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>

scripts/generate_rust_analyzer.py

@@ -67,6 +67,12 @@ def generate_crates(srctree, objtree, sysroot_src):
     )
     crates[-1]["proc_macro_dylib_path"] = "rust/libmacros.so"
 
+    append_crate(
+        "build_error",
+        srctree / "rust" / "build_error.rs",
+        ["core", "compiler_builtins"],
+    )
+
     append_crate(
         "bindings",
         srctree / "rust"/ "bindings" / "lib.rs",
@@ -78,7 +84,7 @@ def generate_crates(srctree, objtree, sysroot_src):
     append_crate(
         "kernel",
         srctree / "rust" / "kernel" / "lib.rs",
-        ["core", "alloc", "macros", "bindings"],
+        ["core", "alloc", "macros", "build_error", "bindings"],
         cfg=cfg,
     )
     crates[-1]["source"] = {