Expose trailing headers

[sagebind]

Provide a way for users to access "trailer headers" as defined in RFC 7230. Trailer headers are headers delivered after the response body has been consumed with chunked encoding.

Accessing the trailer is tricky because it can make APIs look awkward since it violates the traditional flow of a response. Here's one possible API:

// Existing trait
pub trait ResponseExt {
    // New method, returns an object that provides access to the trailer, if a trailer
    // is expected.
    fn trailer(&self) -> Option<&Trailer>;
}

pub struct Trailer {
    // ...
}

impl Trailer {
    // Get the currently available trailer headers. May be None if the trailer has
    // not yet arrived.
    pub fn headers(&self) -> Option<&HeaderMap>;

    // Block the current thread until the trailer is received, or the transfer
    // completes, whichever comes first.
    pub fn wait(&self);

    // Wait for the trailer to arrive asynchronously.
    pub async fn arrive(&self);
}

[sagebind]

This is now available in the 1.4.0 release!