Don't force HTTP2 support

[g2p]

Currently hyper-rustls sets alpn_protocols to advertise HTTP2 (in connector.rs).
It doesn't properly work with current hyper features (which require http2 to be explicitly enabled), and forces the user to enable a feature that has a large dependency footprint and potential security surface (or to be incompatible with some peers).

hyper-rustls should give the caller some control over that, prior to building the connector.
And the support should be conditional on an hyper-rustls/http2 feature, so that the dependency set is always correct at compile time. It could work similar to how the root certificates are determined.

[Arnavion]

To save other people the time to investigate this: If you're using a hyper::Client, and you only enabled the http1 feature of hyper and not the http2 feature, and you see a channel closed error from hyper from executing some (but not necessarily all) of your client requests, this issue may be why. To be sure, enable trace logging for rustls and see if there's a log indicating it selected HTTP/2 during ALPN right before the request failed:

[DEBUG rustls::client::hs] ALPN protocol is Some(b"h2")

For requests that succeed, you'd instead see ALPN protocol is None or ALPN protocol is Some(b"http/1.1")

[kahlil29]

Just to clarify, if I'm using the rusoto crate with the rustls feature enabled (I think it depends on hyper-rustls), is there a way for me to know if I'm hitting this issue?
i.e. (Can I) How do I enable logging that would enable me to narrow down the issue or confirm that this issue affects me?