See the next section for details.

1. Add to dependencies:

   [dependencies]
   # ...
   anysc-rustls = { version = "0.1", optional = true }

2. Activate one of io_* feature flags:

   • io_tokio ( tokio::io interface, used by: tokio, nio, etc. )
   • io_futures ( futures-io interface, used by: async-std, smol, glommio, etc. )

3. Depending on the use case, enable some inheritied features:

   • aws-lc-rs
   • aws_lc_rs
   • early-data
   • fips
   • logging
   • ring
   • tls12

4. Write your code with anysc-rustls as with {tokio, futures}-rustls.

Just reexporting all the items of one of

• tokio-rustls
• futures-rustls

based on the io_* feature flag selected.

The point is that this is a crate: it enables, for some (maybe niche) crates that

• support multiple async runtimes over different async IO interfaces (tokio::io, futures::io)
• AND optionally provide rustls-powered TLS functionality behind a feature flag (like tls)

, to switch {async crate}-rustls dependencies without any needless dependencies.

That's impossible by other way: if simply having a submodule reexporting tokio-rustls and futures-rustls conditionally with respective feature flags (like tokio-io, futures-io), indeed it works, but the crate's [dependencies] will be like

[dependencies]
tokio-rustls = { optional = true, version = ... }
futures-rustls = { optional = true, version = ... }

[features]
tokio-io = ...
futures-io = ...
tls = ...

Here, how we setup the features?

1. tokio-io = if "tls" ["dep:tokio-rustls"] + futures-io = if "tls" ["dep:futures-tls"]

   impossible.

2. tls = if "tokio-io" ["dep:tokio-rustls"] else if "futures-io" ["dep:futures-tls"]

   impossible.

3. tls = ["dep:tokio-rustls", "dep:futures-rustls"]

   works, but one of them must be needless.

So it's impossible to avoid undesired dependencies in this way.

However, it's enabled by a crate-level shim layer as this crate:

[dependencies]
anysc-rustls = { version = "0.1", optional = true }

[features]
tls = ["dep:anysc-rustls"]
tokio-io = ["anysc-rustls?/io_tokio", ...]
futures-io = ["anysc-rustls?/io_futures", ...]

Yes, that's done by the <crate>?/<feature> syntax!