Re: [PATCH 5/5] rust: error: Add from_kernel_result!() macro

[Miguel Ojeda]

On Sun, Feb 26, 2023 at 7:17 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> My preference to function instead of macro here is because I want to
> avoid the extra level of abstraction and make things explict, so that
> users and reviewers can understand the API behavior solely based on
> Rust's types, functions and closures: they are simpler than macros, at
> least to me ;-)

There is one extra problem with the macro: `rustfmt` does not format
the contents if called with braces (as we currently do).

So when I was cleaning some things up for v8, one of the things I did
was run manually `rustfmt` on the blocks by removing the macro
invocation, in commit 77a1a8c952e1 ("rust: kernel: apply `rustfmt` to
`from_kernel_result!` blocks").

Having said that, it does format it when called with parenthesis
wrapping the block, so we could do that if we end up with the macro.

> First, I think the macro version here is just a poor-man's try block, in
> other words, I'd expect explicit use of try blocks intead of
> `from_kernel_result` when the feature is ready. If that's the case, we
> need to change the use sites anyway.

Yeah, if we eventually get a better language feature that fits well,
then we should use it.

> Do both implementation share the same behavior?

Yeah, a `return` will return to the outer caller in the case of a
`try` block, while it returns to the closure (macro) in the other
case. Or do you mean something else?

In that case, I think one could use use a labeled block to `break`
out, not sure if `try` blocks will allow an easier way.

We have a case of such a `return` within the closure at `rust/rust` in
`file.rs`:

    from_kernel_result! {
        let off = match whence as u32 {
            bindings::SEEK_SET => SeekFrom::Start(offset.try_into()?),
            bindings::SEEK_CUR => SeekFrom::Current(offset),
            bindings::SEEK_END => SeekFrom::End(offset),
            _ => return Err(EINVAL),
        };
        ...
        Ok(off as bindings::loff_t)
    }

Cheers,
Miguel