How are rustc -Z flags enabled during build?

How are rustc -Z flags enabled during build?

[Daniel Xu]

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

Re: How are rustc -Z flags enabled during build?

[Gary Guo]

On Wed, 1 Sep 2021 16:07:15 -0700
Daniel Xu <dxu@dxuuu.xyz> wrote:

> 
> 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?
> 

You can set environment variable `RUSTC_BOOTSTRAP=1`.

- Gary Guo

Re: How are rustc -Z flags enabled during build?

[Miguel Ojeda]

Hi Daniel,

On Thu, Sep 2, 2021 at 1:11 AM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> 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?

As Gary points out, it is an environment variable being set in the
top-level `Makefile` nearby the other flags. I considered adding it to
the command-line instead so that copy-pasting the lines is easier,
even if it means being a bit more verbose.

In any case, note that it is an escape hatch intended to be internal
to the compiler (which explains the error message not mentioning it)
and thus should be avoided unless a project has very special
constraints.

Our goal is to drop its usage as soon as it is feasible, but it will
take a while. It is also why we only maintain support for a given
`rustc` version for the moment.

Cheers,
Miguel

Re: How are rustc -Z flags enabled during build?

[Daniel Xu]

Hi Gary, Miguel,

On Thu, Sep 02, 2021 at 07:50:44AM +0200, Miguel Ojeda wrote:
> Hi Daniel,
> 
> On Thu, Sep 2, 2021 at 1:11 AM Daniel Xu <dxu@dxuuu.xyz> wrote:
> >
> > 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?
> 
> As Gary points out, it is an environment variable being set in the
> top-level `Makefile` nearby the other flags. I considered adding it to
> the command-line instead so that copy-pasting the lines is easier,
> even if it means being a bit more verbose.
> 
> In any case, note that it is an escape hatch intended to be internal
> to the compiler (which explains the error message not mentioning it)
> and thus should be avoided unless a project has very special
> constraints.
> 
> Our goal is to drop its usage as soon as it is feasible, but it will
> take a while. It is also why we only maintain support for a given
> `rustc` version for the moment.
> 
> Cheers,
> Miguel

Thanks! I knew there must've been some magic somewhere :) .

Daniel