rustc_middle/ty/inhabitedness/

mod.rs

//! 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 std::assert_matches;

use rustc_data_structures::fx::FxHashSet;
use rustc_span::def_id::LocalModId;
use rustc_type_ir::TyKind::*;
use tracing::instrument;

use crate::query::Providers;
use crate::ty::{self, DefId, Ty, TyCtxt, TypeVisitableExt, TypingEnv, VariantDef, Visibility};

pub mod inhabited_predicate;

pub use inhabited_predicate::InhabitedPredicate;

pub(crate) fn provide(providers: &mut Providers) {
    *providers = Providers {
        inhabited_predicate_adt,
        inhabited_predicate_type,
        is_opsem_inhabited_raw,
        ..*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() {
        tcx.ensure_ok().check_representability(def_id);
    }

    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());
        InhabitedPredicate::all(
            tcx,
            self.fields.iter().map(|field| {
                let pred = tcx
                    .type_of(field.did)
                    .instantiate_identity()
                    .skip_norm_wip()
                    .inhabited_predicate(tcx);
                if adt.is_enum() {
                    return pred;
                }
                match field.vis {
                    Visibility::Public => pred,
                    Visibility::Restricted(from) => {
                        InhabitedPredicate::NotInModule(from).or(tcx, pred)
                    }
                }
            }),
        )
    }
}

impl<'tcx> Ty<'tcx> {
    #[instrument(level = "debug", skip(tcx), ret)]
    pub fn inhabited_predicate(self, tcx: TyCtxt<'tcx>) -> InhabitedPredicate<'tcx> {
        debug_assert!(!self.has_infer());
        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.variant_list_has_applicable_non_exhaustive() => {
                InhabitedPredicate::True
            }
            Never => InhabitedPredicate::False,
            // FIXME(#155345): This should only encounter rigid aliases with the new solver.
            Param(_)
            | Alias(
                _,
                ty::AliasTy {
                    kind: ty::Inherent { .. } | ty::Projection { .. } | ty::Free { .. },
                    ..
                },
            ) => InhabitedPredicate::GenericType(self),
            &Alias(_, ty::AliasTy { kind: ty::Opaque { def_id }, args, .. }) => {
                match def_id.as_local() {
                    // Foreign opaque is considered inhabited.
                    None => InhabitedPredicate::True,
                    // Local opaque type may possibly be revealed.
                    Some(local_def_id) => {
                        let key = ty::OpaqueTypeKey { def_id: local_def_id, args };
                        InhabitedPredicate::OpaqueType(key)
                    }
                }
            }
            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: LocalModId,
        typing_env: ty::TypingEnv<'tcx>,
    ) -> bool {
        self.inhabited_predicate(tcx).apply(tcx, typing_env, module)
    }

    /// Returns true if the type is uninhabited without regard to visibility.
    ///
    /// This is still conservative; for instance, a `#[non_exhaustive]` enum *in another crate*
    /// is always considered inhabited.
    pub fn is_privately_uninhabited(
        self,
        tcx: TyCtxt<'tcx>,
        typing_env: ty::TypingEnv<'tcx>,
    ) -> bool {
        !self.inhabited_predicate(tcx).apply_ignore_module(tcx, typing_env)
    }

    /// Returns whether `self` is considered inhabited on the opsem level, i.e., its validity
    /// invariant might be satisfiable. `self` is expected to be monomorphic and normalized.
    ///
    /// Key constraints are:
    /// - if a type's validity invariant is satisfiable, it must be opsem-inhabited.
    /// - if a type's layout is marked uninhabited, it must be opsem-uninhabited.
    ///
    /// Beyond that, the value returned by this function is not a stable guarantee.
    pub fn is_opsem_inhabited(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> bool {
        // Handle simple cases directly, use the query with its cache for the rest.
        OpsemInhabitedCtx { tcx, typing_env, seen: None, stop_at_ref: false }.is_inhabited_ty(self)
    }
}

/// 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`"),
    }
}

/// Context for computing whether a type is inhabited on the opsem level.
/// See `is_opsem_inhabited` above for the spec of what we compute.
struct OpsemInhabitedCtx<'tcx> {
    tcx: TyCtxt<'tcx>,
    typing_env: TypingEnv<'tcx>,
    /// IDs of ADTs that have been encountered in the current stack.
    /// It's `None` unless we are inside the `is_opsem_inhabited_raw` query,
    /// which is only invoked for more complex types.
    seen: Option<FxHashSet<DefId>>,
    /// If an ADT is encountered recursively within itself, then `stop_at_ref`
    /// is set to `true`, and then any nested references are considered inhabited.
    stop_at_ref: bool,
}

impl<'tcx> OpsemInhabitedCtx<'tcx> {
    /// See `is_opsem_inhabited` above for the spec of what we compute.
    fn is_inhabited_ty(&mut self, ty: Ty<'tcx>) -> bool {
        let tcx = self.tcx;
        match *ty.kind() {
            // Trivially (un)inhabited types
            ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Bool
            | ty::Char
            | ty::Str
            | ty::Foreign(..)
            | ty::RawPtr(..)
            | ty::FnPtr(..)
            | ty::FnDef(..) => true,
            ty::Dynamic(..) => true, // We can't reason about traits, assume they are inhabited
            ty::Slice(..) => true,   // Slices can always be empty
            ty::Never => false,

            // Types where we recurse
            ty::Ref(_, pointee, _) => {
                if self.stop_at_ref {
                    // Bailing out here is safe as the layout code always considers references
                    // inhabited, so the implication ("layout uninhabited => opsem uninhabited")
                    // is upheld.
                    return true;
                }
                self.is_inhabited_ty(pointee)
            }
            ty::Tuple(tys) => tys.iter().all(|ty| self.is_inhabited_ty(ty)),
            ty::Array(elem, len) => {
                len.try_to_target_usize(tcx).unwrap() == 0 || self.is_inhabited_ty(elem)
            }
            ty::Pat(inner, _pat) => self.is_inhabited_ty(inner),
            ty::Closure(_def, args) => {
                let args = args.as_closure();
                args.upvar_tys().iter().all(|ty| self.is_inhabited_ty(ty))
            }
            ty::Coroutine(_def, args) => {
                let args = args.as_coroutine();
                args.upvar_tys().iter().all(|ty| self.is_inhabited_ty(ty))
            }
            ty::CoroutineClosure(_def, args) => {
                let args = args.as_coroutine_closure();
                args.upvar_tys().iter().all(|ty| self.is_inhabited_ty(ty))
            }
            ty::UnsafeBinder(base) => {
                let base = tcx.instantiate_bound_regions_with_erased((*base).into());
                self.is_inhabited_ty(base)
            }
            ty::Adt(..) => self.is_inhabited_adt_ty(ty),

            ty::Error(_error_guaranteed) => {
                // We have a token proving there was an error, so we can return a dummy value.
                true
            }

            ty::Infer(..)
            | ty::Placeholder(..)
            | ty::Bound(..)
            | ty::Param(..)
            | ty::Alias(..)
            | ty::CoroutineWitness(..) => {
                bug!("non-normalized type in `is_opsem_uninhabited`: `{ty}`")
            }
        }
    }

    fn is_inhabited_adt_ty(&mut self, ty: Ty<'tcx>) -> bool {
        let ty::Adt(adt_def, adt_args) = *ty.kind() else {
            unreachable! {}
        };
        let Self { tcx, typing_env, .. } = *self;

        if adt_def.is_union() {
            // Unions are always inhabited.
            return true;
        }

        let Some(seen) = self.seen.as_mut() else {
            // stop recursing, invoke the query.
            return tcx.is_opsem_inhabited_raw(typing_env.as_query_input(ty));
        };

        let new_adt = seen.insert(adt_def.did());
        // If we have seen this ADT before, stop at the next reference to avoid infinite
        // recursion. We can't stop here since we have to ensure that "layout uninhabited"
        // implies "opsem uninhabited". References are always layout-inhabited so the
        // implication is vacuously true.
        let stop_at_ref_prev = self.stop_at_ref;
        self.stop_at_ref |= !new_adt;

        // We are inhabited if in some variant all fields are inhabited.
        let inhabited = adt_def.variants().iter().any(|variant| {
            variant.fields.iter().all(|field| {
                let ty = field.ty(tcx, adt_args);
                let ty = tcx.normalize_erasing_regions(typing_env, ty);
                self.is_inhabited_ty(ty)
            })
        });

        self.stop_at_ref = stop_at_ref_prev;
        // Remove the type again so that we allow it to appear on other branches.
        if new_adt {
            self.seen.as_mut().unwrap().remove(&adt_def.did());
        }

        inhabited
    }
}

fn is_opsem_inhabited_raw<'tcx>(
    tcx: TyCtxt<'tcx>,
    env: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>,
) -> bool {
    let (ty, typing_env) = (env.value, env.typing_env);
    assert_matches!(
        ty.kind(),
        ty::Adt(..),
        "the query should only be invoked by `Ty::is_opsem_inhabited`"
    );

    OpsemInhabitedCtx { tcx, typing_env, seen: Some(FxHashSet::default()), stop_at_ref: false }
        .is_inhabited_adt_ty(ty)
}