Struct rustc::ty::ClosureSubsts

pub struct ClosureSubsts<'tcx> {
    pub substs: &'tcx Substs<'tcx>,
}

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A closure can be modeled as a struct that looks like:

struct Closure<'l0...'li, T0...Tj, CK, CS, U0...Uk> {
    upvar0: U0,
    ...
    upvark: Uk
}

where:

• 'l0...'li and T0...Tj are the lifetime and type parameters in scope on the function that defined the closure,
• CK represents the closure kind (Fn vs FnMut vs FnOnce). This is rather hackily encoded via a scalar type. See TyS::to_opt_closure_kind for details.
• CS represents the closure signature, representing as a fn() type. For example, fn(u32, u32) -> u32 would mean that the closure implements CK<(u32, u32), Output = u32>, where CK is the trait specified above.
• U0...Uk are type parameters representing the types of its upvars (borrowed, if appropriate; that is, if Ui represents a by-ref upvar, and the up-var has the type Foo, then Ui = &Foo).

So, for example, given this function:

fn foo<'a, T>(data: &'a mut T) {
     do(|| data.count += 1)
}

the type of the closure would be something like:

struct Closure<'a, T, U0> {
    data: U0
}

Note that the type of the upvar is not specified in the struct. You may wonder how the impl would then be able to use the upvar, if it doesn't know it's type? The answer is that the impl is (conceptually) not fully generic over Closure but rather tied to instances with the expected upvar types:

impl<'b, 'a, T> FnMut() for Closure<'a, T, &'b mut &'a mut T> {
    ...
}

You can see that the impl fully specified the type of the upvar and thus knows full well that data has type &'b mut &'a mut T. (Here, I am assuming that data is mut-borrowed.)

Now, the last question you may ask is: Why include the upvar types as extra type parameters? The reason for this design is that the upvar types can reference lifetimes that are internal to the creating function. In my example above, for example, the lifetime 'b represents the scope of the closure itself; this is some subset of foo, probably just the scope of the call to the to do(). If we just had the lifetime/type parameters from the enclosing function, we couldn't name this lifetime 'b. Note that there can also be lifetimes in the types of the upvars themselves, if one of them happens to be a reference to something that the creating fn owns.

OK, you say, so why not create a more minimal set of parameters that just includes the extra lifetime parameters? The answer is primarily that it would be hard --- we don't know at the time when we create the closure type what the full types of the upvars are, nor do we know which are borrowed and which are not. In this design, we can just supply a fresh type parameter and figure that out later.

All right, you say, but why include the type parameters from the original function then? The answer is that codegen may need them when monomorphizing, and they may not appear in the upvars. A closure could capture no variables but still make use of some in-scope type parameter with a bound (e.g., if our example above had an extra U: Default, and the closure called U::default()).

There is another reason. This design (implicitly) prohibits closures from capturing themselves (except via a trait object). This simplifies closure inference considerably, since it means that when we infer the kind of a closure or its upvars, we don't have to handle cycles where the decisions we make for closure C wind up influencing the decisions we ought to make for closure C (which would then require fixed point iteration to handle). Plus it fixes an ICE. :P

Generators

Perhaps surprisingly, ClosureSubsts are also used for generators. In that case, what is written above is only half-true -- the set of type parameters is similar, but the role of CK and CS are different. CK represents the "yield type" and CS represents the "return type" of the generator.

It'd be nice to split this struct into ClosureSubsts and GeneratorSubsts, I believe. -nmatsakis

Fields

substs: &'tcx Substs<'tcx>

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Lifetime and type parameters from the enclosing function, concatenated with the types of the upvars.

These are separated out because codegen wants to pass them around when monomorphizing.

Methods

impl<'tcx> ClosureSubsts<'tcx>

fn split(self, def_id: DefId, tcx: TyCtxt) -> SplitClosureSubsts<'tcx>

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Divides the closure substs into their respective components. Single source of truth with respect to the ordering.

pub fn upvar_tys(     self,     def_id: DefId,     tcx: TyCtxt ) -> impl Iterator<Item = Ty<'tcx>> + 'tcx

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pub fn closure_kind_ty(self, def_id: DefId, tcx: TyCtxt) -> Ty<'tcx>

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Returns the closure kind for this closure; may return a type variable during inference. To get the closure kind during inference, use infcx.closure_kind(def_id, substs).

pub fn closure_sig_ty(self, def_id: DefId, tcx: TyCtxt) -> Ty<'tcx>

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Returns the type representing the closure signature for this closure; may contain type variables during inference. To get the closure signature during inference, use infcx.fn_sig(def_id).

pub fn closure_kind(     self,     def_id: DefId,     tcx: TyCtxt<'_, 'tcx, 'tcx> ) -> ClosureKind

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Returns the closure kind for this closure; only usable outside of an inference context, because in that context we know that there are no type variables.

If you have an inference context, use infcx.closure_kind().

pub fn closure_sig(     self,     def_id: DefId,     tcx: TyCtxt<'_, 'tcx, 'tcx> ) -> PolyFnSig<'tcx>

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Extracts the signature from the closure; only usable outside of an inference context, because in that context we know that there are no type variables.

If you have an inference context, use infcx.closure_sig().

Trait Implementations

impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ClosureSubsts<'tcx>

fn hash_stable<W: StableHasherResult>(     &self,     __ctx: &mut StableHashingContext<'a>,     __hasher: &mut StableHasher<W> )

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impl<'tcx> Relate<'tcx> for ClosureSubsts<'tcx>

fn relate<'a, 'gcx, R>(     relation: &mut R,     a: &ClosureSubsts<'tcx>,     b: &ClosureSubsts<'tcx> ) -> RelateResult<'tcx, ClosureSubsts<'tcx>> where     R: TypeRelation<'a, 'gcx, 'tcx>,     'gcx: 'a + 'tcx,     'tcx: 'a,

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impl<'a, 'tcx> Lift<'tcx> for ClosureSubsts<'a>

type Lifted = ClosureSubsts<'tcx>

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fn lift_to_tcx<'b, 'gcx>(     &self,     tcx: TyCtxt<'b, 'gcx, 'tcx> ) -> Option<Self::Lifted>

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impl<'tcx> TypeFoldable<'tcx> for ClosureSubsts<'tcx>

fn super_fold_with<'gcx: 'tcx, V: TypeFolder<'gcx, 'tcx>>(     &self,     folder: &mut V ) -> Self

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fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool

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fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(     &self,     folder: &mut F ) -> Self

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fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool

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fn has_regions_bound_at_or_above(&self, binder: DebruijnIndex) -> bool

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True if self has any late-bound regions that are either bound by binder or bound by some binder outside of binder. If binder is ty::INNERMOST, this indicates whether there are any late-bound regions that appear free.

fn has_regions_bound_above(&self, binder: DebruijnIndex) -> bool

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True if this self has any regions that escape binder (and hence are not bound by it).

fn has_escaping_regions(&self) -> bool

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fn has_type_flags(&self, flags: TypeFlags) -> bool

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fn has_projections(&self) -> bool

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fn references_error(&self) -> bool

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fn has_param_types(&self) -> bool

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fn has_self_ty(&self) -> bool

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fn has_infer_types(&self) -> bool

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fn needs_infer(&self) -> bool

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fn has_skol(&self) -> bool

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fn needs_subst(&self) -> bool

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fn has_re_skol(&self) -> bool

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fn has_closure_types(&self) -> bool

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fn has_free_regions(&self) -> bool

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"Free" regions in this context means that it has any region that is not (a) erased or (b) late-bound.

fn has_erasable_regions(&self) -> bool

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True if there any any un-erased free regions.

fn is_global(&self) -> bool

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Indicates whether this value references only 'global' types/lifetimes that are the same regardless of what fn we are in. This is used for caching.

fn has_late_bound_regions(&self) -> bool

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True if there are any late-bound regions

fn visit_tys_shallow(     &self,     visit: impl FnMut(Ty<'tcx>) -> bool ) -> bool

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A visitor that does not recurse into types, works like fn walk_shallow in Ty.

impl<'tcx> Copy for ClosureSubsts<'tcx>

impl<'tcx> Clone for ClosureSubsts<'tcx>

fn clone(&self) -> ClosureSubsts<'tcx>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'tcx> PartialEq for ClosureSubsts<'tcx>

fn eq(&self, other: &ClosureSubsts<'tcx>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &ClosureSubsts<'tcx>) -> bool

This method tests for !=.

impl<'tcx> Eq for ClosureSubsts<'tcx>

fn assert_receiver_is_total_eq(&self)

impl<'tcx> PartialOrd for ClosureSubsts<'tcx>

fn partial_cmp(&self, other: &ClosureSubsts<'tcx>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &ClosureSubsts<'tcx>) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &ClosureSubsts<'tcx>) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &ClosureSubsts<'tcx>) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &ClosureSubsts<'tcx>) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<'tcx> Ord for ClosureSubsts<'tcx>

fn cmp(&self, other: &ClosureSubsts<'tcx>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

impl<'tcx> Hash for ClosureSubsts<'tcx>

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where     H: Hasher,

Feeds a slice of this type into the given [Hasher].

impl<'tcx> Debug for ClosureSubsts<'tcx>

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.

impl<'tcx> Encodable for ClosureSubsts<'tcx>

fn encode<__S: Encoder>(&self, s: &mut __S) -> Result<(), __S::Error>

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impl<'tcx> Decodable for ClosureSubsts<'tcx>

fn decode<__D: Decoder>(d: &mut __D) -> Result<ClosureSubsts<'tcx>, __D::Error>

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