curl::easy::Easy2::default_configure registers an error buffer like this:

self.setopt_ptr(
    curl_sys::CURLOPT_ERRORBUFFER,
    self.inner.error_buf.borrow().as_ptr() as *const _,
)
.expect("failed to set error buffer");

CURLOPT_ERRORBUFFER requires mutable access over the buffer, and it is important that the buffer pointer is still valid later on when further library calls are made. However, error_buf is borrowed immutably and only for a short time (until the end of the statement).

(A mutable borrow might look like this:

self.inner.error_buf.borrow_mut().as_mut_ptr() as *const u8 as *const _

That does not solve the lifetime problem, though.)