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 | 38 +++++++++++++++++++++----------------- 1 file changed, 21 insertions(+), 17 deletions(-)
diff --git a/rust/Makefile b/rust/Makefile index b361bfedfdf0..2a5428a5503d 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 $(subst rustdoc-,,$@) \ @@ -231,6 +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 = $(pin_init-flags) \ + --extern pin_init_internal=$(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 @@ -520,23 +524,23 @@ 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 +549,20 @@ 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