Meson: host and target toolchain files should define `rust`

[maleadt]

Otherwise compilation of proc macros, which relies on a functional host Rust compiler, fails. For example, compiling for aarch64-linux-gpu:

Called: `rustc -C linker=/opt/bin/x86_64-linux-musl-cxx11/x86_64-linux-musl-gcc -o /workspace/srcdir/mesa/build/meson-private/rusttest.exe /workspace/srcdir/mesa/build/meson-private/sanity.rs` -> 1
stderr:
error: linking with `/opt/bin/x86_64-linux-musl-cxx11/x86_64-linux-musl-gcc` failed: exit status: 1
  |
  = note:  "/opt/bin/x86_64-linux-musl-cxx11/x86_64-linux-musl-gcc" "/tmp/rustcGMmpnD/symbols.o" "<2 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "<sysroot>/lib/rustlib/aarch64-unknown-linux-gnu/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,libcfg_if-*,liblibc-*,liballoc-*,librustc_std_workspace_core-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/tmp/rustcGMmpnD/raw-dylibs" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/aarch64-unknown-linux-gnu/lib" "-o" "/workspace/srcdir/mesa/build/meson-private/rusttest.exe" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: Relocations in generic ELF (EM: 183)
          /opt/x86_64-linux-musl/bin/../lib/gcc/x86_64-linux-musl/8.1.0/../../../../x86_64-linux-musl/bin/ld: /workspace/srcdir/mesa/build/meson-private/rusttest.sanity.425e246c73776925-cgu.0.rcgu.o: error adding symbols: file in wrong format
          collect2: error: ld returned 1 exit status
          

error: aborting due to 1 previous error
-----------
Compiler for language rust for the build machine not found.

The relocation mismatch happens because the target rustc compiler is used instead of the host one.

Normally it'd be sufficient to pass --native-file="${MESON_HOST_TOOLCHAIN}" next to --cross-file="${MESON_TARGET_TOOLCHAIN}", however, the toolchain files don't define rust in the [binaries] section. The reason we don't run into this for the target toolchain file, is that the target rustc binary is on PATH. When that binary is used to compile proc macros, the above mismatch occurs.

Workaround:

host=$MACHTYPE
host_rustc=$(echo /opt/bin/${host}*/rustc)
target_rustc=$(echo /opt/bin/${target}*/rustc)
sed -i "/^\[binaries\]/a rust = '$host_rustc'" "${MESON_HOST_TOOLCHAIN}"
sed -i "/^\[binaries\]/a rust = '$target_rustc'" "${MESON_TARGET_TOOLCHAIN}"

[giordano]

The Meson file is generated in https://github.com/JuliaPackaging/BinaryBuilderBase.jl/blob/99594c5cae8f67b9cdc776418744d88e574d4e05/src/BuildToolchains.jl.