rust: alloc: introduce the VecExt trait

Make `try_with_capacity`, `try_push`, and `try_extend_from_slice`
methods available in `Vec` even though it doesn't implement them. It is
implemented with `try_reserve` and `push_within_capacity`.

This is in preparation for switching to the upstream `alloc` crate.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Suggested-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20240328013603.206764-3-wedsonaf@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc.rs

@@ -5,3 +5,4 @@
 #[cfg(not(test))]
 #[cfg(not(testlib))]
 mod allocator;
+pub mod vec_ext;

rust/kernel/alloc/vec_ext.rs

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Extensions to [`Vec`] for fallible allocations.
+
+use alloc::{collections::TryReserveError, vec::Vec};
+use core::result::Result;
+
+/// Extensions to [`Vec`].
+pub trait VecExt<T>: Sized {
+    /// Creates a new [`Vec`] instance with at least the given capacity.
+    fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError>;
+
+    /// Appends an element to the back of the [`Vec`] instance.
+    fn try_push(&mut self, v: T) -> Result<(), TryReserveError>;
+
+    /// Pushes clones of the elements of slice into the [`Vec`] instance.
+    fn try_extend_from_slice(&mut self, other: &[T]) -> Result<(), TryReserveError>
+    where
+        T: Clone;
+}
+
+impl<T> VecExt<T> for Vec<T> {
+    fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError> {
+        let mut v = Vec::new();
+        v.try_reserve(capacity)?;
+        Ok(v)
+    }
+
+    fn try_push(&mut self, v: T) -> Result<(), TryReserveError> {
+        if let Err(retry) = self.push_within_capacity(v) {
+            self.try_reserve(1)?;
+            let _ = self.push_within_capacity(retry);
+        }
+        Ok(())
+    }
+
+    fn try_extend_from_slice(&mut self, other: &[T]) -> Result<(), TryReserveError>
+    where
+        T: Clone,
+    {
+        self.try_reserve(other.len())?;
+        for item in other {
+            self.try_push(item.clone())?;
+        }
+
+        Ok(())
+    }
+}

rust/kernel/lib.rs

@@ -18,6 +18,7 @@
 #![feature(new_uninit)]
 #![feature(receiver_trait)]
 #![feature(unsize)]
+#![feature(vec_push_within_capacity)]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.

rust/kernel/prelude.rs

@@ -14,6 +14,8 @@
 #[doc(no_inline)]
 pub use core::pin::Pin;
 
+pub use crate::alloc::vec_ext::VecExt;
+
 #[doc(no_inline)]
 pub use alloc::{boxed::Box, vec::Vec};