Struct rustc::infer::type_variable::TypeVariableTable

pub struct TypeVariableTable<'tcx> {
    values: SnapshotVec<Delegate>,
    eq_relations: UnificationTable<InPlace<TyVidEqKey<'tcx>>>,
    sub_relations: UnificationTable<InPlace<TyVid>>,
}

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Fields

values: SnapshotVec<Delegate>

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eq_relations: UnificationTable<InPlace<TyVidEqKey<'tcx>>>

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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>>

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Two variables are unified in eq_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.

Methods

impl<'tcx> TypeVariableTable<'tcx>

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

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pub fn var_diverges<'a>(&'a self, vid: TyVid) -> bool

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Returns the diverges flag given when vid was created.

Note that this function does not return care whether vid has been unified with something else or not.

pub fn var_origin(&self, vid: TyVid) -> &TypeVariableOrigin

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Returns the origin that was given when vid was created.

Note that this function does not return care whether vid has been unified with something else or not.

pub fn equate(&mut self, a: TyVid, b: TyVid)

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Records that a == b, depending on dir.

Precondition: neither a nor b are known.

pub fn sub(&mut self, a: TyVid, b: TyVid)

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Records that a <: b, depending on dir.

Precondition: neither a nor b are known.

pub fn instantiate(&mut self, vid: TyVid, ty: Ty<'tcx>)

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Instantiates vid with the type ty.

Precondition: vid must not have been previously instantiated.

pub fn new_var(     &mut self,     universe: UniverseIndex,     diverging: bool,     origin: TypeVariableOrigin ) -> TyVid

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Creates a new type variable.

• diverging: indicates if this is a "diverging" type variable, e.g. one created as the type of a return expression. The code in this module doesn't care if a variable is diverging, but the main Rust type-checker will sometimes "unify" such variables with the ! or () types.
• origin: indicates why the type variable was created. The code in this module doesn't care, but it can be useful for improving error messages.

pub fn num_vars(&self) -> usize

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Returns the number of type variables created thus far.

pub fn root_var(&mut self, vid: TyVid) -> TyVid

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Returns the "root" variable of vid in the eq_relations equivalence table. All type variables that have been equated will yield the same root variable (per the union-find algorithm), so root_var(a) == root_var(b) implies that a == b (transitively).

pub fn sub_root_var(&mut self, vid: TyVid) -> TyVid

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Returns the "root" variable of vid in the sub_relations equivalence table. All type variables that have been are related via equality or subtyping will yield the same root variable (per the union-find algorithm), so sub_root_var(a) == sub_root_var(b) implies that:

exists X. (a <: X || X <: a) && (b <: X || X <: b)

pub fn sub_unified(&mut self, a: TyVid, b: TyVid) -> bool

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True if a and b have same "sub-root" (i.e., exists some type X such that forall i in {a, b}. (i <: X || X <: i).

pub fn probe(&mut self, vid: TyVid) -> TypeVariableValue<'tcx>

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Retrieves the type to which vid has been instantiated, if any.

pub fn replace_if_possible(&mut self, t: Ty<'tcx>) -> Ty<'tcx>

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If t is a type-inference variable, and it has been instantiated, then return the with which it was instantiated. Otherwise, returns t.

pub fn snapshot(&mut self) -> Snapshot<'tcx>

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Creates a snapshot of the type variable state. This snapshot must later be committed (commit()) or rolled back (rollback_to()). Nested snapshots are permitted, but must be processed in a stack-like fashion.

pub fn rollback_to(&mut self, s: Snapshot<'tcx>)

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Undoes all changes since the snapshot was created. Any snapshots created since that point must already have been committed or rolled back.

pub fn commit(&mut self, s: Snapshot<'tcx>)

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Commits all changes since the snapshot was created, making them permanent (unless this snapshot was created within another snapshot). Any snapshots created since that point must already have been committed or rolled back.

pub fn types_created_since_snapshot(     &mut self,     s: &Snapshot<'tcx> ) -> TypeVariableMap

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Returns a map {V1 -> V2}, where the keys {V1} are ty-variables created during the snapshot, and the values {V2} are the root variables that they were unified with, along with their origin.

Important traits for Vec<u8>

impl Write for Vec<u8>

pub fn types_escaping_snapshot(&mut self, s: &Snapshot<'tcx>) -> Vec<Ty<'tcx>>

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Find the set of type variables that existed before s but which have only been unified since s started, and return the types with which they were unified. So if we had a type variable V0, then we started the snapshot, then we created a type variable V1, unifed V0 with T0, and unified V1 with T1, this function would return {T0}.

Important traits for Vec<u8>

impl Write for Vec<u8>

pub fn unsolved_variables(&mut self) -> Vec<TyVid>

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Returns indices of all variables that are not yet instantiated.