[PATCH V14 1/7] rust: Fix "multiple candidates for rmeta dependency core" error

Mukesh Kumar Chaurasiya (IBM)
2026-04-26T08:57:54Z

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) <mkchauras@gmail.com>
---
rust/Makefile | 39 +++++++++++++++++++++------------------
1 file changed, 21 insertions(+), 18 deletions(-)

diff --git a/rust/Makefile b/rust/Makefile
index b361bfedfdf0..f6369d121c50 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

@@ -31,7 +34,7 @@ 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
@@ -146,7 +149,7 @@ quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
cmd_rustdoc = \
OBJTREE=$(abspath $(objtree)) \
$(RUSTDOC) $(filter-out $(skip_flags) --remap-path-scope=%,$(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 $(subst rustdoc-,,$@) \
@@ -231,7 +234,7 @@ rustdoc-pin_init_internal: $(src)/pin-init/internal/src/lib.rs \

rustdoc-pin_init: private rustdoc_host = yes
rustdoc-pin_init: private rustc_target_flags = $(pin_init-flags) \
- --extern alloc --cfg feature=\"alloc\"
+ -L$(objtree)/$(obj) --extern alloc --cfg feature=\"alloc\"
rustdoc-pin_init: $(src)/pin-init/src/lib.rs rustdoc-pin_init_internal \
rustdoc-macros FORCE
+$(call if_changed,rustdoc)
@@ -520,23 +523,23 @@ quiet_cmd_rustc_procmacrolibrary = $(RUSTC_OR_CLIPPY_QUIET) PL $@
$(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) \
+ --out-dir $(objtree)/$(obj)/host -L$(objtree)/$(obj)/host \
--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 = $(quote-flags)
+$(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 = $(syn-flags)
+$(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 $@
@@ -545,20 +548,20 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
-Clinker-flavor=gcc -Clinker=$(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 $<

# 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
--
2.53.0