rust: proc-macro2: enable support in kbuild

With all the new files in place and ready from the new crate, enable
the support for it in the build system.

`proc_macro_byte_character` and `proc_macro_c_str_literals` were
stabilized in Rust 1.79.0 [1] and were implemented earlier than our
minimum Rust version (1.78) [2][3]. Thus just enable them instead of using
the `cfg` that `proc-macro2` uses to emulate them in older compilers.

In addition, skip formatting for this vendored crate and take the chance
to add a comment mentioning this.

Link: https://github.com/rust-lang/rust/pull/123431 [1]
Link: https://github.com/rust-lang/rust/pull/112711 [2]
Link: https://github.com/rust-lang/rust/pull/119651 [3]
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-11-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -1826,10 +1826,15 @@ rusttest: prepare
 	$(Q)$(MAKE) $(build)=rust $@
 
 # Formatting targets
+#
+# Generated files as well as vendored crates are skipped.
 PHONY += rustfmt rustfmtcheck
 
 rustfmt:
 	$(Q)find $(srctree) $(RCS_FIND_IGNORE) \
+		\( \
+			-path $(srctree)/rust/proc-macro2 \
+		\) -prune -o \
 		-type f -a -name '*.rs' -a ! -name '*generated*' -print \
 		| xargs $(RUSTFMT) $(rustfmt_flags)
 

rust/Makefile

@@ -27,6 +27,8 @@ endif
 
 obj-$(CONFIG_RUST) += exports.o
 
+always-$(CONFIG_RUST) += libproc_macro2.rlib
+
 always-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated.rs
 always-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated_kunit.c
 
@@ -76,6 +78,17 @@ core-flags := \
     --edition=$(core-edition) \
     $(call cfgs-to-flags,$(core-cfgs))
 
+proc_macro2-cfgs := \
+    feature="proc-macro" \
+    wrap_proc_macro \
+    $(if $(call rustc-min-version,108800),proc_macro_span_file proc_macro_span_location)
+
+# Stable since Rust 1.79.0: `feature(proc_macro_byte_character,proc_macro_c_str_literals)`.
+proc_macro2-flags := \
+    --cap-lints=allow \
+    -Zcrate-attr='feature(proc_macro_byte_character,proc_macro_c_str_literals)' \
+    $(call cfgs-to-flags,$(proc_macro2-cfgs))
+
 # `rustdoc` did not save the target modifiers, thus workaround for
 # the time being (https://github.com/rust-lang/rust/issues/144521).
 rustdoc_modifiers_workaround := $(if $(call rustc-min-version,108800),-Cunsafe-allow-abi-mismatch=fixed-x18)
@@ -125,10 +138,15 @@ rustdoc: rustdoc-core rustdoc-macros rustdoc-compiler_builtins \
 	$(Q)for f in $(rustdoc_output)/static.files/rustdoc-*.css; do \
 		echo ".logo-container > img { object-fit: contain; }" >> $$f; done
 
+rustdoc-proc_macro2: private rustdoc_host = yes
+rustdoc-proc_macro2: private rustc_target_flags = $(proc_macro2-flags)
+rustdoc-proc_macro2: $(src)/proc-macro2/lib.rs rustdoc-clean FORCE
+	+$(call if_changed,rustdoc)
+
 rustdoc-macros: private rustdoc_host = yes
 rustdoc-macros: private rustc_target_flags = --crate-type proc-macro \
     --extern proc_macro
-rustdoc-macros: $(src)/macros/lib.rs rustdoc-clean FORCE
+rustdoc-macros: $(src)/macros/lib.rs rustdoc-clean rustdoc-proc_macro2 FORCE
 	+$(call if_changed,rustdoc)
 
 # Starting with Rust 1.82.0, skipping `-Wrustdoc::unescaped_backticks` should
@@ -185,6 +203,10 @@ rusttestlib-build_error: $(src)/build_error.rs FORCE
 rusttestlib-ffi: $(src)/ffi.rs FORCE
 	+$(call if_changed,rustc_test_library)
 
+rusttestlib-proc_macro2: private rustc_target_flags = $(proc_macro2-flags)
+rusttestlib-proc_macro2: $(src)/proc-macro2/lib.rs FORCE
+	+$(call if_changed,rustc_test_library)
+
 rusttestlib-macros: private rustc_target_flags = --extern proc_macro
 rusttestlib-macros: private rustc_test_library_proc = yes
 rusttestlib-macros: $(src)/macros/lib.rs FORCE
@@ -431,6 +453,11 @@ quiet_cmd_rustc_procmacrolibrary = $(RUSTC_OR_CLIPPY_QUIET) PL $@
 	mv $(objtree)/$(obj)/$(patsubst lib%.rlib,%,$(notdir $@)).d $(depfile); \
 	sed -i '/^\#/d' $(depfile)
 
+$(obj)/libproc_macro2.rlib: private skip_clippy = 1
+$(obj)/libproc_macro2.rlib: private rustc_target_flags = $(proc_macro2-flags)
+$(obj)/libproc_macro2.rlib: $(src)/proc-macro2/lib.rs FORCE
+	+$(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) \
@@ -442,7 +469,7 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
 		@$(objtree)/include/generated/rustc_cfg $<
 
 # Procedural macros can only be used with the `rustc` that compiled it.
-$(obj)/$(libmacros_name): $(src)/macros/lib.rs FORCE
+$(obj)/$(libmacros_name): $(src)/macros/lib.rs $(obj)/libproc_macro2.rlib FORCE
 	+$(call if_changed_dep,rustc_procmacro)
 
 $(obj)/$(libpin_init_internal_name): private rustc_target_flags = --cfg kernel
@@ -465,6 +492,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 rust-analyzer:
 	$(Q)MAKEFLAGS= $(srctree)/scripts/generate_rust_analyzer.py \
 		--cfgs='core=$(core-cfgs)' $(core-edition) \
+		--cfgs='proc_macro2=$(proc_macro2-cfgs)' \
 		$(realpath $(srctree)) $(realpath $(objtree)) \
 		$(rustc_sysroot) $(RUST_LIB_SRC) $(if $(KBUILD_EXTMOD),$(srcroot)) \
 		> rust-project.json

scripts/generate_rust_analyzer.py

@@ -86,6 +86,13 @@ def generate_crates(srctree, objtree, sysroot_src, external_src, cfgs, core_edit
         [],
     )
 
+    append_crate(
+        "proc_macro2",
+        srctree / "rust" / "proc-macro2" / "lib.rs",
+        ["core", "alloc", "std", "proc_macro"],
+        cfg=crates_cfgs["proc_macro2"],
+    )
+
     append_crate(
         "macros",
         srctree / "rust" / "macros" / "lib.rs",