[PATCH V9 1/4] rust: Fix "multiple candidates for rmeta dependency core" error
From: Mukesh Kumar Chaurasiya (IBM)
Date: Sat Apr 04 2026 - 08:16:41 EST
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
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$(objtree)/rust`, which
would reintroduce the conflict.
The rust/host/ directory is added to clean-files to ensure it's removed
during `make clean`.
As there is a lot going on, here's a summary of changes:
- Add clean-files := host/ to clean the generated directory
- Change host library targets from lib*.rlib to host/lib*.rlib
- Update cmd_rustc_procmacrolibrary to create host/ directory
- Update cmd_rustc_procmacro to use -L$(objtree)/rust/host
- Update cmd_rustdoc to use -L$(objtree)/rust/host for host builds
- Update rustdoc-pin_init to use explicit --extern paths for proc macros
as pin-init also needs alloc crate from -L$(objtree)/rust. So the proc
macros needs an absolute path
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 | 57 +++++++++++++++++++++++++++++++--------------------
1 file changed, 35 insertions(+), 22 deletions(-)
diff --git a/rust/Makefile b/rust/Makefile
index 9801af2e1e02..5f726a332d9e 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 +196,15 @@ 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
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 --extern quote
rustdoc-syn: $(src)/syn/lib.rs rustdoc-clean rustdoc-quote FORCE
+$(call if_changed,rustdoc)
@@ -236,7 +241,10 @@ 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
@@ -525,48 +533,53 @@ $(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 = $(quote-flags) \
+ -L$(objtree)/$(obj)/host --extern proc_macro2
+$(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 = $(syn-flags) \
+ -L$(objtree)/$(obj)/host --extern proc_macro2 --extern quote
+$(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
--
2.53.0