`Allocator`s, such as `Kmalloc`, will be used by e.g. `Box` and `Vec` in
subsequent patches, and hence this dependency propagates throughout the
whole kernel.

Add the `allocator_test` module that provides an empty implementation
for all `Allocator`s in the kernel, such that we don't break the
`rusttest` make target in subsequent patches.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/alloc.rs                |  9 +++++++--
 rust/kernel/alloc/allocator_test.rs | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)
 create mode 100644 rust/kernel/alloc/allocator_test.rs

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 535ed1527034..da025faf07f8 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -2,12 +2,17 @@
 
 //! Extensions to the [`alloc`] crate.
 
-#[cfg(not(test))]
-#[cfg(not(testlib))]
+#[cfg(not(any(test, testlib)))]
 pub mod allocator;
 pub mod box_ext;
 pub mod vec_ext;
 
+#[cfg(any(test, testlib))]
+pub mod allocator_test;
+
+#[cfg(any(test, testlib))]
+pub use self::allocator_test as allocator;
+
 /// Indicates an allocation error.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct AllocError;
diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
new file mode 100644
index 000000000000..4785efc474a7
--- /dev/null
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#![allow(missing_docs)]
+
+use super::{AllocError, Allocator, Flags};
+use core::alloc::Layout;
+use core::ptr::NonNull;
+
+pub struct Kmalloc;
+
+unsafe impl Allocator for Kmalloc {
+    unsafe fn realloc(
+        _ptr: Option<NonNull<u8>>,
+        _layout: Layout,
+        _flags: Flags,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        panic!();
+    }
+}
-- 
2.46.0