[PATCH v2] rust: add PROCMACROLDFLAGS

[HONG Yifan <elsk@google.com>]

These are additional flags to be passed when linking proc macros for the
Rust toolchain. If unset, it defaults to $(HOSTLDFLAGS).

This is needed because the list of flags to link hostprogs is not
necessarily the same as the list of flags used to link libmacros.so.
When we build proc macros, we need the latter, not the
former. To distinguish between the two, introduce this new variable
to stand out from HOSTLDFLAGS used to link other host progs.

Signed-off-by: HONG Yifan <elsk@google.com>
---
v1 -> v2: Updated text in kbuild.rst as suggested by aliceryhl@google.com.

 Documentation/kbuild/kbuild.rst | 7 +++++++
 Makefile                        | 1 +
 rust/Makefile                   | 2 +-
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index 1796b3eba37b..5ba8c4afd44b 100644
--- a/Documentation/kbuild/kbuild.rst
+++ b/Documentation/kbuild/kbuild.rst
@@ -91,6 +91,13 @@ HOSTRUSTFLAGS
 -------------
 Additional flags to be passed to $(HOSTRUSTC) when building host programs.

+PROCMACROLDFLAGS
+-------------
+Additional flags to be passed when linking Rust proc macros. Since proc macros
+are loaded by rustc at build time, they must be linked in a way that is
+compatible with the rustc toolchain being used. If unset, it defaults to
+$(HOSTLDFLAGS).
+
 HOSTLDFLAGS
 -----------
 Additional flags to be passed when linking host programs.
diff --git a/Makefile b/Makefile
index b77ac70f8be4..89cdf0eca7de 100644
--- a/Makefile
+++ b/Makefile
@@ -469,6 +469,7 @@ KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS) \
 		       -I $(srctree)/scripts/include
 KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
 			-Zallow-features= $(HOSTRUSTFLAGS)
+KBUILD_PROCMACROLDFLAGS := $(or $(PROCMACROLDFLAGS),$(HOSTLDFLAGS))
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
 KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)

diff --git a/rust/Makefile b/rust/Makefile
index 3678e79317f1..95ceaba35975 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -322,7 +322,7 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
       cmd_rustc_procmacro = \
 	$(RUSTC_OR_CLIPPY) $(rust_common_flags) \
 		-Clinker-flavor=gcc -Clinker=$(HOSTCC) \
-		-Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
+		-Clink-args='$(call escsq,$(KBUILD_PROCMACROLDFLAGS))' \
 		--emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \
 		--crate-type proc-macro \
 		--crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
--
2.47.0.rc1.288.g06298d1525-goog