linux-kernel - [PATCH 1/4] rust: Import upstream `alloc::vec::set_len_on

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Date:   Fri, 24 Feb 2023 18:11:47 +0900
From:   Asahi Lina <lina@...hilina.net>
To:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>
Cc:     linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
        asahi@...ts.linux.dev, Asahi Lina <lina@...hilina.net>
Subject: [PATCH 1/4] rust: Import upstream `alloc::vec::set_len_on_drop`
 module

This is a subset of the Rust standard library `alloc` crate,
version 1.66.0, licensed under "Apache-2.0 OR MIT", from:

    https://github.com/rust-lang/rust/tree/1.66.0/library/alloc/src

The file is copied as-is, with no modifications whatsoever
(not even adding the SPDX identifiers).

For copyright details, please see:

    https://github.com/rust-lang/rust/blob/1.66.0/COPYRIGHT

Signed-off-by: Asahi Lina <lina@...hilina.net>
---
 rust/alloc/vec/set_len_on_drop.rs | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/rust/alloc/vec/set_len_on_drop.rs b/rust/alloc/vec/set_len_on_drop.rs
new file mode 100644
index 000000000000..8b66bc812129
--- /dev/null
+++ b/rust/alloc/vec/set_len_on_drop.rs
@@ -0,0 +1,28 @@
+// Set the length of the vec when the `SetLenOnDrop` value goes out of scope.
+//
+// The idea is: The length field in SetLenOnDrop is a local variable
+// that the optimizer will see does not alias with any stores through the Vec's data
+// pointer. This is a workaround for alias analysis issue #32155
+pub(super) struct SetLenOnDrop<'a> {
+    len: &'a mut usize,
+    local_len: usize,
+}
+
+impl<'a> SetLenOnDrop<'a> {
+    #[inline]
+    pub(super) fn new(len: &'a mut usize) -> Self {
+        SetLenOnDrop { local_len: *len, len }
+    }
+
+    #[inline]
+    pub(super) fn increment_len(&mut self, increment: usize) {
+        self.local_len += increment;
+    }
+}
+
+impl Drop for SetLenOnDrop<'_> {
+    #[inline]
+    fn drop(&mut self) {
+        *self.len = self.local_len;
+    }
+}

-- 
2.35.1