Struct rustc_trait_selection::infer::region_constraints::RegionConstraintData

pub struct RegionConstraintData<'tcx> {
    pub constraints: BTreeMap<Constraint<'tcx>, SubregionOrigin<'tcx>>,
    pub member_constraints: Vec<MemberConstraint<'tcx>>,
    pub verifys: Vec<Verify<'tcx>>,
    pub givens: HashSet<(&'tcx RegionKind, RegionVid), BuildHasherDefault<FxHasher>>,
}

The full set of region constraints gathered up by the collector. Describes constraints between the region variables and other regions, as well as other conditions that must be verified, or assumptions that can be made.

Fields

constraints: BTreeMap<Constraint<'tcx>, SubregionOrigin<'tcx>>

Constraints of the form A <= B, where either A or B can be a region variable (or neither, as it happens).

member_constraints: Vec<MemberConstraint<'tcx>>

Constraints of the form R0 member of [R1, ..., Rn], meaning that R0 must be equal to one of the regions R1..Rn. These occur with impl Trait quite frequently.

verifys: Vec<Verify<'tcx>>

A "verify" is something that we need to verify after inference is done, but which does not directly affect inference in any way.

An example is a A <= B where neither A nor B are inference variables.

givens: HashSet<(&'tcx RegionKind, RegionVid), BuildHasherDefault<FxHasher>>

A "given" is a relationship that is known to hold. In particular, we often know from closure fn signatures that a particular free region must be a subregion of a region variable:

foo.iter().filter(<'a> |x: &'a &'b T| ...)

In situations like this, 'b is in fact a region variable introduced by the call to iter(), and 'a is a bound region on the closure (as indicated by the <'a> prefix). If we are naive, we wind up inferring that 'b must be 'static, because we require that it be greater than 'a and we do not know what 'a is precisely.

This hashmap is used to avoid that naive scenario. Basically we record the fact that 'a <= 'b is implied by the fn signature, and then ignore the constraint when solving equations. This is a bit of a hack but seems to work.