Struct ClosureRegionRequirements

pub struct ClosureRegionRequirements<'tcx> {
    pub num_external_vids: usize,
    pub outlives_requirements: Vec<ClosureOutlivesRequirement<'tcx>>,
}

After we borrow check a closure, we are left with various requirements that we have inferred between the free regions that appear in the closure’s signature or on its field types. These requirements are then verified and proved by the closure’s creating function. This struct encodes those requirements.

The requirements are listed as being between various RegionVid. The 0th region refers to 'static; subsequent region vids refer to the free regions that appear in the closure (or coroutine’s) type, in order of appearance. (This numbering is actually defined by the UniversalRegions struct in the NLL region checker. See for example UniversalRegions::closure_mapping.) Note the free regions in the closure’s signature and captures are erased.

Example: If type check produces a closure with the closure args:

ClosureArgs = [
    'a,                                         // From the parent.
    'b,
    i8,                                         // the "closure kind"
    for<'x> fn(&'<erased> &'x u32) -> &'x u32,  // the "closure signature"
    &'<erased> String,                          // some upvar
]

We would “renumber” each free region to a unique vid, as follows:

ClosureArgs = [
    '1,                                         // From the parent.
    '2,
    i8,                                         // the "closure kind"
    for<'x> fn(&'3 &'x u32) -> &'x u32,         // the "closure signature"
    &'4 String,                                 // some upvar
]

Now the code might impose a requirement like '1: '2. When an instance of the closure is created, the corresponding free regions can be extracted from its type and constrained to have the given outlives relationship.

Fields

num_external_vids: usize

The number of external regions defined on the closure. In our example above, it would be 3 – one for 'static, then '1 and '2. This is just used for a sanity check later on, to make sure that the number of regions we see at the callsite matches.

outlives_requirements: Vec<ClosureOutlivesRequirement<'tcx>>

Requirements between the various free regions defined in indices.