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.

Link: rust-lang/rust#123431 [1]
Link: rust-lang/rust#112711 [2]
Link: rust-lang/rust#119651 [3]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Makefile

@@ -1830,6 +1830,9 @@ 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
 
@@ -65,6 +67,16 @@ core-cfgs = \
 
 core-edition := $(if $(call rustc-min-version,108700),2024,2021)
 
+proc_macro2-cfgs := \
+    --cfg 'feature="proc-macro"' \
+    --cfg wrap_proc_macro
+
+# `feature(...)`s stable since Rust 1.79.0.
+proc_macro2-flags := \
+    --cap-lints=allow \
+    -Zcrate-attr='feature(proc_macro_byte_character,proc_macro_c_str_literals)' \
+    $(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)
@@ -114,10 +126,15 @@ 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
@@ -174,6 +191,10 @@ 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
@@ -420,6 +441,11 @@ quiet_cmd_rustc_hostlibrary = $(RUSTC_OR_CLIPPY_QUIET) H $@
 	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_hostlibrary)
+
 quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
       cmd_rustc_procmacro = \
 	$(RUSTC_OR_CLIPPY) $(rust_common_flags) $(rustc_target_flags) \
@@ -431,7 +457,7 @@ 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