[PATCH V11 1/4] rust: Fix "multiple candidates for rmeta dependency core" error
From: Mukesh Kumar Chaurasiya (IBM)
Date: Fri Apr 17 2026 - 11:28:38 EST
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@xxxxxxxxx>
---
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))) \
- $(rustc_target_flags) -L$(objtree)/$(obj) \
+ $(rustc_target_flags) -L$(objtree)/$(obj)$(if $(rustdoc_host),/host) \
-Zunstable-options --generate-link-to-definition \
--output $(rustdoc_output) \
--crate-name $(subst rustdoc-,,$@) \
@@ -237,6 +240,7 @@ 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 = $(pin_init-flags) \
+ --extern pin_init_internal=$(objtree)/$(obj)/$(libpin_init_internal_name) \
--extern alloc --cfg feature=\"alloc\"
rustdoc-pin_init: $(src)/pin-init/src/lib.rs rustdoc-pin_init_internal \
rustdoc-macros FORCE
@@ -528,23 +532,23 @@ 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 $@
@@ -553,20 +557,20 @@ 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