rust: kbuild: expand rusttest target for macros

Previously, the rusttest target for the macros crate did not specify
the dependencies necessary to run the rustdoc tests. These tests rely on
the kernel crate, so add the dependencies.

Signed-off-by: Ethan D. Twardy <ethan.twardy@gmail.com>
Link: https://github.com/Rust-for-Linux/linux/issues/1076
Link: https://lore.kernel.org/r/20240704145607.17732-2-ethan.twardy@gmail.com
[ Rebased (`alloc` is gone nowadays, sysroot handling is simpler) and
  simplified (reused `rustdoc_test` rule instead of adding a new one,
  no need for `rustdoc-compiler_builtins`, removed unneeded `macros`
  explicit path). Made `vtable` example fail (avoiding to increase
  the complexity in the `rusttest` target). Removed unstable
  `-Zproc-macro-backtrace` option. Reworded accordingly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -129,6 +129,14 @@ rusttestlib-macros: private rustc_test_library_proc = yes
 rusttestlib-macros: $(src)/macros/lib.rs FORCE
 	+$(call if_changed,rustc_test_library)
 
+rusttestlib-kernel: private rustc_target_flags = \
+    --extern build_error --extern macros \
+    --extern bindings --extern uapi
+rusttestlib-kernel: $(src)/kernel/lib.rs \
+    rusttestlib-bindings rusttestlib-uapi rusttestlib-build_error \
+    $(obj)/libmacros.so $(obj)/bindings.o FORCE
+	+$(call if_changed,rustc_test_library)
+
 rusttestlib-bindings: $(src)/bindings/lib.rs FORCE
 	+$(call if_changed,rustc_test_library)
 
@@ -181,19 +189,20 @@ quiet_cmd_rustc_test = RUSTC T  $<
 
 rusttest: rusttest-macros rusttest-kernel
 
-rusttest-macros: private rustc_target_flags = --extern proc_macro
+rusttest-macros: private rustc_target_flags = --extern proc_macro \
+	--extern macros --extern kernel
 rusttest-macros: private rustdoc_test_target_flags = --crate-type proc-macro
-rusttest-macros: $(src)/macros/lib.rs FORCE
+rusttest-macros: $(src)/macros/lib.rs \
+    rusttestlib-macros rusttestlib-kernel FORCE
 	+$(call if_changed,rustc_test)
 	+$(call if_changed,rustdoc_test)
 
 rusttest-kernel: private rustc_target_flags = \
     --extern build_error --extern macros --extern bindings --extern uapi
-rusttest-kernel: $(src)/kernel/lib.rs \
+rusttest-kernel: $(src)/kernel/lib.rs rusttestlib-kernel \
     rusttestlib-build_error rusttestlib-macros rusttestlib-bindings \
     rusttestlib-uapi FORCE
 	+$(call if_changed,rustc_test)
-	+$(call if_changed,rustc_test_library)
 
 ifdef CONFIG_CC_IS_CLANG
 bindgen_c_flags = $(c_flags)

rust/macros/lib.rs

@@ -132,7 +132,7 @@ pub fn module(ts: TokenStream) -> TokenStream {
 /// calls to this function at compile time:
 ///
 /// ```compile_fail
-/// # use kernel::error::VTABLE_DEFAULT_ERROR;
+/// # // Intentionally missing `use`s to simplify `rusttest`.
 /// kernel::build_error(VTABLE_DEFAULT_ERROR)
 /// ```
 ///