This provides a clear and consistent interface for selecting TLS-related
features on both (xmpp and tokio-xmpp) crates. All feature checks have
been revamped. All working combinations have been tested (including a
connectivity test + --all-features docs build) using:
```
set -xeuo pipefail
features=(aws_lc_rs ring ktls,aws_lc_rs ktls,ring aws_lc_rs,ring aws_lc_rs,ring,ktls native-tls rustls-any-backend)
export RUSTFLAGS=" -D warnings"
cargo test --no-default-features
cargo test
for feature in ${features[@]}; do
echo ">>> BUILDING with $feature" 2>&1
# Running code or building examples cannot succeed with rustls-any-backend.
features="starttls,$feature"
if [ "$feature" != 'rustls-any-backend' ]; then
if ! cargo test --no-default-features --features="$features"; then
echo ">>> BUILD FAILED for tls feature set: $features" >&2
exit 1
fi
set +e
timeout -sINT -p -k 2 3 cargo run --no-default-features --features="$features" --example keep_connection -- test@hub.sotecware.net "$(pass xmpp/test@hub.sotecware.net)"
status="$?"
set -e
if [ $status -ne 0 ]; then
echo ">>> keep_connection did not shut down cleanly! (status: $status)" >&2
exit 1
fi
else
if ! cargo build --no-default-features --features="$feature"; then
echo ">>> BUILD FAILED for tls feature set: $features" >&2
exit 1
fi
fi
done
RUSTDOCFLAGS="--cfg docsrs" RUSTFLAGS="--cfg xmpprs_doc_build" cargo +nightly doc -Zrustdoc-map --all-features
```
For this, we had to:
- Make the choice(s) of backend explicit in tokio-xmpp features
- Avoid picking a backend through transitive dependencies in xmpp
(reqwest->hyper->rustls).
In addition, we choose the default rustls backend by default and adapt
the examples so that they can cope with either situation.
This way, we don't end up with breakage if rustls is built with more
than one provider enabled. This can (and does) happen if a transitive
dependency (such as reqwest *hinthint*) depends on a specific,
non-default rustls backend.
This is still far from being a full fix. Still, we can avoid the lang
attribute (by allowing discarding the `xml:lang` value via the feature
introduced a couple commits ago) and we can avoid emitting `xml:lang=""`
where no language was previously set using the mechanism introduced for
<message/> in 5172fb5e.
