a

    /// Invoked to create the lifetime argument(s) for an elided trait object
    /// bound, like the bound in `Box<dyn Debug>`. This method is not invoked
    /// when the bound is written, even if it is written with `'_` like in
    /// `Box<dyn Debug + '_>`. In those cases, `lower_lifetime` is invoked.
    fn elided_dyn_bound(&mut self, span: Span) -> hir::Lifetime {
        match self.anonymous_lifetime_mode {
            // NB. We intentionally ignore the create-parameter mode here.
            // and instead "pass through" to resolve-lifetimes, which will apply
            // the object-lifetime-defaulting rules. Elided object lifetime defaults
            // do not act like other elided lifetimes. In other words, given this:
            //
            //     impl Foo for Box<dyn Debug>
            //
            // we do not introduce a fresh `'_` to serve as the bound, but instead
            // ultimately translate to the equivalent of:
            //
            //     impl Foo for Box<dyn Debug + 'static>
            //
            // `resolve_lifetime` has the code to make that happen.
            AnonymousLifetimeMode::CreateParameter => {}

            AnonymousLifetimeMode::ReportError => {
                // ReportError applies to explicit use of `'_`.
            }

            // This is the normal case.
            AnonymousLifetimeMode::PassThrough => {}
        }

        self.new_implicit_lifetime(span)
    }

    fn new_implicit_lifetime(&mut self, span: Span) -> hir::Lifetime {
        let LoweredNodeId { node_id, hir_id } = self.next_id();

        hir::Lifetime {
            id: node_id,
            hir_id,
            span,
            name: hir::LifetimeName::Implicit,
        }
    }