rust: use host dylib naming convention to support macOS

Because the `macros` crate exposes procedural macros, it must be
compiled as a dynamic library (so it can be loaded by the compiler at
compile-time).

Before this change the resulting artifact was always named
`libmacros.so`, which works on hosts where this matches the naming
convention for dynamic libraries. However the proper name on macOS would
be `libmacros.dylib`.

This turns out to matter even when the dependency is passed with a path
(`--extern macros=path/to/libmacros.so` rather than `--extern macros`)
because rustc uses the file name to infer the type of the library (see
link). This is because there's no way to specify both the path to and
the type of the external library via CLI flags. The compiler could
speculatively parse the file to determine its type, but it does not do
so today.

This means that libraries that match neither rustc's naming convention
for static libraries nor the platform's naming convention for dynamic
libraries are *rejected*.

The only solution I've found is to follow the host platform's naming
convention. This patch does that by querying the compiler to determine
the appropriate name for the artifact. This allows the kernel to build
with CONFIG_RUST=y on macOS.

Link: https://github.com/rust-lang/rust/blob/d829780/compiler/rustc_metadata/src/locator.rs#L728-L752
Tested-by: Daniel Gomez <da.gomez@samsung.com>
Co-developed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20241216-b4-dylib-host-macos-v7-1-cfc507681447@gmail.com
[ Added `MAKEFLAGS=`s to avoid jobserver warnings. Removed space.
  Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

.gitignore

@@ -22,6 +22,7 @@
 *.dtb.S
 *.dtbo.S
 *.dwo
+*.dylib
 *.elf
 *.gcno
 *.gcda

Makefile

@@ -1571,7 +1571,7 @@ MRPROPER_FILES += include/config include/generated          \
 		  certs/x509.genkey \
 		  vmlinux-gdb.py \
 		  rpmbuild \
-		  rust/libmacros.so
+		  rust/libmacros.so rust/libmacros.dylib
 
 # clean - Delete most, but leave enough to build external modules
 #

rust/Makefile

@@ -11,9 +11,6 @@ always-$(CONFIG_RUST) += exports_core_generated.h
 obj-$(CONFIG_RUST) += helpers/helpers.o
 CFLAGS_REMOVE_helpers/helpers.o = -Wmissing-prototypes -Wmissing-declarations
 
-always-$(CONFIG_RUST) += libmacros.so
-no-clean-files += libmacros.so
-
 always-$(CONFIG_RUST) += bindings/bindings_generated.rs bindings/bindings_helpers_generated.rs
 obj-$(CONFIG_RUST) += bindings.o kernel.o
 always-$(CONFIG_RUST) += exports_helpers_generated.h \
@@ -38,9 +35,14 @@ obj-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated_kunit.o
 
 always-$(subst y,$(CONFIG_RUST),$(CONFIG_JUMP_LABEL)) += kernel/generated_arch_static_branch_asm.rs
 
-# Avoids running `$(RUSTC)` for the sysroot when it may not be available.
+# Avoids running `$(RUSTC)` when it may not be available.
 ifdef CONFIG_RUST
 
+libmacros_name := $(shell MAKEFLAGS= $(RUSTC) --print file-names --crate-name macros --crate-type proc-macro - </dev/null)
+libmacros_extension := $(patsubst libmacros.%,%,$(libmacros_name))
+
+always-$(CONFIG_RUST) += $(libmacros_name)
+
 # `$(rust_flags)` is passed in case the user added `--sysroot`.
 rustc_sysroot := $(shell MAKEFLAGS= $(RUSTC) $(rust_flags) --print sysroot)
 rustc_host_target := $(shell $(RUSTC) --version --verbose | grep -F 'host: ' | cut -d' ' -f2)
@@ -109,10 +111,10 @@ rustdoc-ffi: $(src)/ffi.rs rustdoc-core FORCE
 	+$(call if_changed,rustdoc)
 
 rustdoc-kernel: private rustc_target_flags = --extern ffi \
-    --extern build_error --extern macros=$(objtree)/$(obj)/libmacros.so \
+    --extern build_error --extern macros \
     --extern bindings --extern uapi
 rustdoc-kernel: $(src)/kernel/lib.rs rustdoc-core rustdoc-ffi rustdoc-macros \
-    rustdoc-compiler_builtins $(obj)/libmacros.so \
+    rustdoc-compiler_builtins $(obj)/$(libmacros_name) \
     $(obj)/bindings.o FORCE
 	+$(call if_changed,rustdoc)
 
@@ -362,10 +364,10 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
 		-Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
 		--emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \
 		--crate-type proc-macro \
-		--crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
+		--crate-name $(patsubst lib%.$(libmacros_extension),%,$(notdir $@)) $<
 
 # Procedural macros can only be used with the `rustc` that compiled it.
-$(obj)/libmacros.so: $(src)/macros/lib.rs FORCE
+$(obj)/$(libmacros_name): $(src)/macros/lib.rs FORCE
 	+$(call if_changed_dep,rustc_procmacro)
 
 quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L $@
@@ -382,7 +384,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 	$(cmd_objtool)
 
 rust-analyzer:
-	$(Q)$(srctree)/scripts/generate_rust_analyzer.py \
+	$(Q)MAKEFLAGS= $(srctree)/scripts/generate_rust_analyzer.py \
 		--cfgs='core=$(core-cfgs)' \
 		$(realpath $(srctree)) $(realpath $(objtree)) \
 		$(rustc_sysroot) $(RUST_LIB_SRC) $(if $(KBUILD_EXTMOD),$(srcroot)) \
@@ -443,7 +445,7 @@ $(obj)/uapi.o: $(src)/uapi/lib.rs \
 $(obj)/kernel.o: private rustc_target_flags = --extern ffi \
     --extern build_error --extern macros --extern bindings --extern uapi
 $(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/build_error.o \
-    $(obj)/libmacros.so $(obj)/bindings.o $(obj)/uapi.o FORCE
+    $(obj)/$(libmacros_name) $(obj)/bindings.o $(obj)/uapi.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
 ifdef CONFIG_JUMP_LABEL

scripts/generate_rust_analyzer.py

@@ -8,6 +8,7 @@ import json
 import logging
 import os
 import pathlib
+import subprocess
 import sys
 
 def args_crates_cfgs(cfgs):
@@ -35,8 +36,7 @@ def generate_crates(srctree, objtree, sysroot_src, external_src, cfgs):
     crates_cfgs = args_crates_cfgs(cfgs)
 
     def append_crate(display_name, root_module, deps, cfg=[], is_workspace_member=True, is_proc_macro=False):
-        crates_indexes[display_name] = len(crates)
-        crates.append({
+        crate = {
             "display_name": display_name,
             "root_module": str(root_module),
             "is_workspace_member": is_workspace_member,
@@ -47,7 +47,15 @@ def generate_crates(srctree, objtree, sysroot_src, external_src, cfgs):
             "env": {
                 "RUST_MODFILE": "This is only for rust-analyzer"
             }
-        })
+        }
+        if is_proc_macro:
+            proc_macro_dylib_name = subprocess.check_output(
+                [os.environ["RUSTC"], "--print", "file-names", "--crate-name", display_name, "--crate-type", "proc-macro", "-"],
+                stdin=subprocess.DEVNULL,
+            ).decode('utf-8').strip()
+            crate["proc_macro_dylib_path"] = f"{objtree}/rust/{proc_macro_dylib_name}"
+        crates_indexes[display_name] = len(crates)
+        crates.append(crate)
 
     # First, the ones in `rust/` since they are a bit special.
     append_crate(
@@ -70,7 +78,6 @@ def generate_crates(srctree, objtree, sysroot_src, external_src, cfgs):
         [],
         is_proc_macro=True,
     )
-    crates[-1]["proc_macro_dylib_path"] = f"{objtree}/rust/libmacros.so"
 
     append_crate(
         "build_error",