Tweak Rust package guidelines

[alerque]

I don't have any RISC boards, but poking around in here one thing that seems to need patching a lot is something I put in the Rust packaging guidelines.

https://github.com/felixonmars/archriscv-packages/blob/82d8d619e862478a076ae657069c9caf2f1caed4/onefetch/riscv64.patch

This platform limited fetch can mean a huge savings in downloads when building packages (in some cases from hundreds of MB down to less than 10), so I am loath to drop it altogether.

Would it be helpful to modify the Rust recommendations like so:

- cargo fetch --target "$CARCH-unknown-linux-gnu"
+ cargo fetch --target "$CHOST"

...or is the issue here that you are cross compiling and need to fetch far an architecture other than the host? If so is there a variable we could suggest people use to get the benefit of only downloading required sources while not making so much work patching downstream?

[r-value]

Unfortunately, neither $CARCH nor $CHOST provides correct triple for Rust. Rust needs something like riscv64gc but these variables are riscv64. So we'd like to just trim them off.

Ref: https://github.com/felixonmars/archriscv-packages/blob/master/devtools-riscv64/makepkg-riscv64.conf#L37

[alerque]

Bummer. Also a bummer this never got flushed out. Also a bummer my patch fixing the cargo fetch --target documentation that fixes the current contradiction in the man page was dropped. I need to resubmit that at least...

So why is Rust coming up with a different architecture name? Is this something it's impossible to normalize?

[XieJiSS]

Hi @alerque ,

https://github.com/laanwj/cc-rs/blob/4981b3cdf070c5f1f8ead508d5b56ea60ec657c2/src/lib.rs#L1431-L1441

I think that's partially because the compiler need to construct the -march argument to be passed to the underlying clang/llvm stuff from the --target argument. The -march uses the format similar to rv64gc / rv64imac.

[r-value]

Is this something it's impossible to normalize?

I think that there can be two possible solutions. One is submitting patch to Rust and change the naming scheme to something like arch=riscv64 and put extensions like G/C/V/K in target-feature just like what they did with x86 extensions. The other is to make a new environment variable like $RSARCH which contains riscv64gc on RISC-V and x86_64 on x86.

[alerque]

Bah. So basically right now the only way to solve both my concern and eliminate the need for downstream patching would be something like this:

cargo fetch --locked --target "$(rustc -vV | awk '/^host:/ { print $2 }')"

Obviously that's a bit convoluted so I'm not recommending it yet, just trying to get my head around the problem here.

It seems to me the best course of action would be to actually fix cargo to work as documented:

--target triple
Fetch for the given architecture. The default is the host architecture.

As far as I can see there is no way to actually use this so called "default" in fetch, it only defaults to "all architectures" (which for some dependency chains is becoming pretty heavy!).

[piggynl]

Hi @alerque, I recreated your PR for cargo's doc change as rust-lang/cargo#10398 👀

[Xeonacid]

Closing since there has been a working Rust package guidelines for a while. :)