rustc_middle/infer/

canonical.rs

//! **Canonicalization** is the key to constructing a query in the
//! middle of type inference. Ordinarily, it is not possible to store
//! types from type inference in query keys, because they contain
//! references to inference variables whose lifetimes are too short
//! and so forth. Canonicalizing a value T1 using `canonicalize_query`
//! produces two things:
//!
//! - a value T2 where each unbound inference variable has been
//!   replaced with a **canonical variable**;
//! - a map M (of type `CanonicalVarValues`) from those canonical
//!   variables back to the original.
//!
//! We can then do queries using T2. These will give back constraints
//! on the canonical variables which can be translated, using the map
//! M, into constraints in our source context. This process of
//! translating the results back is done by the
//! `instantiate_query_result` method.
//!
//! For a more detailed look at what is happening here, check
//! out the [chapter in the rustc dev guide][c].
//!
//! [c]: https://rust-lang.github.io/chalk/book/canonical_queries/canonicalization.html

use std::collections::hash_map::Entry;

use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync::Lock;
use rustc_macros::{HashStable, TypeFoldable, TypeVisitable};
pub use rustc_type_ir as ir;
use smallvec::SmallVec;

use crate::mir::ConstraintCategory;
use crate::ty::{self, GenericArg, List, Ty, TyCtxt, TypeFlags, TypeVisitableExt};

pub type CanonicalQueryInput<'tcx, V> = ir::CanonicalQueryInput<TyCtxt<'tcx>, V>;
pub type Canonical<'tcx, V> = ir::Canonical<TyCtxt<'tcx>, V>;
pub type CanonicalVarKind<'tcx> = ir::CanonicalVarKind<TyCtxt<'tcx>>;
pub type CanonicalVarValues<'tcx> = ir::CanonicalVarValues<TyCtxt<'tcx>>;
pub type CanonicalVarKinds<'tcx> = &'tcx List<CanonicalVarKind<'tcx>>;

/// When we canonicalize a value to form a query, we wind up replacing
/// various parts of it with canonical variables. This struct stores
/// those replaced bits to remember for when we process the query
/// result.
#[derive(Clone, Debug)]
pub struct OriginalQueryValues<'tcx> {
    /// Map from the universes that appear in the query to the universes in the
    /// caller context. For all queries except `evaluate_goal` (used by Chalk),
    /// we only ever put ROOT values into the query, so this map is very
    /// simple.
    pub universe_map: SmallVec<[ty::UniverseIndex; 4]>,

    /// This is equivalent to `CanonicalVarValues`, but using a
    /// `SmallVec` yields a significant performance win.
    pub var_values: SmallVec<[GenericArg<'tcx>; 8]>,
}

impl<'tcx> Default for OriginalQueryValues<'tcx> {
    fn default() -> Self {
        let mut universe_map = SmallVec::default();
        universe_map.push(ty::UniverseIndex::ROOT);

        Self { universe_map, var_values: SmallVec::default() }
    }
}

/// After we execute a query with a canonicalized key, we get back a
/// `Canonical<QueryResponse<..>>`. You can use
/// `instantiate_query_result` to access the data in this result.
#[derive(Clone, Debug, HashStable, TypeFoldable, TypeVisitable)]
pub struct QueryResponse<'tcx, R> {
    pub var_values: CanonicalVarValues<'tcx>,
    pub region_constraints: QueryRegionConstraints<'tcx>,
    pub certainty: Certainty,
    pub opaque_types: Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)>,
    pub value: R,
}

#[derive(Clone, Debug, Default, PartialEq, Eq, Hash)]
#[derive(HashStable, TypeFoldable, TypeVisitable)]
pub struct QueryRegionConstraints<'tcx> {
    pub outlives: Vec<QueryOutlivesConstraint<'tcx>>,
    pub assumptions: Vec<ty::ArgOutlivesPredicate<'tcx>>,
}

impl QueryRegionConstraints<'_> {
    /// Represents an empty (trivially true) set of region constraints.
    ///
    /// FIXME(higher_ranked_auto): This could still just be true if there are only assumptions?
    /// Because I don't expect for us to get cases where an assumption from one query would
    /// discharge a requirement from another query, which is a potential problem if we did throw
    /// away these assumptions because there were no constraints.
    pub fn is_empty(&self) -> bool {
        self.outlives.is_empty() && self.assumptions.is_empty()
    }
}

pub type CanonicalQueryResponse<'tcx, T> = &'tcx Canonical<'tcx, QueryResponse<'tcx, T>>;

/// Indicates whether or not we were able to prove the query to be
/// true.
#[derive(Copy, Clone, Debug, HashStable)]
pub enum Certainty {
    /// The query is known to be true, presuming that you apply the
    /// given `var_values` and the region-constraints are satisfied.
    Proven,

    /// The query is not known to be true, but also not known to be
    /// false. The `var_values` represent *either* values that must
    /// hold in order for the query to be true, or helpful tips that
    /// *might* make it true. Currently rustc's trait solver cannot
    /// distinguish the two (e.g., due to our preference for where
    /// clauses over impls).
    ///
    /// After some unification and things have been done, it makes
    /// sense to try and prove again -- of course, at that point, the
    /// canonical form will be different, making this a distinct
    /// query.
    Ambiguous,
}

impl Certainty {
    pub fn is_proven(&self) -> bool {
        match self {
            Certainty::Proven => true,
            Certainty::Ambiguous => false,
        }
    }
}

impl<'tcx, R> QueryResponse<'tcx, R> {
    pub fn is_proven(&self) -> bool {
        self.certainty.is_proven()
    }
}

pub type QueryOutlivesConstraint<'tcx> = (ty::ArgOutlivesPredicate<'tcx>, ConstraintCategory<'tcx>);

#[derive(Default)]
pub struct CanonicalParamEnvCache<'tcx> {
    map: Lock<
        FxHashMap<
            ty::ParamEnv<'tcx>,
            (Canonical<'tcx, ty::ParamEnv<'tcx>>, &'tcx [GenericArg<'tcx>]),
        >,
    >,
}

impl<'tcx> CanonicalParamEnvCache<'tcx> {
    /// Gets the cached canonical form of `key` or executes
    /// `canonicalize_op` and caches the result if not present.
    ///
    /// `canonicalize_op` is intentionally not allowed to be a closure to
    /// statically prevent it from capturing `InferCtxt` and resolving
    /// inference variables, which invalidates the cache.
    pub fn get_or_insert(
        &self,
        tcx: TyCtxt<'tcx>,
        key: ty::ParamEnv<'tcx>,
        state: &mut OriginalQueryValues<'tcx>,
        canonicalize_op: fn(
            TyCtxt<'tcx>,
            ty::ParamEnv<'tcx>,
            &mut OriginalQueryValues<'tcx>,
        ) -> Canonical<'tcx, ty::ParamEnv<'tcx>>,
    ) -> Canonical<'tcx, ty::ParamEnv<'tcx>> {
        if !key.has_type_flags(
            TypeFlags::HAS_INFER | TypeFlags::HAS_PLACEHOLDER | TypeFlags::HAS_FREE_REGIONS,
        ) {
            return Canonical {
                max_universe: ty::UniverseIndex::ROOT,
                var_kinds: List::empty(),
                value: key,
            };
        }

        assert_eq!(state.var_values.len(), 0);
        assert_eq!(state.universe_map.len(), 1);
        debug_assert_eq!(&*state.universe_map, &[ty::UniverseIndex::ROOT]);

        match self.map.borrow().entry(key) {
            Entry::Occupied(e) => {
                let (canonical, var_values) = e.get();
                if cfg!(debug_assertions) {
                    let mut state = state.clone();
                    let rerun_canonical = canonicalize_op(tcx, key, &mut state);
                    assert_eq!(rerun_canonical, *canonical);
                    let OriginalQueryValues { var_values: rerun_var_values, universe_map } = state;
                    assert_eq!(universe_map.len(), 1);
                    assert_eq!(**var_values, *rerun_var_values);
                }
                state.var_values.extend_from_slice(var_values);
                *canonical
            }
            Entry::Vacant(e) => {
                let canonical = canonicalize_op(tcx, key, state);
                let OriginalQueryValues { var_values, universe_map } = state;
                assert_eq!(universe_map.len(), 1);
                e.insert((canonical, tcx.arena.alloc_slice(var_values)));
                canonical
            }
        }
    }
}