[PATCH V9 1/4] rust: Fix "multiple candidates for rmeta dependency core" error

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

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@gmail.com>
---
 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