self.kind() {
// For now, unions are always considered inhabited
Adt(adt, _) if adt.is_union() => InhabitedPredicate::True,
// Non-exhaustive ADTs from other crates are always considered inhabited
Adt(adt, _) if adt.is_variant_list_non_exhaustive() && !adt.did().is_local() => {
InhabitedPredicate::True
}
Never => InhabitedPredicate::False,
Param(_) | Alias(ty::Projection, _) => InhabitedPredicate::GenericType(self),
// FIXME(inherent_associated_types): Most likely we can just map to `GenericType` like above.
// However it's unclear if the args passed to `InhabitedPredicate::instantiate` are of the correct
// format, i.e. don't contain parent args. If you hit this case, please verify this beforehand.
Alias(ty::Inherent, _) => {
bug!("unimplemented: inhabitedness checking for inherent projections")
}
Tuple(tys) if tys.is_empty() => InhabitedPredicate::True,
// use a query for more complex cases
Adt(..) | Array(..) | Tuple(_) => tcx.inhabited_predicate_type(self),
// references and other types are inhabited
_ => InhabitedPredicate::True,
}
}
/// 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_inhabited_from(
self,
tcx: TyCtxt<'tcx>,
module: DefId,
param_env: ty::ParamEnv<'tcx>,
) -> bool {
self.inhabited_predicate(tcx).apply(tcx, param_env, module)
}
/// Returns true if the type is uninhabited without regard to visibility
pub fn is_privately_uninhabited(
self,
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> bool {
!self.inhabited_predicate(tcx).apply_ignore_module(tcx, param_env)
}
}
/// N.B. this query should only be called through `Ty::inhabited_predicate`
fn inhabited_predicate_type<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> InhabitedPredicate<'tcx> {
match *ty.kind() {
Adt(adt, args) => tcx.inhabited_predicate_adt(adt.did()).instantiate(tcx, args),
Tuple(tys) => {
InhabitedPredicate::all(tcx, tys.iter().map(|ty| ty.inhabited_predicate(tcx)))
}
// If we can evaluate the array length before having a `ParamEnv`, then
// we can simplify the predicate. This is an optimization.
Array(ty, len) => match len.try_to_target_usize(tcx) {
Some(0) => InhabitedPredicate::True,
Some(1..) => ty.inhabited_predicate(tcx),
None => ty.inhabited_predicate(tcx).or(tcx, InhabitedPredicate::ConstIsZero(len)),
},
_ => bug!("unexpected TyKind, use `Ty::inhabited_predicate`"),
}
}