How are rustc -Z flags enabled during build?

[Daniel Xu <dxu@dxuuu.xyz>]

Hi,

Context: While hacking on some rust modules, I ran into a bug where
kbuild was trying to generate BTF (from DWARF) for rust kernel modules.
I'm guessing one cannot generate valid BTF from rust debuginfo b/c at
module load time the btf parser complains and errors out the module
load. I know how to fix this and am planning on sending a fix.

In trying to fix the BTF issue, I'm trying to get more familiar w/
kbuild. Specifically, I've run `make V=1 LLVM=1 -j32` to try and figure
out exactly where the .o's are being created. I see the following
command:

    RUST_MODFILE=samples/rust/rust_minimal rustc
    --emit=dep-info,obj,metadata --edition=2018 -Cpanic=abort
    -Cembed-bitcode=n -Clto=n -Crpath=n -Cforce-unwind-tables=n
    -Ccodegen-units=1 -Zbinary_dep_depinfo=y
    -Zsymbol-mangling-version=v0 -Dunsafe_op_in_unsafe_fn
    -Drust_2018_idioms -Wmissing_docs -Cdebug-assertions=y
    -Coverflow-checks=y -Copt-level=2 -Cdebuginfo=2 --cfg MODULE
    @./include/generated/rustc_cfg
    --target=/home/dxu/dev/linux/arch/x86/rust/target.json
    -Zallow-features=allocator_api,bench_black_box,concat_idents,global_asm,try_reserve
    --extern alloc --extern kernel --crate-type rlib --out-dir /tmp/rust
    -L ./rust/ --crate-name rust_minimal samples/rust/rust_minimal.rs

When I try to run it outside of kbuild, I get:

    error: the option `Z` is only accepted on the nightly compiler

How does kbuild use the -Z features on a stable rustc?


Thanks,
Daniel