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>

lib/Kconfig.debug

@@ -2801,6 +2801,22 @@ config RUST_OVERFLOW_CHECKS
 
 	  If unsure, say Y.
 
+config RUST_BUILD_ASSERT_ALLOW
+	bool "Allow unoptimized build-time assertions"
+	depends on RUST
+	help
+	  Controls how are `build_error!` and `build_assert!` handled during build.
+
+	  If calls to them exist in the binary, it may indicate a violated invariant
+	  or that the optimizer failed to verify the invariant during compilation.
+
+	  This should not happen, thus by default the build is aborted. However,
+	  as an escape hatch, you can choose Y here to ignore them during build
+	  and let the check be carried at runtime (with `panic!` being called if
+	  the check fails).
+
+	  If unsure, say N.
+
 endmenu # "Rust"
 
 source "Documentation/Kconfig"