rust: add build_error crate

The `build_error` crate provides a function `build_error` which
will panic at compile-time if executed in const context and,
by default, will cause a build error if not executed at compile
time and the optimizer does not optimise away the call.

The `CONFIG_RUST_BUILD_ASSERT_ALLOW` kernel option allows to
relax the default build failure and convert it to a runtime
check. If the runtime check fails, `panic!` will be called.

Its functionality will be exposed to users as a couple macros in
the `kernel` crate in the following patch, thus some documentation
here refers to them for simplicity.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -19,6 +19,12 @@ obj-$(CONFIG_RUST) += alloc.o bindings.o kernel.o
 always-$(CONFIG_RUST) += exports_alloc_generated.h exports_bindings_generated.h \
     exports_kernel_generated.h
 
+ifdef CONFIG_RUST_BUILD_ASSERT_ALLOW
+obj-$(CONFIG_RUST) += build_error.o
+else
+always-$(CONFIG_RUST) += build_error.o
+endif
+
 obj-$(CONFIG_RUST) += exports.o
 
 # Avoids running `$(RUSTC)` for the sysroot when it may not be available.
@@ -108,7 +114,7 @@ rustdoc-alloc: $(src)/alloc/lib.rs rustdoc-core rustdoc-compiler_builtins FORCE
 	$(call if_changed,rustdoc)
 
 rustdoc-kernel: private rustc_target_flags = --extern alloc \
-    --extern macros=$(objtree)/$(obj)/libmacros.so \
+    --extern build_error --extern macros=$(objtree)/$(obj)/libmacros.so \
     --extern bindings
 rustdoc-kernel: $(src)/kernel/lib.rs rustdoc-core rustdoc-macros \
     rustdoc-compiler_builtins rustdoc-alloc $(obj)/libmacros.so \
@@ -126,6 +132,9 @@ quiet_cmd_rustc_test_library = RUSTC TL $<
 		-L$(objtree)/$(obj)/test \
 		--crate-name $(subst rusttest-,,$(subst rusttestlib-,,$@)) $<
 
+rusttestlib-build_error: $(src)/build_error.rs rusttest-prepare 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 rusttest-prepare FORCE
@@ -216,9 +225,9 @@ rusttest-macros: $(src)/macros/lib.rs rusttest-prepare FORCE
 	$(call if_changed,rustdoc_test)
 
 rusttest-kernel: private rustc_target_flags = --extern alloc \
-    --extern macros --extern bindings
+    --extern build_error --extern macros --extern bindings
 rusttest-kernel: $(src)/kernel/lib.rs rusttest-prepare \
-    rusttestlib-macros rusttestlib-bindings FORCE
+    rusttestlib-build_error rusttestlib-macros rusttestlib-bindings FORCE
 	$(call if_changed,rustc_test)
 	$(call if_changed,rustc_test_library)
 
@@ -366,6 +375,9 @@ $(obj)/alloc.o: private rustc_target_flags = $(alloc-cfgs)
 $(obj)/alloc.o: $(src)/alloc/lib.rs $(obj)/compiler_builtins.o FORCE
 	$(call if_changed_dep,rustc_library)
 
+$(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
+	$(call if_changed_dep,rustc_library)
+
 $(obj)/bindings.o: $(src)/bindings/lib.rs \
     $(obj)/compiler_builtins.o \
     $(obj)/bindings/bindings_generated.rs \
@@ -373,8 +385,8 @@ $(obj)/bindings.o: $(src)/bindings/lib.rs \
 	$(call if_changed_dep,rustc_library)
 
 $(obj)/kernel.o: private rustc_target_flags = --extern alloc \
-    --extern macros --extern bindings
-$(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/alloc.o \
+    --extern build_error --extern macros --extern bindings
+$(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/alloc.o $(obj)/build_error.o \
     $(obj)/libmacros.so $(obj)/bindings.o FORCE
 	$(call if_changed_dep,rustc_library)
 

rust/build_error.rs

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Build-time error.
+//!
+//! This crate provides a [const function][const-functions] `build_error`, which will panic in
+//! compile-time if executed in [const context][const-context], and will cause a build error
+//! if not executed at compile time and the optimizer does not optimise away the call.
+//!
+//! It is used by `build_assert!` in the kernel crate, allowing checking of
+//! conditions that could be checked statically, but could not be enforced in
+//! Rust yet (e.g. perform some checks in [const functions][const-functions], but those
+//! functions could still be called in the runtime).
+//!
+//! For details on constant evaluation in Rust, please see the [Reference][const-eval].
+//!
+//! [const-eval]: https://doc.rust-lang.org/reference/const_eval.html
+//! [const-functions]: https://doc.rust-lang.org/reference/const_eval.html#const-functions
+//! [const-context]: https://doc.rust-lang.org/reference/const_eval.html#const-context
+
+#![no_std]
+
+/// Panics if executed in [const context][const-context], or triggers a build error if not.
+///
+/// [const-context]: https://doc.rust-lang.org/reference/const_eval.html#const-context
+#[inline(never)]
+#[cold]
+#[export_name = "rust_build_error"]
+#[track_caller]
+pub const fn build_error(msg: &'static str) -> ! {
+    panic!("{}", msg);
+}

rust/exports.c

@@ -19,3 +19,8 @@
 #include "exports_alloc_generated.h"
 #include "exports_bindings_generated.h"
 #include "exports_kernel_generated.h"
+
+// For modules using `rust/build_error.rs`.
+#ifdef CONFIG_RUST_BUILD_ASSERT_ALLOW
+EXPORT_SYMBOL_RUST_GPL(rust_build_error);
+#endif