Disable Rust doctests on Darwin.

[danieldk]

Motivation for this change

Since Rust 1.30 (?) some Rust crates do not build, see e.g. #49693 and #49708
and fail with:

dyld: Symbol not found: __ZTIN4llvm2cl18GenericOptionValueE

So far, the common pattern seems to be that the linking fails in doctests. This
change is a temporary workaround that disables doctests for Rust crates on
Darwin. This avoids the need to patch individual derivations and still runs
library tests.

Tested: building alacritty and nix-index on Darwin with tests enabled.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

---

[danieldk]

@GrahamcOfBorg build alacritty nix-index

[GrahamcOfBorg]

Success on x86_64-linux (full log)

Attempted: alacritty, nix-index

Partial log (click to expand)

copying path '/nix/store/6amr35ylmd648qcif3migqcq1yxcl8v8-libglvnd-1.0.0' from 'https://nix-cache.s3.amazonaws.com'...
copying path '/nix/store/9xhc56gqhrsy1p0xhwzw6j79ddh31ikc-libpng-apng-1.6.34' from 'https://nix-cache.s3.amazonaws.com'...
copying path '/nix/store/z0bh4cgaj6z1g2gkw7i54g3hspbb64rf-libGL-1.0.0' from 'https://nix-cache.s3.amazonaws.com'...
copying path '/nix/store/g9p94w6fx4hckhalmsk3rk9anapckfwp-freetype-2.9' from 'https://nix-cache.s3.amazonaws.com'...
copying path '/nix/store/rzsazacxnj78fiwm15v804ykpkpm4dyp-nix-index-0.1.2' from 'https://nix-cache.s3.amazonaws.com'...
copying path '/nix/store/c394zcr40gx8x97iwyc4xng6glbk02by-fontconfig-2.12.6-lib' from 'https://nix-cache.s3.amazonaws.com'...
copying path '/nix/store/kvadmbyjd8lg86rz29b1w8n7vzhd9akn-xclip-0.13' from 'https://nix-cache.s3.amazonaws.com'...
copying path '/nix/store/q812836bwhcyd0535pfn1zxha80b5qbq-alacritty-unstable-0.2.1' from 'https://nix-cache.s3.amazonaws.com'...
/nix/store/q812836bwhcyd0535pfn1zxha80b5qbq-alacritty-unstable-0.2.1
/nix/store/rzsazacxnj78fiwm15v804ykpkpm4dyp-nix-index-0.1.2

[GrahamcOfBorg]

Success on aarch64-linux (full log)

Attempted: nix-index

The following builds were skipped because they don't evaluate on aarch64-linux: alacritty

Partial log (click to expand)

post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2
shrinking /nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2/bin/nix-locate
shrinking /nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2/bin/nix-index
strip is /nix/store/p9akxn2sfy4wkhqdqa3li97pc6jaz3r1-binutils-2.30/bin/strip
stripping (with command strip and flags -S) in /nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2/bin
patching script interpreter paths in /nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2
/nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2/etc/profile.d/command-not-found.sh: interpreter directive changed from "/bin/sh" to "/nix/store/dsyc1z7ck08ga7l0b1jcxx35wj69qcii-bash-4.4-p23/bin/sh"
checking for references to /build in /nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2...
/nix/store/299bcy5rp0qg8q5iihbbfj7rlahc739l-nix-index-0.1.2

[danieldk]

This fails when there are no library targets in a Rust project.