[PATCH 02/10] rust: alloc: introduce the `VecExt` trait

[Wedson Almeida Filho <wedsonaf@gmail.com>]

From: Wedson Almeida Filho <walmeida@microsoft.com>

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.

Suggested-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
---
 rust/kernel/alloc.rs        |  1 +
 rust/kernel/alloc/vecext.rs | 52 +++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs          |  1 +
 rust/kernel/prelude.rs      |  2 ++
 4 files changed, 56 insertions(+)
 create mode 100644 rust/kernel/alloc/vecext.rs

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index ccd4149932c3..8ad57a2e693e 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -5,3 +5,4 @@
 #[cfg(not(test))]
 #[cfg(not(testlib))]
 mod allocator;
+pub mod vecext;
diff --git a/rust/kernel/alloc/vecext.rs b/rust/kernel/alloc/vecext.rs
new file mode 100644
index 000000000000..59e92bab534e
--- /dev/null
+++ b/rust/kernel/alloc/vecext.rs
@@ -0,0 +1,52 @@
+// 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,
+    {
+        let extra_cap = self.capacity() - self.len();
+        if extra_cap > 0 {
+            self.try_reserve(extra_cap)?;
+        }
+
+        for item in other {
+            self.try_push(item.clone())?;
+        }
+
+        Ok(())
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 51f30e55bd00..7f2841a18d05 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -19,6 +19,7 @@
 #![feature(offset_of)]
 #![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.
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index ae21600970b3..a0177f195dec 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -14,6 +14,8 @@
 #[doc(no_inline)]
 pub use core::pin::Pin;
 
+pub use crate::alloc::vecext::VecExt;
+
 #[doc(no_inline)]
 pub use alloc::{boxed::Box, vec::Vec};
 
-- 
2.34.1