Cannot find xcrun when building with Nix flakes on Darwin/MacOS

[josephst]

On latest version of llama.cpp, building from flake (ie nix build github:ggerganov/llama.cpp) produces error with latest commit (d01b3c4 at time of writing).

error: builder for '/nix/store/9qrdgaz8s1wfz9fwci01j2vi97y45cfy-llama-cpp-metalkit-0.0.0.drv' failed with exit code 1;
       last 25 log lines:
       > -- Generating done (0.0s)
       > CMake Warning:
       >   Manually-specified variables were not used by the project:
       >
       >     CMAKE_EXPORT_NO_PACKAGE_REGISTRY
       >     CMAKE_POLICY_DEFAULT_CMP0025
       >
       >
       > -- Build files have been written to: /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build
       > cmake: enabled parallel building
       > cmake: enabled parallel installing
       > Running phase: buildPhase
       > build flags: -j8
       > [1/132] Compiling Metal kernels
       > FAILED: bin/default.metallib /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/default.metallib
       > cd /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build && xcrun -sdk macosx metal -O3 -c /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/ggml-metal.metal -o /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/ggml-metal.air && xcrun -sdk macosx metallib /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/ggml-metal.air -o /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/default.metallib && rm -f /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/ggml-metal.air && rm -f /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/ggml-common.h && rm -f /tmp/nix-build-llama-cpp-metalkit-0.0.0.drv-0/source/build/bin/ggml-metal.metal
       > /bin/sh: xcrun: command not found
       > [2/132] Building C object CMakeFiles/ggml.dir/ggml-alloc.c.o
       > [3/132] Building C object CMakeFiles/ggml.dir/ggml-backend.c.o
       > [4/132] Building C object CMakeFiles/ggml.dir/ggml-metal.m.o
       > [5/132] Building C object CMakeFiles/ggml.dir/ggml-quants.c.o
       > [6/132] Building C object CMakeFiles/ggml.dir/ggml.c.o
       > [7/132] Building CXX object CMakeFiles/llama.dir/unicode.cpp.o
       > [8/132] Building CXX object CMakeFiles/llama.dir/llama.cpp.o
       > ninja: build stopped: subcommand failed.
       For full logs, run 'nix log /nix/store/9qrdgaz8s1wfz9fwci01j2vi97y45cfy-llama-cpp-metalkit-0.0.0.drv'.

The key error is /bin/sh: xcrun: command not found -- Nix provides a clean $PATH to CMake which does not include /usr/bin and so Cmake is unable to locate or run /usr/bin/xcrun. This behavior is specific to the combination of MacOS and Nix.

This can be fixed by either manually adding /usr/bin to $PATH prior to configuration & build steps, or by using absolute path (/usr/bin/xcrun) and setting dontFixCmake = true; in package.nix.

System details:
Nix 2.21 (sandbox = off), MacOS 14.4 Sonoma

[josephst]

Suspect it's related to this change #6015