This will allow for e.g. xous on riscv32-imac but not a precursor
target, for example: by using feature selectors we get to have
a bigger range of targets that we support, such as for implementations
that haven't been upstreamed or have official target-triples.
The downsides are:
- patch override libraries don't absorb feature flags. so, for
example, the AES crate that is used to patch other crates won't
be aware of the target features without modifying the referencing
crates.
- features are normally meant to be additive, but target features
are mutually exclusive. The mutex is enforced by a check in
utralib that throws a panic on build if two targets are accidentally
selected at the same time.
This is in a bid to gain CBC crate compatibility which requires
at least an AES 0.8 API level.
It breaks a lot of stuff, which is fixed in the next commit.
Each Cargo.toml now includes a comment which remnids maintainers
of the dependency policy for Xous (that is, nail down every dependency
to the minor number).
Versions are pinned down to the minor number. Yes, this means that
we don't automatically pull in features and bug updates from crates.io,
but it also means we don't pull in new attack surfaces and code that
hasn't been tested or reviewed. I'd like this process to be very deliberate
and not automatic for Xous.
The `rand` crate is almost pinned down to a single version, except
for an odd dependency that's pulled in by hosted mode that depends
on rand 0.8.4 via a subordinate dependency via the `tempfile` crate.
Pondering if it's worth it to try and eliminate that, because the
0.8.4 version doesn't get compiled into any version that touches
real hardware. It just makes it harder to analyze the Cargo.lock
file, mostly. I think I'll leave it as-is.
- inverted text indicates a trusted message. policy is enforced in the GAM.
- we don't have a way to lock the USB port yet; but at least now it's communicated.
