Struct rustc_trait_selection::infer::type_variable::TypeVariableStorage

pub struct TypeVariableStorage<'tcx> {
    values: SnapshotVec<Delegate, Vec<TypeVariableData, Global>, ()>,
    eq_relations: UnificationTable<InPlace<TyVidEqKey<'tcx>, Vec<VarValue<TyVidEqKey<'tcx>>, Global>, ()>>,
    sub_relations: UnificationTable<InPlace<TyVid, Vec<VarValue<TyVid>, Global>, ()>>,
}

Fields

values: SnapshotVec<Delegate, Vec<TypeVariableData, Global>, ()>

eq_relations: UnificationTable<InPlace<TyVidEqKey<'tcx>, Vec<VarValue<TyVidEqKey<'tcx>>, Global>, ()>>

Two variables are unified in eq_relations when we have a constraint ?X == ?Y. This table also stores, for each key, the known value.

sub_relations: UnificationTable<InPlace<TyVid, Vec<VarValue<TyVid>, Global>, ()>>

Two variables are unified in sub_relations when we have a constraint ?X <: ?Y or a constraint ?Y <: ?X. This second table exists only to help with the occurs check. In particular, we want to report constraints like these as an occurs check violation:

?1 <: ?3
Box<?3> <: ?1

This works because ?1 and ?3 are unified in the sub_relations relation (not in eq_relations). Then when we process the Box<?3> <: ?1 constraint, we do an occurs check on Box<?3> and find a potential cycle.

This is reasonable because, in Rust, subtypes have the same “skeleton” and hence there is no possible type such that (e.g.) Box<?3> <: ?3 for any ?3.

Layout

Note: Most layout information is completely unstable and may be different between compiler versions and platforms. The only exception is types with certain repr(...) attributes. Please see the Rust Reference’s “Type Layout” chapter for details on type layout guarantees.

Size: 72 bytes