rust: alloc: remove the `borrow` module (`ToOwned`, `Cow`)

The `Cow` type [1] requires that its generic parameter type implements
the `ToOwned` trait [2], which provides a method to create owned data
from borrowed data, usually by cloning.

However, it is infallible, and thus in most cases it is not useful for
the kernel. [3]

Therefore, introduce `cfg(no_borrow)` to remove the `borrow` module
(which contains `ToOwned` and `Cow`) from `alloc`.

Link: https://doc.rust-lang.org/alloc/borrow/enum.Cow.html [1]
Link: https://doc.rust-lang.org/alloc/borrow/trait.ToOwned.html [2]
Link: https://lore.kernel.org/rust-for-linux/20221204103153.117675b1@GaryWorkstation/ [3]
Cc: Gary Guo <gary@garyguo.net>
Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Finn Behrens <fin@nyantec.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/rust/Makefile b/rust/Makefile
index ff70c4c916f8..d7621fad5b56 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -50,6 +50,7 @@ core-cfgs = \
     --cfg no_fp_fmt_parse
 
 alloc-cfgs = \
+    --cfg no_borrow \
     --cfg no_fmt \
     --cfg no_global_oom_handling \
     --cfg no_macros \