feat(exporter): introduce PredicateId

frontend/exporter/src/types/new/mod.rs

@@ -2,7 +2,9 @@
 //! Rustc.
 
 mod item_attributes;
+mod predicate_id;
 mod typed_constant_kind;
 
 pub use item_attributes::*;
+pub use predicate_id::*;
 pub use typed_constant_kind::*;

frontend/exporter/src/types/new/predicate_id.rs

@@ -0,0 +1,65 @@
+use crate::prelude::*;
+use rustc_middle::ty;
+
+#[derive(
+    Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, Hash, PartialEq, Eq, PartialOrd, Ord,
+)]
+#[serde(transparent)]
+/// A `PredicateId` is a unique identifier for a clause or a
+/// predicate. It is computed by hashing predicates and clause in a
+/// uniform and deterministic way.
+pub struct PredicateId(u64);
+
+/// Implemented by anything that can be assimilated to a predicate.
+pub trait IntoPredicateId<'tcx, S: UnderOwnerState<'tcx>> {
+    /// Compute a consistent `PredicateId`
+    fn predicate_id(&self, s: &S) -> PredicateId;
+}
+
+impl<'tcx, S: UnderOwnerState<'tcx>> IntoPredicateId<'tcx, S> for ty::Clause<'tcx> {
+    fn predicate_id(&self, s: &S) -> PredicateId {
+        self.as_predicate().predicate_id(s)
+    }
+}
+
+impl<'tcx, S: UnderOwnerState<'tcx>> IntoPredicateId<'tcx, S> for ty::Predicate<'tcx> {
+    fn predicate_id(&self, s: &S) -> PredicateId {
+        // Here, we need to be careful about not hashing a `crate::Predicate`,
+        // but `crate::Binder<crate::PredicateKind>` instead,
+        // otherwise we would get into a infinite recursion.
+        let poly_kind: Binder<PredicateKind> = self.kind().sinto(s);
+        PredicateId(deterministic_hash(&poly_kind))
+    }
+}
+
+impl<'tcx, S: UnderOwnerState<'tcx>> IntoPredicateId<'tcx, S> for ty::PolyTraitPredicate<'tcx> {
+    fn predicate_id(&self, s: &S) -> PredicateId {
+        use ty::ToPredicate;
+        let predicate: ty::Predicate<'tcx> = (*self).to_predicate(s.base().tcx);
+        predicate.predicate_id(s)
+    }
+}
+
+/// A `PredicateId` can be mapped to itself via SInto. This is useful
+/// for mirroring the type [`traits::search_clause::PathChunk`] as
+/// [`traits::ImplExprPathChunk`].
+impl<'tcx, S: UnderOwnerState<'tcx>> SInto<S, PredicateId> for PredicateId {
+    fn sinto(&self, _s: &S) -> PredicateId {
+        *self
+    }
+}
+
+/// We need identifiers that are stable across different
+/// architectures, different paths (which are observable from
+/// `Span`s), etc.
+/// Rustc's stable hash is not doing what we want here: it is sensible
+/// to the environment. Instead, we first `sinto` and then hash with
+/// `deterministic_hash` below.
+fn deterministic_hash<T: std::hash::Hash>(x: &T) -> u64 {
+    use crate::deterministic_hash::DeterministicHasher;
+    use std::collections::hash_map::DefaultHasher;
+    use std::hash::Hasher;
+    let mut hasher = DeterministicHasher::new(DefaultHasher::new());
+    x.hash(&mut hasher);
+    hasher.finish()
+}