From: "Gary Guo" <gary@garyguo.net>
To: "Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com>,
<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>
Subject: Re: [PATCH V8 1/4] rust: Fix "multiple candidates for rmeta dependency core" error
Date: Fri, 03 Apr 2026 16:09:04 +0100 [thread overview]
Message-ID: <DHJM52JEY7D3.MGKXTZB319ZL@garyguo.net> (raw)
In-Reply-To: <20260403145308.1042622-2-mkchauras@gmail.com>
On Fri Apr 3, 2026 at 3:53 PM BST, Mukesh Kumar Chaurasiya (IBM) wrote:
> 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
> 3. Test builds use `-L$(objtree)/rust/test` for their dependencies
> 4. Target builds continue to use `-L$(objtree)/rust` as before
>
> 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$(objtree)/rust`, which
> would reintroduce the conflict.
>
> The rust/host/ directory is added to clean-files to ensure it's removed
> during `make clean`.
>
> Changes:
> - Add clean-files := host/ to clean the generated directory
> - Change host library targets from lib*.rlib to host/lib*.rlib
> - Update cmd_rustc_procmacrolibrary to create host/ directory
> - Update cmd_rustc_procmacro to use -L$(objtree)/rust/host
> - Update cmd_rustdoc to use -L$(objtree)/rust/host for host builds
> - Update cmd_rustc_test_library to use -L$(objtree)/rust/test
> - Update rustdoc-pin_init to use explicit --extern paths for proc macros
This should go after the tags and ---, so it's not part of the commit message.
>
> 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 | 63 ++++++++++++++++++++++++++++++++-------------------
> 1 file changed, 40 insertions(+), 23 deletions(-)
>
> diff --git a/rust/Makefile b/rust/Makefile
> index 9801af2e1e02..762bddc868e4 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 +196,16 @@ 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 = $(quote-flags) \
> + --extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib)
Why not just add `-L$(objtree)/$(obj)/host`? If we go with the host directory
approach we shouldn't need to explicitly specify the path?
Best,
Gary
> 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 = $(syn-flags) \
> + --extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib) \
> + --extern quote=$(abspath $(objtree)/$(obj)/host/libquote.rlib)
> rustdoc-syn: $(src)/syn/lib.rs rustdoc-clean rustdoc-quote FORCE
> +$(call if_changed,rustdoc)
>
> @@ -236,7 +242,10 @@ 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: $(src)/pin-init/src/lib.rs rustdoc-pin_init_internal \
> rustdoc-macros FORCE
> @@ -369,7 +378,9 @@ rusttest: rusttest-macros
>
> rusttest-macros: private rustc_target_flags = --extern proc_macro \
> --extern macros --extern kernel --extern pin_init \
> - --extern proc_macro2 --extern quote --extern syn
> + --extern proc_macro2=$(abspath $(objtree)/$(obj)/test/libproc_macro2.rlib) \
> + --extern quote=$(abspath $(objtree)/$(obj)/test/libquote.rlib) \
> + --extern syn=$(abspath $(objtree)/$(obj)/test/libsyn.rlib)
> rusttest-macros: private rustdoc_test_target_flags = --crate-type proc-macro
> rusttest-macros: $(src)/macros/lib.rs \
> rusttestlib-macros rusttestlib-kernel rusttestlib-pin_init FORCE
> @@ -525,48 +536,54 @@ $(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 = $(quote-flags) \
> + --extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib)
> +$(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 = $(syn-flags) \
> + --extern proc_macro2=$(abspath $(objtree)/$(obj)/host/libproc_macro2.rlib) \
> + --extern quote=$(abspath $(objtree)/$(obj)/host/libquote.rlib)
> +$(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
next prev parent reply other threads:[~2026-04-03 15:09 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-03 14:53 [PATCH V8 0/4] Rust support for powerpc Mukesh Kumar Chaurasiya (IBM)
2026-04-03 14:53 ` [PATCH V8 1/4] rust: Fix "multiple candidates for rmeta dependency core" error Mukesh Kumar Chaurasiya (IBM)
2026-04-03 15:09 ` Gary Guo [this message]
2026-04-04 3:39 ` Mukesh Kumar Chaurasiya
2026-04-04 20:28 ` Miguel Ojeda
2026-04-03 14:53 ` [PATCH V8 2/4] powerpc/jump_label: adjust inline asm to be consistent Mukesh Kumar Chaurasiya (IBM)
2026-04-03 14:53 ` [PATCH V8 3/4] rust: Add PowerPC support Mukesh Kumar Chaurasiya (IBM)
2026-04-03 14:53 ` [PATCH V8 4/4] powerpc: Enable Rust for ppc64le Mukesh Kumar Chaurasiya (IBM)
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