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Sat, 04 Apr 2026 05:16:33 -0700 (PDT) From: "Mukesh Kumar Chaurasiya (IBM)" To: maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, chleroy@kernel.org, peterz@infradead.org, jpoimboe@kernel.org, jbaron@akamai.com, aliceryhl@google.com, rostedt@goodmis.org, ardb@kernel.org, ojeda@kernel.org, boqun@kernel.org, gary@garyguo.net, bjorn3_gh@protonmail.com, lossin@kernel.org, a.hindborg@kernel.org, tmgross@umich.edu, dakr@kernel.org, nathan@kernel.org, nick.desaulniers+lkml@gmail.com, morbo@google.com, justinstitt@google.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, llvm@lists.linux.dev Cc: "Mukesh Kumar Chaurasiya (IBM)" Subject: [PATCH V9 1/4] rust: Fix "multiple candidates for rmeta dependency core" error Date: Sat, 4 Apr 2026 17:46:07 +0530 Message-ID: <20260404121610.1956528-2-mkchauras@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260404121610.1956528-1-mkchauras@gmail.com> References: <20260404121610.1956528-1-mkchauras@gmail.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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) --- 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