Enum RegionKind 

pub enum RegionKind<I>
where
    I: Interner,
{
    ReEarlyParam(<I as Interner>::EarlyParamRegion),
    ReBound(BoundVarIndexKind, BoundRegion<I>),
    ReLateParam(<I as Interner>::LateParamRegion),
    ReStatic,
    ReVar(RegionVid),
    RePlaceholder(Placeholder<I, BoundRegion<I>>),
    ReErased,
    ReError(<I as Interner>::ErrorGuaranteed),
}

Representation of regions. Note that the NLL checker uses a distinct representation of regions. For this reason, it internally replaces all the regions with inference variables – the index of the variable is then used to index into internal NLL data structures. See rustc_const_eval::borrow_check module for more information.

Note: operations are on the wrapper Region type, which is interned, rather than this type.

The Region lattice within a given function

In general, the region lattice looks like

empty(Un) --------                      (smallest)
|                 \
...                \
|                   \
empty(U1) --         \
|           \         placeholder(Un)
|            \                  |
empty(root)   placeholder(U1)   |
|                |              |
|                |              |
|                |              |
param regions    |              |
|                |              |
static ----------+-----...------+       (greatest)

Lifetimes in scope from a function declaration are represented via RegionKind::ReEarlyParam/RegionKind::ReLateParam. They have relationships to one another and 'static based on the declared relationships from the function.

Note that inference variables and bound regions are not included in this diagram. In the case of inference variables, they should be inferred to some other region from the diagram. In the case of bound regions, they are excluded because they don’t make sense to include – the diagram indicates the relationship between free regions.

You can read more about the distinction between early and late bound parameters in the rustc dev guide: Early vs Late bound parameters.

A note on subtyping: If we assume that references take their region covariantly, and use that to define the subtyping relationship of regions, it may be somewhat surprising that 'empty is Top and 'static is Bottom, and that “'a is a subtype of 'b” is defined as “'a is bigger than 'b” - good to keep in mind.

Inference variables

During region inference, we sometimes create inference variables, represented as RegionKind::ReVar. These will be inferred by the code in infer::lexical_region_resolve to some free region from the lattice above (the minimal region that meets the constraints).

During NLL checking, where regions are defined differently, we also use RegionKind::ReVar – in that case, the index is used to index into the NLL region checker’s data structures. The variable may in fact represent either a free region or an inference variable, in that case.

Bound Regions

These are regions that are stored behind a binder and must be instantiated with some concrete region before being used. A type can be wrapped in a Binder, which introduces new type/const/lifetime variables (e.g., for<'a> fn(&'a ())). These parameters are referred to via RegionKind::ReBound. You can instantiate them by the likes of liberate_late_bound_regions.

Unlike Params, bound regions are not supposed to exist “in the wild” outside their binder, e.g., in types passed to type inference, and should first be instantiated (by placeholder regions, free regions, or region variables).

Placeholder and Free Regions

One often wants to work with bound regions without knowing their precise identity. For example, when checking a function, the lifetime of a borrow can end up being assigned to some region parameter. In these cases, it must be ensured that bounds on the region can’t be accidentally assumed without being checked.

To do this, we replace the bound regions with placeholder markers, which don’t satisfy any relation not explicitly provided.

There are two kinds of placeholder regions in rustc: ReLateParam and RePlaceholder. When checking an item’s body, ReLateParam is supposed to be used. These also support explicit bounds: both the internally-stored scope, which the region is assumed to outlive, as well as other relations stored in the FreeRegionMap. Note that these relations aren’t checked when you make_subregion (or eq_types), only by resolve_regions_and_report_errors.

When working with higher-ranked types, some region relations aren’t yet known, so you can’t just call resolve_regions_and_report_errors. RePlaceholder is designed for this purpose. In these contexts, there’s also the risk that some inference variable laying around will get unified with your placeholder region: if you want to check whether for<'a> Foo<'_>: 'a, and you instantiate your bound region 'a with a placeholder region '%a, the variable '_ would just be instantiated to the placeholder region '%a, which is wrong because the inference variable is supposed to satisfy the relation for every value of the placeholder region. To ensure that doesn’t happen, you can use leak_check. This is more clearly explained by the rustc dev guide.

Variants

ReEarlyParam(<I as Interner>::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(BoundVarIndexKind, BoundRegion<I>)

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(<I as Interner>::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_parameters.html for more info about early and late bound lifetime parameters.

ReStatic

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

ReVar(RegionVid)

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

RePlaceholder(Placeholder<I, BoundRegion<I>>)

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(<I as Interner>::ErrorGuaranteed)

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

Trait Implementations

impl<I> Clone for RegionKind<I>

where I: Interner,

fn clone(&self) -> RegionKind<I>

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl<I> Debug for RegionKind<I>

where I: Interner,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<I, __D> Decodable<__D> for RegionKind<I>

where I: Interner, __D: Decoder, <I as Interner>::EarlyParamRegion: Decodable<__D>, BoundRegion<I>: Decodable<__D>, <I as Interner>::LateParamRegion: Decodable<__D>, Placeholder<I, BoundRegion<I>>: Decodable<__D>, <I as Interner>::ErrorGuaranteed: Decodable<__D>,

fn decode(__decoder: &mut __D) -> RegionKind<I>

impl<I, __E> Encodable<__E> for RegionKind<I>

where I: Interner, __E: Encoder, <I as Interner>::EarlyParamRegion: Encodable<__E>, BoundRegion<I>: Encodable<__E>, <I as Interner>::LateParamRegion: Encodable<__E>, Placeholder<I, BoundRegion<I>>: Encodable<__E>, <I as Interner>::ErrorGuaranteed: Encodable<__E>,

fn encode(&self, __encoder: &mut __E)

impl<I> Hash for RegionKind<I>

where I: Interner,

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

where __H: Hasher,

Feeds this value into the given Hasher.

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

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<CTX, I> HashStable<CTX> for RegionKind<I>

where I: Interner, <I as Interner>::EarlyParamRegion: HashStable<CTX>, <I as Interner>::LateParamRegion: HashStable<CTX>, <I as Interner>::DefId: HashStable<CTX>, <I as Interner>::Symbol: HashStable<CTX>,

Available on crate feature nightly only.

fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher<SipHasher128>)

impl<I> PartialEq for RegionKind<I>

where I: Interner,

fn eq(&self, __other: &RegionKind<I>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<I> Copy for RegionKind<I>

where I: Interner,

impl<I> Eq for RegionKind<I>

where I: Interner,

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.