My interest is in this is for Yocto. For Yocto the --build and --target are different but they're the same for Rust (I'm using x86_64 Ubuntu and building an x86_64 Yocto image). So what that means is that Yocto uses the distro compiler to produce a build sysroot using the triple x86_64-linux and the target sysroot uses the triple x86_64-poky-linux. A binary built for the build sysroot needs different libraries and a different rpath then what a binary built for the target sysroot. Now the Rust built-in target that would match both of these would be x86_64-unknown-linux-gnu and that's where the problem lies. Since these need different sysroots and rpath's then I need to be able to pass flags to the linker (or what I really use is a custom linker that's a wrapper to provide the right args). Now gcc-rs has the ability to pass HOST_CC and TARGET_CC along with other HOST_ and TARGET_ options but Cargo does not. Now if I built from x86_64 for an arm target and have the following config:

[target.x86_64-unknown-linux-gnu]
linker = "custom-build-linker"
[target.aarch64-unknown-linux-gnu]
linker = "custom-target-linker"

Cargo will build build-script-build with the custom-build-linker but when I'm building from x86_64 for x86_64 then there's no way for me to supply two different linkers. I focused my examples on linkers here but rustflags work the same way as well.

I propose adding support to Cargo to read a [host] section that would behave like a [target] section but instead would be used for any items that Cargo needs to build to run on the build machine (e.g. compiler plugins?). It would look something like:

[host]
linker = "custom-build-linker"
[target.aarch64-unknown-linux-gnu]
linker = "custom-target-linker"