Function build 

pub fn build<'a, V: Verify, W: Watch, R: Recognizer>(
    target: &'a Certificate<'a>,
    pool: &[&'a Certificate<'a>],
    context: &Context<'a, '_, V, W, R>,
) -> BuildResult<Outcome<'a>, V::Error, R::Error>

Builds the paths from one certificate to the anchors the caller trusts.

The pool is where issuers are looked for, and holding the target itself changes nothing: a certificate never appears twice in one path (RFC 5280 6.1), so it cannot be its own issuer.

Errors

A spent budget, which says this run gave up rather than that the certificates are wrong.