crossbeam-utils: check only the architecture, not the whole target string

[kanavin]

There can be custom targets in use, and it's not possible to make a list
of them; for the check only the first item in the target string is actually
relevant (the architecture of the target, which is generally consistent
between standard and custom targets).

Signed-off-by: Alexander Kanavin alex@linutronix.de

[taiki-e]

In some cases, the kinds of atomic operations supported depend on the OS. For example:

• x86_64-*: cmpxchg16b is available for all x86_64 macOS devices, so Atomic128 is available for x86_64 macOS but not for x86_64 Linux.
• riscv32imc-*: riscv32imc-esp-espidf has atomic, but riscv32imc-unknown-none-elf does not.

Currently, the recommended way to build for custom targets is to use rustflags to pass the cfgs.
I would accept a patch to improve the situation, but we need to handle the edge cases mentioned above (in a way that does not manually maintain a list of such targets, whenever possible).

[kanavin]

Currently, the recommended way to build for custom targets is to use rustflags to pass the cfgs. I would accept a patch to improve the situation, but we need to handle the edge cases mentioned above (in a way that does not manually maintain a list of such targets, whenever possible).

Thanks. The context is that this patch comes from the efforts to cross-compile librsvg in the Yocto project, where crossbeam is included into the librsvg tarball as a vendored library. For various reasons Yocto cannot use the standard rust targets (e.g. the compiler binary is hardcoded in them to 'cc' which is something we cannot use).

I guess I need to find out how to convince librsvg's build system to pass those custom flags to crossbeam, instead of running build.rs - if you can take the librsvg tarball and check what it does, I'd appreciate:
https://download.gnome.org/sources/librsvg/2.52/

Just for reference, you can see our configuration ('recipe' in yocto parlance) for librsvg here:
http://git.yoctoproject.org/cgit/cgit.cgi/poky-contrib/tree/meta/recipes-gnome/librsvg/librsvg_2.52.3.bb?h=akanavin/package-version-updates

[kanavin]

So yes, it's actually easier to just pass the needed flags via RUSTFLAGS on architectures where it's needed:

RUSTFLAGS:append:mips = " --cfg crossbeam_no_atomic_64"
RUSTFLAGS:append:powerpc = " --cfg crossbeam_no_atomic_64"

Thanks!