Subject: Re: [PATCH V11 1/4] rust: Fix "multiple candidates for rmeta dependency core" error

From: Link Mauve <[email-masked]>
Date: Tue, 21 Apr 2026 11:38:08 +0200

Hi Mukesh,

This patch doesn’t apply on top of mainline, which tree did you base it
off?

On Fri, Apr 17, 2026 at 08:52:50PM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:
> When building Rust code for powerpc64le with LLVM=1 and -j1, rustc
> encounters an error: "multiple candidates for `rmeta` dependency `core`
> found", with two candidates:
> 1. The host's standard library from the rustup toolchain
> 2. The kernel's custom libcore.rmeta in the rust/ directory
> 
> This occurs because the build system uses `-L$(objtree)/rust` for host
> library builds (proc_macro2, quote, syn), which causes rustc to search
> the rust/ directory. During this search, rustc finds both the kernel's
> custom libcore.rmeta and gains access to the host's standard library,
> creating a conflict.
> 
> The solution is to separate host libraries into a dedicated rust/host/
> subdirectory and use `-L$(objtree)/rust/host` for host builds instead
> of `-L$(objtree)/rust`. This ensures that:
> 
> 1. Host library builds (proc_macro2, quote, syn) only search rust/host/
>    and never encounter the kernel's libcore.rmeta
> 2. Proc macro builds use `-L$(objtree)/rust/host` to find their
>    dependencies
> 
> Special handling is added for rustdoc-pin_init, which is a host build
> (to access the alloc crate) but depends on proc macros from the main
> rust/ directory. It uses explicit `--extern` paths to reference the
> proc macros without adding `-L$(objtree)/rust`, which would reintroduce
> the conflict.
> 
> The rust/host/ directory is added to clean-files to ensure it's removed
> during `make clean`.
> 
> Link: https://github.com/Rust-for-Linux/linux/issues/105
> Link: https://github.com/linuxppc/issues/issues/451
> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <[email-masked]>
> ---
>  rust/Makefile | 38 +++++++++++++++++++++-----------------
>  1 file changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/rust/Makefile b/rust/Makefile
> index 9801af2e1e02..e234b8a39358 100644
> --- a/rust/Makefile
> +++ b/rust/Makefile
> @@ -3,6 +3,9 @@
>  # Where to place rustdoc generated documentation
>  rustdoc_output := $(objtree)/Documentation/output/rust/rustdoc
>  
> +# Clean generated host directory
> +clean-files := host/
> +
>  obj-$(CONFIG_RUST) += core.o compiler_builtins.o ffi.o
>  always-$(CONFIG_RUST) += exports_core_generated.h
>  
> @@ -27,7 +30,7 @@ endif
>  
>  obj-$(CONFIG_RUST) += exports.o
>  
> -always-$(CONFIG_RUST) += libproc_macro2.rlib libquote.rlib libsyn.rlib
> +always-$(CONFIG_RUST) += host/libproc_macro2.rlib host/libquote.rlib host/libsyn.rlib
>  
>  always-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated.rs
>  always-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated_kunit.c
> @@ -150,7 +153,7 @@ quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
>  	OBJTREE=$(abspath $(objtree)) \
>  	$(RUSTDOC) $(filter-out $(skip_flags) --remap-path-prefix=% --remap-path-scope=%, \
>  			$(if $(rustdoc_host),$(rust_common_flags),$(rust_flags))) \

The issue is here ↑, --remap-path-prefix=% got removed and the two
previous lines got merged into one.

> -		$(rustc_target_flags) -L$(objtree)/$(obj) \
> +		$(rustc_target_flags) -L$(objtree)/$(obj)$(if $(rustdoc_host),/host) \
>  		-Zunstable-options --generate-link-to-definition \
>  		--output $(rustdoc_output) \
>  		--crate-name $(subst rustdoc-,,$@) \
[…]

Thanks anyway for iterating on this series!  I’ve started writing a DRM
driver based off a previous version. :)

-- 
Link Mauve