Type Alias RegionKind

pub type RegionKind<'tcx> = RegionKind<TyCtxt<'tcx>>;

Aliased Type

pub enum RegionKind<'tcx> {
    ReEarlyParam(EarlyParamRegion),
    ReBound(DebruijnIndex, BoundRegion),
    ReLateParam(LateParamRegion),
    ReStatic,
    ReVar(RegionVid),
    RePlaceholder(Placeholder<BoundRegion>),
    ReErased,
    ReError(ErrorGuaranteed),
}

Variants

ReEarlyParam(EarlyParamRegion)

A region parameter; for example 'a in impl<'a> Trait for &'a ().

There are some important differences between region and type parameters. Not all region parameters in the source are represented via ReEarlyParam: late-bound function parameters are instead lowered to a ReBound. Late-bound regions get eagerly replaced with ReLateParam which behaves in the same way as ReEarlyParam. Region parameters are also sometimes implicit, e.g. in impl Trait for &().

ReBound(DebruijnIndex, BoundRegion)

A higher-ranked region. These represent either late-bound function parameters or bound variables from a for<'a>-binder.

While inside of a function, e.g. during typeck, the late-bound function parameters can be converted to ReLateParam by calling tcx.liberate_late_bound_regions.

Bound regions inside of types must not be erased, as they impact trait selection and the TypeId of that type. for<'a> fn(&'a ()) and fn(&'static ()) are different types and have to be treated as such.

ReLateParam(LateParamRegion)

Late-bound function parameters are represented using a ReBound. When inside of a function, we convert these bound variables to placeholder parameters via tcx.liberate_late_bound_regions. They are then treated the same way as ReEarlyParam while inside of the function.

See https://rustc-dev-guide.rust-lang.org/early-late-bound-params/early-late-bound-summary.html for more info about early and late bound lifetime parameters.

ReStatic

Static data that has an “infinite” lifetime. Top in the region lattice.

ReVar(RegionVid)

A region variable. Should not exist outside of type inference.

RePlaceholder(Placeholder<BoundRegion>)

A placeholder region – the higher-ranked version of ReLateParam. Should not exist outside of type inference.

Used when instantiating a forall binder via infcx.enter_forall.

ReErased

Erased region, used by trait selection, in MIR and during codegen.

ReError(ErrorGuaranteed)

A region that resulted from some other error. Used exclusively for diagnostics.

Layout

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.