Struct rustc_infer::infer::type_variable::TypeVariableStorage

pub struct TypeVariableStorage<'tcx> {
    values: SnapshotVecStorage<Delegate>,
    eq_relations: UnificationTableStorage<TyVidEqKey<'tcx>>,
    sub_relations: UnificationTableStorage<TyVid>,
}

Fields

values: SnapshotVecStorage<Delegate>

eq_relations: UnificationTableStorage<TyVidEqKey<'tcx>>

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: UnificationTableStorage<TyVid>

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.

Implementations

impl<'tcx> TypeVariableStorage<'tcx>

pub fn new() -> TypeVariableStorage<'tcx>

pub(crate) fn with_log<'a>(
    &'a mut self,
    undo_log: &'a mut InferCtxtUndoLogs<'tcx>
) -> TypeVariableTable<'a, 'tcx>

Trait Implementations

impl<'tcx> Rollback<UndoLog<'tcx>> for TypeVariableStorage<'tcx>

fn reverse(&mut self, undo: UndoLog<'tcx>)