Enum rustc_trait_selection::infer::region_constraints::VerifyBound

pub enum VerifyBound<'tcx> {
    IfEq(&'tcx TyS<'tcx>, Box<VerifyBound<'tcx>, Global>),
    OutlivedBy(&'tcx RegionKind),
    IsEmpty,
    AnyBound(Vec<VerifyBound<'tcx>, Global>),
    AllBounds(Vec<VerifyBound<'tcx>, Global>),
}

Describes the things that some GenericKind value G is known to outlive. Each variant of VerifyBound can be thought of as a function:

fn(min: Region) -> bool { .. }

where true means that the region min meets that G: min. (False means nothing.)

So, for example, if we have the type T and we have in scope that T: 'a and T: 'b, then the verify bound might be:

fn(min: Region) -> bool {
   ('a: min) || ('b: min)
}

This is described with a AnyRegion('a, 'b) node.

Variants

IfEq(&'tcx TyS<'tcx>, Box<VerifyBound<'tcx>, Global>)

Given a kind K and a bound B, expands to a function like the following, where G is the generic for which this verify bound was created:

fn(min) -> bool {
    if G == K {
        B(min)
    } else {
        false
    }
}

In other words, if the generic G that we are checking is equal to K, then check the associated verify bound (otherwise, false).

This is used when we have something in the environment that may or may not be relevant, depending on the region inference results. For example, we may have where <T as Trait<'a>>::Item: 'b in our where-clauses. If we are generating the verify-bound for <T as Trait<'0>>::Item, then this where-clause is only relevant if '0 winds up inferred to 'a.

So we would compile to a verify-bound like

IfEq(<T as Trait<'a>>::Item, AnyRegion('a))

meaning, if the subject G is equal to <T as Trait<'a>>::Item (after inference), and 'a: min, then G: min.

OutlivedBy(&'tcx RegionKind)

Given a region R, expands to the function:

fn(min) -> bool {
    R: min
}

This is used when we can establish that G: R – therefore, if R: min, then by transitivity G: min.

IsEmpty

Given a region R, true if it is 'empty.

AnyBound(Vec<VerifyBound<'tcx>, Global>)

Given a set of bounds B, expands to the function:

fn(min) -> bool {
    exists (b in B) { b(min) }
}

In other words, if we meet some bound in B, that suffices. This is used when all the bounds in B are known to apply to G.

AllBounds(Vec<VerifyBound<'tcx>, Global>)

Given a set of bounds B, expands to the function:

fn(min) -> bool {
    forall (b in B) { b(min) }
}

In other words, if we meet all bounds in B, that suffices. This is used when some bound in B is known to suffice, but we don’t know which.