[PATCH V16 1/7] rust: Fix "multiple candidates for rmeta dependency core" error

["Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com>]

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 b9e9f512cec3..6d7ed7a1ff18 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
@@ -522,23 +526,23 @@ 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 $@
@@ -547,20 +551,20 @@ 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
-- 
2.54.0