Struct rustc::ty::ClosureSubsts

pub struct ClosureSubsts<'tcx> {
    pub func_substs: &'tcx Substs<'tcx>,
    pub upvar_tys: &'tcx [Ty<'tcx>],
}

Unstable (rustc_private)

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

struct Closure<'l0...'li, T0...Tj, 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, and U0...Uk are type parameters representing the types of its upvars (borrowed, if appropriate).

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 extent of the closure itself; this is some subset of foo, probably just the extent 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 trans 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

Fields

func_substs: &'tcx Substs<'tcx>

Unstable (rustc_private)

Lifetime and type parameters from the enclosing function. These are separated out because trans wants to pass them around when monomorphizing.

upvar_tys: &'tcx [Ty<'tcx>]

Unstable (rustc_private)

The types of the upvars. The list parallels the freevars and upvar_borrows lists. These are kept distinct so that we can easily index into them.

Trait Implementations

impl<'gcx> TransNormalize<'gcx> for ClosureSubsts<'gcx>

fn trans_normalize<'a, 'tcx>(&self, infcx: &InferCtxt<'a, 'gcx, 'tcx>) -> Self

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

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

type Lifted = ClosureSubsts<'tcx>

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

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

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

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

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

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

fn has_regions_escaping_depth(&self, depth: u32) -> bool

fn has_escaping_regions(&self) -> bool

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

fn has_projection_types(&self) -> bool

fn references_error(&self) -> bool

fn has_param_types(&self) -> bool

fn has_self_ty(&self) -> bool

fn has_infer_types(&self) -> bool

fn needs_infer(&self) -> bool

fn needs_subst(&self) -> bool

fn has_closure_types(&self) -> bool

fn has_erasable_regions(&self) -> bool

fn is_global(&self) -> bool

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. Errs on the side of returning false.

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

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

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

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

Derived Implementations

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

fn fmt(&self, __arg_0: &mut Formatter) -> Result

Formats the value using the given formatter.

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

fn hash<__H: Hasher>(&self, __arg_0: &mut __H)

Feeds this value into the state given, updating the hasher as necessary.

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

Feeds a slice of this type into the state provided.

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

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

fn eq(&self, __arg_0: &ClosureSubsts<'tcx>) -> bool

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

fn ne(&self, __arg_0: &ClosureSubsts<'tcx>) -> bool

This method tests for !=.

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> Copy for ClosureSubsts<'tcx>