Basic Constraints certificate extension

For Basic Constraints certificate extension currently rcgen support two options:

pub enum IsCa {
    SelfSignedOnly,
    Ca(BasicConstraints)
}

And if we look into spec we will see exactly these two options:

id-ce-basicConstraints OBJECT IDENTIFIER ::=  { id-ce 19 }

   BasicConstraints ::= SEQUENCE {
        cA                      BOOLEAN DEFAULT FALSE,
        pathLenConstraint       INTEGER (0..MAX) OPTIONAL }
and extension identifies two aspects:

Subject type: is the certificate a CA certificate or it is an end entity certificate;
[Optionally] How many CAs are allowed in the chain below current CA certificate. This setting has no meaning for end entity certificates.

And for sure Ca(x) case generate something like:

SubjectType=CA
Path length Constraint = x

But in case of SelfSignedOnly it didn't generate anything.

Empty

Despite spec said: "If Basic Constraints extension is not included in certificate, it is automatically treated as end entity certificate."

Is it possible to force writing this field?

SubjectType=End Entity
Path length Constraint = None

Start by locating the IsCa and BasicConstraints definitions and the certificate-extension encoding path, then compare how SelfSignedOnly and Ca(BasicConstraints) are represented. The work is done when SelfSignedOnly can explicitly encode the end-entity Basic Constraints result, with tests covering the generated extension and its absence or presence as appropriate.