mbedtls-rs-sys should expose `include` metadata for downstream crates

[scootermon]

It's a somewhat established convention for -sys crates to expose a cargo::metadata=include=path/to/dir metadata. Say you're creating a sys crate for a C library that requires mbedtls.
Instead of building or sourcing a separate copy of mbedtls, it's much nicer to add the mbedtls-rs-sys crate as a dependency, which then gives you access to the exposed include dir metadata in the build script.

See for example the openssl-sys crate: https://github.com/rust-openssl/rust-openssl/blob/87bf0e2e364976453fd6ae335f9c5ff34a314244/openssl-sys/build/main.rs#L214

This came up while I was hacking around on the openthread crate. Since openthread requires mbedtls I wanted it to use this crate. See:
https://github.com/inomotech-foss/openthread/blob/556e76558d68f676d65ae8d98a1f902e75f8dcc3/openthread-sys/gen/builder.rs#L434-L435

Small tangent: The dep_metadata function is from build-rs, which provides a clean interface around the build script protocol and is maintained by the Cargo team. Of course there's always the trade-off of compile times, but IMO that's entirely offset by how much more readable the build scripts are.

[ivmarkov]

This came up while I was hacking around on the openthread crate. Since openthread requires mbedtls I wanted it to use this crate. See: https://github.com/inomotech-foss/openthread/blob/556e76558d68f676d65ae8d98a1f902e75f8dcc3/openthread-sys/gen/builder.rs#L434-L435

It is interesting that you are mentioning this, because one of my goals is precisely to use mbedtls-rs-sys WITH openthread (which currently has its own embedded mbedtls). Or at least to have that possibility.

Putting aside the fact that this would provide HW accel for OpenThread crypto, that would be the most natural setup for rs-matter+Thread, because in the meantime rs-matter can use mbedtls-rs-sys as its Crypto provider (in addition to / instead of RustCrypto or OpenSSL).

So the TL;DR is:

• Patches to mbedtls-rs-sys for that are welcome!
• Patches to openthread for using mbedtls-rs-sys also very welcome!

[ivmarkov]

@scootermon Can you also BTW contribute this: inomotech-foss/openthread@556e765

(I.e. the working sysroot for Clang?)

How did you manage to cheat it with that sysroot? I had great difficulties with the STD C++ lib (the templates stuff).

[ivmarkov]

Ah, just noticed you left a relevant comment on the sysroot thing in my sysroot issue. Let's continue there.