linux-kernel - [PATCH 1/2] rust: arc: add explicit `drop()` around `Box::from

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Message-ID: <20230823160244.188033-2-ojeda@kernel.org>
Date:   Wed, 23 Aug 2023 18:02:42 +0200
From:   Miguel Ojeda <ojeda@...nel.org>
To:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>
Cc:     Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev
Subject: [PATCH 1/2] rust: arc: add explicit `drop()` around `Box::from_raw()`

`Box::from_raw()` is `#[must_use]`, which means the result cannot
go unused.

In Rust 1.71.0, this was not detected because the block expression
swallows the diagnostic [1]:

    unsafe { Box::from_raw(self.ptr.as_ptr()) };

It would have been detected, however, if the line had been instead:

    unsafe { Box::from_raw(self.ptr.as_ptr()); }

i.e. the semicolon being inside the `unsafe` block, rather than
outside.

In Rust 1.72.0, the compiler started warning about this [2], so
without this patch we will get:

        error: unused return value of `alloc::boxed::Box::<T>::from_raw` that must be used
        --> rust/kernel/sync/arc.rs:302:22
        |
    302 |             unsafe { Box::from_raw(self.ptr.as_ptr()) };
        |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`
        = note: `-D unused-must-use` implied by `-D warnings`
    help: use `let _ = ...` to ignore the resulting value
        |
    302 |             unsafe { let _ = Box::from_raw(self.ptr.as_ptr()); };
        |                      +++++++                                 +

Thus add an add an explicit `drop()` as the `#[must_use]`'s
annotation suggests (instead of the more general help line).

Link: https://github.com/rust-lang/rust/issues/104253 [1]
Link: https://github.com/rust-lang/rust/pull/112529 [2]
Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
---
 rust/kernel/sync/arc.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 172f563976a9..4c14d540a581 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -299,7 +299,7 @@ fn drop(&mut self) {
             // The count reached zero, we must free the memory.
             //
             // SAFETY: The pointer was initialised from the result of `Box::leak`.
-            unsafe { Box::from_raw(self.ptr.as_ptr()) };
+            unsafe { drop(Box::from_raw(self.ptr.as_ptr())) };
         }
     }
 }
-- 
2.42.0