Allow to build without specifying a target triple

[adrienballsonos]

Description of the problem

I did not find a way to use setuptools_rust without having it passing a target triple flag (--target) to the rust compiler. The problem is that, as explained in the rust doc, using the --target flag changes the build directory layout and build outputs are no longer under target/debug or target/release but instead they are stored in target/<triple>/debug/ or target/<triple>/release/.

In my case this is problematic because I want to share the cargo build cache and avoid re-compiling the rust library when I build the python wheel, after having built the rust library using cargo build. But to do so, I need the target dir to be target/debug/ or target/release/.

[adamreichold]

Just an idea that might be helpful immediately: Maybe you can pass --target (matching that of the host) to your previous cargo build invocation as well so that the native build and the build for the wheel both end up using $CARGO_TARGET_DIR/$TARGET/{debug,release}.

[davidhewitt]

I very much agree with this. Perhaps we can invoke rustc to detect the default target and skip adding the --target flag if it already matches?

I'm quite busy atm so won't get around to implementing this soon, however a PR would be very welcome 👍