[PATCH v4] rust: Respect HOSTCC when linking for host

[Matthew Maurer <mmaurer@google.com>]

Currently, rustc defaults to invoking `cc`, even if `HOSTCC` is defined,
resulting in build failures in hermetic environments where `cc` does not
exist. This includes both hostprogs and proc-macros.

Since we are setting the linker to `HOSTCC`, we set the linker flavor to
`gcc` explicitly. The linker-flavor selects both which linker to search
for if the linker is unset, and which kind of linker flags to pass.
Without this flag, `rustc` would attempt to determine which flags to
pass based on the name of the binary passed as `HOSTCC`. `gcc` is the
name of the linker-flavor used by `rustc` for all C compilers, including
both `gcc` and `clang`.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---

Edited to use escsq in both Makefiles, as per Masahiro Yamada's
suggestion.

 rust/Makefile         | 2 ++
 scripts/Makefile.host | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/rust/Makefile b/rust/Makefile
index 87958e864be0..2ddd821d9435 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -383,6 +383,8 @@ $(obj)/exports_kernel_generated.h: $(obj)/kernel.o FORCE
 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))' \
 		--emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \
 		--crate-type proc-macro \
 		--crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index 8f7f842b54f9..08d83d9db31a 100644
--- a/scripts/Makefile.host
+++ b/scripts/Makefile.host
@@ -91,6 +91,8 @@ hostcxx_flags  = -Wp,-MMD,$(depfile) \
 # current working directory, which may be not accessible in the out-of-tree
 # modules case.
 hostrust_flags = --out-dir $(dir $@) --emit=dep-info=$(depfile) \
+		 -Clinker-flavor=gcc -Clinker=$(HOSTCC) \
+		 -Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
                  $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
                  $(HOSTRUSTFLAGS_$(target-stem))
 
-- 
2.42.0.609.gbb76f46606-goog