Re: [PATCH V8 1/4] rust: Fix "multiple candidates for rmeta dependency core" error

[Gary Guo]

On Fri Apr 3, 2026 at 3:53 PM BST, Mukesh Kumar Chaurasiya (IBM) wrote:
> When building Rust code with LLVM=1 with -j1, rustc was encountering
> 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 occurred because the build system was using `-L$(objtree)/rust`
> for host library builds (proc_macro2, quote, syn), which caused rustc
> to search the rust/ directory. During this search, rustc would find
> both the kernel's custom libcore.rmeta and gain access to the host's
> standard library, creating a conflict.
>
> The solution is to separate host libraries into a dedicated rust/host/
> subdirectory and use `-L$(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
> 3. Test builds use `-L$(objtree)/rust/test` for their dependencies
> 4. Target builds continue to use `-L$(objtree)/rust` as before
>
> 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 with absolute paths
> to reference the proc macros without adding `-L$(objtree)/rust`, which
> would reintroduce the conflict.
>
> The rust/host/ directory is added to clean-files to ensure it's removed
> during `make clean`.
>
> Changes:
> - Add clean-files := host/ to clean the generated directory
> - Change host library targets from lib*.rlib to host/lib*.rlib
> - Update cmd_rustc_procmacrolibrary to create host/ directory
> - Update cmd_rustc_procmacro to use -L$(objtree)/rust/host
> - Update cmd_rustdoc to use -L$(objtree)/rust/host for host builds
> - Update cmd_rustc_test_library to use -L$(objtree)/rust/test
> - Update rustdoc-pin_init to use explicit --extern paths for proc macros

This should go after the tags and ---, so it's not part of the commit message.

>
> 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) <mkchauras@xxxxxxxxx>
> ---
>  rust/Makefile | 63 ++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 40 insertions(+), 23 deletions(-)
>
> diff --git a/rust/Makefile b/rust/Makefile
> index 9801af2e1e02..762bddc868e4 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))) \
> -		$(rustc_target_flags) -L$(objtree)/$(obj) \
> +		$(rustc_target_flags) -L$(objtree)/$(obj)$(if $(rustdoc_host),/host) \
>  		-Zunstable-options --generate-link-to-definition \
>  		--output $(rustdoc_output) \
>  		--crate-name $(subst rustdoc-,,$@) \
> @@ -193,13 +196,16 @@ rustdoc-proc_macro2: $(src)/proc-macro2/lib.rs rustdoc-clean FORCE
>  	+$(call if_changed,rustdoc)
>  
>  rustdoc-quote: private rustdoc_host = yes
> -rustdoc-quote: private rustc_target_flags = $(quote-flags)
> +rustdoc-quote: private rustc_target_flags = $(quote-flags) \
> +    --extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib)

Why not just add `-L$(objtree)/$(obj)/host`? If we go with the host directory
approach we shouldn't need to explicitly specify the path?

Best,
Gary

>  rustdoc-quote: private skip_flags = $(quote-skip_flags)
>  rustdoc-quote: $(src)/quote/lib.rs rustdoc-clean rustdoc-proc_macro2 FORCE
>  	+$(call if_changed,rustdoc)
>  
>  rustdoc-syn: private rustdoc_host = yes
> -rustdoc-syn: private rustc_target_flags = $(syn-flags)
> +rustdoc-syn: private rustc_target_flags = $(syn-flags) \
> +    --extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib) \
> +    --extern quote=$(abspath $(objtree)/$(obj)/host/libquote.rlib)
>  rustdoc-syn: $(src)/syn/lib.rs rustdoc-clean rustdoc-quote FORCE
>  	+$(call if_changed,rustdoc)
>  
> @@ -236,7 +242,10 @@ rustdoc-pin_init_internal: $(src)/pin-init/internal/src/lib.rs \
>  	+$(call if_changed,rustdoc)
>  
>  rustdoc-pin_init: private rustdoc_host = yes
> -rustdoc-pin_init: private rustc_target_flags = $(pin_init-flags) \
> +rustdoc-pin_init: private rustc_target_flags = \
> +    --extern pin_init_internal=$(abspath $(objtree)/$(obj)/$(libpin_init_internal_name)) \
> +    --extern macros=$(abspath $(objtree)/$(obj)/$(libmacros_name)) \
> +    $(call cfgs-to-flags,$(pin_init-cfgs)) \
>      --extern alloc --cfg feature=\"alloc\"
>  rustdoc-pin_init: $(src)/pin-init/src/lib.rs rustdoc-pin_init_internal \
>      rustdoc-macros FORCE
> @@ -369,7 +378,9 @@ rusttest: rusttest-macros
>  
>  rusttest-macros: private rustc_target_flags = --extern proc_macro \
>      --extern macros --extern kernel --extern pin_init \
> -    --extern proc_macro2 --extern quote --extern syn
> +    --extern proc_macro2=$(abspath $(objtree)/$(obj)/test/libproc_macro2.rlib) \
> +    --extern quote=$(abspath $(objtree)/$(obj)/test/libquote.rlib) \
> +    --extern syn=$(abspath $(objtree)/$(obj)/test/libsyn.rlib)
>  rusttest-macros: private rustdoc_test_target_flags = --crate-type proc-macro
>  rusttest-macros: $(src)/macros/lib.rs \
>      rusttestlib-macros rusttestlib-kernel rusttestlib-pin_init FORCE
> @@ -525,48 +536,54 @@ $(obj)/exports_kernel_generated.h: $(obj)/kernel.o FORCE
>  
>  quiet_cmd_rustc_procmacrolibrary = $(RUSTC_OR_CLIPPY_QUIET) PL $@
>        cmd_rustc_procmacrolibrary = \
> +	mkdir -p $(dir $@); \
>  	$(if $(skip_clippy),$(RUSTC),$(RUSTC_OR_CLIPPY)) \
>  		$(filter-out $(skip_flags),$(rust_common_flags) $(rustc_target_flags)) \
>  		--emit=dep-info=$(depfile) --emit=link=$@ --crate-type rlib -O \
> -		--out-dir $(objtree)/$(obj) -L$(objtree)/$(obj) \
>  		--crate-name $(patsubst lib%.rlib,%,$(notdir $@)) $<
>  
> -$(obj)/libproc_macro2.rlib: private skip_clippy = 1
> -$(obj)/libproc_macro2.rlib: private rustc_target_flags = $(proc_macro2-flags)
> -$(obj)/libproc_macro2.rlib: $(src)/proc-macro2/lib.rs FORCE
> +$(obj)/host/libproc_macro2.rlib: private skip_clippy = 1
> +$(obj)/host/libproc_macro2.rlib: private rustc_target_flags = $(proc_macro2-flags)
> +$(obj)/host/libproc_macro2.rlib: $(src)/proc-macro2/lib.rs FORCE
>  	+$(call if_changed_dep,rustc_procmacrolibrary)
>  
> -$(obj)/libquote.rlib: private skip_clippy = 1
> -$(obj)/libquote.rlib: private skip_flags = $(quote-skip_flags)
> -$(obj)/libquote.rlib: private rustc_target_flags = $(quote-flags)
> -$(obj)/libquote.rlib: $(src)/quote/lib.rs $(obj)/libproc_macro2.rlib FORCE
> +$(obj)/host/libquote.rlib: private skip_clippy = 1
> +$(obj)/host/libquote.rlib: private skip_flags = $(quote-skip_flags)
> +$(obj)/host/libquote.rlib: private rustc_target_flags = $(quote-flags) \
> +	--extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib)
> +$(obj)/host/libquote.rlib: $(src)/quote/lib.rs $(obj)/host/libproc_macro2.rlib FORCE
>  	+$(call if_changed_dep,rustc_procmacrolibrary)
>  
> -$(obj)/libsyn.rlib: private skip_clippy = 1
> -$(obj)/libsyn.rlib: private rustc_target_flags = $(syn-flags)
> -$(obj)/libsyn.rlib: $(src)/syn/lib.rs $(obj)/libquote.rlib FORCE
> +$(obj)/host/libsyn.rlib: private skip_clippy = 1
> +$(obj)/host/libsyn.rlib: private rustc_target_flags = $(syn-flags) \
> +	--extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib) \
> +	--extern quote=$(abspath $(objtree)/$(obj)/host/libquote.rlib)
> +$(obj)/host/libsyn.rlib: $(src)/syn/lib.rs $(obj)/host/libquote.rlib FORCE
>  	+$(call if_changed_dep,rustc_procmacrolibrary)
>  
>  quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
>        cmd_rustc_procmacro = \
> -	$(RUSTC_OR_CLIPPY) $(rust_common_flags) $(rustc_target_flags) \
> +	$(RUSTC_OR_CLIPPY) $(rust_common_flags) \
>  		-Clinker-flavor=gcc -Clinker=$(HOSTCC) \
>  		-Clink-args='$(call escsq,$(KBUILD_PROCMACROLDFLAGS))' \
>  		--emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \
> -		--crate-type proc-macro -L$(objtree)/$(obj) \
> +		--crate-type proc-macro \
> +		-L$(objtree)/$(obj)/host \
>  		--crate-name $(patsubst lib%.$(libmacros_extension),%,$(notdir $@)) \
> -		@$(objtree)/include/generated/rustc_cfg $<
> +		@$(objtree)/include/generated/rustc_cfg \
> +		$(rustc_target_flags) \
> +		$<
>  
>  # Procedural macros can only be used with the `rustc` that compiled it.
>  $(obj)/$(libmacros_name): private rustc_target_flags = \
>      --extern proc_macro2 --extern quote --extern syn
> -$(obj)/$(libmacros_name): $(src)/macros/lib.rs $(obj)/libproc_macro2.rlib \
> -    $(obj)/libquote.rlib $(obj)/libsyn.rlib FORCE
> +$(obj)/$(libmacros_name): $(src)/macros/lib.rs $(obj)/host/libproc_macro2.rlib \
> +    $(obj)/host/libquote.rlib $(obj)/host/libsyn.rlib FORCE
>  	+$(call if_changed_dep,rustc_procmacro)
>  
>  $(obj)/$(libpin_init_internal_name): private rustc_target_flags = $(pin_init_internal-flags)
>  $(obj)/$(libpin_init_internal_name): $(src)/pin-init/internal/src/lib.rs \
> -    $(obj)/libproc_macro2.rlib $(obj)/libquote.rlib $(obj)/libsyn.rlib FORCE
> +    $(obj)/host/libproc_macro2.rlib $(obj)/host/libquote.rlib $(obj)/host/libsyn.rlib FORCE
>  	+$(call if_changed_dep,rustc_procmacro)
>  
>  # `rustc` requires `-Zunstable-options` to use custom target specifications