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
Co-developed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
V5 -> V6: Removed setting `no-clean-files`. Rewrote description.
e4 -> V5: Added missing `shell` in rust/Makefile.
V3 -> V4: Added motivation. Added missing Signed-off-by.
V2 -> V3: Added .strip() to rustc output to remove errant newline.
V1 -> V2: De-duplicated and sorted imports. Changed Signed-off-by to
Co-developed-by.
---
 .gitignore                        |  1 +
 Makefile                          |  2 +-
 rust/Makefile                     | 20 +++++++++++---------
 scripts/generate_rust_analyzer.py | 15 +++++++++++----
 4 files changed, 24 insertions(+), 14 deletions(-)

diff --git a/.gitignore b/.gitignore
index a61e4778d011cf706e6784818a1357f392f3a669..088696a6a46a12fdb77eb9ccab5b6b7b11ef4707 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,6 +22,7 @@
 *.dtb.S
 *.dtbo.S
 *.dwo
+*.dylib
 *.elf
 *.gcno
 *.gcda
diff --git a/Makefile b/Makefile
index a9e723cb05961877d5e6b50920dcabc78cf4988f..470e6f20c513bf0f90a42c586aa92dd7a8b16fb0 100644
--- a/Makefile
+++ b/Makefile
@@ -1513,7 +1513,7 @@ 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
 #
diff --git a/rust/Makefile b/rust/Makefile
index 3678e79317f12d7116ad0c2ac1ca416ee5b969aa..0d882103375aa45be78e22f59c64739c8722fd98 100644
--- a/rust/Makefile
+++ b/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 \
@@ -36,9 +33,14 @@ always-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated_kunit.c
 obj-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated.o
 obj-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated_kunit.o
 
-# 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 $(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)
@@ -104,10 +106,10 @@ rustdoc-compiler_builtins: $(src)/compiler_builtins.rs rustdoc-core FORCE
 	+$(call if_changed,rustdoc)
 
 rustdoc-kernel: private rustc_target_flags = \
-    --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-macros \
-    rustdoc-compiler_builtins $(obj)/libmacros.so \
+    rustdoc-compiler_builtins $(obj)/$(libmacros_name) \
     $(obj)/bindings.o FORCE
 	+$(call if_changed,rustdoc)
 
@@ -325,10 +327,10 @@ 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 $@
@@ -401,7 +403,7 @@ $(obj)/uapi.o: $(src)/uapi/lib.rs \
 $(obj)/kernel.o: private rustc_target_flags = \
     --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)
 
 endif # CONFIG_RUST
diff --git a/scripts/generate_rust_analyzer.py b/scripts/generate_rust_analyzer.py
index 09e1d166d8d236fcf8c2b2375624b243ebf6e7f7..aa8ea1a4dbe5f9037c8c231d87ddc8d95c297f12 100755
--- a/scripts/generate_rust_analyzer.py
+++ b/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",

---
base-commit: 6ce162a002657910104c7a07fb50017681bc476c
change-id: 20241016-b4-dylib-host-macos-16757c350b49

Best regards,
-- 
Tamir Duberstein <tamird@gmail.com>