Re: [PATCH 1/2] rust: arc: add explicit `drop()` around `Box::from_raw()`

From: Andreas Hindborg
Date: Thu Aug 24 2023 - 02:42:57 EST



Miguel Ojeda <ojeda@xxxxxxxxxx> writes:

> `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://protect2.fireeye.com/v1/url?k=9e9028e1-ff1b3dd7-9e91a3ae-74fe485cbff1-f2bfda5f7702c68e&q=1&e=66bd90a6-86db-48e7-8538-c118c3f45baa&u=https%3A%2F%2Fgithub.com%2Frust-lang%2Frust%2Fissues%2F104253 [1]
> Link: https://protect2.fireeye.com/v1/url?k=89300613-e8bb1325-89318d5c-74fe485cbff1-fcf76e46b008b25a&q=1&e=66bd90a6-86db-48e7-8538-c118c3f45baa&u=https%3A%2F%2Fgithub.com%2Frust-lang%2Frust%2Fpull%2F112529 [2]
> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>

> 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())) };
> }
> }
> }