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rust: arc: add explicit drop() around Box::from_raw()
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`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: rust-lang/rust#104253 [1]
Link: rust-lang/rust#112529 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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ojeda authored and intel-lab-lkp committed Aug 23, 2023
1 parent 4af84c6 commit 32348c3
Showing 1 changed file with 1 addition and 1 deletion.
2 changes: 1 addition & 1 deletion rust/kernel/sync/arc.rs
Original file line number Diff line number Diff line change
Expand Up @@ -299,7 +299,7 @@ impl<T: ?Sized> Drop for Arc<T> {
// 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())) };
}
}
}
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